A short, stereocontrolled, and practical synthesis of α-methylomuralide, a potent inhibitor of proteasome function

Article abstract of DOI:10.1021/jo0268916

An efficient and practical synthesis of α-methylomuralide (3), a selective inhibitor of proteasomes, has been developed as outlined in Scheme 1. Among the advantages of this route of synthesis over previously described approaches are (1) ease of scale-up

Full text of DOI:10.1021/jo0268916

A Sh or t, Ster eocon tr olled , a n d P r a ctica l Syn th esis of
r-Meth ylom u r a lid e, a P oten t In h ibitor of P r otea som e F u n ction
P. Saravanan and E. J . Corey*
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138
corey@chemistry.harvard.edu
Received December 20, 2002
An efficient and practical synthesis of R-methylomuralide (3), a selective inhibitor of proteasomes,
has been developed as outlined in Scheme 1. Among the advantages of this route of synthesis over
previously described approaches are (1) ease of scale-up and (2) high yields (28% overall yield of
R-methylomuralide from 6) and stereocontrol (including high enantiocontrol). The synthesis is well
suited to the production of 3 in the quantities needed for material-intensive in vivo investigations.
Lactacystin (1), a microbial product first isolated by
Omura1,2 et al. from a screening project to detect neu-
rotrophic (nerve growth factor-like) activity, was later
found to be a highly selective and potent irreversible
inactivator of proteasomes.3,4 The proteasome is a cylin-
drical, multiprotein assembly which effects the degrada-
tion of ubiquitin-tagged proteins. Proteasome action is
crucial to the cleavage of a wide variety of proteins
including not only misfolded and denatured molecules5
but also proteins involved in the cell cycle,6 gene tran-
scription,7 and cell function. Inactivation of the protea-
some by acylation of a critical catalytic threonine subunit
is actually mediated by the â-lactone 2,4,8,11,13 which we
have previously designated as “omuralide”11 because of
its key role in this inactivation. Synthetic 1 and 211-13
have been used in many hundreds of biological labora-
tories as a reagent to evaluate the role of the proteasome
in determining the concentrations and mode of disposal
of many proteins.
During the last several years, a new and important
chapter of proteasome inhibition has developed from the
finding that proteasome inhibitors increase the sensitiv-
ity of cancer cells to apoptosis (biochemically regulated
cell death) through their effect on the NF-κ B nuclear
signaling pathway.14 Because proteasome inactivation
prevents the degradation of I-κ B, the natural inhibitor
of NF-κ B, it prevents NF-κ B migration into the nucleus
and subsequent transcriptional activation. This results
in cell cycle arrest and a decrease in anti-apoptotic
protein formation. Increases in pro-apoptotic Bcl-2 and
stress kinase J NK proteins ensue which then lead to the
release of cytochrome c from mitochondria, activation of
the Apaf-1-containing aptosome complex, and formation
of caspases 3, 7, and 9, direct effectors of apoptosis.15-20
Various peptidomimetic proteasome inhibitors are cur-
rently being evaluated as novel anticancer agents in
clinical trials involving hematological malignancies.16,21,22
The peptidomimetic proteasome inhibitors currently
known are generally chemically reactive electrophiles
that carry subunits such as aldehydic formyl, R-halo-
ketone, vinyl sulfone, R,â-epoxycarbonyl, boronic acid,
and trifluoroacetyl, groups that can interact with many
proteins and cause toxicity problems.23
(1) Omura, S.; Fujimoto, T.; Otoguro, K.; Matsuzaki, K.; Moriguchi,
R.; Tanaka, H.; Sasaki, Y. J . Antibiot. 1991, 44, 113.
(2) Omura, S.; Matsuzaki, K.; Fujimoto, T.; Kosuge, K.; Furuya, T.;
Fujita, S.; Nakagawa, A. J . Antibiot. 1991, 44, 117.
(3) Fenteany, G.; Standaert, R. F.; Lane, W. S.; Choi, S.; Corey, E.
J .; Schreiber, S. L. Science 1995, 268, 726.
(4) (a) Fenteany, G.; Standaert, R. F.; Reichard, G. A.; Corey, E. J .;
Schreiber, S. L. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 3358. (b) 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.
(5) Rock, K. L.; Gramm, C.; Rothstein, L.; Clark, K.; Stein, R.; Dick,
L.; Hwang, D.; Goldberg, A. L. Cell 1994, 78, 761.
(6) King, R. W.; Deshaies, R. J .; Peters, J .-M.; Kirschner, M. W.
Science 1996, 274, 1652.
(7) Pahl, H. L.; Baeuerle, P. A. Curr. Opin. Cell Biol. 1996, 8, 340.
(8) Corey, E. J .; Reichard, G. A.; Kania, R. Tetrahedron Lett. 1993,
34, 6977.
(9) Groll, M.; Ditzel, L.; Lo¨we, J .; Stock, D.; Bochtler, M.; Bartunik,
H. D.; Huber, R. Nature 1997, 386, 463.
(10) See also: Bogyo, M.; McMaster, J . S.; Gaczynska, M.; Tortorella,
D.; Goldberg, A. L.; Ploegh, H. Proc. Natl. Acad. Sci. U.S.A. 1997, 94,
6629.
