Stereoselective synthesis of constrained azacyclic hydroxyethylene isosteres as aspartic protease inhibitors: Dipolar cycloaddition and related methodologies toward branched pyrrolidine and pyrrolidinone carboxylic acids

Article abstract of DOI:10.1021/jo050740w

The synthesis of three vicinally substituted azacyclic carboxylic acids in enantiopure form was achieved from a common α-amino aldehyde originating from L-leucine. Pyrrolidines and pyrrolidinones were elaborated from α,β-unsaturated γ-hydroxy-δ-amino acid

Full text of DOI:10.1021/jo050740w

Stereoselective Synthesis of Constrained Azacyclic
Hydroxyethylene Isosteres as Aspartic Protease Inhibitors:
Dipolar Cycloaddition and Related Methodologies toward
Branched Pyrrolidine and Pyrrolidinone Carboxylic Acids
Stephen Hanessian,*,† Hongying Yun,† Yihua Hou,† and Marina Tintelnot-Blomley‡
Department of Chemistry, Universite´ de Montre´al, C.P. 6128, Station Centre-ville, Montre´al,
P.Q., H3C 3J7 Canada, and Novartis Institutes for Biomedical Research, Novartis Pharma AG,
Postfach, CH-4002, Basel, Switzerland
stephen.hanessian@umontreal.ca
Received April 13, 2005
The synthesis of three vicinally substituted azacyclic carboxylic acids in enantiopure form was
achieved from a common R-amino aldehyde originating from L-leucine. Pyrrolidines and pyrroli-
dinones were elaborated from R,â-unsaturated γ-hydroxy-δ-amino acids via azomethine ylide 1,3-
dipolar addition and conjugate addition/cyclization strategies, respectively. The azacyclic amino
acids were incorporated in a pseudopeptide now encompassing a hydroxyethylene isostere. Low
nanomolar inhibition of BACE1, an enzyme implicated in the cascade of events leading to plaque
formation in Alzheimer’s disease, was found with a pyrrolidinone analogue.
Introduction
amide bond corresponds to an S-hydroxyethylene isostere
(Figure 1B). X-ray crystallographic studies3,4 further
demonstrated the specific interactions of P4-P4′ subsites
with corresponding S-S′ sites in the enzyme. In particu-
lar, two Asp residues at the catalytic site of the enzyme
interact with the S-hydroxyl group in the P1-P1′ 4-hy-
droxy-5-amino octanoic acid subunit. Extensive analogue
work aimed at understanding the role of individual amino
acids at the P′ and P sites has greatly advanced our
knowledge of functional prerequisites for inhibitory activ-
ity.5 These studies have also been validated by X-ray
cocrystal structures of new analogues.4,5 On the basis of
this valuable structural information, several groups have
contributed toward the design and synthesis of inhibitors
In the preceding article,1 we described the methodology
for the synthesis of oxacyclic hydroxyethylene isosteres
that encompass a δ-amino-γ-hydroxy carboxylic acid
motif as a constrained unnatural amino acid. Hydroxy-
ethylene isosteres of peptidic substrates have gained
popularity in conjunction with the design and synthesis
of aspartic protease inhibitors.2 Tang, Ghosh, and co-
workers3 have described the synthesis of a pseudo-
octapeptide (OM99-2) that is a potent inhibitor of â-secre-
tase, memapsin-2 (BACE1) (Figure 1A). The unnatural
2-alkyl-4-hydroxy-5-amino-7-methyl octanoic acid for-
mally replaces a central Leu-Ala dipeptide in which the
* To whom correspondence should be addressed. Phone: (514) 343-
6738. Fax: (514) 343-5728.
(4) (a) Turner, R. T.; Koelsch, G.; Hong, L.; Castaheira, P.; Ghosh,
A. K.; Tang, J. Biochemistry 2001, 40, 10001. (b) Ghosh, A. K.; Shin,
D.; Downs, D.; Koelsch, G.; Lin, X.; Ermolieff, J.; Tang, J. J. Am. Chem.
Soc. 2000, 122, 3522. (c) Ghosh, A. K.; Bilcer, G.; Harewood, C.;
Kawahama, R.; Shin, D.; Hussain, K. A.; Hong, L.; Loy, J. A.; Nguyen,
C.; Koelsch, G.; Ermolieff, J.; Tang, J. J. Med. Chem. 2001, 44, 2865.
(5) Coburn, C. A.; Stachel, S. J.; Li, Y.-M.; Rush, D. M.; Steele, T.
G.; Chen-Dodson, E.; Holloway, M. K.; Xu, M.; Huang, Q.; Lai, M.-T.;
DiMuzio, J.; Crouthamel, M.-C.; Shi, X.-P.; Sardana, V.; Chen, Z.;
Munshi, S.; Kuo, L.; Makara, G. M.; Annis, D. A.; Tadikonda, P. K.;
Nash, H. M.; Vacca, J. P.; Wang, T. J. Med. Chem. 2004, 47, 6117.
† Universite´ de Montre´al.
‡ Novartis Pharma AG.
(1) Hanessian, S.; Hou, Y.; Bayrakdarian, M.; Tintelnot-Blomley,
M. J. Org. Chem. 2005, 70, 6735.
(2) For excellent reviews, see: (a) Cooper, J. B. Curr. Drug Targets
2002, 3, 155. (b) Leung, D.; Abbenante, G.; Fairlie, D. P. J. Med. Chem.
2000, 43, 305. (c) Bender, S. L.; Babine, R. E. Chem. Rev. 1997, 97,
1359.
(3) Hong, L.; Koelsch, G.; Lin, X.; Wu, W. S.; Terzyan, S.; Ghosh, A.
K.; Zhang, X. C.; Tang, J. Science 2000, 290, 150.
