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International Edition: DOI: 10.1002/anie.201601834
German Edition:
DOI: 10.1002/ange.201601834

Total Synthesis

Synthesis of (À)-Cannabimovone and Structural Reassignment of
Anhydrocannabimovone through Gold(I)-Catalyzed
Cycloisomerization
Javier Carreras, Mariia S. Kirillova, and Antonio M. Echavarren*
Dedicated to Professor Miquel A. Pericàs on the occasion of his 65th birthday
Abstract: The first total synthesis of cannabimovone from
Cannabis sativa and anhydrocannabimovone was achieved by
means of a highly stereoselective gold(I)-catalyzed cycloisomerization. The results led to reassignment of the structure
of anhydrocannabimovone.

The herbaceous plant Cannabis sativa has been used in

medicine for centuries and still attracts significant interest due
to the biological and pharmaceutical activity of many of its
metabolites.[1] More than 60 compounds, known as cannabinoids (a group of C21 terpenophenolic compounds), are
exclusively found in Cannabis sativa.[2] Owing to the development of synthetic cannabinoids,[3, 4] the unique components of
Cannabis sativa are known as phytocannabinoids. The most
abundant compound is D9-tetrahydrocannabinol (THC, 1;
Figure 1), which shows interesting pharmacological activity as
an analgesic, antiemetic, and appetite stimulant, among
others, besides its well-known psychotropic effects.[5] Several
total syntheses of 1 have been accomplished to date.[6]

Figure 1. Cannabinoids THC (1), CBD (2), and cannabimovone (3).

[*] Dr. J. Carreras, M. S. Kirillova, Prof. A. M. Echavarren
Institute of Chemical Research of Catalonia (ICIQ)
Barcelona Institute of Science and Technology
Av. Països Catalans 16, 43007 Tarragona (Spain)
E-mail: aechavarren@iciq.es
Prof. A. M. Echavarren
Departament de Química Analítica i Química
Orgànica, Universitat Rovira i Virgili
C/ Marcel·li Domingo s/n, 43007 Tarragona (Spain)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201601834.
 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution-NonCommercial License, which permits use,
distribution and reproduction in any medium, provided the original
work is properly cited and is not used for commercial purposes.
Angew. Chem. Int. Ed. 2016, 55, 7121 –7125

Cannabidiol (CBD, 2) is another important phytocannabinoid
with great potential as a drug[7] since it modulates the
undesired effects of THC when they are administrated
together.[8]
A structurally different cannabinoid named cannabimovone (3) has recently been isolated by the groups of
Taglialatela-Scafati and Appendino from a nonpsychotropic
variety of hemp (Cannabis sativa L.; Figure 1).[9] In their
attempt at preparing 3 from CBD (2) through an intramolecular aldol reaction of keto aldehyde 4 under mild acidic
conditions, the product of dehydration (5) was formed instead
(Scheme 1). Under basic conditions, the novel cannabinoid

Scheme 1. Synthesis of anhydrocannabimovone (6) from cannabidiol
(CBD, 2).[9]

anhydrocannabimovone (6) was directly formed through an
intramolecular oxy-Michael addition of one of the phenol
groups to the intermediate enone. Synthetic 6 was found to be
active against metabotropic and ionotropic cannabinoid
receptors, showing a similar biological profile to THC,
whereas cannabimovone (3) has affinity only for ionotropic
receptors.[9]
The unprecedented abeo-menthane terpenoid structure of
cannabimovone (3) includes a densely functionalized cyclopentane with four contiguous stereocenters. The novel
structure of 3, coupled with its lability towards dehydration
under acidic or basic conditions and the interesting biological
profiles of both 3 and 6, inspired us to develop a total
synthesis that could allow access to a wide variety of

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Scheme 2. Retrosynthetic analysis for 3 and 6.

analogues. Herein, we report the first total synthesis of
enantiopure cannabimovone (3) and also revise the structure
originally assigned to anhydrocannabimovone from transfused 6’ to cis-tetrahydro-1H-cyclopenta[b]benzofuran 6. Our
’
approach to the synthesis of these compounds relies on
a gold(I)-catalyzed cycloisomerization[10–15] of aryl-substituted
1,5-enyne 7, which could be obtained in a few steps from
commercially available (+)-methyl (S)-3-hydroxybutyrate (9;
+
Scheme 2).
The synthesis commenced with alkylation of the lithium
enolate of 9 with prenyl bromide to provide known compound
10 with excellent diastereoselectivity (98:2) by following
a slight modification of the reported procedure[16] (Scheme 3).
Protection of the alcohol of 10 as a silyl ether, conversion of
the ester into an aldehyde by a two-step procedure (DIBAL
reduction/Swern oxidation), and subsequent homologation
with the Ohira–Bestmann reagent led to 1,5-enyne 11 (31 %
over 5 steps). Sonogashira coupling of 11 with iodo arene 12,
prepared in two steps from olivetol, gave 7 in 83 % yield on
a multi-gram scale. The gold(I)-catalyzed cyclization of 1,5enyne 7 was highly solvent dependent. Exposing 7 to the
cationic gold(I) complex [(JohnPhos)Au(MeCN)]SbF6 in
CH2Cl2 led to bicyclic compound 13 (49 %). A similar result
was obtained using other solvents such as Et2O or toluene.
Reaction in MeOH afforded methyl ether 14 (93 %). However, when the reaction was performed in DMSO, cyclopentene 8 was obtained in excellent yield (88 %). This
reaction was performed up to a 2.1 g scale. A similar result
was observed when the reaction was performed in DMF
(79 %). Presumably, the initial intermediate of the gold(I)catalyzed cyclization (Int) undergoes proton elimination
assisted by the solvent to give 8 after protodeauration.
Notably, the gold-catalyzed cyclization led exclusively to the
product with the correct relative configuration, thereby
setting two of the final four stereocenters.
Although deprotection of the TBS group of 8 followed by
oxidation of the alcohol to the methyl ketone could be carried
out uneventfully, isomerization to form the a,b-unsaturated
ketone failed under all the conditions we examined with this
and with other intermediates with different phenol protecting
groups. Fortunately, the desired functionality in the fivemembered ring could be introduced by epoxidation with mCPBA and NaHCO3 to exclusively form 15, followed by
Meinwald rearrangement with stoichiometric BF3·Et2O to

