A novel enantioselective synthetic route to omuralide analogues with the potential for species selectivity in proteasome inhibition.

Article abstract of DOI:10.1021/ol015757p

[reaction in text] The building blocks shown can be combined for an enantioselective construction of the simplified omuralide analogue 4 in nine steps, with the use of (R)-atrolactic acid as a recoverable chiral controller.

Full text of DOI:10.1021/ol015757p

ORGANIC
LETTERS
2001
Vol. 3, No. 9
1395-1397
A Novel Enantioselective Synthetic
Route to Omuralide Analogues with the
Potential for Species Selectivity in
Proteasome Inhibition
Sheldon N. Crane and E. J. Corey*
Department of Chemistry and Chemical Biology, HarVard UniVersity,
12 Oxford Street, Cambridge, Massachusetts 02138
corey@chemistry.harVard.edu
Received February 23, 2001
ABSTRACT
The building blocks shown can be combined for an enantioselective construction of the simplified omuralide analogue 4 in nine steps, with
the use of (R)-atrolactic acid as a recoverable chiral controller.
Lactacystin (1) and the corresponding â-lactone, omuralide
(2), exhibit a remarkably selective and potent irreversible
inhibition of proteasome function.1,2 Since proteasomes
degrade many, if not most, of the proteins in cells, including
misfolded proteins,3 and proteins involved in cell cycle
progression4 and regulation of gene transcription,5 1 and 2
have emerged as important biochemical tools. Totally
synthetic 1 and 26,7 have been used as reagents in hundreds
(1) (a) Fenteany, G.; Standaert, R. F.; Lane, W. S.; Choi, S.; Corey, E.
J.; Schreiber, S. L. Science 1995, 268, 726. (b) Fenteany, G.; Standaert, R.
F.; Reichard, G. A.; Corey, E. J.; Schreiber, S. L. Proc. Natl. Acad. Sci.
U.S.A. 1994, 91, 3358. (c) See also Dick, L. R.; Cruikshank, A. A.; Grenier,
L.; Melandri, F. D.; Nunes, S. L.; Stein, R. L. J. Biol. Chem. 1996, 271,
7273.
(2) For general reviews on proteasomes, see: (a) Voges, D.; Zwickl, P.;
of laboratories because of their unequalled selectivity and
Baumeister, W. Annu. ReV. Biochem. 1999, 69, 1015. (b) Coux, O.; Tanaka,
effectiveness. The â-lactone 2 inactivates the 20 S protea-
some at a much faster rate than does lactacystin (1), with a
major source of inactivation being through acylation of the
N-terminal threonine subunit, a key catalytic component.1b,8
An extensive series of synthetic and biological studies has
K.; Goldberg, A. L. Annu. ReV. Biochem. 1996, 65, 801. (c) Bogyo, M.;
Gaczynska, M.; Ploegh, H. L. Biopolymers Peptide Sci. 1997, 43, 269.
(3) Rock, K. L.; Gramm, C.; Rothstein, L.; Clark, K.; Stein, R.; Dick,
L.; Hwang, D.; Goldberg, A. L. Cell 1994, 78, 761.
(4) King, R. W.; Deshaies, R. J.; Peters, J.-M.; Kirschner, M. W. Science
1996, 274, 1652.
(5) Pahl, H. L.; Baeuerle, P. A. Curr. Opin. Cell Biol. 1996, 8, 340.
(6) Corey, E. J.; Reichard, G. A. J. Am. Chem. Soc. 1992, 114, 10677.
(7) Corey, E. J.; Li. W.; Reichard, G. A. J. Am. Chem. Soc. 1998, 120,
2330.
(8) Groll, M.; Ditzel, L.; Lowe, J.; Stock, D.; Bochtler, M.; Bartunik,
H. D.; Huber, R. Nature 1997, 386, 463.
10.1021/ol015757p CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/27/2001