10.1021/jo050740w CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/21/2005
6746
J. Org. Chem. 2005, 70, 6746-6756

Synthesis of Azacyclic Hydroxyethylene Isosteres
FIGURE 1. (A) Tang-Ghosh inhibitor of BACE1. (B) δ-Amino-γ-hydroxyoctanoic acid hydroxyethylene isostere. (C, D, E) P1-
P1′ constrained azacyclic hydroxyethylene isosteres.
of BACE16 in the quest of a therapeutic agent for
Alzheimer’s disease.7 We have recently investigated the
backbone replacement of the P1′ Ala subunit in OM99-2
with a rigid variant by introducing a cyclopentane ring
spanning the Ala methyl group and the adjacent meth-
ylene in the chain.8
Herein, we report on the synthesis of azacyclic con-
strained hydroxyethylene isosteres encompassed in the
4-hydroxy-5-amino-7-methyl octanoic acid structure (Fig-
ure 1C,D,E) of OM99-2. Relying on the available X-ray
parameters,4 we envisaged a ligation of the P1′ C-methyl
group with the adjacent methylene to form an N-alkyl
pyrrolidine or pyrrolidinone. Preliminary molecular mod-
eling of an energy-minimized hypothetical azacyclic
structure did not show adverse interactions with the
enzyme. It was also hoped that the basic nitrogen in the
pyrrolidine and the lactam carbonyl in the corresponding
azacycles would be favorably positioned for productive
interactions with suitable donor/acceptor sites in the
enzyme. We therefore embarked on the stereoselective
synthesis of the three azacyclic disubstituted amino acids
shown in Figure 1, in which the absolute configurations
of three of the four stereogenic centers in the hydroxy-
ethylene substructure correspond to the original Tang-
Ghosh motif.3
There are ample methods for the construction of
vicinally substituted pyrrolidines and pyrrolidinones.9
However, the intended novel azacyclic targets presented
several challenges, not the least of which was controlling
the relative stereochemistry of four contiguous stereo-
genic centers. In considering various approaches to the
vicinally substituted pyrrolidine and pyrrolidinone car-
boxylic acid motifs such as C, D, and E (Figure 1), we
explored various heterocyclization methods relying on
asymmetric induction by resident chirality from a com-
mon intermediate. Dipolar [3 + 2] cycloaddition reac-
tions10 and stereocontrolled conjugate addition of suitable
carbon nucleophiles11 were considered to be logical syn-
thetic strategies.
(6) For selected examples, see: (a) Stachel, S. J.; Coburn, C. A.;
Steele, T. G.; Jones, K. G.; Loutzenhiser, E. F.; Gregro, A. R.;
Rajapakse, H. A.; Lai, M.-T.; Crouthamel, M.-C.; Xu, M.; Tugushevo,
K.; Lineberger, J. E.; Pietrak, B. L.; Espeseth, A. S.; Shi, X.-P.; Chen-
Dodson, E.; Holloway, M. K.; Munshi, S.; Simon, A. J.; Kuo, L.; Vacca,
I. P. J. Med. Chem. 2004, 47, 6447. (b) Hom, R. K.; Gailunas, A. F.;
Mamo, S.; Fang, L. Y.; Tung, J. S.; Walker, D. E.; Davis, D.; Thorsett,
E. D.; Jewett, N.; Moon, J. B.; John, V. J. Med. Chem. 2004, 47, 158.
(c) Chen, S.-H.; Lamar, J.; Guo, D.; Kohn, T.; Yang, H.-C.; McGee, J.
E.; Timm, D.; Erickson, J. A.; Yip, Y.; May, P.; McCarthy, J. R. Bioorg.
Med. Chem. Lett. 2004, 14, 245. (d) Lamar, J.; Hu, J.; Bueno, A. B.;
Yang, H.-C.; Guo, D.; Copp, J. D.; McGee, J. E.; Gitter, B.; Timm, D.;
May, P. C.; McCarthy, J. R.; Chem., S.-H. Bioorg. Med. Chem. Lett.
2003, 13, 4335. (e) Shuto, D.; Kasai, S.; Kimura, T.; Liu, P.; Hidaka,
K.; Hamada, T.; Shibakawa, S.; Hayashi, Y.; Hattori, C.; Szabo, B.;
Ishiura, S.; Kiso, Y. Bioorg. Med. Chem. Lett. 2003, 13, 4273. (f) Jeon,
S.-Y.; Back, K.; Seong, Y.-H.; Song, K.-S. Bioorg. Med. Chem. Lett. 2003,
13, 3905. (g) Hom, R. K.; Fang, L. Y.; Mamo, S.; Tung, J. S.; Guinn, A.
C.; Walker, D. E.; Davis, D. L. Gailunas, A. F.; Thorsett, E. D.; Sinha,
S.; Knops, J. E.; Jewett, N. E.; Anderson, J. P.; John, V. J. Med. Chem.
2003, 46, 1799. (h) Tung, T. S.; Davis, D. L.; Anderson, J. P.; Walker,
D. E.; Mamo, S.; Jewett, N.; Hom, R. K.; Sinha, S.; Thorsett, E. D.;
John, V. J. Med. Chem. 2002, 45, 259.
(7) For recent reviews, see: (a) Vassar, R. J. Mol. Neurosci. 2004,
23, 105. (b) Bullock, R. Expert Opin. Invest. Drugs 2004, 13, 303. (c)
Citron, M. Nat. Rev. Neurosci. 2004, 5, 677. (d) Cumming, J. N.; Iserloh,
U.; Kennedy, M. E. Curr. Opin. Drug Discovery Dev. 2004, 7, 536. (e)
John, V.; Beck, J. P.; Bienkowski, M. J.; Sinha, S.; Heinrikson, R. L.
J. Med. Chem. 2003, 46, 4626. (f) Selkoe, D. J.; Schenk, D. Annu. Rev.
Pharmacol. Toxicol. 2003, 43, 545. (g) Golde, T. E. Curr. Pharm. Des.
2003, 9, 427. (h) Conway, K. A.; Baxter, E. W.; Felsenstein, K. M.;
Reitz, A. B. Curr. Pharm. Des. 2003, 9, 427.
(8) Hanessian, S.; Yun, H.; Hou, Y.; Yang, G.; Baryakdarian, M.;
Therrien, E.; Moitessier, N.; Roggo, S.; Veenstra, S.; Tintelnot-Blomley,
M.; Rondeau, J.-M.; Paganetti, P.; Neumann, U.; Betschart, C. J. Med.
Chem., in press.
Results
Treatment of N-Boc-leucinal 1 with the Li salt of
methyl propiolate, as described by Fray and Kleinman,12
gave a 25-30% yield of a mixture in which the desired
4S-hydroxy isomer 2 was the major product (Scheme 1).
Using the less basic and more oxophilic Ce salt of methyl
propiolate13 resulted in a significant improvement in yield
without affecting the ratio of epimeric products (72%).
