Jacob Bell
New Member
BY ALEXANDER T. SHULGIN, Ph.D.
The marijuana plant Cannabis sativa contains a bewildering
array of organic chemicals. As is true with other botanic
species, there are representatives of almost all chemical
classes present, including mono- and sesquiterpenes, carbohydrates,
aromatics, and a variety of nitrogenous compounds.
Interest in the study of this plant has centered primarily on
the resinous fraction, as it is this material that is invested
with the pharmacological activity that is peculiar to the
plant. This resin is secreted by the female plant as a
protective agent during seed ripening, although it can be
found as a microscopic exudate through the aerial portions of
plants of either sex. The pure resin, hashish or charas, is the
most potent fraction of the plant, and has served as the
source material for most of the chemical studies.
The family of chemicals that has been isolated from this
source has been referred to as the cannabinoid group. It is
unique amongst psychotropic materials from plants in that
there are no alkaloids present. The fraction is totally nitrogen-
free. Rather, the set of compounds can be considered as
analogs of the parent compound cannabinol (I), a fusion
product of terpene and a substituted resorcinol. Beyond the
scope of this present review are such questions as the
distribution of these compounds within the plant, the botanic
variability resulting from geographic distribution, the
diversity of pharmacological action assignable to the several
distinct compounds present, and the various preparations and
customs of administration. This presentation will be limited
to chemical structure and synthesis, with only passing comments
on the topics of biosynthesis and human use.
A brief review of the early analytical and synthetic studies m the area of cannabis chemistry is necessary as a background
for the discussion of recent developments. Cannabin-
01 (I) was the first compound isolated from the resin of
Cannabis sativa as a pure chemical substance. Its synthesis
established the carbon skeleton that is common to the entire
Fig. 1
(Cll, )4CH,
cannabinol (1)
group of cannabinoids. This 21-carbon system can be best
described as an amalgamation of a lo-carbon monoterpene
.and 5amylresorcinol (olivetol). The terpene half is shown to
the left of the dotted line above. In cannabinol, it is
completely aromatic and represents a molecule of cymene. In
all of the remaining cannabinoids isolated from the native
resin it is found in a partially hydrogenated state, usually
with a single double bond remaining. The resorcinol moiety,
olivetol, is the portion that is represented to the right of the
dotted line. It is to be found as an invariable component of
all the constituents of the resin, although as will be described
below, it often appears as the corresponding carboxylic acid,
olivetolic acid. Synthetic variations on the amyl group have
proved to be one of the most rewarding series of studies in
this area of chemistry, and represents most of the structureactivity
investigations that have been pursued. This early
synthetic work will be described only briefly, for it has
involved analogs of tetrahydrocannabinol which display the
terpene double bond in an unnatural position. The most
recent chemical advances represent syntheses of the exact
natural products, and these will be the body of this discussion.
A brief comment is desirable concerning the various modes
of ring numbering that have been employed in this area. Four
separate and distinct methods can be found in the chemical
literature. Each has a virtue over the others, but each carries
with it limitations. These complications arise from the fact
that, in many of the cannabinoids present in the natural
resin, the oxygen-containing ring (the pyran ring in the
cannabinol above) is not present. These materials as isolated
are dihydroxybiphenyls, with no heterocyclic ring.
The first of these numbering systems was introduced by
Todd in England. It considers these substances best referred
to as variously substituted pyrans. Thus the base numbers are
Fig. 2
4'
pyran numbering
assigned to the pyran ring, and the 4,5-bond between the two
remaining rings defines the l-position of each. The first of
these, the "prime" set, is applied to the terpene ring and
proceeds clockwise. The second, or "double prime" set,
refers to the aromatic ring and proceeds counterclockwise.
Thus the carbon atoms common to the pyran ring are
numbered one and two in each case.
A related numbering system is one that employs the
Chemical Abstracts convention. Here these substances are
considered as substituted dibenzopyrans, and are numbered
starting with the first unfused position of the aromatic ring.
The obvious disadvantage of both of these systems is that the
numbering must be totally changed in those isomers in which
the central pyran ring is open. .
To compensate for this latter limitation, a biphenyl numbering
system has come into usage, primarily in Europe.
4
F biphenyl numbering
Here, the various substitution positions are sequentially
numbered from the central carbon bond of biphenyl. The
terpene ring is fundamental, and the aromatic ring commands
the prime numbers. The advantages of this system are
reciprocal to those mentioned above. The open ring compounds
are easily numbered, but there is no general convention
that extends to modifications that include the pyran ring.
Note should be made of the fact that the course of numbers
in the terpene ring is opposite to that of the other systems.
Most broadly used today is a numbering system that
recognizes both the terpene nature and the aromatic nature
of the two different parts of the molecule. Thus the terpene
is numbered in a manner that is conventional for it, i.e., from
the ring carbon that carries the branched methyl group. This
is in turn numbered seven, and the remaining three carbons
6 K
terpene numbering
of the isopropyl group are then numbered sequentially. The
aromatic ring assignments are straightforward. The overwhelming advantage here is that this numbering system is
applicable whether the center ring is open or closed, and
further it can be extended to new compounds that may be
isolated as long as they can be represented as a combination
of a terpene and an aromatic ring. The only exception is in
the instance that the terpene portion is an open chain.
Examples of this are known, and their numbering system will
be mentioned later.
A brief discussion of the early synthetic efforts in this area
is informative, as it provides the only systematic correlation
between chemical structure and biological activity. Early in
these chemical studies, at about the time of World War II,
two compounds were isolated from the red oil fraction of
cannabis. One was an optically active tetrahydrocannabinol
which carried a double bond in the terpene ring. The other
was the open-ring counterpart; it contained two phenolic
groups and two double bonds. It was also optically active,
/\ CH, CH, A'"'-tetrahydrocannabinol A
CH, CH, A'"'-cannabidiol
and was named cannabidiol.