(11) Corey, E. J .; Li, W.-D. Z. Chem. Pharm. Bull. 1999, 47, 1.
(12) Corey, E. J .; Reichard, G. A. J . Am. Chem. Soc. 1992, 114,
10677.
Extensive experience in these laboratories with the
determination of proteasome inhibitory activity of many
structural analogues of lactacystin11 suggested that
(14) (a) Zhang, L.; Yu, J .; Park, B. H.; Kinzler, K. W.; Vogelstein,
B. Science 2000, 290, 989. (b) Soengas, M. S.; Capodieci, P.; Polski,
D.; Mora, J .; Esteller, M.; Opitz-Araya, X.; McCombie, R.; Herman, J .
G.; Gerald, W. L.; Lazebnik, Y. A.; Cordon-Cardo, C.; Lowe, S. W.
Nature 2001, 409, 207.
(15) Adams, J . Trends Molec. Med. 2002, 8 (4 Suppl.), S49.
(16) Pajonk, F.; Himmelsbach, J .; Riess, K.; Sommer, A.; McBride,
W. H. Cancer Res. 2002, 62, 5230.
(17) Almond, J . B.; Cohen, G. M. Leukemia 2002, 16, 433.
(18) Rivett, A. J .; Gardner, R. C. J . Peptide Sci. 2000, 6, 478.
(19) Murray, R. Z.; Norbury, C. Anti-Cancer Drugs 2000, 11, 407.
(20) Li, B.; Dou, Q. P. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 3850.
(21) Shah, S. A.; Potter, M. W.; Callery, M. P. Surg. Oncol. 2001,
10, 43.
(22) Nicholson, D. W. Nature 2000, 407, 810.
(13) Corey, E. J .; Li, W.; Reichard, G. A. J . Am. Chem. Soc. 1998,
120, 2330. Synthetic 1 and 2 are available from Calbiochem-Novabio-
chem Corp. and Biomol Research Labs, Inc.
(23) For example, see: Spaltenstein, A.; Leban, J . J .; Huang, J . J .;
Reinhardt, K. R.; Viveros, O. H.; Sigafoos, J .; Crouch, R. Tetrahedron
Lett. 1996, 37, 1343.
10.1021/jo0268916 CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/20/2003
2760
J . Org. Chem. 2003, 68, 2760-2764

Synthesis of R-Methylomuralide
SCHEME 1. Syn th esis of r-Meth ylom u r a lid e (3)
R-methylomuralide (3) might be a superior candidate for
study as a proteasome-selective anticancer agent. In a
previous study, 3 was synthesized and found to be nearly
as potent as omuralide (2) (kinactivation 2300 M-1 s-1 for 3
vs 3060 M-1 s-1 for 2 as irreversible inhibitors of the
mammalian proteasome).24 In addition, 3 has the advan-
tage over 2 of superior hydrolytic stability. We therefore
have developed an improved synthesis of 3 that is
enantio- and diastereoselective, short, and easily ex-
ecuted on large scale. The pathway of synthesis is
summarized in Scheme 1.
The E-ester 425 was subjected to Sharpless asymmetric
dihydroxylation26 at 5 °C for 3 days with a catalytic
amount of (DHQ)2PHAL ligand (1 mol %, Aldrich) and
K2OsO2(OH)4 (0.5 mol %) and stoichiometric K3Fe(CN)6
in 1:1 t-BuOH-H2O containing CH3SO2NH2 to give after
recrystallization from EtOAc enantiomerically pure crys-
talline diol 5 in 92% yield, mp 33-35 °C,27 [R]23 -4.8 (c
D
) 2.7, CHCl3). Cyclic sulfite formation from 5 and
subsequent oxidation with catalytic RuO4 and stoichio-
28
metric NaIO4 provided the crystalline cyclic sulfate 6,
(24) Corey, E. J .; Li, W.-D. Z. Tetrahedron Lett. 1998, 39, 7475.
(25) Prepared from the corresponding acid by heating with HCl in
CH3OH; see: Fadnavis, N. W.; Sharfuddin, M.; Vadival, S. K.;
Bhalerao, U. T. J . Chem. Soc., Perkin Trans. 1 1997, 3577.
(26) Sharpless, K. B.; Amberg, W.; Bennani, Y. L.; Crispino, G. A.;
J eong, K.-S.; Kwong, H.-L.; Morikawa, K.; Wang, Z.-M.; Xu, D.; Zhang,
X.-L. J . Org. Chem. 1992, 57, 2768.
(27) Soucy, F.; Grenier, L.; Behnke, M. L.; Destree, A. T.; McCor-
mack, T. A.; Adams, J .; Plamondon, L. J . Am. Chem. Soc. 1999, 121,
9967.
(28) (a) Lohray, B. B. Synthesis 1992, 1035-1052. (b) Gao, Y.;
Sharpless, K. B. J . Am. Chem. Soc. 1988, 110, 7538.