Protection as the N,O-cyclic acetonide, followed by sepa-
(9) See, for example: O’Hagan, D. Nat. Prod. Chem. 2000, 17, 435.
(10) For reviews, see: (a) Synthetic Applications of 1,3-Dipolar
Cycloaddition Chemistry toward Heterocycles and Natural Products;
Padwa, A., Pearson, W., Eds.; Wiley-VCH: Weinheim, Germany, 2002;
p 169. (b) Kanemasa, S. Synlett 2002, 1371. (c) Tsuge, O.; Kanemasa,
S. In Advances in Heterocyclic Chemistry; Katristsky, A. R., Ed.;
Academic Press: San Diego, CA, 1989; Vol. 45, p 323. (d) Grigg, R.
Chem. Soc. Rev. 1987, 16, 89. (e) 1,3-Dipolar Cycloaddition Chemistry;
Padwa, A., Ed.; Wiley-Interscience: New York, 1984.
(11) (a) Hanessian, S.; Wang, W.; Gai, Y. Tetrahedron Lett. 1996,
37, 7477. (b) Hanessian, S.; Sumi, K. Synthesis 1991, 1083. See also:
(c) Reetz, M. J.; Ro¨hrig, D. Angew. Chem., Int. Ed. Engl. 1989, 28,
1706. (d) Iako, I.; Uiber, P.; Mann, A.; Taddei, M.; Wermuth, C.-G.
Tetrahedron Lett. 1990, 31, 1011. (e) Cardani, S.; Poli, G.; Scolastico,
C.; Villa, R. Tetrahedron 1988, 44, 5929.
(12) Fray, A. H.; Kaye, R. L.; Kleinman, E. F. J. Org. Chem. 1986,
51, 4828.
J. Org. Chem, Vol. 70, No. 17, 2005 6747

Hanessian et al.
SCHEME 1a
a Reagents and conditions: (a) i. LDA, methyl propiolate, THF, -78 °C; ii. CeCl3, -78 °C then aldehyde, 72% (ratio of epimers is 5:1);
(b) i. DMP, TsOH (cat.), acetone, 64% (yield of the separable major isomer); ii. Lindlar Pd, quinoline, C6H6, H2 (1 atm), 90%; (c)
MeNHCH2COOH, (HCHO)n, 4 Å MS, C6H6, reflux, 96% (4/5 ) 4:1); (d) , NaOMe, MeOH, 99%; (e) i. Ba(OH)2, EtOH-H2O; H2SO4; ii.
BuNH2, EDC, HOBt, DMAP, DCM, 88%; (f) i. Ba(OH)2, MeOH-H2O and then H2SO4; ii. PyBOP, DIEA, DCM, H-Val-NHBu, 76%; (g) i.
4 M HCl in dioxane; ii. Boc-Met-OH, PyBOP, DIEA, DCM, 44%; (h) i. 4 M HCl in dioxane; ii. Ac-Leu-OH, EDC, HOBt, DCM-H2O, 40%.
ration of the desired major isomer, and Lindlar catalytic
hydrogenation led to the cis-ester 3 in excellent yield.
Treatment with N-methyl glycine in the presence of
formaldehyde14 and molecular sieves afforded the azo-
methine ylide cyclization product 4 and its diastereomer
5 as a 4:1 mixture, respectively, in 96% yield. The easily
separable cis-isomer 4 was equilibrated with NaOMe to
the desired trans-ester in quantitative yield. Hydrolysis
of the corresponding ester to the acid and coupling with
n-butylamine gave the amide 6. A single-crystal X-ray
analysis provided definitive evidence for the structural
and configurational assignments of diastereomer 5 pro-
duced in the cycloaddition reaction (Scheme 1). Previous
reports have shown that P2′-P4′ truncated pseudopep-
tides containing the acyclic hydroxyethylene isostere unit
exhibited good inhibitory activity against BACE 1.4
Furthermore, the replacement of the asparagine subunit
with methionine was found to be acceptable. We therefore
continued with the synthesis of a prototype inhibitor that
took advantage of these structural simplifications. Cou-
pling of the acid from 5 with L-Val n-butyramide gave 7,
which was subsequently treated with TFA in CH2Cl2 to
cleave the N,O-acetal. Peptide coupling with Boc-Met-
OH gave 8, which was further elaborated to the intended
prototype pseudopeptide inhibitor 9.
The synthesis of the 2-pyrrolidinone analogue D (Fig-
ure 1) was initiated from the common precursor 3
(Scheme 2). Treatment with vinylmagnesio cuprate in the
presence of TMSCl11a,b led to a >9:1 mixture of vinyl
adducts 10 and 11. On the basis of our previous results
with γ-N-Boc R,â-unsaturated esters11a,b and correspond-
ing N,N-dibenzyl esters,11c we had anticipated a prepon-
derance of the syn-adduct 11. Instead, the major product
was found to be the undesired anti-isomer 10 as con-
firmed by a single-crystal X-ray structure of an n-
butylamide derivative (Scheme 2). In the event, the
mixture of vinyl adducts 10 and 11 was subjected to
ozonolysis to give the corresponding aldehydes 12. Re-
ductive amination with ethylamine resulted in concomi-
tant lactam formation to afford 13a and its epimer 13b
in a ratio of >9:1 after chromatographic separation.
Treatment of 13a with LDA and acylation of the enolate
with methyl chloroformate afforded the trans-isomer 14.
Hydrolysis of the ester group in a suspension of barium
hydroxide, followed by amide formation with n-butyl-
amine, gave the crystalline amide 15 (Scheme 2).
We sought an alternative synthesis of the desired
trans-3,4 disubstituted 2-pyrrolidinone motif D (Figure
1). Thus, conjugate addition of nitromethane to 3 in the
presence of a bulky bicyclic guanidine base15 led to the
corresponding 3-nitromethyl adduct 16 in quantitative
yield. Reduction of the nitro group with Raney-Ni in the
presence of H2PtCl616 afforded the lactam 17, which was
N-ethylated with NaH and ethyl iodide to give 18.
(13) (a) Takeda, K.; Yano, S.; Yoshii, E. Tetrahedron Lett. 1988, 29,
6951. See also: (b) Imamoto, T.; Kusumoto, T.; Tawarayamo, Y.;
Sugiura, Y.; Mita, T.; Hatanaka, Y.; Yokoyama, M. J. Org. Chem. 1984,
49, 3904. (c) Fox, C. M.; Hiner, R. N.; Warrier, U.; White, J. D.