The location of the exocyclic double bond in the latter
compound was readily established, both by its easy conversion
into tetrahydrocannabinol (THC), and by the generation
of formaldehyde on ozonolysis. The endocyclic double bond
proved to be extremely difficult to locate. It was known not
to be in conjugation either with the exocyclic counterpart of
with the aromatic ring. This still left three possibilities, the
A1 , the A', and the A' (6) -THCs.
Although this problem has only recently been solved,
during the period of these initial isolations and characterizations,
synthetic explorations were numerous. As two generally
different synthetic methods were employed, and two
different biological assays as well, it is quite difficult to
interrelate these studies. The first of the tetrahydrocannabinol
syntheses was that of Todd and co-workers in England.
Their scheme consisted in the fusion of a terpene such as
pulegone with olivetol, thus producing the three ring product.
Pharmacological evaluations were made employing
Fig. 8
pulegone
+
other products
rabbit cornea1 areflexia; unfortunately there are no available
correlations between this response and human intoxicative
potency. Two serious complications have appeared in this
approach. The pulegone employed has been shown to be of
uncertain optical purity, thus leading to optically active
products of inconsistent composition. Further, the actual
nature of the condensation leads to structural isomers. This
was due in part to the contamination of pulegone as isolated
from natural sources with isopulegone, and in part to a
sensitivity to the specific nature of the condensation agent.
A more satisfactory scheme was developed by Adams and
his co-workers at Illinois. In this process a completely
synthetic keto-ester was condensed with olivetol, and the
resulting lactone converted in a separate step to the gemdimethyl
product. Again, as with the pulegone synthesis
above, the unnatural A3 isomer of THC was the principle
product. This, and related homologs, were titrated pharmacologically
by dog ataxia assay. They were found to be
qualitatively similar although quantitatively less active than
natural THC isolated from the red oil. This unnatural but
reproducibly available isomer was taken as a reference standard
for an extensive study of structural modifications.
A complete review of these studies would be out of place
in a presentation designed to emphasize recent developments.
Comment should be made, however, on the importance of
the amyl group on the aromatic ring. It was found that, in
the straight chain series, a maximum activity was observed at
the 6-carbon (hexyl) substitution. This material was about
twice as active as the reference amyl compound in the ataxia
analysis, and has undergone extensive clinical study under the
name of Synhexyl or Pyrahexyl. The replacement of the
straight chain with one branched at the alpha- carbon, led to
the alpha-methylhexyl counterpart, with the increase of
potency of a full order of magnitude. The studies that have
resulted in the development of alpha, beta- dimethyl analogs,
have led to the dimethylheptyl-analog, a compound known as
DMHP or Adams' nine-carbon compound. It is yet a full
Adams' 9-carbon compound Pars' nitrogen analog
order of magnitude more potent than the methyl hexyl
material mentioned above, i.e., five hundred times more
potent than the reference A3 -THC. Its activity has been
confirmed in human subjects, and just recently the tedious
task of its separation into the eight possible isomers has been
reported by Aaron. Quite recently a nitrogen analog of this
compound has been prepared by Pars, and appears also to be
biologically active.
In recent years, immense strides have been made in the area of the chemistry of the cannabinoids of C. sativa. The development of sophisticated spectroscopic instruments, particularly
in nuclear magnetic resonance, has settled the
question of the exact isomeric configuration of the elusive
double bond in the terpene ring, and has established the
stereoconfiguration about the 3,4-position. The principle
isomer present in the red oil is A' -tetrahydrocannabinol, II,
in which the 3,4-hydrogens are oriented trans- to one
another. The open-ring counterpart to A' -THC is consequently
A' -3,4-trans-cannabidiol (III). It has been reported
that the A' (6) -isomer of THC (IV) is also present in the
Fig. 11 CH,
(=$&W&f&
CH,CH,
A'-3,4-trans-tetrahydrocannabinol (II)
@(CH,),CH,
A
CH, CH,
A'-3,4-trans-cannabidiol (111)
CH, HO
F-O
CH, CH,
A'c6'-trans-tetrahydrocannabinol (IV)
native resin, but this is uncertain as it could have arisen as an
artifact of isolation.
None of these fine structural assignments could have been
possible, however, without the development of elegant
methods of fractionation and isomer separation concurrently
with the instrument techniques. The procedures of column
and thin layer chromatography have made possible not only
the isolation of characterizable amounts of isomerically pure
materials, but have led to the discovery of a host of additional chemicals that had heretofore been unknown.
An acidic fraction has long been known to be present in
the red oil of C. sat&a. Some ten years ago an acid was
isolated which proved to be, after structural correction for
the now-known location of the endocyclic double bond of
A' -THC, the benzoic acid that corresponds to III, i.e.,
cannabidiolic acid, V. This material corresponds to cannabidiol
both in the double bond location and in the u-ansconfiguration
about the 3,4-bond. The presence of this and
other acidic materials in the resins isolated from marijuana
grown in the colder northern latitudes suggests their roles as
biosynthetic presursors to the more neutral aromatic, active,
fractions. Careful chromatographic separation of this acidic
fraction into individual components has afforded three more
aromatic carboxylic acids. These are cannabigerolic acid (VI)
that upon decarboxylation could yield cannabigerol (v. i. ),
cannabinolic acid (VII) which can give rise to cannabinol (I),
and A1 -3,4-trans-tetrahydrocannabinolic acid (VIII) which
can be converted to, and which may well be argued as being a
normal biosynthetic precursor to, A1 THC. The possible roles
of these acids as biological intermediates which could lead to
the neutral (phenolic) cannabinoids, will be discussed below.
In the chromatographic analysis of the less plentiful
components of the resinous fraction of C. sativa several
additional phenolic components have been isolated and assigned
tentative chemical structures.
A one-ring resorcinol has been separated that, upon spectroscopic
analysis, appeared to be the simple fusion of an
open-chain terpene and olivetol. This material, cannabigerol
(IX) has had its structure proven by synthesis. The fusion of
this terpene with olivetolic acid would then give rise to the
above-mentioned acid, cannabigerolic acid (VI). This type of
open-ring terpene compound presents yet another numbering
system, being determined by the eight-carbon chain attached
to the olivetol nucleus. The numbering by convention starts
at the distal end of the chain, as shown in IX and X.
