Efficient construction of the clerodane decalin core by an asymmetric Morita-Baylis-Hillman reaction/Lewis acid promoted annulation strategy

Article abstract of DOI:10.1002/anie.200601076

(Chemical Equation Presented) Linking rings: A general route toward the clerodane diterpene core using an asymmetric Morita-Baylis-Hillman (MBH)/ Lewis acid mediated ring-annulation process has been developed. The scope of the asymmetric Bronsted acid cat

Full text of DOI:10.1002/anie.200601076

Angewandte
Chemie
Asymmetric Catalysis
DOI: 10.1002/anie.200601076
Efficient Construction of the Clerodane Decalin
Core by an Asymmetric Morita–Baylis–Hillman
Reaction/Lewis Acid Promoted Annulation
Strategy**
Stacy A. Rodgen and Scott E. Schaus*
Dedicated to Professor James S. Panek
on the occasion of his 50th birthday.
The clerodane class of natural products are diterpenes that
exhibit wide-ranging structural diversity.[1] Over 150 new
bioactive clerodanes have been reported since 2002.[2] Of
particular interest are asmarines A (1) and B (2)[3] and
popolohuanone E (3),[4] members of this class of natural
products that exhibit potent antiproliferative activity against
several types of human-cancer-cell lines (Scheme 1).[5] Popo-
lohuanone E is a topoisomerase II inhibitor,[4] whereas the
biological target of asmarine A or B is not known. Given their
biological activity and the prevalence of the structural motif
they display, a general and efficient strategy towards the core
structure of the clerodane would be attractive.
[*] S. A. Rodgen, Prof. Dr. S. E. Schaus
Department of Chemistry
Metcalf Center for Science and Engineering
Boston University, 590 Commonwealth Avenue
Boston, Massachusetts, 02215 (USA)
Fax: (+1)617-353-6466
E-mail: seschaus@bu.edu
[**] The authors acknowledge Dr. J. P. Lee (Boston University) for
assistance with key NMR experiments and Dr. E. B. Lobkovsky
(Cornell University) for X-ray crystallographic analysis. This research
was supported by a NSF CAREER grant (CHE-0349206) and Amgen,
Inc.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Table 1: Brønsted acid catalyzed asymmetric MBH reactions.[a]
Scheme 1. Biologically active clerodane natural products.
Entry
1
Aldehyde
Yield [%][b]
ee[c] [%]
Synthesis of the diterpene core structure has focused on
elaboration of the Wieland–Miescher ketone.[6] Complemen-
tary approaches have elegantly utilized diastereoselective
ring-annulation strategies towards substituted decalin struc-
tures; however, these approaches have mainly been race-
mic.[7] Recently, we reported the asymmetric Morita-Baylis-
Hillman (MBH) reaction of cyclohexenone with aldehydes
promoted by trialkyl phosphines and catalyzed by binapthol-
derived Brønsted acids.[8] We envisioned an asymmetric
synthetic strategy toward the clerodane decalin core through
a two-step ring-annulation procedure (Scheme 2).[9] The first
8a (75)
86
2
3
4
5
8b (94)
8c (96)
8d (80)
8e (75)
98
93[d]
90
91[d]
6
7
8 f (94)
8g (97)
93
98
[a] Reactions were run with 6 (1 mmol), cyclohexanone (2 mmol), PEt3
(2 mmol), and (R)-7 (0.1 mmol) in THF (1m) at ꢀ108C for 48 h under
argon followed by flash chromatography on silica gel. [b] Yield of the
isolated product. [c] Determined by chiral HPLC analysis. [d] Enantio-
meric excess of the major olefin isomer. TBS=tributyldimethylsilyl,
TMS=trimethylsilyl.
Scheme 2. Retrosynthetic analysis of asmarine A (1), thus illustrating
the key MBH building block 5.
We first considered alkynyl and vinyl silanes in the
reaction (Table 1, entries 1 and 2). Although the general
reaction conditions afforded the alkyne-containing product
8a in only 86% ee, the vinyl silane containing aldehyde
underwent a more selective reaction (98% ee). The MBH
reaction conditions proved general for allyl silane containing
aldehydes 6c–g (Table 1, entries 3–7). The reaction of these
aldehydes with cyclohexenone promoted by PEt3 and
10 mol% of catalyst 7 in THF at ꢀ108C afforded the
corresponding MBH products 8c–8g in good yields (75–
97%) and with high enantioselectivities (90–99% ee). The
successful MBH reactions of this substrate class illustrated
that acid-sensitive, multifunctional aldehydes of this type
could be tolerated in the reaction. With the successful
production of these MBH products, we began our investiga-
tion of the Lewis acid promoted ring annulation as a way to
access the desired decalin ring system.
step would be an asymmetric MBH reaction of cyclohex-
enone with an aldehyde functionalized with an appropriate
nucleophile[10] followed by a Lewis acid promoted ring
formation.[11] The ring-annulation strategy we chose was an
intramolecular Hosomi–Sakauri reaction[12] that required the
synthesis and use of aldehydes containing allyl silanes in the
asymmetric MBH reaction. Herein, we report the construc-
tion of the clerodane decalin core through an asymmetric
MBH reaction/Lewis acid promoted annulation strategy.
The strategy relies on two key experimental observations.
First, the allyl silyl containing aldehyde must afford the MBH
product with high enantioselectivity. Second, the enantiomer-
ic excess of the product must be maintained during the ring-