Tetrahedron Lett 1998, 29, 2926. (d) For a review, see: Liu, H.; Shia,
K.; Shang, X.; Zhu, B. Tetrahedron 1999, 55, 3803.
(14) (a) Joucla, M.; Mortier, J. Bull. Soc. Chim. Fr. 1988, 579. See
also: (b) Wee, A. G. H. J. Chem. Soc., Perkin Trans. I 1989, 1363. (c)
Terao, Y.; Kotaki, H.; Imai, N.; Achiwa, K. Chem. Pharm. Bull. 1985,
33, 2762. (d) Chen, C.; Li, X.; Schreiber, S. L. J. Am. Chem. Soc. 2003,
125, 10174. (e) Padwa, A.; Chen, Y.-Y.; Dent, W.; Nimmesgren, H. J.
Org. Chem. 1985, 50, 4006.
(15) For related examples, see: (a) Simoni, D.; Invidiata, F. P.;
Manfredini, S.; Ferroni, R.; Lampronti, I.; Roberti, M.; Pollini, G. P.
Tetrahedron Lett. 1997, 38, 2749. (b) Simoni, D.; Rondanin, R.; Morini,
M.; Baruchello, R.; Invidiata, F. P. Tetrahedron Lett. 2000, 41, 1607.
(c) Davis, A. P.; Dempsey, K. J. Tetrahedron: Asymmetry 1995, 6, 2829.
For an excellent review, see: (d) Ballini, R.; Bosica, G.; Fiorini, D.;
Palmieri A.; Petrini, M. Chem. Rev. 2005, 105, 933.
6748 J. Org. Chem., Vol. 70, No. 17, 2005

Synthesis of Azacyclic Hydroxyethylene Isosteres
SCHEME 2a
a Reagents and conditions: (a) i. CH2CHMgBr, CuI, TMSCl, THF, -78 °C; ii. aqueous NH4Cl, 91%; (b) O3, DCM, -78 °C and then
PPh3, 99%; (c) i. EtNH2, Na2SO4; ii. NaBH(OAc)3 and then aqueous NaHCO3, separate 13a from diastereomer (13a/13b ) 9:1), 91%; (d)
i. LDA, THF, -78 °C; ii. ClCOOMe, 85%; (e) i. Ba(OH)2, H2O-EtOH then H2SO4; ii. EDC, HOBt, DMAP, BuNH2, DCM, 70%.
SCHEME 3a
Introduction of a methoxycarbonyl group via enolate
chemistry followed the same protocol as described above
to give 19. Elaboration to the intended P1′-P1 motif was
accomplished by peptide coupling of 20 as previously
described1 to give the prototypical azacyclic pseudopep-
tide 21 (Scheme 3).
The synthesis of the last pyrrolidinone carboxylic acid
motif E (Figure 1) is shown in Scheme 4. The common
precursor 3 was oxidatively cleaved to the aldehyde 22,
and the latter was subjected to a Wittig olefination to
give the bromo ester 23. Michael addition and concomi-
tant lactam formation with tolysulfonyl N-methylacet-
amide17 in the presence of sodium hydride in THF led to
an inseparable 1.3:1 mixture of the tosyl lactams 24a and
24b. Cleavage of the tosyl group with sodium amalgam
in sodium dihydrogen phosphate led to a mixture of the
corresponding lactams 25a,b.
Cleavage of the methyl ester and peptide coupling with
H-Val-NHBu, gave the two amides 26 and 27, which
a Reagents and conditions: (a) MeNO2, TBD, 99%; (b) H2PtCl6,
could be separated by column chromatography. Chain
Raney-Ni, MeOH, H2, 72%; (c) NaH, THF; EtI, 80%; (d) LDA, THF,
extension of 26 and 27 as described above gave the
respective lactams 28 and 29. We did not secure chemical
proof for their configurational identity due to the dif-
ficulty of distinguishing between the precursor diaster-
eomeric adducts 24a and 24b resulting from the original
addition/cyclization. However, potent inhibition of BACE1
by compound 29 by analogy with a carbocyclic analogue8
(see below) argued heavily in favor of its configurational
assignment, since the isomeric 28 was inactive.
-78 °C and then ClCO2Me, 98%; (e) NaOH, MeOH-H2O, 65 °C;
then 1 N HCl; and then H-Val-NHBu, PyBOP, DIEA, DCM, 86%;
(f) TFA, DCM; then NaHCO3; and then Boc-Met-OH, PyBOP,
DIEA, DCM, 63%; (g) TMSI, DCM; then Na2S2O3, NaHCO3; and
then Ac-Leu-OH EDC, HOBt, DCM/H2O, 83%.
Discussion
The addition of propiolate esters to N-Boc aldehydes
derived from amino acids has many precedents.12,13 In
general, selectivities in favor of the syn-amino alcohols
are good, although the basicity of the Li propiolates can
lead to side reactions and racemization in some cases.
Imamoto and co-workers12a have shown that addition of
CeCl3 to Grignard and organolithium reagents greatly
improves yields. Indeed, inclusion of CeCl3 effectively
(16) (a) Lieber, E.; Smith, G.-B. L. J. Am. Chem. Soc. 1936, 58, 1417.
(b) Levering, D. R.; Morritz, F. L.; Lieber, E. J. J. Am. Chem. Soc. 1950,
72, 1190. (c) Samuelsen, G. S.; Garik, V. L.; Smith, G.-B. L. J. Am.
Chem. Soc. 1950, 72, 3872.
(17) Sun, P.-P.; Chang, M.-Y.; Chiang, M. Y.; Chang, N.-C. Org. Lett.
2003, 5, 1761.
J. Org. Chem, Vol. 70, No. 17, 2005 6749

Hanessian et al.
SCHEME 4a
FIGURE 3. Transition state model for azomethine ylide
cycloaddition.