Two additional phenolic components have recently been
described. Cannabichromene, X, is an open-chain benzopyran
containing a carbon system closely related to the material
cannabigerol but with heterocyclic ring closure. A phenol
named cannabicyclol (XI) has also been isolated which,
lacking any unsaturation whatsoever, has been assigned the
structure shown. These two hypothetical structures lack
support from synthetic studies. These fusions of terpenes and
aromatics have suggested several of the resent synthetic
approaches into this area, and also provide the basis of
biosynthetic paths, to be discussed.
An expected correlary to the establishment of the tools of
structural analysis of C. sativa, was their use in the development
and evaluation of synthetic techniques. It must be
noted that, in this same period of time-the last four years or
so-no less than six separate and mutually confirmatory
syntheses within this family of compounds have appeared in
the chemical literature. Three of these represent modifications
of the olivetol ring providing the basis for the construction
of the terpenacious half of the cannabinoid molecule.
The remaining three syntheses employ the reactivity of the
resorcinol system itself and, in effect, bring a terpene into
reaction with it.
The first of these procedures can be illustrated by the
general reaction between citral and the lithio-derivative of the
dimethyl ether of olivetol. The first description of this
reaction was advanced by Mechoulam and Gaoni. The
coupling product shown undergoes an internal rearrangement
to yield the trans- isomer of the dimethyl ether of cannabidiol.
Demethylation leads to cannabidiol itself, and cyclization
provides a mixture of the A' and the A' (6) -isomers of THC.
The over-all yield in this procedure is small.
1,3-dimethoxy-2-lithio-
+
:,)='Cl1'0
2~p-mentha-l,8-dien-4,8-trans-3-y[)-
5-n-pentyl-1,3-dimerhoxybenzene
Taylor, Lenard and Shvo have confirmed this reaction
scheme and have found that the A1 W) -trans isomer of THC
to be a principle product. They did obtain after chromatographic
separation, a reasonable yield of the A' -THC isomer.
Very recent modifications of the procedure of Taylor have
been investigated by Gaoni and Mechoulam, in which they
use BF, rather than HCI as the condensation agent between
the terpene and olivetol. They have observed a 20 percent
yield of the stereo-specifically proper isomer of A' -THC.
The details of these most recent studies have not yet been
published.
Two more syntheses have been described which employ
modifications of the olivetol molecule. Both effect the
construction of the terpene ring through some form of the
Diels-Alder reaction, but the necessary intermediates are
derived in different ways.
In one of these processes, the diene is attached to the
olivetol nucleus, between the two oxygen functions. This
diene has been obtained through two separate procedures. In
one, Korte, Dlugosch, and Claussen converted the appropriate
ketonic intermediate directly to the diene through a
Wittig reaction. In the other, Kochi and Matsui dehydrated
the carbinol that resulted from a Grignard reaction on this
ketone. In both cases, they achieved an identical diene that
cyclized readily with methyl vinyl ketone. This reaction led
to a stereo-specifically correct bicyclic methyl ketone which,
upon a Wittig methylene replacement, led to the dimethyl
ether of cannabidiol. The yields are poor, but both the
stereo-configuration and the double-bond location are correct
for the natural orientation.
The other Diels-Alder synthesis involves a scheme in which
the dieneophyle is itself attached to the olivetol nucleus.
Here either the cinnamic acid or the corresponding methyl
ketone is employed as a condensing agent with isoprene.
Korte, Hackel and Sieper have reported this reaction with the
styryl methyl ketone and have found that the isoprene
orientation is proper. After the necessary Wittig reaction
they found that the resulting A ' (6 ) -cannabidiol did not have
an assignable configuration about the 3,4-bond. A modification
of this approach has been described by Jen, Hughes and
Smith in which, through the employment of the free cinnamic
acid itself, a product is obtained that is not only properly oriented with regard to the isoprene molecule, but is
also appropriately trans- about the eventual 3,4-terpene
bond. This carboxylic acid has been resolved into its optical
isomers. These separate enantiomorphs have been appropriately
methylated, cyclized, and finally demethylated to
provide both the natural and the unnatural optical isomers of
A' N) -THC. In neither of these reactions is the yield good,
and it is only in the second example that the appropriate
stereoconfiguration is obtained. This must be isomerized at
an additional expense in yield, to the natural A' -isomer.
The three remaining synthetic procedures all employ oliveto1
as a free resorcinol, not protected or activated in any
manner. The first of these examples represents a complete
construction of the terpene ring through synthetic procedures,
but the last two involve reactions with natural or
near-natural terpenes, and so might cast light on those
biosynthetic pathways actually effected in the plant in the in
viva production of these cannabinoids.
Fahrenholtz, Lurie and Kierstead have reported the
syntheses of both the A' - and the A' (6)-3,4-trans-tetrahydrocannabinol
through a synthetic process that represents
a complete construction of the terpene ring. The condensation
of olivetol with acetoglutarate yields a cyclic product, a
benzopyran. This lactone is converted to a three-ring system
which carries a ketonic group at the one-location of the
terpene ring. This lactone carries a 3,4-double bond, but it
can be converted through appropriate protection and methylation,
to the 2,3- conjugated counterpart with a geminal
methylation. The resulting (trans) cyclohexanone is easily
methylated and dehydrated to a mixture of THC's which can
be separated chromatographically. The stereoconfiguration is
correct but the over-all yields are poor.
Mechoulam, Braun and Gaoni, have reported a total synthesis
that starts from the readily available terpene, pinene.
This is oxidized to the ally1 alcohol, verbinol, and then
condensed with olivetol to produce a heroic mixture of
products. Olive@ pinene can be chromatographically separated,
and converted into the A' (6)-THC product, which
can in turn be isomerized and so converted into the natural
A'-counterpart. In this reaction both the stereo-specificity
and the absolute optical configuration can be controlled, but
the necessary separations again limit the procedure to the
preparation of only small amounts of end-products.
