Total Synthesis of (+)-Acanthodoral by the Use of a Pd-Catalyzed Metal-ene Reaction and a Nonreductive 5-exo-Acyl Radical Cyclization

Article abstract of DOI:10.1021/ol0363063

(Equation presented) The first total synthesis of the antibiotic acanthodoral (1) has been achieved from 3-methyl-2-cyclohexen-1-one in 19 steps in 2.1% overall yield. The synthesis features the use of a Pd-ene reaction in the presence of CO to form the endocyclic alkene 8, a nonreductive acyl radical cyclization reaction, and a ring contraction reaction by the Wolff rearrangement. (+)-Acanthodoral has also been synthesized starting from (+)-S-2,2-dimethyl-6-methylenecyclohexanecarboxylic acid.

Full text of DOI:10.1021/ol0363063

ORGANIC
LETTERS
2004
Vol. 6, No. 4
537-540
Total Synthesis of (+)-Acanthodoral by
the Use of a Pd-Catalyzed Metal-ene
Reaction and a Nonreductive 5-exo-Acyl
Radical Cyclization
Liming Zhang† and Masato Koreeda*
,†,‡
Departments of Chemistry and Medicinal Chemistry, UniVersity of Michigan,
Ann Arbor, Michigan 48109-1055
koreeda@umich.edu
Received November 25, 2003
ABSTRACT
The first total synthesis of the antibiotic acanthodoral (1) has been achieved from 3-methyl-2-cyclohexen-1-one in 19 steps in 2.1% overall
yield. The synthesis features the use of a Pd-ene reaction in the presence of CO to form the endocyclic alkene 8, a nonreductive acyl radical
cyclization reaction, and a ring contraction reaction by the Wolff rearrangement. (+)-Acanthodoral has also been synthesized starting from
(+)-S-2,2-dimethyl-6-methylenecyclohexanecarboxylic acid.
As in the case of countless secondary metabolites produced
by other marine organisms,1 those from the dorid nudibranchs
Acanthodoris nanaimoensis include terpenoids with un-
precedented carbon skeletons.2
of acanthodoral was postulated as shown for structure 1.2,3
The absolute stereochemistry at C-8 of nanaimoal (2) has
been validated as R by its total synthesis from a compound
with a known configuration.4,5
Of these, the structure of acanthodoral (1) contains the
highly strained bicyclo[3.1.1]heptane framework. Although
acanthodoral contaminated with the other two coexisting
sesquiterpene aldehydes 2 and 3 was shown to exhibit strong
antibiotic activity, the inherent activity of acanthodoral itself
could not be assessed due to the difficulty in its isolation in
pure form.2 The structure and relative stereochemistry of
acanthodoral were established by the X-ray analysis of the
urethane derivative, 4.2 Although its optical rotation has not
been determined, on the basis of biosynthetic studies
demonstrating that acanthodoral is derived from nanaimoal
(2) and/or isoacanthodoral (3), the absolute stereochemistry
In connection with our continued interest in the develop-
ment of approaches toward the synthesis of strained natural
products, we set out to investigate the total synthesis of
acanthodoral (1).6,7 Our initial attempts at directly construct-
† Department of Medicinal Chemistry.
‡ Department of Chemistry.
(3) Graziani, E. I.; Andersen, R. J. J. Am. Chem. Soc. 1996, 118, 4701-
4702.
(4) Ayer, S. W.; Hellou, J.; Tischler, M.; Andersen, R. J. Tetrahedron
Lett. 1984, 25, 141-144.
(5) Omodani, T.; Shishido, K. J. Chem. Soc., Chem. Commun. 1994,
2781-2782.
(1) For a recent review, see: Blunt, J. W.; Copp, B. R.; Munro, M. H.
G.; Northcote, P. T.; Prinsep, M. R. Nat. Prod. Rep. 2003, 20, 1-48.
(2) (a) Ayer, S. W.; Andersen, R. J.; He, C. H.; Clardy, J. J. Org. Chem.