FIGURE 4. Transition state model for cuprate addition.
precursor.10 The N-ylides can be generated in situ with
N-alkylamino glycine and formaldehyde as shown in
preparatively useful examples by Joucla and Mortier.14
Alternatively, N-alkyl-N-trimethylsilylmethyl methoxy-
methylamines14b can be direct precursors of the azo-
methine. Usually electron poor or activated alkene di-
polarophiles harboring functional groups on stereogenic
carbons react with high facial selectivity, due to internal
induction (resident chirality).14b It has been reported that
a cis-R,â-unsaturated γ-alkoxy ester gives much higher
facial selectivity compared to the trans-isomer.14a To the
best of our knowledge, azomethine ylide 1,3-dipolar
cycloaddition with R,â-unsaturated γ-amino acid esters
have little if any precedent. The facial selectivity found
in the case of 3 can be rationalized based on a transition
state model where the A1,3 strain is minimized,18 as
depicted in Figure 3. Thus, cycloaddition is more likely
to occur from the least hindered face of the enoate leading
to 5 as the major isomer.
a Reagents and conditions: (a) O3, DCM, -78 °C; and then PPh3,
99%; (b) Ph3PC(Br)CO2Me, toluene, DCE, 80 °C, 3h, 94% (Z-
isomer, Z/E > 14:1); (c) TsCH2CONHMe, NaH, THF, 20 min; and
then 23, room temperature 12h, 81%; (d) Na amalgam, Na2HPO4,
MeOH, 0 °C, 1 h, 99%; (e) NaOH, MeOH-H2O, 65 °C; then 1 N
HCl; and then H-Val-NHBu, PyBOP, DIEA, DCM, 50% (26/27 )
1.3:1); (f) TFA, DCM; then NaHCO3; and then Boc-Met-OH,
PyBOP, DIEA, DCM, 73% from 26 and 73% from 27; (g) TMSI,
DCM; then Na2S2O3, NaHCO3; then Ac-Leu-OH, EDC, HOBt,
DCM/H2O, 28 (75%) and 29 (68%).
Addition of vinyl cuprates to γ-ureido trans-enoates
leads to the syn-adducts as major isomers.11a,b However,
in the case of the cis-enoate 3, the major product was in
fact the anti-isomer 10. Considering the effect of the A1,3
strain in the transition state of the conjugate addition,
the syn-product 11 should have predominated. The
results can be rationalized on the basis of a chelated
intermediate of 3, where the normally preferred facial
attack due to minimization of the A1,3 strain is counter-
balanced by the steric bulk of the chelate, leading to the
anti-isomer 10 as the major product (Figure 4).
FIGURE 2. Transition state model of a Ce3+ coordinated
intermediate.
improved the efficiency and selectivity of the addition,
affording 2 in 72% yield (5:1 ratio). A proposed transition
state model in which a chelation-controlled addition of
Ce methyl propiolate may be operative as shown in
Figure 2.
The azomethine ylide cycloaddition reaction is a ver-
satile method for the synthesis of 3,4-substituted pyrro-
lidines from an olefin and a suitable nitrogen dipole
The exclusive formation of the desired syn-nitromethyl
isomer 16 in the conjugate addition to 3 can be rational-
(18) For a review, see: Hoffmann, R. W. Chem. Rev. 1989, 89, 1841.
6750 J. Org. Chem., Vol. 70, No. 17, 2005

Synthesis of Azacyclic Hydroxyethylene Isosteres
FIGURE 5. Transition state model for Michael addition with
nitromethane.
ized based on the minimization of the A1,3 strain in the
transition state and the approach of the bulky nitro-
methyl anion from the more favored Si face of the enoate
to give 16 (Figure 5).
In conclusion, we have described methods for the
synthesis of 3,4-substituted pyrrolidine, 2-pyrrolidinone
3-carboxylic acids as well as 5-pyrrolidine 2,3-substituted-
3-carboxylic acids utilizing internal asymmetric induction
in cycloaddition and Michael addition reactions from a
common precursor 3. The azacyclic motifs were used as
constrained replacements of a hydroxyethylene dipeptide
mimic found in OM99-2. The latter is a potent inhibitor
of BACE1 and is a pivotal enzyme in the formation of
plaque, a hallmark event in the progression of Alzhei-
mer’s disease. The azacyclic motifs were elaborated to
prototypical inhibitor-type molecules and tested for activ-
ity against BACE1, which uncovered a low nanomolar
inhibitor (IC50 < 10 nM, 79% inhibition at 10 µM) for
compound 29. In contrast, the diastereomeric 28 and the
pyrrolidine 9 or positional lactam isomer, 21, respec-
tively, were considerably less active.8 The potent activity
of 29 follows a similar level of inhibition by an analogous
cyclopentanone analogue8 30, where the carbonyl group
points in the same direction in the enzyme as the lactam
carbonyl in 29 and interacts with two water molecules
as evidenced by a cocrystal structure. Figure 6 depicts
the superposition of the modeled structure of 29 on the
cocrystal structure of the cyclopentanone analogue in the
presence of BACE1.
FIGURE 6. Superposition of docked structure of 29 (orange)
on the X-ray cocrystal structure of a cyclopentanone analogue8
(green). Red circles correspond to water molecules.
extracted with Et2O (100 mL) and washed with 10% citric acid
(2 × 40 mL), and the separated organic layer was washed with
saturated NaHCO3 (3 × 50 mL) and brine (50 mL), dried (Na2-
SO4), filtered, and concentrated. The residue was purified by
column chromatography (30% EtOAc in hexanes) to afford 2
and its epimer (5:1) (3.1 g, 72%) as a yellow oil; 1H NMR
(CDCl3) δ 4.74 (m, 1H), 4.50 (m, 1H), 3.84-3.70 (m, 1H), 3.76
(s, 3H), 1.67 (m, 1H), 1.58-1.20 (m, 2H), 1.43 (s, 9H), 0.93 (m,
6H); 13C NMR (CDCl3) δ 156.6, 153.0, 86.2, 80.0, 65.1, 52.9,
52.7, 38.6, 28.1, 24.6, 23.2, 21.7, 21.5.