J . Org. Chem, Vol. 68, No. 7, 2003 2761

Saravanan and Corey
mp 63-65 °C, [R]23 -22.8 (c ) 1.0, CHCl3). Selective
Exp er im en ta l Section
D
displacement of sulfate at C(R) of 6 by NaN3 in 4:1
acetone-water generated the corresponding R-azido
â-monosulfate ester which was stirred with ether-2 N
H2SO4 at 23 °C for 16 h to form the oily azido alcohol 7,
(2R,3S)-2,3-Dih yd r oxy-4-m eth ylp en ta n oic Acid Meth yl
Ester , 5. A solution of (DHQ)2PHAL (1.76 g, 2.24 mmol), K2-
OsO2(OH)4 (412 mg, 1.12 mmol), K3[Fe(CN)6] (220 g, 0.67 mol),
K2CO3 (92 g, 0.67 mol), NaHCO3 (56 g, 0.67 mol), and MeSO2-
NH2 (21.3 g, 0.224 mol) in t-BuOH-H2O (1:1, 2 L) was stirred
at room temperature for 10 min. The resulting orange homo-
geneous solution was cooled to 0 °C and treated with (E)-4-
methylpent-2-enoic methyl ester (28.8 g, 0.224 mol). The
mixture was stirred vigorously for 3 days at 5 °C using a
mechanical stirrer. After completion, the reaction mixture was
treated with Na2S2O5 (180 g, 1.0 mol), and stirring was
continued for 1 h at room temperature. The t-BuOH layer was
separated and kept aside; the aqueous layer was extracted
with EtOAc (4 × 250 mL). The combined organic extracts were
washed with 1 N NaOH (300 mL) and brine (300 mL), dried
over Na2SO4, and concentrated in vacuo. The residue was
purified by a short silica gel column (using eluent 50-100%
EtOAc) and further recrystallized in hot EtOAc (∼100 mL) to
give pure diol 5 as white solid (33 g, 92%). Data for 5: mp
[R]23 -47.6 (c ) 1.1, CHCl3) in 90% yield. Catalytic
D
hydrogenation of 7 over Pd-C at 23 °C and 1 atm H2 for
24 h provided in 99% yield (2S,3S)-3-hydroxyleucine
methyl ester 8, [R]23 +8.6 (c ) 0.7, CHCl3). Treatment
D
of 8 with trimethylorthobenzoate (3 equiv) and p-tolu-
enesulfonic acid (1 equiv) in dimethoxyethane at reflux
for 4 h29 gave the oily cis-oxazoline 9, [R]23 -74.4 (c )
D
1.8 CHCl3), in 85% yield. Exposure of 9 to 5 mol % of
1,8-diazobicyclo[5.4.0]undec-7-ene at 23 °C for 12 h
effected complete isomerization to the more stable trans-
oxazoline 10, [R]23 -107 (c ) 3.0, CHCl3) (96% yield).
D
Deprotonation of 10 in THF at -78 °C with lithium
hexamethyldisilazane for 1 h and treatment with ethe-
real formaldehyde30 at -78 °C for 3 h produced the
R-hydroxymethylated ester oxazoline 11 diastereoselec-
33-35 °C; Rf 0.50 (Hex-EtOAc 50:50); [R]23 -4.8 (c 2.7,
D
CHCl3); FTIR (film) νmax 3456, 2954, 1746, 1390, 1256, 1049
1
cm-1; H NMR (CDCl3, 400 MHz) δ 4.23 (1H, d, J ) 7.2 Hz),
tively in 81% yield, mp 76-78 °C, [R]23 -4.4 (c ) 1 in
D
3.75 (3H, s), 3.43 (1H, dd, J ) 8.6, 1.2 Hz), 3.19 (1H, d, J )
5.6 Hz), 1.80 (1H, m), 0.98 (3H, d, J ) 6.8 Hz), 0.91 (3H, d, J
) 6.5 Hz); 13C NMR (CDCl3, 100 MHz) δ 174.8, 78.1, 71.6, 53.0,
31.3, 19.3, 19.2; LRMS (m/z)162.9 (M + H)+ (100); EIMS m/z
calcd for C7H14O4 162.1837, found 162.1832.