The most recent synthesis in this area has been reported by
Petrzilka, Haefliger, Sikemeier, Ohloff and Eschenmoser.
They have described the reaction between unsubstantiated
olivetol and the easily synthesized terpenol (+)-trans-p-menthadien-
2 J-01. This leads directly to the stereospecifically correct isomer of cannabidiol. The yields are quite good (ca.
25 percent) and the general process suggests an easy entry to
the study of structural isomers that carry the naturally
correct optical and stereo-configurations.
From the onset of this review it has been emphasized that
the entire family of the cannabinoids can be considered as a
combination of the structures of a terpene and a resorcinol.
In fact, many of the recently evolved syntheses within this
family have employed just such a combination, with chemical
modifications as might be dictated by the reaction conditions.
It has been mentioned that the several carboxylic acid
components of the resin may serve as precursors to this
family. This is supported by the observation that in the
colder climates, in the Northern regions which provide
shorter growing periods and generally cooler conditions, the
marijuana that is harvested is known to be more raw, to
contain a greater percentage of acidic components, and to be
less biologically active.
Mechoulam and Gaoni have presented an argument that
not only olivetol but olivetolic acid may serve as the
condensing moiety for the terpene component. It can be
argued that geranol, upon appropriate activation, could condense with olivetol or with olivetolic acid to yield cannabigerol.
This reaction has been achieved in vitro. This compound
can then undergo an alpha-oxidation to yield cannabidiol
through hydroxy elimination, or cannabichromene through
addition. The remaining components of the cannabis resin are
then explainable by appropriate steps of cyclization, dehydrogenation,
and decarboxylation. These transformations are
outlined in the flow diagram in which chemicals known to be
present are numbered in accordance with their presentation
above. The one material of established structure missing from
this scheme is the A' (6)-3,4-trans-THC, IV. It can be readily
prepared in the laboratory by the acid-catalysed isomerization
of the A' -counterpart, and such a conversion could
certainly occur in the intact plant.
Until recently the only laws that pertained to the possession and the use of marijuana were contained in the Federal Marijuana Tax Act of 1937, and the many state laws which
were based primarily on it. All of these were concerned with
the plant and its components. The exact wording of the
California Narcotic Act, Health and Safety Code, is as
follows:
1103.1 "Marijuana" as used in this division means all
parts of the plant Cannabis sativa L. (commonly known
as marijuana), whether growing or not; the seeds thereof,
the resin extracted from any part of such a plant;
and every compound, manufacture, salt, derivative, mixture,
or preparation of such plant, its seeds or resin.
Although not explicitly stated, it has been accepted that
the term "all parts of the plant" should be limited to those
materials that are presumably capable of producing biological
action similar to that of the entire plant. Such obvious
components as chlorophyll and water, ubiquitous to the
plant world, were excluded. Until recently chemicals such as
tetrahydrocannabinol or cannabidiol could only have arisen
from the plant and therefore were covered in this statute.
The question as to whether a material might have been
synthetically produced was moot, because until recently
there had been no successful syntheses of these compounds.
The recent syntheses of tetrahydrocannabinol and its congeners,
coupled with the appearance through illicit channels
of materials claimed to be synthetic "THC," has reopened
the question as to whether a totally synthetic component of
the plant should legally be considered the same as an isolated
component of the plant.
This question was tacitly answered by recent modifications
of the dangerous drugs section of the Federal Food, Drug,
and Cosmetic Act, an entirely separate law. This section was
designed to control the improper distribution of dangerous
drugs. In 1968 the law was amended with the following
insertion :
Listing of drugs defined in section 201 (v) of the Act
[shall be amended to include] synthetic equivalents of
the substances contained in the plant, or in the resinous
extracts of Cannabis sp. and/or synthetic substances,
derivatives, and their isomers with similar chemical
structure and activity such as the following:
A' -cis- or trans-tetrahydrocannabinol, and their
optical isomers,
A6 -cis- or trans -tetrahydrocannabinol, and their
optical isomers,
A314 tetrahydrocannabinol, and its optical isomers.
It is recognized in this amendment that the nomenclature
of these substances has not been internationally standardized
and it is the specific compounds (and their racemic combinations)
that are specified, regardless of the numerical designation
employed. As these ten specific compounds are handled
by a separate law, they are considered synthetic substances
and not materials of plant origin. Whether the terms "isomers
with similar chemical structure and activity" will probably
prove to be worthless for similarity between things, unless
specified in detail, is a matter of individual opinion.
Because of these distinctions, it will be the responsibility
of the legal authorities not only to demonstrate that a
contraband drug is tetrahydrocannabinol, but also to determine
its origin. Since the two laws define two completely
different tetrahydrocannabinols (synthetic or natural), they
cannot be applied in the same instance. The tasks of identification are thus multiplied several-fold by the necessity of
identifying trace congeners present in the seized sample, for
if these contaminants were not detectable the origin could
not be established.
Although there have been many recent reports of the
appearance of synthetic tetrahydrocannabinol in the illicit
drug trade (as "Synthetic THC"), as of the present time these
all appear to be false. The complexity of the currently
reported syntheses of tetrahydrocannabinol makes it seem
unlikely that a synthetic material, as a pure single isomer,
could be inexpensively made. It would seem that this recent
legislation might work hardships on legitimate investigators
rather than on the enterprising amateur chemist.
As has been shown, marijuana contains a wealth of
complex organic molecules, many of which have structures
that are now well defined. Further it is known to present a
complex spectrum of pharmacological properties in the
human subject. It is certainly possible that some of these
properties can eventually be assigned specifically to some of
these compounds. Only within the last year has the first such
experiment been attempted, in which an isolated component,
A ' -tetrahydrocannabinol, has been studied in clinical
experiments. It has been shown to account in part for the
intoxicative properties of total marijuana.
The chemical and physical tools needed to implement an
extensive research program in this area are now at hand, and
the only remaining difficulties to a proper study of marijuana
are administrative. There is obviously much pharmacological
potential in Cannabis sutiva. It is axiomatic that when this is
revealed and appreciated, there will be valid contributions to
the science of medicine.