(4S)-Isobutyl-(5S)-(2-methoxycarbonyl-vinyl)-2,2-di-
methyl-oxazolidine-3-carboxylic Acid tert-Butyl Ester
(3). A solution of 2 (1.27 g, 3.74 mmol), 2,2-dimethoxypropane
(15 mL, 0.12 mol), and dry acetone (70 mL) was heated to
reflux for 5 min under argon atmosphere. Then TsOH was
added until a permanent dark red solution was obtained. The
reaction mixture was refluxed for an additional 2 h, cooled to
room temperature, and concentrated. Saturated NaHCO3 (20
mL) was added to the residue and extracted with EtOAc (3 ×
60 mL). The combined organic layer was dried (Na2SO4),
filtered, and concentrated to give a crude product as a yellow
oil, which was purified by column chromatography (4% EtOAc
in hexanes) to afford a yellow oil (0.92 g, 64%); [R]D -25.6 (c
1.5, CHCl3); IR (neat) 2960, 2874, 2238, 1723, 1703, 1457, 1436,
Experimental Section
1
1381, 1254, 1175, 1128, 1089, 1068, 1027, 851, 752; H NMR
(5S)-tert-Butoxycarbonylamino-(4S,R)-hydroxy-7-meth-
yl-oct-2-ynoic Acid Methyl Ester (2).13 To a solution of i-Pr2-
NEt (2.55 g, 25.2 mmol) in 20 mL of THF at -78 °C was added
butyllithium (2.5 M in hexanes, 8.4 mL, 21 mmol), and the
solution was warmed to 0 °C for 0.5 h and then cooled to -78
°C. To the LDA solution was added methyl propiolate (2.55 g,
25.2 mmol) dropwise, and the reaction mixture was stirred at
-78 °C for 0.5 h (solution A).
To a 250-mL flame-dried flask was added anhydrous CeCl3
(5.16 g, 21 mmol) and THF (40 mL). The reaction mixture was
vigorously stirred at room temperature for 2 h and then cooled
to -78 °C. To this suspension was added solution A (-78 °C)
in one portion. The reaction mixture was stirred at the same
temperature for 1 h, and then a solution of Boc-leucinal (3.0
g, 14 mmol) in THF (12 mL) was added. The resulting mixture
was stirred at -78 °C for 3 h, quenched by addition of a
solution of acetic acid (4 mL) in THF (16 mL) at -78 °C, and
allowed to warm to room temperature. The mixture was
(CDCl3) δ 4.58 (s, 1H), 4.28-3.90 (m, 1H), 3.77 (s, 3H), 1.73
(s, 3H), 1.62-1.30 (m, 6H), 1.48 (s, 9H), 0.95 (m, 6H); 13C NMR
(CDCl3) δ 153.9, 151.6, 118.1, 96.4, 91.0, 86.6, 68.8, 62.5, 53.1,
43.4, 42.5, 28.8, 28.0, 26.3, 24.0, 21.9; MS (FAB): m/z 340 [M
+ 1]+; HRMS calcd for C18H30NO5 [M + 1]+ 340.4325; found
340.2124.
Benzene (30 mL), Lindlar catalyst (60 mg, 5 wt %), and
quinoline (0.24 mL) were placed in a 100-mL round-bottom
flask, and the suspension was stirred for 20 min at room
temperature. A solution of the yellow oil (1.2 g, 3.54 mmol) in
benzene (20 mL) was added to the flask, and the mixture was
stirred for an additional 20 min at room temperature. The flask
was then charged with H2 (balloon) and stirred vigorously
overnight. The catalyst was removed by filtration through a
pad of Celite, washed with EtOAc, and filtered. The filtrate
was concentrated, and the residual yellow oil was purified by
column chromatography (5% Et2O in CH2Cl2) to give 3 (1.1 g,
90%) as a yellow oil; [R]D +24.0 (c 1.2, CHCl3); IR (neat) 2959,
J. Org. Chem, Vol. 70, No. 17, 2005 6751

Hanessian et al.
2873, 1743, 1700, 1388, 1367, 1256, 1175, 1092, 922, 870, 770
sodium sulfate, filtered, and concentrated. The residue was
purified by column chromatography (20% MeOH in EtOAc)
to afford 6 as a colorless oil; 1H NMR (CDCl3) δ 6.83 (br s,
1H), 3.83 (br s, 1H), 3.67 (br s, 1H), 3.21 (q, J ) 6.9 Hz, 2H),
3.02 (br, 1H), 2.86 (m, 1H), 2.58-2.46 (m, 1H), 2.34 (s, 3H),
2.46-2.17 (m, 4H), 1.48 (s, 9H), 1.2-1.72 (m, 14H), 1.27 (m,
2H), 0.92 (m, 6H); 13C NMR (CDCl3) δ 175.0, 94.9, 84.4, 79.7,
59.6, 59.2, 48.9, 41.5, 38.7, 31.6, 28.4, 27.5, 25.4, 24.1, 19.9,
13.6; MS (FAB): m/z 440 [M + 1]+; HRMS calcd for C24H46N3O4
[M + 1]+ 440.3489; found 440.3476.
Compound 7. Boc-Val-NHBu (52 mg, 0.19 mmol) was
treated with HCl in dioxane (4 M, 1.5 mL) at room temperature
for 1 h. After the solvent was removed under reduced pressure,
the residue was dissolved in CH2Cl2 (2 mL) and cooled to 0
°C. The acid (from epimerization and hydrolysis of methyl ester
4, 40 mg, 0.10 mmol) was added followed by PyBOP (54 mg,
0.10 mmol) and i-Pr2NEt (74 µL, 0.42 mmol). The reaction
mixture was stirred from 0 °C to room temperature for 3 h
before it was diluted with EtOAc (10 mL), washed with 1 N
aqueous HCl and saturated NaHCO3. The organic phase was
dried and concentrated, and the residue was purified by
column chromatography (10% MeOH in EtOAc) to afford 7 (43
mg, 76%) as a white solid; [R]D -28.0 (c 0.50, CHCl3); 1H NMR
(CDCl3) δ 8.28 (br, 1H), 7.27 (br, 1H), 6.58 (br, 1H), 5.26 (d, J
) 6.9 Hz, 1H), 5.0 (br, 1H), 4.31 (t, J ) 6.4 Hz, 1H), 4.22 (q, J
) 7.0 Hz, 1H), 3.40 (m, 1H), 3.28 (m, 1H), 3.21 (m, 2H), 2.57
(m, 2H), 2.38 (m, 1H), 2.36 (m, 1H), 2.10 (s, 3H), 1.91 (m, 2H),
1.88 (m, 2H), 1.74 (m, 2H), 1.63 (m, 4H), 1.47 (m, 2H), 1.43 (s,
9H), 1.37 (d, J ) 7.2 Hz, 3H), 1.34 (m, 3H), 0.91 (m, 9H); 13C
NMR (CDCl3) δ 175.7, 171.4, 152.0, 83.4, 80.2, 59.9, 59.7, 58.9,
49.1, 47.6, 46.7, 41.9, 39.5, 32.0, 30.6, 28.9, 28.0, 26.9, 26.8,
25.9, 24.6, 21.5, 20.4, 20.0, 18.5, 14.05; IR (film) 3285, 2960,
2776, 1701, 1638, 1547; MS (FAB): m/z 539.3 [M + 1]+; HRMS
calcd for C29H55N4O5 [M + 1]+ 539.4172; found 539.4131.