CHCl3). Swern oxidation of 11 (2 equiv of ClCOCOCl, 3
equiv of dimethyl sulfoxide reagent preformed in CH2-
Cl2 at -78 °C for 30 min and added via cannula to a
mixture of 11 and excess Et3N in CH2Cl2 at -78 °C
followed by addition of pentane, filtration, and nonaque-
ous isolation) gave the sensitive aldehyde 12 in 99%
yield.31 Mukaiyama aldol coupling32 of 12 with the TMS
enol ether of ethyl isobutyrate in CH2Cl2 at 0 °C in the
presence of lithium perchlorate (1.1 equiv)33 at -20 °C
for 2 h afforded in 67% yield and diastereoselectively the
required isomer 13, [R]23D -18.2 (c ) 1.8, CHCl3). Heating
of 13 in 1:1 6 N HCl-CH3OH at reflux for 5 h produced
Cyclic Su lfa te of (2R,3S)-2,3-Dih yd r oxy-4-m eth ylp en -
ta n oic Acid Meth yl Ester , 6. To an ice-cold solution of diol
5 (30.8 g, 0.19 mol) in CH2Cl2 (400 mL) was added pyridine
(62 mL, 0.76 mol) and the mixture stirred for 5 min. SOCl2
(20.5 mL, 0.285 mol) was added to the above solution dropwise
over a period of 15 min, maintaining the temperature below 5
°C. Stirring was continued for another 30 min at 0 °C, and
the mixture was diluted with a cold ether-water (1:1, 400 mL)
mixture. The aqueous layer was separated and extracted with
ether (2 × 200 mL), and the combined organic layers were
washed with cold water and brine. The organic phase was
dried (Na2SO4) and condensed under reduced pressure to give
the crude sulfite (crude ∼39.5 g) as colorless oil in quantitative
yield. Thus obtained cyclic sulfite analytically was pure enough
for the next step. The crude cyclic sulfite was dissolved in a
mixture of CCl4-MeCN (1:1, 180 mL) and treated with NaIO4
(60.8 g, 0.28 mol) followed by RuCl3 (59 mg, 0.28 mmol) and
water (240 mL). The resulting biphasic reaction medium was
stirred vigorously for 90 min at room temperature. The
mixture was diluted with ether (250 mL), and the aqueous
layer was separated and extracted with ether (2 × 250 mL).
The combined organic layers were washed with water, satu-
rated NaHCO3, and brine and dried over Na2SO4. The solution
was filtered and concentrated in vacuo to give cyclic sulfate 6
(41.5 g, 97%) as a colorless solid upon cooling. The crude
material was pure enough for further reaction, although a
small amount of 6 (0.5 g) was purified by silica gel column
(50% EtOAc as eluent) for analytical purposes. Data for 6: mp
in 89% yield the γ-lactam 14, [R]23 +25.7 (c ) 3.0 in
D
CHCl3). Saponification of 14 in 0.5 N aqueous NaOH at
5 °C for 40 h gave the dihydroxy acid 15, which was
transformed directly into R-methylomuralide 3 (73% from
14) by reaction with bis(2-oxo-3-oxazolidinyl)phosphinic
chloride (BOPCl) and Et3N in CH2Cl2 at 23 °C. Spectral
and physical data for 3, [R]23 -84.6 (c ) 1.0, EtOAc),
D
mp 181-183 °C, matched those of an authentic sample.24,34
In summary, the simple, enantio- and diastereocon-
trolled synthesis of R-methylomuralide (3) described
herein provides access to this stable and potent analogue
of omuralide in quantity. We believe that further biologi-
cal studies with 3 will be facilitated by its ready avail-
ability, especially for more material-intensive in vivo
investigations, and that 3 will allow a more rapid and
complete examination of the therapeutic possibilities of
highly selective proteasome inhibition.
63-65 °C; Rf 0.39 (Hex-EtOAc 50:50); [R]23 -22.8 (c 1.0,
D
CHCl3); FTIR (film) νmax 2974, 1746, 1473, 1216, 1018 cm-1
;
1H NMR (CDCl3, 400 MHz) δ 4.89 (1H, d, J ) 6.5 Hz), 4.73
(1H, apparent t, J ) 6.2 Hz), 3.83 (3H, s), 2.16 (1H, m), 1.05
(3H, d, J ) 6.0 Hz), 1.02 (3H, d, J ) 6.5 Hz); 13C NMR (CDCl3,
100 MHz) δ 166.0, 88.4, 78.1, 53.9, 31.2, 17.9, 16.8; LRMS
(m/z) 242.3 (M + NH4)+ (50), 223.0 (20), 209.1 (100); EIMS
m/z calcd for C7H16NO6S 242.2712, found for (M + NH4)+
242.2714.
(29) Corey, E. J .; Choi, S. Tetrahedron Lett. 1993, 34, 6969.
(30) Schlosser, M.; J enny, T.; Guggisberg, Y. Synlett 1990, 704.
(31) The aldehyde 12 is rapidly deformylated to form 10 upon
exposure to H2O or silica gel.
(32) Mukaiyama, T.; Kobayashi, S. Org. React. 1994, 46, 1.
(33) As Lewis acid catalyst for the conversion of 12 to 13, LiClO4 in
CH2Cl2 was far superior to SnCl4, TiCl4, ZnCl2, Cu(OTf)2, or MgI2. The
face selectivity of coupling at the formyl center of 12 is readily
explained in terms of a bidentate coordination of the formyl and
methoxycarbonyl oxygens with lithium.
(34) Except for the labile aldehyde 12, all synthetic intermediates
were readily purified by column chromatography on silica gel or by
recrystallization (in the case of solids). Satisfactory spectroscopic and
high-resolution mass spectral data were obtained for each reaction
product.