Scheme 1
provided a clear correlation between chemical structure and
somes. Such a molecule with a therapeutic index of ca. 103-
104 could in principle provide a cure for malaria and other
parasite-caused diseases.
rate of irreversible inactivation1a of the mammalian (bovine)
proteasome.9 The (S)-1-hydroxyisobutyl side chain attached
to C(5) was found to be critical to activity, as well as the
γ-lactam and â-lactone subunits. However, the methyl group
attached to C(7) could be replaced by Et, i-Pr, n-Bu, or
CH2Ph without loss of activity. Furthermore, the 7,7-dimethyl
analogue 3 had nearly the same (0.75×) activity as 2.10 The
objective of the present research, the synthesis of the
simplified omuralide analogue 4, derived from the knowledge
of these structure-activity correlations for the mammalian
proteasome and the fact that 1 and 2 block development of
the malaria parasite (Plasmodium sp.)11 and the trypanosomal
parasite (Trypanosoma cruzi).12 Unfortunately, the concentra-
tions of 1 and 2 required to inhibit the development of these
parasites to the pathological stage also inactivate the mam-
malian proteasome and cause unacceptable toxicity. We
therefore undertook the synthesis of omuralide analogues
such as 4 which clearly would be less toxic to humans9 in
the hope of finding compounds which more effectively
exploit the differences between human and parasite protea-
The synthesis of the target molecule 4 is outlined in
Scheme 1. Coupling of N-p-methoxybenzylglycine ethyl ester
(5)13 with (R)-O-acetylatrolactoyl chloride (6)14 in the
presence of pyridine and 4-(dimethylamino)pyridine in
CH2Cl2 at 0-23 °C provided the amide ester 7 which was
deacetylated by NaOH in aqueous ethanol and lactonized
by heating at reflux in benzene with methanesulfonic acid
(8 mol %) for 30 min to afford 8 in 92% overall yield from
5. Oxidative cleavage of the PMB group with ceric am-
monium nitrate in aqueous acetonitrile at 23 °C generated
the lactam 9 (58%). N-Acylation of 9 with the monoester
monoacid chloride of dimethylmalonic acid gave the coupling
product 10 (97%) which underwent Dieckmann cyclization
to 11 (87%) when treated with TiCl4-Et3N-Me3SiCl at -40
°C in CH2Cl2.15 Intermediate 11, which exists in solution
(13) Miknis, G. F.; Williams, R. M. J. Am. Chem. Soc. 1993, 115, 536.
(14) (R)-Atrolactic acid was obtained from the commercially available
racemate (Acros) by the following sequence: (1) precipitation of the (S)-
enantiomer as the salt with (S)-(-)-R-methylbenzylamine from water, (2)
extractive isolation of the remaining enriched (R)-atrolactic acid from the
acidified filtrate, and (3) crystallization of the salt of (R)-atrolactic acid
and (R)-(+)-R-methylbenzylamine from hot water. See: Smith, L. J. Prakt.
Chem. 1911, 115, 731. (R)-Atrolactic acid of >99% ee was obtained from
this salt by extraction from acidic water and a single recrystallization from
hot toluene. (R)-Atrolactic acid was acetylated by stirring with acetyl
chloride at 23 °C for 1.5 h and the resulting acid was converted into the
acid chloride 6 by reaction with oxalyl chloride in benzene with dimethyl-
formamide (3 mol %) as catalyst.
(9) For reviews on structure-activity correlations for a large series of
omuralide analogues, see: (a) Corey, E. J.; Li, W.-D. Z. Chem. Pharm.
Bull. 1999, 47, 1. (b) Corey, E. J.; Li, W.-D. Z.; Nagamitsu, T.; Fenteany,
G. Tetrahedron 1999, 55, 3305. (c) Masse, C. E.; Morgan, A. J.; Adams,
J.; Panek, J. S. Eur. J. Org. Chem. 2000, 2513.
(10) Corey, E. J.; Li, W.-D. Z. Tetrahedron Lett. 1998, 39, 7475.
(11) Gantt, S. M.; Myung, J. M.; Briones, M. R. S.; Li, W.-D.; Corey,
E. J.; Omura, S.; Nussenzweig, V.; Sinnis, P. Antimicrob. Agents Chemother.
1998, 42, 2731.
(12) Gonzalez, J.; Ramalho-Pinto, F. J.; Frevert, U.; Ghiso, J.; Tomlinson,
J.; Scharfstein, J.; Corey, E. J.; Nussenzweig, V. J. Exp. Med. 1996, 184,
1909.
(15) (a) Patek, M. Collect. Czech. Chem. Commun. 1989, 54, 1223. (b)
Yoshida, Y.; Matsumoto, N.; Hamasaki, R.; Tanabe, Y. Tetrahedron Lett.
1999, 68, 1015.
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Org. Lett., Vol. 3, No. 9, 2001

along with the enol tautomer, was fully characterized as the
enol ether A obtained by the action of diazomethane in ether.
Figure 1. Assignment of relative stereochemistry of 12 and B.
Hydroxymethylation of 11 occurred smoothly upon reaction
with 10 equiv of aqueous formaldehyde and pyridine (10
mol %) in aqueous methanol at 23 °C to form 12 stereo-
selectively in 80% yield. The face selectivity of the hy-
droxymethylation is controlled by the difference in steric
screening (C6H5 > CH3) due to the atrolactic derived
stereocenter.
Treatment of 12 with borane-tetrahydrofuran complex
resulted in conversion to diol B of high diastereomeric purity
(>95%). Nuclear Overhauser difference measurements re-
vealed that B possessed the desired configurations at C(8)
and C(8a) (Figure 1). Selective removal of the atrolactic acid
controller without compromising the γ-lactam ring in B was
accomplished by treatment with potassium carbonate in
methanol to afford dihydroxy acid 13 in 67% yield from
12. These two operations were conveniently carried out in
one flask. The separation of 13 and atrolactic acid (for reuse)
was accomplished simply by trituration of the crude product
with chloroform, which dissolves the latter. Treatment of
the triethylammonium salt of 13 with bis(2-oxo-3-oxazoli-
dinonyl)phosphinic chloride (BOPCl) in CH2Cl2 at 23 °C
provided â-lactone 4 in 57% yield after chromatography on
silica gel.
The synthesis of 4 which is outlined in Scheme 1 utilizes
a novel chiral glycine equivalent (9) for enantiocontrol. The
brevity and efficiency of the synthesis allow ready access
to 4 and related analogues. The facile recovery of the chiral
controller (R)-atrolactic acid for reuse adds to the practicality
of this approach.
Acknowledgment. This work was assisted financially by
a research grant from Pfizer Inc. and a Canadian NSERC
postdoctoral fellowship to S.N.C.
Supporting Information Available: Full experimental
procedures for the synthesis of 4. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL015757P
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