1
cm-1; H NMR (CDCl3) δ 6.28 (m, 1H), 5.84 (m, 1H), 5.47 (m,
1H), 3.80-3.62 (m, 1H), 3.68 (s, 3H), 1.70-1.47 (m, 9H), 1.44
(s, 9H), 0.89 (d, J ) 5.9 Hz, 3H), 0.85 (d, J ) 6.2 Hz, 3H); 13
C
NMR (CDCl3) δ 166.2, 152.1, 147.6, 121.5, 80.3, 75.6, 61.9, 53.3,
51.9, 43.6, 28.9, 28.0, 26.3, 25.4, 24.3, 21.9; MS (FAB): m/z
342 [M + 1]+; HRMS calcd for C18H32NO5 [M + 1]+ 342.2281;
found 342.2281.
(4S)-Isobutyl-(5S)-((4R)-methoxycarbonyl-1-methyl-
pyrrolidin-(3S)-yl)-2,2-dimethyl-oxazolidine-3-carboxylic
Acid tert-Butyl Ester (4) and (4S)-Isobutyl-(5S)-((4S)-
methoxycarbonyl-1-methyl-pyrrolidin-(3R)-yl)-2,2-
dimethyl-oxazolidine-3-carboxylic Acid tert-Butyl Ester
(5). Compound 3 (175.1 mg, 0.51 mmol), sarcosine (0.2 g, 1.12
mmol), paraformaldehyde (3.42 mmol), and 0.8 of activated
molecular sieves 4 Å powder were dissolved in dry benzene
(14 mL) and refluxed gently under the argon atmosphere
overnight. The reaction mixture was cooled and filtered, the
filtrate was concentrated, and the residual yellow oil was
purified by column chromatography (10% MeOH in EtOAc)
to give 4 (160 mg, 78%) and 5 (38 mg, 18%) as light yellow
oils; For 4 [R]D +12.0 (c 0.95, CHCl3), 1H NMR (CDCl3) δ 4.07
(m, 1H), 3.66 (s, 3H), 3.60-3.82 (br, 1H), 3.13 (m, 1H), 3.03
(m, 1H), 2.58-2.75 (m, 2H), 2.51 (br, 1H), 2.40 (s, 3H), 1.48-
1.72 (m, 4H), 1.46 (S, 9H), 1.20-1.43 (m, 5H), 0.93 (d, J ) 6.4
Hz, 3H), 0.90 (d, J ) 6.3 Hz, 3H); 13C NMR (CDCl3) δ 173.7,
152.4, 80.1, 59.5, 57.8, 51.9, 45.2, 44.2, 42.5, 28.8, 27.9, 25.0,
24.0, 22.4, 14.0; MS (EI): m/z 398.3, [M+]; HRMS calcd for
C21H38N2O5 398.2781, found 398.2814. For 5 [R]D -35.8 (c 0.54,
CHCl3), 1H NMR (CDCl3) δ 4.09 (m, 1H), 3.65 (s, 3H), 3.56
(m, 1H), 3.10 (m, 2H), 2.88 (m, 1H), 2.75 (m, 1H), 2.56 (m,
1H), 2.33 (s, 3H), 2.10 (m, 1H), 1.50-1.61 (m, 7H), 1.45 (s, 9H),
1.20-1.38 (m, 2H), 0.86 (m, 6H); 13C NMR (CDCl3) δ 175.1,
152.2, 80.1, 60.8, 60.0, 52.0, 47.0, 44.9, 43.4, 42.3, 28.9, 28.5,
28.3, 25.9, 24.4, 21.5; MS (EI) m/z 398.3, [M]+; HRMS calcd
for C21H38N2O5 [M]+ 398.2781; found 398.2790.
(5S)-((4S)-Butylcarbamoyl-1-methyl-pyrrolidin-(3S)-
yl)-(4S)-isobutyl-2,2-dimethyl-oxazolidine-3-carboxylic
Acid tert-Butyl Ester (6). To the solution of 4 (50 mg, 0.13
mmol) in MeOH (1 mL) was added freshly prepared NaOMe
(0.5 M, 0.5 mL in methanol). The reaction mixture was
refluxed under an argon atmosphere for 5 h, cooled, and
quenched by addition of 5 mL of aqueous saturated ammonium
chloride solution. The aqueous layer was extracted by ethyl
acetate (3 × 30 mL), and the combined organic layers were
dried anhydrous sodium sulfate, filtered, and evaporated under
reduced pressure to a yellow syrup. The syrup was purified
by column chromatography (10% MeOH in EtOAc) to afford a
colorless syrup; [R]D -10.9 (c 1.0, MeOH), 1H NMR (CDCl3) δ
3.83 (m, 1H), 3.66 (s, 3H), 3.60-3.75 (m, 1H), 2.72 (d, J ) 6.9
Hz, 2H), 2.61 (br, 3H), 2.47 (br, 1H), 2.26 (s, 3H), 1.48-1.60
(m, 4H), 1.42-1.48 (m, 3H), 1.41 (s, 9H), 1.21-1.40 (m, 2H),
0.88 (d, J ) 6.3 Hz, 6H); 13C NMR (CDCl3) δ 174.2, 151.4, 93.7,
82.9, 79.3, 59.0, 58.7, 58.5, 51.9, 46.1, 43.3, 42.2, 41.6, 28.3,
27.7, 25.4, 24.0, 20.8; MS (EI) m/z 398.2, [M]+; HRMS calcd
for C21H38N2O5 [M + 1]+ 398.2780; found 398.2790.
Compound 8. A solution of 7 (31 mg, 0.06 mmol) in HCl (4
M in dioxane, 1 mL) was stirred at room temperature for 1 h.
After removal of the solvent under reduced pressure, the
residue was dissolved in CH2Cl2 (1.5 mL) and cooled to 0 °C.
