A simple stereocontrolled synthesis of salinosporamide A

Article abstract of DOI:10.1021/ja048613p

A simple and effective stereocontrolled synthesis of salinosporamide A has been developed. This process, the first synthesis of salinosporamide A, is capable of providing the compound in substantial quantities for further biological studies. Salinosporamide A was of special interest as a synthetic target because of its potent in vitro cytotoxic activity against many tumor cell lines (IC₅₀ values of 10 nM or less). Copyright

Full text of DOI:10.1021/ja048613p

Published on Web 04/22/2004
A Simple Stereocontrolled Synthesis of Salinosporamide A
Leleti Rajender Reddy, P. Saravanan, and E. J. Corey*
Department of Chemistry and Chemical Biology, HarVard UniVersity, Cambridge, Massachusetts 02138
Received March 10, 2004; E-mail: corey@chemistry.harvard.edu
Salinosporamide A (1) was recently discovered by Fenical and
his group as a bioactive product of a marine microorganism that is
widely distributed in ocean sediments.1 Structurally it closely
resembles the terrestrial microbial product omuralide2a (2a) that
we synthesized several years ago and demonstrated to be a potent
inhibitor of proteasome function.2 Omuralide is generated by
â-lactonization of the N-acetylcysteine thiolester lactacystin (2b)
that was first isolated by the Omura group as a result of microbial
screening for nerve growth factor-like activity.3 Salinosporamide
A is an even more effective proteasome inhibitor than omuralide,
and, in addition, it displays surprisingly high in vitro cytotoxic
activity against many tumor cell lines (IC50 values of 10 nM or
less). We report herein the first enantiospecific total synthesis
of 1.
The pathway of the synthesis of salinosporamide A is outlined
in Scheme 1. (S)-Threonine methyl ester was N-acylated with
Scheme 1
9
6230
J. AM. CHEM. SOC. 2004, 126, 6230-6231
10.1021/ja048613p CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
4-methoxybenzoyl chloride in CH2Cl2 at 23 °C to form the amide
3 (71%) which was then cyclized to oxazoline 4 (80%) by heating
at reflux in toluene with p-toluenesulfonic acid. Deprotonation of
4 with lithium diisopropylamide in THF and alkylation of the
resulting enolate with chloromethyl benzyl ether afforded the
required tertiary stereocenter of 5 selectively in 69% yield.
Reduction of 5 with NaBH3CN-HOAc gave the N-4-methoxy-
benzylamine 6 (90%) which was then transformed in 96% yield to
the N-acrylyl-N-PMB derivative (PMB ) 4-methoxybenzyl) by the
one-flask sequence: (1) reaction with Me3SiCl and Et3N to form
the TMS ether, (2) acylation with acrylyl chloride at 0 °C, and (3)
acidic work up with aqueous HCl. Dess-Martin periodinane
oxidation of 7 produced the keto amide ester 8 in 96% yield.
Cyclization of 8 to the γ-lactam 9 was accomplished by means of
an internal Baylis-Hillman-aldol reaction4 using quinuclidine as
the catalytic base in dimethoxyethane at 0 °C for 7 d. The
cyclization product, obtained in 90% yield, consisted of 9 and the
C(â) diastereomer (10) in a ratio of 9:1. The N-benzyl analogue of
10 was obtained in crystalline form, mp ) 136-7 °C, and was
demonstrated to possess the stereochemistry shown for 10 by single-
crystal X-ray diffraction analysis. When the internal aldol reaction
of 8 was conducted at 20 °C for 9 h, 9 and 10 were obtained in
90% yield and a ratio of 4:1. Silylation of 9 with bromometh-
yldimethylsilyl chloride afforded 11 in 95% yield. Silyl ether 11
and the diastereomeric silyl ether were easily and conveniently
separated at this stage by silica gel column chromatography on a
preparative scale.
under a wide variety of conditions gave considerably lower yields
than the process shown in Scheme 1 mainly because of competing
O-acylation and subsequent further transformations. So far, qui-
nuclidine has proved superior to other catalytic bases, for example,
1,4-diaza[2.2.2] bicyclooctane, for the cyclization of 8 to 9. As
indicated just above, the attachment of the 2-cyclohexenyl group
to aldehyde 13 to form 14 worked best with the reagent 2-cyclo-
hexenylzinc chloride.7 Attempts to form 14 from 13 using Lewis
acid-catalyzed reaction with tri-n-butyl-2-cyclohexenyltin were
totally unsuccessful. The saponification of methyl ester 16 at
temperatures above +5 °C led to lowered yields of the required
carboxylic acid. Finally, the one-flask â-lactonization and chlorina-
tion reactions leading to 1 were remarkably clean and probably
proceed in >90% yield per step.
In summary, this paper describes an efficient and short total
synthesis of salinosporamide A that is capable of providing
substantial quantities of this currently rare substance for further
biological study, especially to determine its potential as an anti-
