A novel cis-selective cyclohexanone annulation as the key step of a total synthesis of the sesquiterpene lsoacanthodoral

Article abstract of DOI:10.1021/ol9018979

Isoacanthodoral (1) Is a structurally unique sesquiterpene in that it is a bicyclo[4.4.0]dec-1-ene with a cis- rather than the common transjunction between the constituting rings. An efficient construction of this motif has been accomplished by a novel ci

Full text of DOI:10.1021/ol9018979

ORGANIC
LETTERS
2009
Vol. 11, No. 21
4842-4845
A Novel Cis-Selective Cyclohexanone
Annulation as the Key Step of a Total
Synthesis of the Sesquiterpene
Isoacanthodoral
Thomas Hampel and Reinhard Bru¨ckner*
Institut fu¨r Organische Chemie und Biochemie, Albert-Ludwigs-UniVersita¨t Freiburg,
Albertstrasse 21, 79104 Freiburg im Breisgau, Germany
reinhard.brueckner@organik.chemie.uni-freiburg.de
Received August 14, 2009
ABSTRACT
Isoacanthodoral (1) is a structurally unique sesquiterpene in that it is a bicyclo[4.4.0]dec-1-ene with a cis- rather than the common trans-
junction between the constituting rings. An efficient construction of this motif has been accomplished by a novel cis-selective cyclohexanone
annulation, combining the lithium enolate of ester 8, the r,ꢀ-unsaturated ester 6, and vinylmagnesium bromide in a single synthetic operation.
For completing the total synthesis of 1, a Shapiro-olefination/hydrogenation sequence and a reductive cyanation were employed.
Nudibranches are marine mollusks devoid of a protective
shell. Many of them seem to make up for the resulting
vulnerability by excreting strong odorantsslethal both to fish
and crustaceans.1 For humans, the scent of nudibranches is
fruity as long as perceived in minute concentrations but so
malodorous at higher concentrations that it causes nausea.
Isoacanthodoral (1) is an odorous sesquiterpene from this
class of compounds (Figure 1). Along with other terpenoids,
bicyclo[4.4.0]dec-1-ene displaying a cis- rather than the
common trans-junction between the constituting rings. While
investigating the total synthesis of a tricyclic terpenoid, which
contains both a cis- and a trans-fused bicyclo[4.4.0]decane
moiety we became interested in accessing such motifs and
their unsaturated analoguesslike terpenoid 1sefficiently.
To date, 1 has been synthesized once, as a racemic mixture
and with an expenditure of 16 steps in the longest linear
sequence.3 The approach featured an intermolecular Diels-Alder
reaction between 2-(methoxycarbonyl)-4,4-dimethylcyclohexa-
2,5-dien-1-one and isoprene as the key step; the dienophile was
derived from 4,4-dimethylcyclohex-2-en-1-one.3a
In our retrosynthetic analysis, the aldehyde-containing cis-
bicyclo[4.4.0]decene 1 was traced back to the chain-
Figure 1. Sesquiterpenes isoacanthodoral (1), acanthodoral (2), and
(1) Andersen, R. Skin chemistry of nudibranchs from the West Coast
of North America. In Molluscs; Cimino, G., Ed.; Springer: Berlin, 2006;
pp 277-301.
nanaimoal (3) isolated from A. nanaimoensis.
(2) Ayer, S. W.; Andersen, R. J.; He, C. H.; Clardy, J. J. Org. Chem.
1984, 49, 2653–2654.
(3) (a) Liu, H.-J.; Browne, E. N. C. Can. J. Chem. 1981, 59, 601–608.
(b) Liu, H.-J.; Ul´ıbarri, G.; Nelson, L. A. K. J. Chem. Soc., Chem. Commun.
1990, 1419–1421. (c) Liu, H.-J.; Ul´ıbarri, G.; Nelson, L. A. K. Can. J. Chem.