1984, 49, 2653-2654. (b) Ayer, S. W. Ph.D. Thesis, University of British
Columbia, 1985.
10.1021/ol0363063 CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/27/2004

ing the bicyclo[3.1.1]heptane system by a tandem 6-endo-
acyl radical/4-exo-alkyl radical cyclization sequence (see,
e.g., structure X above) were not successful presumably due
to its insurmountably severe strain energy (SE).8 We,
therefore, opted for an indirect approach that involves initial
assembly of the substantially less strained bicyclo[3.2.1]-
octane system followed by a ring contraction to the desired
bicyclo[3.1.1]heptane system.
Scheme 2. Synthesis of Hydrindene Carboxylic Acid 8a
a Reagents and conditions: (a) MeMgBr (1.2 mol equiv), CuI
(0.05 mol equiv), LiCl (0.1 mol equiv)/THF, 0 °C, 0.5 h, then CH2O
(gas), -15 °C, 0.5 h; (b) CH2Br2/Zn/TiCl4 (1:3:0.7 molar ratio)
(excess)/CH2Cl2, rt, 0.5 h; (c) Swern oxidation; (d) isopropenyl-
magnesium bromide (2 mol equiv)/THF, 0 °C, then HMPA (20%
v/v), ClCO2Me (2.2 mol equiv); 0 °C f rt; (e) Pd(OAc)2 (0.1 mol
equiv), PPh3 (0.4 mol equiv)/HOAc, CO (1 atm), reflux, 12 h.
As summarized in the retrosynthetic analysis (Scheme 1),
tricyclic ketone 5 is converted into acanthodoral using a
Wolff rearrangement reaction.9 The synthesis of ketone 5
was envisaged as being obtainable from either 6 by a
nonreductive acyl radical recyclization10 or 7 by an intra-
molecular Prins reaction.11 The intermediates 6 and 7 could
readily be accessed from hydrindene acid 8 by ring expansion
of its dibromocyclopropane derivative. Hydrindene acid 8
in turn was predicted to be prepared by the use of a
palladium-mediated metal ene-reaction.
carbonate 12. Treatment of carbonate 12 with Pd(OAc)2/
Ph3P in wet acetic acid at 118 °C under CO13 resulted in the
formation of the five-membered endo-alkene acid 8 in
50% yield. The reaction may be viewed as a Pd-mediated
metalene reaction, and it is likely to proceed through the
σ-allyl complex intermediate Y (see Scheme 2).14,15
The ring expansion of the hydrindene to the 6,6-cis-fused,
functionalized decalin system was achieved by the solvolysis
of the gem-dibromocyclopropane derivative 13, which in turn
was prepared from acid 8 in two steps [(1) K2CO3, MeI/
acetone, rt (92%); (2) CHBr3, CH2Cl2, BnEt3NCl, rt, 50%
aq NaOH]. Thus, simply exposing 13 to a sulfur (16 or 17)
or hydride nucleophile (Et3SiH) in (CF3)2CHOH16 resulted
in the stereo- and regioselective formation of the correspond-
ing decalin product in excellent yield, presumably via the
π-allylic cation intermediate Z (Figure 1). Interestingly, use
of an Ag+ salt17 led to the formation of complex mixtures
of products.
Scheme 1. Retrosynthetic Analysis of Acanthodoral (1)
The key intermediate acid 8 was synthesized starting from
3-methyl-2-cyclohenxen-1-one (9) in five steps (Scheme 2).
Alkenol 11 obtained from ketone 10 by the Takai/Oshima-
Lombardo methylenation12 was transformed into the methyl
(6) See, e.g.: Zhang, L.; Koreeda, M. Org. Lett. 2002, 4, 3755-3758.
(7) For a previous synthetic study toward acanthodoral, see: Engler, T.
A.; Ali, M. H.; Takusagawa, F. J. Org. Chem. 1996, 61, 8456-8463.