Source: Recent Developments in Cannabis Chemistry
The marijuana plant Cannabis sativa contains a bewildering
array of organic chemicals. As is true with other botanic
species, there are representatives of almost all chemical
classes present, including mono- and sesquiterpenes, carbohydrates,
aromatics, and a variety of nitrogenous compounds.
Interest in the study of this plant has centered primarily on
the resinous fraction, as it is this material that is invested
with the pharmacological activity that is peculiar to the
plant. This resin is secreted by the female plant as a
protective agent during seed ripening, although it can be
found as a microscopic exudate through the aerial portions of
plants of either sex. The pure resin, hashish or charas, is the
most potent fraction of the plant, and has served as the
source material for most of the chemical studies.
The family of chemicals that has been isolated from this
source has been referred to as the cannabinoid group. It is
unique amongst psychotropic materials from plants in that
there are no alkaloids present. The fraction is totally nitrogen-
free. Rather, the set of compounds can be considered as
analogs of the parent compound cannabinol (I), a fusion
product of terpene and a substituted resorcinol. Beyond the
scope of this present review are such questions as the
distribution of these compounds within the plant, the botanic
variability resulting from geographic distribution, the
diversity of pharmacological action assignable to the several
distinct compounds present, and the various preparations and
customs of administration. This presentation will be limited
to chemical structure and synthesis, with only passing comments
on the topics of biosynthesis and human use.
A brief review of the early analytical and synthetic studies m the area of cannabis chemistry is necessary as a background
for the discussion of recent developments. Cannabin-
01 (I) was the first compound isolated from the resin of
Cannabis sativa as a pure chemical substance. Its synthesis
established the carbon skeleton that is common to the entire
Fig. 1
(Cll, )4CH,
cannabinol (1)
group of cannabinoids. This 21-carbon system can be best
described as an amalgamation of a lo-carbon monoterpene
.and 5amylresorcinol (olivetol). The terpene half is shown to
the left of the dotted line above. In cannabinol, it is
completely aromatic and represents a molecule of cymene. In
all of the remaining cannabinoids isolated from the native
resin it is found in a partially hydrogenated state, usually
with a single double bond remaining. The resorcinol moiety,
olivetol, is the portion that is represented to the right of the
dotted line. It is to be found as an invariable component of
all the constituents of the resin, although as will be described
below, it often appears as the corresponding carboxylic acid,
olivetolic acid. Synthetic variations on the amyl group have
proved to be one of the most rewarding series of studies in
this area of chemistry, and represents most of the structureactivity
investigations that have been pursued. This early
synthetic work will be described only briefly, for it has
involved analogs of tetrahydrocannabinol which display the
terpene double bond in an unnatural position. The most
recent chemical advances represent syntheses of the exact
natural products, and these will be the body of this discussion.
A brief comment is desirable concerning the various modes
of ring numbering that have been employed in this area. Four
separate and distinct methods can be found in the chemical
literature. Each has a virtue over the others, but each carries
with it limitations. These complications arise from the fact
that, in many of the cannabinoids present in the natural
resin, the oxygen-containing ring (the pyran ring in the
cannabinol above) is not present. These materials as isolated
are dihydroxybiphenyls, with no heterocyclic ring.
The first of these numbering systems was introduced by
Todd in England. It considers these substances best referred
to as variously substituted pyrans. Thus the base numbers are
Fig. 2
4'
pyran numbering
assigned to the pyran ring, and the 4,5-bond between the two
remaining rings defines the l-position of each. The first of
these, the "prime" set, is applied to the terpene ring and
proceeds clockwise. The second, or "double prime" set,
refers to the aromatic ring and proceeds counterclockwise.
Thus the carbon atoms common to the pyran ring are
numbered one and two in each case.
A related numbering system is one that employs the
Chemical Abstracts convention. Here these substances are
considered as substituted dibenzopyrans, and are numbered
starting with the first unfused position of the aromatic ring.
The obvious disadvantage of both of these systems is that the
numbering must be totally changed in those isomers in which
the central pyran ring is open. .
To compensate for this latter limitation, a biphenyl numbering
system has come into usage, primarily in Europe.
4
F biphenyl numbering
Here, the various substitution positions are sequentially
numbered from the central carbon bond of biphenyl. The
terpene ring is fundamental, and the aromatic ring commands
the prime numbers. The advantages of this system are
reciprocal to those mentioned above. The open ring compounds
are easily numbered, but there is no general convention
that extends to modifications that include the pyran ring.
Note should be made of the fact that the course of numbers
in the terpene ring is opposite to that of the other systems.
Most broadly used today is a numbering system that
recognizes both the terpene nature and the aromatic nature
of the two different parts of the molecule. Thus the terpene
is numbered in a manner that is conventional for it, i.e., from
the ring carbon that carries the branched methyl group. This
is in turn numbered seven, and the remaining three carbons
6 K
terpene numbering
of the isopropyl group are then numbered sequentially. The
aromatic ring assignments are straightforward. The overwhelming advantage here is that this numbering system is
applicable whether the center ring is open or closed, and
further it can be extended to new compounds that may be
isolated as long as they can be represented as a combination
of a terpene and an aromatic ring. The only exception is in
the instance that the terpene portion is an open chain.
Examples of this are known, and their numbering system will
be mentioned later.
A brief discussion of the early synthetic efforts in this area
is informative, as it provides the only systematic correlation
between chemical structure and biological activity. Early in
these chemical studies, at about the time of World War II,
two compounds were isolated from the red oil fraction of
cannabis. One was an optically active tetrahydrocannabinol
which carried a double bond in the terpene ring. The other
was the open-ring counterpart; it contained two phenolic
groups and two double bonds. It was also optically active,
/\ CH, CH, A'"'-tetrahydrocannabinol A
CH, CH, A'"'-cannabidiol
and was named cannabidiol.
The location of the exocyclic double bond in the latter
compound was readily established, both by its easy conversion
into tetrahydrocannabinol (THC), and by the generation
of formaldehyde on ozonolysis. The endocyclic double bond
proved to be extremely difficult to locate. It was known not
to be in conjugation either with the exocyclic counterpart of
with the aromatic ring. This still left three possibilities, the
A1 , the A', and the A' (6) -THCs.