1994, 72, 2416–2427.
it was isolated from the nudibranch Acanthodoris nan-
aimoensis, which thrives on the Canadian Pacific coast.2 The
compound is structurally unique in that it constitutes a
10.1021/ol9018979 CCC: $40.75
Published on Web 09/23/2009
 2009 American Chemical Society

shortened ester-containing cis-bicyclo[4.4.0]decenone 4c
(Figure 2). An obvious disconnection of 4c would be into
Other than the mentioned enones, enone 11c and its less
hindered analogues 11a and b (Scheme 1, bottom) self-
Scheme 1
.
Anionic Diels-Alder Reaction (or Michael-Michael
Addition)
Figure 2. Two retrosynthetic analyses of isoacanthodoral (1).
silyldienol ether 7c and dienophile 6. This would allow for
a cis-selective cyclohexanone annulation either by an anionic
Diels-Alder reaction of the corresponding lithium dienolate
5c or by an equivalent sequence of two Michael additions.
Alternatively, we envisioned that the cis-bicyclo-
[4.4.0]decenone 4c results from three components and two
Michael additions, i.e., from a novel domino process.4 In its
course, the enolate of ester 8, the previously mentioned R,ꢀ-
unsaturated ester 6, and vinylmagnesium bromide (9) would
combine in a series of steps, which could probably be
performed as a one-pot operation.
Anionic Diels-Alder reactions, i.e., [4 + 2]-additions of
oxyanion-substituted dienes (“dienolates”) to dienophiles do
not represent widely used approaches to cyclohexanones.5
Their best known variants employ cyclohexenones6 or alk-
3-en-2-ones with an electron-withdrawing 1-substituent
(“Nazarov reagents”7)8 as dienolate precursors, combined
with a base (LDA6 and cesium carbonate,8 respectively).
condensed during attempted dienolate generation by depro-
tonation. This problem was avoided when we transformed
these compounds into silyldienol ethers 7a-c (11-27%
yield, not optimized9) and treated the latter with methyl-
lithium for releasing the corresponding dienolates. Dienophile
6 was prepared from m-toluic acid (10) in three steps10 and
73% overall yield (Scheme 1, top).
The anionic Diels-Alder reaction (or synthetically equiva-
lent sequence of two Michael additions) worked best for the
dienolate (5a) with the fewest 1-substituents: following this
pathway, silyldienol ether 7a rendered cis-bicyclo-
[4.4.0]decenone 4a in 67% yield. Annulation yields were
limited to 39-53% and 18%, however, when the sterically
more hindered silyldienol ethers 7b or 7c were employed.
Faced with the latter result and aggravating purification and
yield problems during the preparation of enone 11c (cf.
Supporting Information), we abandoned this route toward 1
as unexpedient.
A better working approach to the bicyclo[4.4.0]decenone
4c was established by the cis-selective cyclohexanone
annulation shown in Scheme 2. It is a five-step, one-pot
process, novel (to the best of our knowledge), and entails
the following course of events: A Michael addition (1) of
the 8-derived lithium enolate to the cyclohexadienoate 6
delivers the methyl ester enolate 12. The latter undergoes
O-acylation intramolecularly (2) so that the ketene acylal
13 is formed. The addition of vinylmagnesium bromide (3)
generates hemiacylal 14, which ring-opens (4) upon warming
and delivers the enone-substituted ester enolate 15, which
in turn ring-closes by an intramolecular Michael addition
(5).
(4) (a) Tietze, L. F.; Brasche, G.; Gericke, K. M. Domino Reactions in
Organic Synthesis; Wiley-VCH: Weinheim, 2006. (b) Nicolaou, K. C.;
Edmonds, D. J.; Bulger, P. G. Angew. Chem. 2006, 118, 7292–7344.
Nicolaou, K. C.; Edmonds, D. J.; Bulger, P. G. Angew. Chem., Int. Ed.