(8) Liebman, J. F.; Greenberg, A. Chem. ReV. 1976, 76, 311-365.
(9) Reviews: (a) Gill, G. B. In ComprehensiVe Organic Synthesis; Trost,
B. M., Fleming, I., Eds.; Pergamon Press: Oxford, UK, 1991; Vol. 3, pp
887-912. (b) Ye, T.; McKervey, M. A. Chem. ReV. 1994, 94, 1091-1160.
(c) Kirmse, W. Eur. J. Org. Chem. 2002, 2193-2256.
(10) Review: Chatgilialoglu, C.; Crich, D.; Komatsu, M.; Ryu, I. Chem.
ReV. 1999, 99, 1991-2069.
(11) Reviews: (a) Adams, D. R.; Bhatnagar, S. P. Synthesis 1977, 661-
672. (b) Snider, B. B. In ComprehensiVe Organic Synthesis; Trost, B. M.;
Fleming, I., Eds.: Pergamon Press: Oxford, UK., 1991; Vol. 2, pp 661-
672.
Figure 1. Construction of the Decalin System from 13.
Org. Lett., Vol. 6, No. 4, 2004
538

In an effort to access the bicyclo[3.2.1]octane skeleton by
an intramolecular Prins reaction, aldehyde 18, obtained from
15 in two steps ((1) LiAlH4; (2) Dess-Martin periodinane)
in 95% overall yield, was treated with a catalytic amount of
Et2AlCl in CH2Cl2 at room temperature (Figure 2). While
Scheme 3. Synthesis of Tricyclic Ketone 23a
a Reagents and conditions: (a) MCPBA (2.3 mol equiv)/CH2Cl2,
rt, 4 h (97%); (b) 50% aq NaOH (8/1), PhSH (4 mol equiv), reflux,
6 h (99%); (c) PhOP(dO)Cl2 (2 mol equiv), Et3N (3 mol equiv),
0 °C, 1 h, then PhSeH (3 mol equiv)/Et3N (5 mol equiv), 0 °C, 0.5
h (70%); (d) (Bu3Sn)2 (2 mol equiv)/PhH, sun lamp, reflux, 2 h;
(e) Raney nickel/MeOH, -20 °C, 5 min; (f) H2 (55 psi), PtO2 (cat)/
EtOAc, 2.5 h.
Figure 2. Prins Reaction of Aldehyde 18.
the reaction produced the bicyclo[2.2.2]octane 19 as a 9:1
regioisomeric alkene mixture in 90% yield within 30 min,
favoring the endocyclic alkene, the formation of the desired
tricyclic alcohol with the bicyclo[3.2.1]octane skeleton, 20,
was not observed. In view of the similar SEs of these two
bicyclooctane systems,8 exclusive formation of 19 may be
attributable, at least partially, to the relative carbocation-
stabilizing abilities of the methyl and bromine groups in
intermediates i and ii, respectively.18
Our efforts were then directed to the use of a nonreductive
acyl radical cyclization to construct the bicyclo[3.2.1]octane
skeleton, 23. To obviate the complications resulting from
the direct interaction between the acyl radical and the S atom
introduced as a nucleophile during the solvolysis reaction
of 13, the 4-methoxyphenyl sulfide in 14 was first oxidized
to the corresponding sulfone (Scheme 3). In addition, it was
envisaged that, in view of the electrophilic nature of an acyl
radical, the presence of a bromine atom should make the
radical cyclization less favorable, and a means of replacing
the Br with an electron-donating group was sought. This
group would also have to be readily reductively removable.
Fortuitously, during an attempt to hydrolyze the methyl ester
of 14 with the 4-methoxyphenyl sulfone group (MeOH, aq
NaOH, reflux), it was observed that the bromine atom was
replaced with a methoxyl group in addition to hydrolysis of
the methyl ester. This presumably involved deprotonation
of the R-H to the sulfonyl group followed by replacement
of the Br with MeO in the vinyl sulfone intermediate.