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Scheme 3. a) LDA, HMPA, prenyl bromide, THF, À70 8C to À10 8C,
3 h, 81 %; b) TBSCl, DBU, CH2Cl2, 25 8C, 14 h, 89 %; c) DIBAL-H,
Toluene, À78 8C to À50 8C, 4 h, 84 %; d) Oxalyl chloride, DMSO, Et3N,
CH2Cl2, À60 8C to 25 8C, 1 h; e) Ohira–Bestmann reagent, K2CO3
MeOH, 25 8C, 5 h, 51 % (2 steps); f) Pd(PPh3)2Cl2 (5 mol %), CuI
(10 mol %), Et3N/iPr2EtN (1:1), 25 8C, 16 h, 83 %; g) [(JohnPhos)Au(MeCN)]SbF6 (5 mol %), CH2Cl2 1 m, 25 8C, 30 min, 49 %;
h) [(JohnPhos)Au(MeCN)]SbF6 (5 mol %), MeOH 1 m, 25 8C, 30 min,
93 %. i) [(JohnPhos)Au(MeCN)]SbF6 (5 mol %), DMSO 0.5 m, 25 8C,
3 h, 88 %. LDA = lithium diisopropylamide, HMPA = hexamethylphosphoramide, THF = tetrahydrofuran, TBSCl = tert-butylsilyl chloride,
DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene, DIBAL-H = diisobutylalane,
DMSO = dimethyl sulfoxide, JohnPhos = (2-biphenyl)-di-tert-butylphosphine.

give ketone 16 (2,3-cis). Epimerization and cleavage of the
silyl ether was achieved with aqueous HCl to give 17
(Scheme 4). The relative configuration of 17 was determined
by NMR studies and was confirmed by preparation of the
same compound by a different route, namely cleavage of the
TBS group of 8 followed by epoxidation to give 18, which
underwent Meinwald rearrangement to provide 17. Diastereoselective reduction of b-hydroxy ketone 17 by Saksena–
Evans reaction with NaBH(OAc)3 in CH2Cl2 afforded diol 19.
Protection with AllocCl, which proceeded with moderate
selectivity, followed by Dess–Martin oxidation gave protected
cannabimovone 20. Cleavage of the MOM groups using
MgBr2 and BnSH,[17] followed by Pd0 deprotection of the allyl
carbonate provided 3 (53 % over 2 steps). The spectral data
and optical rotation of the synthetic cannabimovone (3)
matched those reported for the natural compound.[18]
Whereas the synthesis of 3 fully supported the assigned
configuration for all of the intermediates, a crystalline oxabicycle intermediate 21 was obtained through treatment of
epoxide 18 with a Brønsted acid, which led to opening of the

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Scheme 6. a) Ac2O, Et3N, DMAP, CH2Cl2, À30 8C, 1 h, 50 % (56 %
brsm); b) DMP, NaHCO3, CH2Cl2, 25 8C, 1 h, 73 %; c) MgBr2, BnSH,
Et2O, 25 8C, 24 h, 82 %; d) K2CO3, MeOH, 25 8C, 15 min, 60 % (6) and
19 % (6’’).
’

Scheme 4. a) mCPBA, NaHCO3, CH2Cl2, 0 8C to 25 8C, 3 h, 57 %;
b) BF3·OEt2, THF, 25 8C, 30 min, 93 %; c) HCl dil./THF, 25 8C, 1 h,
88 %; d) TBAF, THF, 25 8C, 14 h, 83 %; e) mCPBA, NaHCO3, CH2Cl2,
0 8C to 25 8C, 4 h, 58 %; f) BF3·OEt2, THF, 25 8C, 1 h, 48 %; g) NaBH(OAc)3, CH2Cl2, 25 8C, 48 h, 48 % (77 % brsm); h) AllocCl, TMEDA,
CH2Cl2, À40 8C, 1.5 h, 41 % (72 % brsm); i) DMP, NaHCO3, CH2Cl2,
25 8C, 1.5 h, 87 %; j) MgBr2, BnSH, Et2O, 25 8C, 30 h, 62 %; k) Pd(PPh3)4
(5 mol %), dimedone, THF, 25 8C, 1 h, 85 %. mCPBA = m-chloroperbenzoic acid, TBAF = tetrabutylammonium fluoride, Alloc = allyloxycarbonyl, TMEDA = tetramethylethylenediamine, DMP = Dess–Martin periodinane.