2006, 45, 7134–7186. (c) Nicolaou, K. C.; Montagnon, T.; Snyder, S. A.
Chem. Commun. 2003, 551–564.
(5) Reviews about Diels-Alder reactions: (a) Marsault, E.; Toro, A.;
Nowak, P.; Deslongchamps, P. Tetrahedron 2001, 57, 4243–4260. (b) Corey,
E. J. Angew. Chem. 2002, 114, 1724–1741. Corey, E. J. Angew. Chem.,
Int. Ed. Engl. 2002, 41, 1650–1667. (c) Nicolaou, K. C.; Snyder, S. A.;
Montagnon, T.; Vassilikogiannakis, G. Angew. Chem. 2002, 114, 1742–
1773. Nicolaou, K. C.; Snyder, S. A.; Montagnon, T.; Vassilikogiannakis,
G. Angew. Chem., Int. Ed. Engl. 2002, 41, 1668–1698. (d) Takao, K.;
Munakata, R.; Tadano, K. Chem. ReV. 2005, 105, 4779–4807. (e) Ess, D. H.;
Jones, G. O.; Houk, K. N. AdV. Synth. Catal. 2006, 348, 2337–2361. (f)
Wessig, P.; Mueller, G. Chem. ReV. 2008, 108, 2051–2063. (g) Reymond,
S.; Cossy, J. Chem. ReV. 2008, 108, 5359–5406.
(6) First report: (a) Lee, R. A. Tetrahedron Lett. 1973, 3333–3336.
Representative applications: (b) Spitzner, D.; Engler, A. Organic Syntheses;
Wiley: New York, 1993; Collect. Vol. 8, pp 219-222, and references cited
therein.
(7) Nazarov, I. N.; Zavyalov, S. I. Zh. Obshch. Khim. 1953, 23, 1703–
1705. Nazarov, I. N.; Zavyalov, S. I. Russ. J. Org. Chem. 1953, 23, 1793–
1794. Nazarov, I. N.; Zavyalov, S. I. Chem. Abstr. 1954, 48, 13667h.
(8) First report: (a) Lavalle´e, J.-F.; Deslongchamps, P. Tetrahedron Lett.
1988, 29, 5117–5118. (b) Lead reference for subsequent studies: Chen, L.;
Deslongchamps, P. Can. J. Chem. 2005, 83, 728–740. Erratum: Chen, L.;
Deslongchamps, P. Can. J. Chem. 2005, 83, 2144.
(9) The considerable volatility of silyldienolethers 7a-c and their
sensitivity toward hydrolysis may account in part for the low yields preparing
them.
(10) Kaliappan, K.; Subba Rao, G. S. R. J. Chem. Soc., Perkin Trans.
1 1997, 3387–3392.
Org. Lett., Vol. 11, No. 21, 2009
4843

reaction mixture gradually warmed to room temperature. This
modus procedendi rendered cis-bicyclo[4.4.0]decenone 4c
in 39% yield.
Scheme 2. Cyclohexanone Annulation by a Domino Process
To complete the total synthesis of (()-isoacanthodoral (1),
bicyclo[4.4.0]decenone 4c had to be defunctionalized at C-2
(cf. Scheme 3) and C1-elongated at C-4a′ (cf. Scheme 4).
Scheme 3.
Defunctionalization of Ketone 4c at C-2a
All in all, the cis-bicyclo[4.4.0]decenone 4c resulted as a
single diastereomer in 38% yield as unambiguously proven
by the crystal structure analysis featured as Figure 3.