(2S,3S)-2-Azid o-3-h yd r oxy-4-m et h ylp en t a n oic Acid
Meth yl Ester , 7. A solution of cyclic sulfate 6 (41.0 g, 0.18
mol) in acetone-water (5:1, 600 mL) was treated with NaN3
(21.9 g, 0.36 mol) at room temperature and stirred vigorously
for 2 h. After completion, the reaction mixture was concen-
trated (150 mL) in vacuo, diluted with ether-2 N H2SO4 (1:1,
1 L), and allowed to stir at room temperature overnight to give
2762 J . Org. Chem., Vol. 68, No. 7, 2003

Synthesis of R-Methylomuralide
the crude azido alcohol 7 (31.2 g, 90%) as a viscous oil. The
crude product was pure enough for next step. A small amount
of azido alcohol was purified by silica gel column to give
analytically pure 7 as a colorless oil. Data for 7: Rf 0.75 (Hex-
32.6, 17.5, 17.4; LRMS (m/z) 248.1 (100) (M + H+),178.2 (50);
EIMS m/z calcd for C14H17NO3 247.2897, found 247.2896.
Con ver sion of tr a n s-(4S, 5R)-P h en yloxa zolin e to P r i-
m a r y Alcoh ol 11. To a flame-dried flask was added LiHMDS
(1.5 m.mol, 2.27 mL, 0.66 M stock solution in THF), which
was then then cooled to -78 °C, immersed in THF (10 mL),
and treated dropwise with a solution of oxazoline 10 (4.0 g,
16.2 mmol) in THF (20 mL) to give the yellow transparent
solution. Stirring was continued for another 1 h. After the
enolate formation was complete, it was subjected to formyla-
tion by adding freshly prepared formaldehyde (160 mL, 1 M
stock solution in ether, through pre cooled cannula with dry
ice)32 solution at -78 °C, and stirring was continued for 3 h.
The mixture was rapidly quenched with water (20 mL) and
warmed to room temperature. The organic phase was sepa-
rated, the aqueous layer was extracted with EtOAc (3 × 200
mL),and the combined organic layers were dried over Na2SO4
and concentrated in vacuo. The pure alcohol 11 (3.59 g, 81%)
was obtained as a white powder by flash column chromatog-
raphy using Hex-EtOAc (25% neat EtOAc in hexane) as
eluent: mp 76-78 °C; Rf 0.18 (Hex-EtOAc 50:50); [R]23D -4.4
(c 1.0, CHCl3); FTIR (film) νmax 3356, 2978, 1742, 1628, 1498,
1253, 1020, 865 cm-1; 1H NMR (CDCl3, 600 MHz) δ 7.84 (2H,
d, J ) 8.4 Hz), 7.41 (1H, m), 7.32 (2H, m), 4.40 (1H, d, J ) 7.2
Hz), 3.97 (1H, dd, J ) 12.0, 6.6 Hz), 3.79 (1H, dd, J ) 11.8,
6.4 Hz), 3.69 (3H, s), 3.10 (1H, t, 6.5 Hz), 1.97 (1H, m), 0.95
(3H, d, J ) 6.6 Hz), 0.94 (3H, d, J ) 6.5 Hz); 13C NMR (CDCl3,
100 MHz) δ 171.6, 166.6, 132.0, 128.7, 128.5, 127.1, 89.3, 80.4,
67.0, 52.6, 29.8, 19.7, 18.9; LRMS (m/z) 277.0 (100) (M+), 250.1
(60), 178.2 (35), 165.2 (25); EIMS m/z calcd for C15H19NO4
277.3157, found 277.3152.
EtOAc 50:50); [R]23 -47.6 (c 1.1, CHCl3); FTIR (film) νmax
D
3498, 2964, 2118, 1744, 1434, 1513, 1256, 980 cm-1; 1H NMR
(CDCl3, 400 MHz) δ 3.90 (1H, d, J ) 6.4 Hz), 3.80 (3H, s),
3.65 (1H, q, J ) 6.2 Hz), 2.56 (1H, br s, -OH), 1.89 (1H, m),
0.97 (3H, d, J ) 6.8 Hz), 0.95 (3H, d, J ) 6.5 Hz); 13C NMR
(CDCl3, 100 MHz) δ 170.3, 76.6, 64.0, 52.9, 30.4, 19.5, 16.7;
LRMS (m/z) 210.0 (70) (M + Na)+, 165.2 (100), 129.4 (40);
EIMS m/z calcd for C7H14N3O3 188.2044, found (M + H)+
188.2039.