annulation process. We initially evaluated the scope of the
MBH reaction of cyclohexenone with unsaturated silane
containing aldehydes (Table 1). We found the Brønsted acid
catalyzed phosphine-promoted MBH reaction conditions
were mild enough to tolerate a variety of silane-containing
aldehydes.[13]
Experiments were carried out to determine the feasibility
of a diastereoselective ring annulation of 8c. A selection of
Lewis acids (BF3·OEt2, [TiCl4], [Yb(OTf)3], [Sc(OTf)3], and
MgBr2; OTf = triflate) were evaluated in the reaction for their
ability to affect the intramolecular ring formation diastereo-
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Angewandte
Chemie
selectively and result in high yields while maintaining the
enantiomeric excess during the reaction.[7a] Although many of
these Lewis acids were capable of affecting ring formation,
BF3·OEt2 was found to be optimal for yield and chemo-
selectivity. Treatment of allyl silane 8c with BF3·OEt2 at ꢀ78
to ꢀ108C resulted in efficient ring formation to afford decalin
9 in 88% yield of the isolated product as a single diastereomer
(Scheme 3). The enantiomeric excess of the product was
Scheme 4. Synthesis of clerodane core 4. a) 1. ClMgCH2SiMe2Ph, Et2O,
08C; 2. IBX, EtOAc, 768C; b) (R)-Me-CBS (0.4 equiv), BH3, THF,
=
ꢀ508C; c) 1. Hg(OAc)2 (0.028 equiv), EtOCH CH2, 358C; 2. chroma-
tography on silica gel; d) cyclohexenone, PEt3, (R)-7 (0.1 equiv), THF,
ꢀ108C, 48 h; e) BF3·OEt2, CH2Cl2, ꢀ78!ꢀ108C. IBX=o-iodoxyben-
zoic acid. (R)-Me-CBS=(R)-methyl oxazaborolidine.
the resulting alcohol with IBX[15] in ethyl acetate afforded the
ketone in 90% yield. Asymmetric reduction of the unsatu-
rated ketone with BH3 catalyzed by the Corey (R)-Me-CBS
catalyst[16] provided the requisite chiral allylic alcohol 12 in
95% ee. Formation of the vinyl ether was carried out in
refluxing ethyl vinyl ether and catalyzed by Hg(OAc)2.[17]
A
stereoselective [3,3] sigmatropic rearrangement was found to
proceed upon chromatography on silica gel to give the
aldehyde in 85% yield.[17] The asymmetric MBH reaction of
aldehyde 13 with cyclohexenone using the Brønsted acid
catalyst (R)-7 afforded alcohol 14 in 86% yield of the isolated
product and 99% de. The intramolecular Hosomi–Sakuari
reaction using BF3·OEt2 resulted in the clean formation of the
desired clerodane core structure 4 in 81% yield of isolated
product and 98% de. Based on our originally proposed
transition state, the new methyl substituent in the six-
membered transition state adopted an equatorial position
that reinforced the chairlike transition state to yield trans
decalin 4. The substituents on the allyl silane work synergisti-
cally to produce high levels of diastereoselectivity; an
approach that has previously been met with mixed suc-
cess.[7c–d]
In summary, we have developed a general route to the
clerodane diterpene core by using an asymmetric MBH/Lewis
acid mediated ring-annulation process. We have expanded the
scope of the asymmetric MBH reaction to include silane-
containing aldehydes that can be utilized in synthesis. We
have elaborated these MBH products into the trans decalin
core by using an intramolecular Lewis acid promoted ring
annulation. Utilization of this synthetic methodology in the
synthesis of bioactive clerodanes is underway and will be
reported in due course.
Scheme 3. Ring-annulation reactions of allyl silane containing MBH
products 1) 8c and 2) 8g. a) BF3·OEt2, CH2Cl2, ꢀ78!ꢀ108C; b) dini-
trophenylhydrazine, EtOH, RT. X-ray structure of 10.
determined to be 93% ee by chiral HPLC chromatography.
The formation of the trans decalin bicylic ring structure was
confirmed by X-ray crystallographic analysis of the corre-
sponding dinitrophenyl hydrazone 10. The observed selectiv-
ity can be rationalized by a chairlike transition state that
places the secondary alcohol in an equatorial position.
Protonation of the resulting enolate after the conjugate
addition affords the thermodynamically favored trans decalin
system. The reaction conditions using BF3·OEt2 proved
equally effective at promoting the ring annulation of allyl
silane 8g. The bicyclic product was formed in 85% yield
without a significant change in the enantiomeric excess. The
formation of trans decalin was confirmed by an observed
ꢀ
NOE interaction between the axial CH3 group and the axial
methine hydrogen atom.
We next set out to construct the chiral aldehyde required
for the synthesis of the clerodane core structure through the
two-step asymmetric MBH reaction/Lewis acid promoted
ring-annulation strategy. Our strategy for the synthesis of 13
relied on an asymmetric reduction followed by a stereoselec-
tive [3,3] sigmatropic rearrangement of the corresponding
vinyl ether (Scheme 4).[14] The Gringard reaction of tiglic
aldehyde with ClMgCH2SiMe2Ph followed by oxidation of
Received: March 19, 2006
Published online: June 27, 2006
Angew. Chem. Int. Ed. 2006, 45, 4929 –4932
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www.angewandte.org
4931

Communications
Keywords: asymmetric catalysis · Brønsted acids · Morita–
.
Baylis–Hillman reaction · organocatalysis · phosphanes ·
synthetic methods
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