a ZrBDC ) Zr(BH4)2Cl2(dabco)2.14
All attempts to defunctionalize bicyclo[4.4.0]decenone 4c
at C-2 by a Wolff-Kishner reduction, i.e., in a single
operation, failed no matter whether under the Huang-Minlon
(N2H4·H2O, KOH, 120 °C; 200 °C)15 or Cram (N2H4·H2O,
KOtBu, 70 °C)16 conditions. As the prelude to another one-
step reduction bicyclo[4.4.0]decenone 4c was converted into
the tosylhydrazone 16 (75% yield). Yet contrary to our
expectations, 16 could not be turned into the half-saturated
ester 18 by a variety of reducing agents: catecholborane,17
NaBH3CN,18 or Zr(BH4)2Cl2(dabco)2.14 Gratifyingly, a Sha-
piro olefination19 of tosylhydrazone 16 rendered dienoic ester
17, which could be hydrogenated completely regioselectively
at the less substituted CdC double bond using 5% Rh on
charcoal as a catalyst. Therefore, the monounsaturated ester
18 resulted in 86% yield over the two steps.
Our initial plan called for transforming the carboxylic
center C-4a′ of ester 18 into the CH2-CHdO moiety of the
target structure (1) either by the attack of an appropriate C1-
nucleophile upon the corresponding alkylating agent 22 or
by advancing to the epoxide 23 and subjecting the latter to
a semipinacol rearrangement (Scheme 4). En route to both
options, ester 18 was reduced with DIBAL to give the
“neopentylic” alcohol 19. Attempted transformations of 19
into alkylating agents 22 failed for a variety of leaving groups
Figure 3. ORTEP plot of bicyclo[4.4.0]decenone 4c (ellipsoids:
50% probability; all H atoms other than 8a-H were omitted for
clarity).11
The phenyl ester of thioisobutyric acid was less suited for
providing ketene acylal 13 and the annulation product 4c
than the sulfur-free analogue 8. The lithium enolates of
methyl or tert-butyl isobutyrate underwent Michael additions
to 6, i.e., step 1 of Scheme 2 without step 2 ensuing.
Employing vinyllithium rather than vinylmagnesium bromide
in step 3 lowered the yield by more than 50%.
In support of the proposed mechanism we were able to
isolate the ketene acylal intermediate 13 after workup at
0 °C.12,13 The purified specimen of 13 continued to react
according to the domino process of Scheme 2 when it was
treated with vinylmagnesium bromide at -78 °C, and the
(11) CCDC 743905 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via the link www.ccdc.cam.ac.uk/
data_request/cif.
(14) Alinezhad, H.; Tajbakhsh, M.; Salehian, F. Synlett 2005, 170–172.
(15) Method : Tenius, B. S. M.; Schroeder, E. K. Synth. Commun. 2000,
30, 1371–1378.
(16) Method: Secci, F.; Frongia, A.; Ollivier, J.; Piras, P. P. Synthesis
2007, 999–1002.
(12) Regarding the oxidation states of the dioxygenated carbon atoms,
ketene acylal 13 can be viewed as a monoenol ether of a carboxylic
anhydride. This structural motif can be found in more than 800 compounds,
most of them aromatic. For selected nonaromatic examples (like 13), see
(17) Method: Chapdelaine, M. J.; Warwick, P. J.; Shaw, A. J. Org.
Chem. 1989, 54, 1218–1221.
(18) Method : Freitas, J. M.; Abrantes, L. M.; Darbre, T. HelV. Chim.
Acta 2005, 88, 2470–2478.
ref 13
.
(13) (a) Reid, E. B. J. Am. Chem. Soc. 1950, 72, 2853–2855. (b) White,
E. H.; Steinmetz, M. G.; Miano, J. D.; Wildes, P. D.; Morland, R. J. Am.
Chem. Soc. 1980, 102, 3199–3208. (c) Mezger, F.; Simchen, G.; Fischer,
(19) Reviews about Shapiro olefinations: (a) Shapiro, R. H. Org. React.
1976, 23, 405–507. (b) Adlington, R. M.; Barrett, A. G. M. Acc. Chem.
Res. 1983, 16, 55–59. (c) Chamberlin, A. R.; Bloom, S. H. Org. React.