Scheme 5. a) m-ClC6H4CO2H, CH2Cl2, 25 8C, 1 h, 77 %; b) ZnI2,
(NaBH4), (CH2Cl)2, 25 8C, 14 h, 63 %. X-Ray structure for 21.

epoxide and trapping of the benzylic carbocation by the free
alcohol (Scheme 5). The molecular structure of 21 was
determined by X-ray diffraction, which confirmed the relative
configuration between the isopropenyl and the hydroxyethyl
substituents.[19] Treatment of 21 with ZnI2 effected a pinacol
rearrangement to give ketone 17.

Angew. Chem. Int. Ed. 2016, 55, 7121 –7125

Figure 2. X-Ray structures for 6 and 23.

The synthesis of anhydrocannabimovone (6) was carried
out from acetate 22 by MOM cleavage followed by treatment
with K2CO3 to promote the oxy-Michael addition. Under
these conditions, two separable epimers (6 and 6’’) were
formed in a 4:1 ratio (Scheme 6). Surprisingly, although
1
H NMR of the major isomer 6 was identical to that reported
for anhydrocannabimovone, very significant differences were
observed in the 13C NMR spectrum.[20] Furthermore, the
optical rotation of 6 ([a]22 =+ 40.68 (c = 0.29, CHCl3)) was
8
D
very different to that reported ([a]22 = À178 (c = 0.02,
8
D
CHCl3)).[9] The structure of anhydrocannabimovone (6) was
finally confirmed by X-ray diffraction[19] and its absolute
configuration was assigned on the basis of the X-ray structure
of
anhydrocannabimovone
2-bromobenzoate
(23;
Figure 2).[19]
In order to clarify the discrepancy between our structural
assignment and that originally reported,[9] we performed DFT
calculations to study the oxy-Michael cyclization. Under basic
conditions, oxy-Michael cyclization of the phenolate anion
leads to cis fusion, which is more favored than the trans
addition by 19.8 Kcal molÀ1 (Figure 3).[21] Furthermore, DFT
calculations were employed to predict the expected 13C NMR
chemical shifts of the different possible products.[22] Our data
were in better agreement with the cis-tetrahydro-1H-

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Figure 3. Energy profile (cis and trans) for the oxy-Michael cyclization.
DFT calculations (M06-2x/6-31G(d,p) (MeOH), DG (kcal molÀ1).

cyclopenta[b]benzofuran structure for anhydrocannabimovone (6).[20]
In conclusion, we have accomplished the first total
synthesis of cannabimovone (3). The four stereogenic centers
of the target molecule were set up starting from the
stereogenic center present in commercially available
(+)-methyl (S)-3-hydroxybutyrate (9) and by using a fully
+
diastereoselective gold(I)-catalyzed cyclization. Interestingly,
this is the first example of the cycloisomerization of simple
1,5-enynes into 3-vinylcyclopent-1-enes catalyzed by gold in
the context of natural product synthesis. We also synthesized
anhydrocannabimovone (6) and revised the stereochemistry
at the ring fusion by X-ray crystallography and DFT
calculations. This synthetic endeavor provides ready access
to 3 and 6, as well as other synthetic cannabinoids, for
biological testing.

Acknowledgements
We thank MINECO (Severo Ochoa Excellence Accreditation 2014-2018 (SEV-2013-0319), and project CTQ201342106-P), the European Research Council (Advanced Grant
No. 321066), the AGAUR (2014 SGR 818 and Beatriu de
Pinós Postdoctoral Fellowship to J. C.), and the ICIQ
Foundation. We thank the ICIQ X-ray Diffraction and
Chromatograpy units. We also thank Nfflria Huguet and
Verónica L. Carrillo (ICIQ) for additional experiments and
Dr. G. JimØnez-OsØs (Universidad de La Rioja) for computational advice.
Keywords: cannabinoids · cycloisomerization · gold ·
oxy-Michael reaction · total synthesis
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[17]
[18]
[19]

[20]
[21]
[22]

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See the Supporting Information for more details. Optical
rotation: [a]28 = À6.88 (c = 0.70, CHCl3); (lit. [a]22 = À108 (c =
8
8
D
D
0.07, CHCl3)).[9] .
CCDC 1454956 (6), CCDC 1454957 (21) and CCDC 1454958
(23) contain the supplementary crystallographic data. These data
can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
See the Supporting Information for more details.
Similarly, the cis addition was also favored for the pyrrolidinepromoted process. See the Supporting Information for more
details.
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Received: February 22, 2016
Published online: April 27, 2016

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