cancer agent.
Acknowledgment. We thank Pfizer Inc. for generous financial
support and Dr. William Fenical for a reference sample of
salinosporamide A.
Supporting Information Available: Experimental procedures for
the synthetic sequences described herein, together with characterization
data for reaction products. X-ray diffraction data (CIF) are provided
for the N-benzyl analogue of 10. This material is available free of charge
via the Internet at http://pubs.acs.org.
The required stereochemical relationship about C(R) and C(â)
of the γ-lactam core was established by tri-n-butyltin hydride-
mediated radical-chain cyclization which transformed 11 cleanly
into the cis-fused γ-lactam 12.5 Cleavage of the benzyl ether of 12
(H2, Pd-C) and Dess-Martin periodinane oxidation provided the
aldehyde 13 in ca. 90% yield from 12. The next step, the attachment
of the 2-cyclohexenyl group to the formyl carbon and the establish-
ment of the remaining two stereocenters, was accomplished in a
remarkably simple way. 2-Cyclohexenyl-tri-n-butyltin (from Pd-
(0)-catalyzed 1,4-addition of tributyltin hydride to 1,3-cyclohexa-
diene)6 was sequentially transmetalated by treatment with 1 equiv
of n-butyllithium and 1 equiv of zinc chloride to form 2-cyclohex-
enylzinc chloride in THF solution. Reaction of this reagent with
the aldehyde 13 furnished the desired formyl adduct stereoselec-
tively (20:1) in 88% yield.7 Tamao-Fleming oxidation8 of 14 gave
the triol 15 in 92% yield. Ce(IV)-induced oxidative cleavage of
the PMB group of 15 afforded the triol ester 16 which was
hydrolyzed to the corresponding γ-lactam-carboxylic acid using 3
N lithium hydroxide in aqueous THF at 4 °C. This acid was
converted to salinosporamide A (1) (65% overall yield) by
successive reaction with 1.1 equiv of bis-(2-oxo-3-oxazolidinyl)
phosphinic chloride (BOPCl) and pyridine at 23 °C for 1 h (to form
the â-lactone) and then 2 equiv of triphenylphosphine dichloride
in CH3CN-pyridine at 23 °C for 1 h. The identity of synthetic 1
and natural salinosporamide A was established by comparison
References
(1) Feling, R. H.; Buchanan, G. O.; Mincer, T. J.; Kauffman, C. A.; Jensen,
P. R.; Fenical, W. Angew. Chem., Int. Ed. 2003, 42, 355-357.
(2) (a) Reviewed in: Corey, E. J.; Li, W.-D. Z. Chem. Pharm. Bull. 1999,
47, 1-10. (b) Corey, E. J.; Reichard, G. A.; Kania, R. Tetrahedron Lett.
1993, 34, 6977-6980. (c) Corey, E. J.; Reichard, G. A. J. Am. Chem.
Soc. 1992, 114, 10677-10678. (d) Fenteany, G.; Standaert, R. F.;
Reichard, G. A.; Corey, E. J.; Schreiber, S. L. Proc. Natl. Acad. Sci. U.S.A.
1994, 91, 3358-3362.
(3) (a) Omura, S.; Fujimoto, T.; Otoguro, K.; Matsuzaki, K.; Moriguchi, R.;
Tanaka, H.; Sasaki, Y. J. Antibiot. 1991, 44, 113-116. (b) Omura, S.;
Matsuzaki, K.; Fujimoto, T.; Kosuge, K.; Furuya, T.; Fujita, S.; Nakagawa,
A. J. Antibiot. 1991, 44, 117-118.
(4) (a) Frank, S. A.; Mergott, D. J.; Roush, W. R. J. Am. Chem. Soc. 2002,
124, 2404-2405. (b) Mergott, D. J.; Frank, S. A.; Roush, W. R. Org.
Lett. 2002, 4, 3157-3160. (c) Aggarwal, V. K.; Emme, I.; Fulford, S. Y.
J. Org. Chem. 2003, 68, 692-700. (d) Yeo, J. E.; Yang, X.; Kim, H. J.;
Koo, S. J. Chem. Soc., Chem. Commun. 2004, 236-237.
(5) (a) Bols, M.; Skrydstrup, T. Chem. ReV. 1995, 95, 1253-1277. (b)
Fleming, I.; Barbero, A.; Walter, D. Chem. ReV. 1997, 97, 2063-2092.
(c) Stork, G.; Mook, R.; Biller, S. A.; Rychnovsky, S. D. J. Am. Chem.
Soc. 1983, 105, 3741-3742. (d) Stork, G.; Sher, P. M.; Chen, H. L. J.
Am. Chem. Soc. 1986, 108, 6384-6385.
(6) Miyake, H.; Yamamura, K. Chem. Lett. 1992, 507-508.
(7) The stereochemistry of the conversion 13 f 14, established by the identity
of totally synthetic 1 with naturally formed salinosporamide A, is that
predicted from a cyclic, chair-formed, six-membered transition state
involving addition of the organozinc reagent to the sterically more
accessible face of the formyl group. The use of 2-cyclohexenylzinc
chloride is critical to successful formation of 14 because none of this
product is obtained with 2-cyclohexenyllithium (probably because the
initial carbonyl adduct undergoes retroaldol cleavage and decomposition;
see: Corey, E. J.; Li, W.; Nagamitsu, T. Angew. Chem., Int. Ed. 1998,
37, 1676-1679).
1
measurements of H and 13C NMR spectra, mp and mixed mp
(168-170 °C), optical rotation, IR and mass spectra, and chro-
matographic mobilities in three different solvent systems.
There are a number of steps in the synthesis of 1 that require
comment. The direct conversion of 6 to 7 with acrylyl chloride
(8) (a) Fleming, I. Chemtracts-Org. Chem. 1996, 9, 1-64. (b) Jones, G. R.;
Landais, Y. Tetrahedron 1996, 52, 7599-7662.
JA048613P
9
J. AM. CHEM. SOC. VOL. 126, NO. 20, 2004 6231