(2S,3S)-3-Hyd r oxyleu cin e Meth yl Ester , 8. A solution
of azido alcohol 7 (24 g, 0.13 mol) in 20% EtOH-EtOAc (0.5
L) at room temperature was treated with 10% Pd-C (5 g)
under an argon atmosphere. The reaction setup was kept
under vacuum and purged with H2 gas (four times); the
resulting dark suspension was stirred vigorously in an atmo-
sphere of H2 (1 atm, H2 balloon). After 24 h, the reaction
mixture was filtered through Celite and concentrated in vacuo
to give the crude amino alcohol 8 (20.8 g, 99%) as a yellow
viscous oil and subjected to the next step without further
purification. Data for 8: Rf 0.10 (neat EtOAc); [R]23 +8.6 (c
D
0.7, CHCl3); FTIR (film) νmax 3494, 2898, 1738, 1513, 1252,
988 cm-1; 1H NMR (CDCl3, 400 MHz) δ 4.10 (1H, m), 3.79 (3H,
s), 3.69 (1H, d, J ) 6.5 Hz), 3.01 (3H, bs), 1.72 (1H, m), 0.98
(3H, d, J ) 4.6 Hz), 0.92 (3H, d, J ) 4.2 Hz); 13C NMR (CDCl3,
100 MHz) δ 175.1, 80.1, 57.6, 52.1, 31.5, 20.4, 18.2; LRMS (m/
z) 162.1 (40) (M + H+), 128.4 (50), 112.1 (30); EIMS m/z calcd
for C7H15NO3 161.1989, found 161.1987.
Met h yl (4S,5S)-5-Isop r op yl-2-p h en yl-4,5-d ih yd r oox-
a zole-4-ca r boxyla te, 9. To a mixture of (2S,3S)-3-hydroxy-
leucine methyl ester (16.7 g, 0.1 mol) and p-TsOH‚H2O (19.0
g, 0.1 mol) in dimethoxyethane (250 mL) was added trimethyl
orthobenzoate (53.7 mL, 0.313 mol), and the mixture was
refluxed for 4 h. After completion, the reaction mixture was
diluted with water (200 mL), and the aqueous layer was
separated and extracted with ether (3 × 150 mL). The
combined organic layers were washed with water and brine
and dried over Na2SO4. The solvent was removed in vacuo to
give crude oxazoline 9 and excess orthobenzoate. Flash column
chromatography on silica gel column (eluent 10-60% EtOAc-
hexane) afforded the pure oxazoline 9 (21.8 g, 85%) as viscous
Con ver sion of P r im a r y Alcoh ol 11 to Ald eh yd e 12
(Mod ified Sw er n Oxid a tion ). To a solution of oxalyl chloride
(875 µL, 10.0 mmol) in CH2Cl2 (50 mL) was added dropwise a
solution of dry DMSO (1.0 mL, 15.0 mmol) in CH2Cl2 (30 mL)
at -78 °C. Stirring was continued for 30 min at this temper-
ature. The above preformed Swern reagent was carefully
transferred through a cannula (wrapped with dry ice alumi-
num foil) to a solution containing the mixture of alcohol 11
(1.38 g, 5.0 mmol) and excess Et3N (3.45 mL, 25.0 mmol) at
-78 °C. After 3 h of vigorous stirring, the excess solvent was
pumped off in vacuo and the reaction mixture was diluted with
pentane (30 mL). The resulting white crystalline Et3N‚HCl salt
was filtered off under argon through a sintered funnel fitted
with a dry round-bottom flask and nitrogen inlet. The flask
was washed with pentane portionwise (3 × 30 mL) and the
supernatant solution decanted through a syringe and filtered.
The combined filtrates were condensed in vacuo under argon
atmosphere (vacuum pump 2 mmHg at rt) to give the aldehyde
12 (pure by 1H NMR analysis) 1.37 g (99%): Rf 0.28-0.35
oil. Data for 9: Rf 0.55 (Hex-EtOAc 50:50); [R]23 -74.4 (c
D
1.8, CHCl3); FTIR (film) νmax 2950, 1746, 1544, 1358, 1186,
1
1010 cm-1; H NMR (CDCl3, 400 MHz) δ 7.92 (2H, d, J ) 8.9
Hz), 7.39 (1H, m), 7.34 (2H, m), 4.87 (1H, d, J ) 8.9 Hz), 4.45
(1H, dd, J ) 10.0, 8.0 Hz), 3.67 (3H, s), 1.94 (1H, m), 0.96 (3H,
d, J ) 6.5 Hz), 0.93 (3H, d, J ) 6.4 Hz); 13C NMR (CDCl3, 100
MHz) δ 170.7, 166.8, 131.9, 128.7, 128.4, 127.4, 87.7, 70.6, 52.3,
29.4, 19.8, 18.9; LRMS (m/z) 248.1 (M + H+) (100), 171.1 (90),
158.3 (20); EIMS m/z calcd for C14H17NO3 247.2897, found
247.2892.
(Hex-EtOAc 50:50, decomposes on TLC); [R]23 -3.4 (c 1.7,
D
CHCl3); FTIR (film) νmax 2943, 2860, 1728, 1608, 1416, 1228,
1
cm-1; H NMR (CDCl3, 400 MHz) δ 9.71 (1H, s), 7.91 (2H, d,
J ) 8.1 Hz), 7.42 (1H, m), 7.34 (2H, m), 4.84 (1H, d, J ) 8.4
Hz), 3.70 (3H, s), 2.01 (1H, m), 1.03 (3H, d, J ) 6.4 Hz), 0.87
(3H, d, J ) 6.6 Hz); 13C NMR (CDCl3, 100 MHz) δ 192.2, 168.3,
167.3, 132.5, 128.8, 128.5, 126.9, 86.8, 85.3, 53.0, 41.0, 29.4,
19.2, 19.1; LRMS (m/z) 276.1 (45) (M+), 249.1 (20), 220.2 (100)
(45), 171.1 (20), 133.3 (35).