Although this problem has only recently been solved,
during the period of these initial isolations and characterizations,
synthetic explorations were numerous. As two generally
different synthetic methods were employed, and two
different biological assays as well, it is quite difficult to
interrelate these studies. The first of the tetrahydrocannabinol
syntheses was that of Todd and co-workers in England.
Their scheme consisted in the fusion of a terpene such as
pulegone with olivetol, thus producing the three ring product.
Pharmacological evaluations were made employing
Fig. 8
pulegone
+
other products
rabbit cornea1 areflexia; unfortunately there are no available
correlations between this response and human intoxicative
potency. Two serious complications have appeared in this
approach. The pulegone employed has been shown to be of
uncertain optical purity, thus leading to optically active
products of inconsistent composition. Further, the actual
nature of the condensation leads to structural isomers. This
was due in part to the contamination of pulegone as isolated
from natural sources with isopulegone, and in part to a
sensitivity to the specific nature of the condensation agent.
A more satisfactory scheme was developed by Adams and
his co-workers at Illinois. In this process a completely
synthetic keto-ester was condensed with olivetol, and the
resulting lactone converted in a separate step to the gemdimethyl
product. Again, as with the pulegone synthesis
above, the unnatural A3 isomer of THC was the principle
product. This, and related homologs, were titrated pharmacologically
by dog ataxia assay. They were found to be
qualitatively similar although quantitatively less active than
natural THC isolated from the red oil. This unnatural but
reproducibly available isomer was taken as a reference standard
for an extensive study of structural modifications.
A complete review of these studies would be out of place
in a presentation designed to emphasize recent developments.
Comment should be made, however, on the importance of
the amyl group on the aromatic ring. It was found that, in
the straight chain series, a maximum activity was observed at
the 6-carbon (hexyl) substitution. This material was about
twice as active as the reference amyl compound in the ataxia
analysis, and has undergone extensive clinical study under the
name of Synhexyl or Pyrahexyl. The replacement of the
straight chain with one branched at the alpha- carbon, led to
the alpha-methylhexyl counterpart, with the increase of
potency of a full order of magnitude. The studies that have
resulted in the development of alpha, beta- dimethyl analogs,
have led to the dimethylheptyl-analog, a compound known as
DMHP or Adams' nine-carbon compound. It is yet a full
Adams' 9-carbon compound Pars' nitrogen analog
order of magnitude more potent than the methyl hexyl
material mentioned above, i.e., five hundred times more
potent than the reference A3 -THC. Its activity has been
confirmed in human subjects, and just recently the tedious
task of its separation into the eight possible isomers has been
reported by Aaron. Quite recently a nitrogen analog of this
compound has been prepared by Pars, and appears also to be
biologically active.
In recent years, immense strides have been made in the area of the chemistry of the cannabinoids of C. sativa. The development of sophisticated spectroscopic instruments, particularly
in nuclear magnetic resonance, has settled the
question of the exact isomeric configuration of the elusive
double bond in the terpene ring, and has established the
stereoconfiguration about the 3,4-position. The principle
isomer present in the red oil is A' -tetrahydrocannabinol, II,
in which the 3,4-hydrogens are oriented trans- to one
another. The open-ring counterpart to A' -THC is consequently
A' -3,4-trans-cannabidiol (III). It has been reported
that the A' (6) -isomer of THC (IV) is also present in the
Fig. 11 CH,
(=$&W&f&
CH,CH,
A'-3,4-trans-tetrahydrocannabinol (II)
@(CH,),CH,
A
CH, CH,
A'-3,4-trans-cannabidiol (111)
CH, HO
F-O
CH, CH,
A'c6'-trans-tetrahydrocannabinol (IV)
native resin, but this is uncertain as it could have arisen as an
artifact of isolation.
None of these fine structural assignments could have been
possible, however, without the development of elegant
methods of fractionation and isomer separation concurrently
with the instrument techniques. The procedures of column
and thin layer chromatography have made possible not only
the isolation of characterizable amounts of isomerically pure
materials, but have led to the discovery of a host of additional chemicals that had heretofore been unknown.
An acidic fraction has long been known to be present in
the red oil of C. sat&a. Some ten years ago an acid was
isolated which proved to be, after structural correction for
the now-known location of the endocyclic double bond of
A' -THC, the benzoic acid that corresponds to III, i.e.,
cannabidiolic acid, V. This material corresponds to cannabidiol
both in the double bond location and in the u-ansconfiguration
about the 3,4-bond. The presence of this and
other acidic materials in the resins isolated from marijuana
grown in the colder northern latitudes suggests their roles as
biosynthetic presursors to the more neutral aromatic, active,
fractions. Careful chromatographic separation of this acidic
fraction into individual components has afforded three more
aromatic carboxylic acids. These are cannabigerolic acid (VI)
that upon decarboxylation could yield cannabigerol (v. i. ),
cannabinolic acid (VII) which can give rise to cannabinol (I),
and A1 -3,4-trans-tetrahydrocannabinolic acid (VIII) which
can be converted to, and which may well be argued as being a
normal biosynthetic precursor to, A1 THC. The possible roles
of these acids as biological intermediates which could lead to
the neutral (phenolic) cannabinoids, will be discussed below.
In the chromatographic analysis of the less plentiful
components of the resinous fraction of C. sativa several
additional phenolic components have been isolated and assigned
tentative chemical structures.
A one-ring resorcinol has been separated that, upon spectroscopic
analysis, appeared to be the simple fusion of an
open-chain terpene and olivetol. This material, cannabigerol
(IX) has had its structure proven by synthesis. The fusion of
this terpene with olivetolic acid would then give rise to the
above-mentioned acid, cannabigerolic acid (VI). This type of
open-ring terpene compound presents yet another numbering
system, being determined by the eight-carbon chain attached
to the olivetol nucleus. The numbering by convention starts
at the distal end of the chain, as shown in IX and X.