Treatment of the sulfone obtained from sulfide 14 [Nu )
4-(MeO)PhS-] with PhSH under basic conditions resulted
in quantitative formation of the hydrolysis product with the
Br cleanly replaced by the PhS group (Scheme 3). Interest-
ingly, as in the case of using MeO- as the nucleophile, the
corresponding conjugated sulfone 24 was not observed. As
expected, the acyl radical generated from seleno ester 21
under nonreductive conditions underwent regioselective
cyclization to provide tricyclic ketone 22 with a bicyclo-
[3.2.1]octane framework. This was subsequently transformed
into tricyclic ketone 23.
The elaboration of the tricyclic ketone 23 into acanthodoral
(1) was achieved by the Wolff rearrangement reaction of
R-diazoketone 26 (Scheme 4), which was prepared from
R-oxime ketone 25. Treatment of ketone 23 with isoamyl
nitrite in the presence of KOBut resulted in the formation of
a mixture of syn- and anti-oxime derivatives 25 (δ 13.19
(12) (a) Takai, K.; Hotta, Y.; Oshima, K.; Nozaki, H. Tetrahedron Lett.
1978, 2417-2420. (b) Takai, K.; Hotta, Y.; Oshima, K.; Nozaki, H. Bull.
Chem. Soc. Jpn. 1980, 53, 1698-1702. (c) Lombardo, L. Tetrahedron Lett.
1982, 23, 4293-4296. See also: (d) Lombardo, L. Organic Syntheses;
Wiley: New York, 1993; Collect. Vol. 8, pp 386-390.
(13) Oppolzer, W.; Bedoya-Zurita, M.; Switzer, C. Y. Tetrahedron Lett.
1988, 29, 6433-6436.
1
and 8.80 ppm, respectively, for the oxime OH H peaks).
The ratio of these two isomers was dependent upon the
amount of base used. If the reaction was quenched with acetic
acid at -78 °C, the formation of syn isomer was favored by
5:1. In contrast, when a large excess (6 mol equiv) of the
base was used and the reaction mixture was allowed to warm
to room temperature over 1 h, the anti isomer was predomi-
nant (2:1 and 88% combined yield). This ratio favoring
formation of the anti isomer of 25 could not be improved.
However, the isolated syn isomer could be equilibrated to a
2:1 anti/syn mixture under photochemical conditions in
MeOH (rt, 1 h), providng the desired anti isomer in 75%
(14) Review: Oppolzer, W. Angew. Chem., Int. Ed. Engl. 1989, 28, 35-
52.
(15) For another example of a palladium-ene reaction that produces an
endocyclic alkene product, see: Zair, T.; Santellirouvier, C.; Santelli, M.
Tetrahedron 1993, 49, 3313-3324.
(16) Schadt, F. L.; v. R. Schleyer, P.; Bentley, T. W. Tetrahedron Lett.
1974, 2335-2338.
(17) (a) Danheiser, R. L.; Morin, J. M.; Yu, M.; Basak, A. Tetrahedron
Lett. 1981, 22, 4205-4208. (b) Banwell, M. G.; Harvey, J. E.; Hockless,
D. C. R.; Wu, A. W. J. Org. Chem. 2000, 65, 4241-4250.
(18) Olah, G. A.; Mo, Y. K. In Carbonium Ions; Olah, G. A., v. R.
Schleyer, P., Eds.; Wiley-Interscience: New York, 1976; Vol. 5, pp 2135-
2262.
Org. Lett., Vol. 6, No. 4, 2004
539

27 (Scheme 4) with the endo-methoxycarbonyl group as a
single stereoisomer. The endo stereochemistry of the ester
was assigned on the basis of a lack of W-type coupling of
Scheme 4. Synthesis of Acanthodoral (1)a
1
the H NMR peak (δ 2.22 ppm) of the H R to the ester
CdO.20 Ester 27 was then converted to acanthodoral (1) in
two steps in 80% overall yield. The NMR spectral data of
the synthetic alcohol 28 (1H and 13C) and its 4-bromophenyl
urethane derivative 4 (1H) were in complete agreement with
those derived from natural acanthodoral,2b thus confirming
the first total synthesis of acanthodoral (1).