Met h yl (4R,5S)-5-Isop r op yl-2-p h en yl-4,5-d ih yd r oox-
a zole-4-ca r boxyla te, 10. A solution of oxazoline 9 (11.8 g,
48.0 mmol) in CH2Cl2 (100 mL) was treated with 1,8-
diazabicyclo[5.4.0]undec-7-ene (360 µL, 2.4 mmol) at room
temperature. After 12 h, the reaction mixture was diluted with
ether (250 mL) and washed with water (3×) and brine. The
organic layer was dried over Na2SO4, and removal of the
solvent in vacuo afforded the crude trans-oxazoline 10 (11.4
g, 96%) as a yellow viscous oil. A small amount of compound
was purified by flash column chromatography (eluent 10-60%
EtOAc-hexane) to give pure compound for analytical pur-
poses. Data for 10: Rf 0.50 (Hex-EtOAc 50:50); [R]23D -106.8
(c 3.0, CHCl3); FTIR (film) νmax 2964, 1746, 1644, 1256, 938
Con ver sion of Ald eh yd e 12 to Ald ol P r od u ct 13. A
solution of LiClO4 (580 mg, 5.5 mmol) in CH2Cl2 (20 mL) was
treated with freshly prepared aldehyde 12 (1.37 g, 5.0 mmol)
at 0 °C, and the dimethyl ketene acetal (10 mL, 1 M solution
in CH2Cl2, 10.0 mmol) was added dropwise at this tempera-
ture. After 5 h, the reaction mixture was quenched with
saturated NaHCO3 (20 mL), the aqueous layer was extracted
with CH2Cl2 (3 × 25 mL), and the combined organic layers
were washed with water and brine and dried over Na2SO4.
The crude product was obtained by solvent removal and
purified by silica gel column to give the pure aldol product 13
(1.3 g, 67%) as a single diastereomer. Data for 13: Rf 0.50
1
cm-1; H NMR (CDCl3, 400 MHz) δ 7.98 (2H, d, J ) 6.8 Hz),
7.47 (1H, m), 7.39 (2H, m), 4.66 (1H, apparent t, J ) 6.4 Hz),
4.56 (1H, d, J ) 6.8 Hz), 3.78 (3H, s), 1.94 (1H, m), 1.02 (3H,
d, J ) 6.8 Hz), 0.98 (3H, d, J ) 6.5 Hz); 13C NMR (CDCl3, 100
MHz) δ 172.3, 165.9, 131.9, 128.7, 128.5, 127.4, 87.3, 71.4, 52.8,
(Hex-EtOAc 60:40); [R]23 -18.2 (c 1.8, CHCl3); FTIR (film)
D
J . Org. Chem, Vol. 68, No. 7, 2003 2763

Saravanan and Corey
1
νmax 2954, 2898, 1746, 1644, 1513, 1256, 838 cm-1; H NMR
the crude acid 15 as a yellow solid. The crude acid was
suspended in dry CH2Cl2 (10 mL), treated with Et3N (830 µL,
6.0 mmol), and stirred vigorously at room temperature for 5
min. To this solution was added BOPCl (560 mg, 2.2 mmol) at
room temperature under argon, and stirring was continued
for 35 min. After completion (TLC), the reaction mixture was
quenched with wet EtOAc (5 mL) and stirred for 5 min. The
mixture was diluted with water (10 mL), and the aqueous layer
was extracted with EtOAc (3 × 20 mL). The combined organic
layers were washed with water and filtered through a short
pad of silica gel and the silica pad washed repeatedly to give
the crude â-lactone. Purification by flash column afforded the
pure lactone 3 (336 mg, 73%) as a white solid. Data for 3: mp
(CDCl3, 400 MHz) δ 7.78 (2H, d, J ) 7.8 Hz), 7.40 (1H, m),
7.31 (2H, m), 4.71 (1H, d, J ) 3.8 Hz), 4.50 (1H, d, J ) 9.8
Hz), 3.98 (1H, d, J ) 10.2 Hz), 3.82 (1H, m), 3.68 (3H, s), 3.60
(1H, m), 1.82 (1H, m), 1.18 (3H, s), 1.06 (3H, s), 1.02 (3H, d, J
) 6.7 Hz), 0.84 (3H, t, J ) 6.3 Hz), 0.71 (3H, d, J ) 6.5 Hz);
13C NMR (CDCl3, 100 MHz) δ 176.08, 174.7, 165.3, 132.1,
128.8, 128.5, 127.2, 92.9, 82.4, 80.5, 60.6, 52.9, 46.9, 30.0, 24.8,
21.5, 19.3, 16.1, 13.9; LRMS (m/z) 392.1 (100) (M + H+), 259.4
(20), 226.1 (30), 197.7 (90); EIMS m/z calcd for C21H30NO6
392.2073, found 392.2068 (M + H)+.