Two additional phenolic components have recently been
described. Cannabichromene, X, is an open-chain benzopyran
containing a carbon system closely related to the material
cannabigerol but with heterocyclic ring closure. A phenol
named cannabicyclol (XI) has also been isolated which,
lacking any unsaturation whatsoever, has been assigned the
structure shown. These two hypothetical structures lack
support from synthetic studies. These fusions of terpenes and
aromatics have suggested several of the resent synthetic
approaches into this area, and also provide the basis of
biosynthetic paths, to be discussed.
An expected correlary to the establishment of the tools of
structural analysis of C. sativa, was their use in the development
and evaluation of synthetic techniques. It must be
noted that, in this same period of time-the last four years or
so-no less than six separate and mutually confirmatory
syntheses within this family of compounds have appeared in
the chemical literature. Three of these represent modifications
of the olivetol ring providing the basis for the construction
of the terpenacious half of the cannabinoid molecule.
The remaining three syntheses employ the reactivity of the
resorcinol system itself and, in effect, bring a terpene into
reaction with it.
The first of these procedures can be illustrated by the
general reaction between citral and the lithio-derivative of the
dimethyl ether of olivetol. The first description of this
reaction was advanced by Mechoulam and Gaoni. The
coupling product shown undergoes an internal rearrangement
to yield the trans- isomer of the dimethyl ether of cannabidiol.
Demethylation leads to cannabidiol itself, and cyclization
provides a mixture of the A' and the A' (6) -isomers of THC.
The over-all yield in this procedure is small.
1,3-dimethoxy-2-lithio-
+
:,)='Cl1'0
2~p-mentha-l,8-dien-4,8-trans-3-y[)-
5-n-pentyl-1,3-dimerhoxybenzene
Taylor, Lenard and Shvo have confirmed this reaction
scheme and have found that the A1 W) -trans isomer of THC
to be a principle product. They did obtain after chromatographic
separation, a reasonable yield of the A' -THC isomer.
Very recent modifications of the procedure of Taylor have
been investigated by Gaoni and Mechoulam, in which they
use BF, rather than HCI as the condensation agent between
the terpene and olivetol. They have observed a 20 percent
yield of the stereo-specifically proper isomer of A' -THC.
The details of these most recent studies have not yet been
published.
Two more syntheses have been described which employ
modifications of the olivetol molecule. Both effect the
construction of the terpene ring through some form of the
Diels-Alder reaction, but the necessary intermediates are
derived in different ways.
In one of these processes, the diene is attached to the
olivetol nucleus, between the two oxygen functions. This
diene has been obtained through two separate procedures. In
one, Korte, Dlugosch, and Claussen converted the appropriate
ketonic intermediate directly to the diene through a
Wittig reaction. In the other, Kochi and Matsui dehydrated
the carbinol that resulted from a Grignard reaction on this
ketone. In both cases, they achieved an identical diene that
cyclized readily with methyl vinyl ketone. This reaction led
to a stereo-specifically correct bicyclic methyl ketone which,
upon a Wittig methylene replacement, led to the dimethyl
ether of cannabidiol. The yields are poor, but both the
stereo-configuration and the double-bond location are correct
for the natural orientation.
The other Diels-Alder synthesis involves a scheme in which
the dieneophyle is itself attached to the olivetol nucleus.
Here either the cinnamic acid or the corresponding methyl
ketone is employed as a condensing agent with isoprene.
Korte, Hackel and Sieper have reported this reaction with the
styryl methyl ketone and have found that the isoprene
orientation is proper. After the necessary Wittig reaction
they found that the resulting A ' (6 ) -cannabidiol did not have
an assignable configuration about the 3,4-bond. A modification
of this approach has been described by Jen, Hughes and
Smith in which, through the employment of the free cinnamic
acid itself, a product is obtained that is not only properly oriented with regard to the isoprene molecule, but is
also appropriately trans- about the eventual 3,4-terpene
bond. This carboxylic acid has been resolved into its optical
isomers. These separate enantiomorphs have been appropriately
methylated, cyclized, and finally demethylated to
provide both the natural and the unnatural optical isomers of
A' N) -THC. In neither of these reactions is the yield good,
and it is only in the second example that the appropriate
stereoconfiguration is obtained. This must be isomerized at
an additional expense in yield, to the natural A' -isomer.
The three remaining synthetic procedures all employ oliveto1
as a free resorcinol, not protected or activated in any
manner. The first of these examples represents a complete
construction of the terpene ring through synthetic procedures,
but the last two involve reactions with natural or
near-natural terpenes, and so might cast light on those
biosynthetic pathways actually effected in the plant in the in
viva production of these cannabinoids.
Fahrenholtz, Lurie and Kierstead have reported the
syntheses of both the A' - and the A' (6)-3,4-trans-tetrahydrocannabinol
through a synthetic process that represents
a complete construction of the terpene ring. The condensation
of olivetol with acetoglutarate yields a cyclic product, a
benzopyran. This lactone is converted to a three-ring system
which carries a ketonic group at the one-location of the
terpene ring. This lactone carries a 3,4-double bond, but it
can be converted through appropriate protection and methylation,
to the 2,3- conjugated counterpart with a geminal
methylation. The resulting (trans) cyclohexanone is easily
methylated and dehydrated to a mixture of THC's which can
be separated chromatographically. The stereoconfiguration is
correct but the over-all yields are poor.
Mechoulam, Braun and Gaoni, have reported a total synthesis
that starts from the readily available terpene, pinene.
This is oxidized to the ally1 alcohol, verbinol, and then
condensed with olivetol to produce a heroic mixture of
products. Olive@ pinene can be chromatographically separated,
and converted into the A' (6)-THC product, which
can in turn be isomerized and so converted into the natural
A'-counterpart. In this reaction both the stereo-specificity
and the absolute optical configuration can be controlled, but
the necessary separations again limit the procedure to the
preparation of only small amounts of end-products.
The most recent synthesis in this area has been reported by
Petrzilka, Haefliger, Sikemeier, Ohloff and Eschenmoser.