Optically active acanthodoral was synthesized starting
from the known (S)-(+)-2,2-dimethyl-6-mehylenecyclo-
hexanecarboxylic acd (29).21 Reduction of this readily
23
availble acid, [R]D +121 (c 0.10, CHCl3), with LiAlH4
afforded homoallylic alcohol, (+)-11, [R]D23 +27.8 (c 0.29,
CHCl3), with >96% optical purity. Alcohol (+)-11 was
22
converted into (+)-acanthodoral (1), [R]D +15.2 (c 0.23,
CHCl3), following the method described above for the
synthesis of its racemate. Although no information pertaining
to the absolute configuration of acanthodoral has been
reported, on the basis of biogenetic considerations (vide ante),
natural acanthodoral is likely to have a dextrorotatory optical
rotation.
In summary, the first total synthesis of the marine
sesquiterpene aldehyde acanthodoral (1) has been achieved
commencing with 3-methyl-2-cyclohexen-1-one (9) in 19
steps in 2.1% overall yield. Construction of the highly
strained bicyclo[3.1.1]heptane skeleton was realized by
employing a Wolff rearrangment of the corresponding
R-diazoketone with the bicyclo[3.2.1]octane system (26). The
tricyclic ketone 23 was accessed using a unique palladium-
mediated ene reaction of allylic carbonate 12 in the presence
of CO, solvolytic ring expansion to the decalin, and a
nonreductive acyl radical cyclization.
a Reagents and conditions: (a) t-BuOK (6 mol equiv), isoamyl
nitrite (1.5 mol equiv), 1 h at -78 °C, then 0.5 h at room
temperature; (b) NaOCl (excess), NH4OH, 0 °C; (c) MeOH, hν, rt,
2 h; (d) LiAlH4 (1.6 mol equiv), Et2O, rt, 2 h; (e) Dess-Martin
periodinane (1.2 mol equiv)/CH2Cl2, rt, 0.5 h; (f) 4-bromophenyl
isocyanate (4 mol equiv)/CCl4, 60 °C, 2 h.
overall yield from 23 after one recycle. Unexpectedly, only
the anti-oxime 25 could be converted into R-diazoketone 26
under the Foster conditions.19 Of the two mechanisms
suggested for the Forster reaction,19b the one involving attack
of NH2Cl by the lone-pair electrons of the oxime N appears
to explain the observed result. Namely, the severe steric
conjestion surrounding the oxime N in the syn isomer 25
might not allow a nucleophilic reaction that involves the
oxime N lone pair to take place.
Acknowledgment. We are grateful to Professor R. J.
Andersen of the University of British Columbia for providing
us with a copy of the NMR spectra of alcohol 28 and
urethane 4. L.Z. thanks the Eli Lilly Company for a 2002-
2003 Industrial Predoctoral Fellowship.
Scheme 5. Synthesis of (+)-Acanthodoral (1)
Supporting Information Available: Experimental details
as well as spectra of all new compounds described. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL0363063
(19) (a) Foster, M. O. J. Chem. Soc. 1915, 107, 260. (b) Meinwald, J.;
Gassman, P. G.; Miller, E. G. J. Am. Chem. Soc. 1959, 81, 4751-4752.
(20) Coates, R. M.; Kang, H.-Y. J. Org. Chem. 1987, 52, 2065-2074.
(21) Tanimoto, H.; Oritani, T. Tetrahedron 1997, 53, 3527-3536.
Irradiation of diazoketone 26 with UV light in dry MeOH
at room temperature cleanly induced a Wolff rearrangement
to provide the ring-contracted bycyclo[3.1.1]heptane system
540
Org. Lett., Vol. 6, No. 4, 2004