Con ver sion of Ald ol P r od u ct 13 to La cta m 14. The aldol
product 13 (1.3 g,) was suspended in MeOH-6 N HCl (1:1, 30
mL) and refluxed for 5 h. The mixture was concentrated under
reduced pressure and diluted with water (20 mL). The result-
ing aqueous phase was basified (pH ∼9.0) with solid NaHCO3.
The aqueous layer was extracted with EtOAc (3 × 50 mL),
and combined organic layers were washed with water and
brine and dried (Na2SO4). The residue was purified by flash
column chromatography to give pure aldol products 14 (1.06
g, 89%) as a yellow semisolid. Data for 14: Rf 0.38 (Hex-
181-183 °C; Rf 0.50 (Hex-EtOAc 20:80); [R]23 -84.6 (c 1.0,
D
EtOAc); FTIR (film) νmax 3450, 2954, 2890, 1836, 1720, 1640,
1513, 1248 cm-1; 1H NMR (CDCl3, 400 MHz) δ 6.97 (1H, br s,
-NH), 4.82 (1H, s), 3.74 (1H, t, J ) 3.8 Hz), 3.59 (1H, d, J )
7.6 Hz), 1.81 (1H, m), 1.20 (6H, s), 1.06 (3H, d, J ) 6.5 Hz),
0.99 (3H, d, J ) 6.7 Hz); 13C NMR (CDCl3, 100 MHz) δ 180.2,
170.9, 81.9, 79.4, 72.8, 42.8, 30.8, 23.6, 20.0, 18.7, 17.6; 1H
NMR (Py-d5, 400 MHz) δ 10.41 (1H, br s, -NH), 7.66 (1H, br
s, -OH), 5.17 (1H, s), 4.05 (1H, t, J ) 7.8 Hz), 1.37 and 1.38
(3H, each s), 1.19 (3H, d, J ) 6.5 Hz), 1.17 (3H, d, J ) 6.7 Hz);
13C NMR (Py-d5, 100 MHz) δ 180.8, 171.0, 82.0, 80.2, 72.5,
42.8, 30.9, 23.8, 20.5, 19.1, 17.9; LRMS (m/z) 244.1 (M + NH3)+
(70), 226.1 (40), 154.3 (20); EIMS m/z calcd for C11H21N2O4
245.1501, found for (M + NH4)+ 245.1508.
EtOAc 20:80); [R]23 +25.7 (c 3.0, CHCl3); FTIR (film) νmax
D
3445, 2938, 1740, 1725, 1710, 1240 cm-1; 1H NMR (CDCl3, 400
MHz) δ 7.98 (2H, d, J ) 7.9 Hz), 7.51 (1H, m), 7.38 (2H, m),
6.97 (1H, bs, -NH), 5.61 (1H, d, J ) 5.2 Hz), 3.99 (1H, s), 3.74
(3H, s), 2.12 (1H, m), 0.99 (3H, s), 0.98 (3H, s), 0.94 (3H, d, J
) 3.5 Hz), 0.93 (3H, d, J ) 3.8 Hz); 13C NMR (CDCl3, 100 MHz)
δ 182.0, 171.9, 166.6, 133.9, 130.1, 129.2, 128.9, 79.0, 77.9, 70.3,
53.0, 43.6, 30.6, 24.7, 20.8, 18.9, 18.6; LRMS (m/z) 363.6 (30)
(M + H)+, 330.8 (100), 278.1 (50), 266.2 (30), 247.9 (20); EIMS
m/z calcd for C19H25NO6 363.4049, found 363.4091.
Ack n ow led gm en t. We are grateful to Pfizer, Inc.,
for generous financial support of this research.
Note Ad d ed a fter ASAP P ostin g. In the text located
above the summary, the physical data for compund 3 was
incorrect in the version posted February 20, 2003. The
corrected version was posted February 25, 2003.
Con ver sion of La cta m 14 to R-Meth ylom u r a lid e (3). A
solution of lactam 14 (1.0 g, 2.6 mmol) in 0.5 N NaOH (20 mL)
was allowed to stir at 5 °C for 40 h. After hydrolysis was
complete, the reaction mixture was acidified with 1 N HCl (30
mL). The aqueous layer was washed with EtOAc (25 mL),
separated, and concentrated in vacuo to give the crude
carboxylic acid. The crude material was triturated with 10%
Et3N in CH2Cl2 (100 mL), filtered through plug of cotton to
remove any trance amount of NaCl precipitated, and washed
with CH2Cl2 (10 mL). The filtrates were concentrated to give
Su p p or t in g In for m a t ion Ava ila b le: 13C NMR spectra
available for 6-13 and 3. This material is available free of
charge via the Internet at http://pubs.acs.org.
J O0268916
2764 J . Org. Chem., Vol. 68, No. 7, 2003