They have described the reaction between unsubstantiated
olivetol and the easily synthesized terpenol (+)-trans-p-menthadien-
2 J-01. This leads directly to the stereospecifically correct isomer of cannabidiol. The yields are quite good (ca.
25 percent) and the general process suggests an easy entry to
the study of structural isomers that carry the naturally
correct optical and stereo-configurations.
From the onset of this review it has been emphasized that
the entire family of the cannabinoids can be considered as a
combination of the structures of a terpene and a resorcinol.
In fact, many of the recently evolved syntheses within this
family have employed just such a combination, with chemical
modifications as might be dictated by the reaction conditions.
It has been mentioned that the several carboxylic acid
components of the resin may serve as precursors to this
family. This is supported by the observation that in the
colder climates, in the Northern regions which provide
shorter growing periods and generally cooler conditions, the
marijuana that is harvested is known to be more raw, to
contain a greater percentage of acidic components, and to be
less biologically active.
Mechoulam and Gaoni have presented an argument that
not only olivetol but olivetolic acid may serve as the
condensing moiety for the terpene component. It can be
argued that geranol, upon appropriate activation, could condense with olivetol or with olivetolic acid to yield cannabigerol.
This reaction has been achieved in vitro. This compound
can then undergo an alpha-oxidation to yield cannabidiol
through hydroxy elimination, or cannabichromene through
addition. The remaining components of the cannabis resin are
then explainable by appropriate steps of cyclization, dehydrogenation,
and decarboxylation. These transformations are
outlined in the flow diagram in which chemicals known to be
present are numbered in accordance with their presentation
above. The one material of established structure missing from
this scheme is the A' (6)-3,4-trans-THC, IV. It can be readily
prepared in the laboratory by the acid-catalysed isomerization
of the A' -counterpart, and such a conversion could
certainly occur in the intact plant.
Until recently the only laws that pertained to the possession and the use of marijuana were contained in the Federal Marijuana Tax Act of 1937, and the many state laws which
were based primarily on it. All of these were concerned with
the plant and its components. The exact wording of the
California Narcotic Act, Health and Safety Code, is as
follows:
1103.1 "Marijuana" as used in this division means all
parts of the plant Cannabis sativa L. (commonly known
as marijuana), whether growing or not; the seeds thereof,
the resin extracted from any part of such a plant;
and every compound, manufacture, salt, derivative, mixture,
or preparation of such plant, its seeds or resin.
Although not explicitly stated, it has been accepted that
the term "all parts of the plant" should be limited to those
materials that are presumably capable of producing biological
action similar to that of the entire plant. Such obvious
components as chlorophyll and water, ubiquitous to the
plant world, were excluded. Until recently chemicals such as
tetrahydrocannabinol or cannabidiol could only have arisen
from the plant and therefore were covered in this statute.
The question as to whether a material might have been
synthetically produced was moot, because until recently
there had been no successful syntheses of these compounds.
The recent syntheses of tetrahydrocannabinol and its congeners,
coupled with the appearance through illicit channels
of materials claimed to be synthetic "THC," has reopened
the question as to whether a totally synthetic component of
the plant should legally be considered the same as an isolated
component of the plant.
This question was tacitly answered by recent modifications
of the dangerous drugs section of the Federal Food, Drug,
and Cosmetic Act, an entirely separate law. This section was
designed to control the improper distribution of dangerous
drugs. In 1968 the law was amended with the following
insertion :
Listing of drugs defined in section 201 (v) of the Act
[shall be amended to include] synthetic equivalents of
the substances contained in the plant, or in the resinous
extracts of Cannabis sp. and/or synthetic substances,
derivatives, and their isomers with similar chemical
structure and activity such as the following:
A' -cis- or trans-tetrahydrocannabinol, and their
optical isomers,
A6 -cis- or trans -tetrahydrocannabinol, and their
optical isomers,
A314 tetrahydrocannabinol, and its optical isomers.
It is recognized in this amendment that the nomenclature
of these substances has not been internationally standardized
and it is the specific compounds (and their racemic combinations)
that are specified, regardless of the numerical designation
employed. As these ten specific compounds are handled
by a separate law, they are considered synthetic substances
and not materials of plant origin. Whether the terms "isomers
with similar chemical structure and activity" will probably
prove to be worthless for similarity between things, unless
specified in detail, is a matter of individual opinion.
Because of these distinctions, it will be the responsibility
of the legal authorities not only to demonstrate that a
contraband drug is tetrahydrocannabinol, but also to determine
its origin. Since the two laws define two completely
different tetrahydrocannabinols (synthetic or natural), they
cannot be applied in the same instance. The tasks of identification are thus multiplied several-fold by the necessity of
identifying trace congeners present in the seized sample, for
if these contaminants were not detectable the origin could
not be established.
Although there have been many recent reports of the
appearance of synthetic tetrahydrocannabinol in the illicit
drug trade (as "Synthetic THC"), as of the present time these
all appear to be false. The complexity of the currently
reported syntheses of tetrahydrocannabinol makes it seem
unlikely that a synthetic material, as a pure single isomer,
could be inexpensively made. It would seem that this recent
legislation might work hardships on legitimate investigators
rather than on the enterprising amateur chemist.
As has been shown, marijuana contains a wealth of
complex organic molecules, many of which have structures
that are now well defined. Further it is known to present a
complex spectrum of pharmacological properties in the
human subject. It is certainly possible that some of these
properties can eventually be assigned specifically to some of
these compounds. Only within the last year has the first such
experiment been attempted, in which an isolated component,
A ' -tetrahydrocannabinol, has been studied in clinical
experiments. It has been shown to account in part for the
intoxicative properties of total marijuana.
The chemical and physical tools needed to implement an
extensive research program in this area are now at hand, and
the only remaining difficulties to a proper study of marijuana
are administrative. There is obviously much pharmacological
potential in Cannabis sutiva. It is axiomatic that when this is
revealed and appreciated, there will be valid contributions to
the science of medicine.
Source: Recent Developments in Cannabis Chemistry