1990, 39, 1–83.
P. Synthesis 1991, 375–378
.
4844
Org. Lett., Vol. 11, No. 21, 2009

(X ) Br, I, OTs, and OMs) or led to a Wagner-Meerwein
rearrangement product. In situ activation of alcohol 19 under
the conditions of a Mukaiyama redox condensation in the
presence of acetone cyanohydrin as a cyanide source20 was
similarly unsuccessful.
(not optimized, vide infra).24 At odds with the formation of
24 from this undesired semipinacol rearrangement, a delib-
erately induced semipinacol rearrangement of the 23/24
mixture by an established procedure, i.e., exposure to 1
mol % of Er(OTf)3,25 did not convert its fraction of epoxide
23 into the desired (target!) aldehyde as expected,25 but
delivered another rearrangement product.
After extensive experimentation, it was found that alde-
hyde 20 could be homologized in 95% yield to give the title
compound (1):26 by an initial reductive cyanation with
tosylmethyl isocyanide21 and a subsequent reduction of nitrile
21 (Scheme 4). This procedure allowed a remarkably facile
C1-elongation in a neopentylic position in a remarkably good
yield.
Scheme 4
.
C1-Elongation of Ester 18 at C-4′ and Completion of
the Synthesisa
In summary, we have developed a novel cis-selective
cyclohexanone annulationscombining the enolate of ester
8, the R,ꢀ-unsaturated ester 6, and vinylmagnesium bromide
in a domino reactionsand applied it to a total synthesis of
(()-isoacanthodoral (1). Eleven steps in the longest linear
sequence and 12% overall yield make our approach signifi-
cantly shorter than its predecessor. Studies toward an
application of our cyclohexanone annulation in the total
synthesis of tricyclic terpenoids as well as toward an
asymmetric annulation variant are underway in our labora-
tory.
a TosMIC ) tosylmethyl isocyanide.21
Acknowledgment. We thank Dr. J. Geier (Institut fu¨r
Organische Chemie und Biochemie, Albert-Ludwigs-Uni-
versita¨t Freiburg) for solving the X-ray structure of 4c, Dr.
M. Keller (Institut fu¨r Organische Chemie und Biochemie,
Albert-Ludwigs-Universita¨t Freiburg) for NMR experiments,
and BASF SE for generous gifts of chemicals.
Coming closer to success, oxidation of alcohol 19 with
the Dess-Martin periodinane22 furnished aldehyde 20 in
91% yield (Scheme 4). When 20 was treated with methyl-
lithium in the presence of diiodomethane,23 the desired
epoxide 23 became accessible in 42% yield as a 60:40
mixture with ketone 24. The latter must have arisen from
an over-reaction of 23 through a semipinacol rearrangement
Note Added after ASAP Publication. There were errors
in the TOC/abstract graphic in the version published ASAP
September 23, 2009; the correct version published ASAP
September 24, 2009.
(20) Method: Ghosh, A. K.; Moon, D. K. Org. Lett. 2007, 9, 2425–
2427.
(21) Modification of the method developed by: van Leusen, A. M.;
Oomkes, P. G. Synth. Commun. 1980, 10, 399–403.
(22) Method: Dess, D. B.; Martin, J. C. J. Org. Chem. 1983, 48, 4155–
4156.
Supporting Information Available: Experimental pro-
cedures, characterization data, and copies of NMR spectra.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(23) Method: Tam, N. T.; Chang, J.; Jung, E.-J.; Cho, C.-C. J. Org.
Chem. 2008, 73, 6258–6264.
(24)
OL9018979
(25) Procopio, A.; Dalpozzo, R.; De Nino, A.; Nardi, M.; Sindona, G.;
Tagarelli, A. Synlett 2004, 2633–2635.
(26) All spectroscopic and analytical data of 1 were identical to those
reported previously.3c
Org. Lett., Vol. 11, No. 21, 2009
4845