Boc-methionine (14 mg, 0.06 mmol) was added followed by
PyBOP (30 mg, 0.06 mmol) and i-Pr2NEt (39.7 µL, 0.23 mmol).
The solution was stirred from 0 °C to room temperature for 2
h and diluted with EtOAc to 3 mL. The reaction mixture was
kept in the cold room (5 °C) overnight before it was diluted
with EtOAc (10 mL) and washed with 1 N aqueous HCl and
saturated NaHCO3. The organic phase was dried and concen-
trated, and the residue was purified by column chromatogra-
phy (10% MeOH in CH2Cl2) to afford 8 (13 mg, 33%) as a
1
colorless oil; [R]D -20.75 (c 1.60, CHCl3); H NMR (CDCl3) δ
8.28 (br, 1H), 7.27 (br, 1H), 6.58 (br, 1H), 5.26 (d, J ) 6.9 Hz,
1H), 5.0 (br, 1H), 4.31 (t, J ) 6.4 Hz, 1H), 4.22 (q, J ) 7.0 Hz,
1H), 3.40 (m, 1H), 3.28 (m, 1H), 3.21 (m, 2H), 2.57 (m, 2H),
2.38 (m, 1H), 2.36 (m, 1H), 2.10 (s, 3H), 1.91 (m, 2H), 1.88 (m,
2H), 1.74 (m, 4H), 1.63 (m, 4H), 1.47 (m, 2H), 1.43 (s, 9H),
1.37 (d, J ) 7.2 Hz, 3H), 1.34 (m, 3H), 0.91 (m, 9H); 13C NMR
(CDCl3) δ 175.0, 173.8, 156.2, 80.7, 60.5, 57.9, 54.8, 49.2, 47.3,
40.0, 39.7, 32.0, 31.7, 30.8, 30.7, 28.7, 25.3, 23.6, 22.3, 20.5,
19.9, 19.3, 15.8, 14.2; MS (FAB): m/z [M + 1]+ 630.3; HRMS
calcd for C31H60N5O6S [M + 1]+ 630.4219; found 630.4251.
To a solution of the colorless syrup (40 mg, 0.1 mmol) in
ethanol (1 mL) was added 1 mL of 0.1 M aqueous barium
hydroxide. The resulting suspension was stirred for 1 h and
then heated to 50 °C for 3 h. The mixture was quenched by
addition of 2 mL of 0.1 N sulfuric acid dropwise at -5 °C,
stirred for 30 min, and filtered, and the filtrate was concen-
trated. The residue was dissolved in CH2Cl2 (2 mL), the
solution was cooled to 0 °C, and N-(3-dimethylaminopropyl)-
N′-ethylcarbodiimide hydrochloride (EDC) (43 mg, 0.22 mmol)
was added, followed by hydroxybenzotriazole (HOBt) (30 mg,
0.22 mmol). n-Butylamine (22 µL, 0.22 mmol) and 4-(dimethyl-
amino)-pyridine (27 mg, 0.22 mmol) were added, and the
reaction mixture was stirred at 0 °C for 0.5 h, allowed to warm
to room temperature overnight, quenched by addition of
saturated NaHCO3 (10 mL), and extracted with ethyl acetate
(3 × 30 mL). The combined organic layers were dried over
Compound 9. A solution of 8 (10 mg, 0.02 mmol) in HCl (4
M in dioxane, 1 mL) was stirred at room temperature for 1 h.
After removal of the solvent under reduced pressure, the
residue was dissolved in EtOAc (10 mL) and washed with
NaHCO3 (1 N, 5 mL) and brine (5 mL). The solvent was
evaporated, and the residue was dissolved in CH2Cl2/H2O (1:
1) (1.0 mL) and cooled to 0 °C. Ac-Leu-OH (5.42 mg, 0.03 mmol)
was added, followed by EDC (7 mg, 0.03 mmol) and HOBt (4
mg, 0.03 mmol). The reaction mixture was stirred from 0 to 5
°C for 24 h. The reaction mixture was extracted with EtOAc
(3 × 5 mL). The organic phase was dried and concentrated,
and the residue was purified by column chromatography (20%
MeOH in CH2Cl2) to afford 9 (4 mg, 40%) as a white solid;
[R]D -19.56 (c 0.23, MeOH); 1H NMR (CD3OD) δ 7.69 (m, 1H),
6752 J. Org. Chem., Vol. 70, No. 17, 2005

Synthesis of Azacyclic Hydroxyethylene Isosteres
7.27 (m, 1H), 4.46 (q, J ) 4.7 Hz, 1H), 4.30 (t, J ) 6.5 Hz,
1H), 4.09 (d, J ) 7.5 Hz, 1H), 3.97 (m, 1H), 3.61 (t, J ) 3.9
1H), 3.35 (s, 3H), 3.30 (s, 3H), 3.22 (m, 1H), 3.13 (m, 2H), 3.08
(m, 2H), 2.65 (s, 3H), 2.61 (m, 1H), 2.51 (m, 1H), 2.10 (s, 3H),
2.08 (m, 1H), 2.00 (s. 3H), 1.58 (m, 1H), 1.48 (m, 4H), 1.35 (m,
2H), 1.27 (m, 7H), 0.91 (m, 16H); 13C NMR (CD3OD) δ 174.7,
174.2, 172.6, 172.3, 172.2, 73.2, 59.8, 59.3, 56.6, 52.7, 48.6, 48.4,
46.3, 40.8, 39.1, 31.5, 30.2, 24.9, 24.8, 22.8, 22.4, 21.4, 21.1,
20.1, 18.8, 18.2, 13.1; MS (FAB): m/z [M + 1]+ 685.4; HRMS
calcd for C34H65N6O6S [M + 1]+ 685.4686; found 685.4688.
48.6, 42.0, 36.8, 35.3, 33.6, 28.3, 27.4, 25.4, 23.9, 21.0, 12.3;
MS (FAB): m/z 368.3 [M]+; HRMS calcd for C20H36N2O4 [M]+
368.2675; found 368.2663. For 13b [R]D -32.2 (c 3.6, CHCl3);
1H NMR (CDCl3) δ 3.83-3.55 (m, 2H), 3.41 (m, 2H), 3.32 (m,
2H), 2.53 (m, 2H), 1.98 (m, 1H), 1.70-1.35 (m, 9H), 1.46 (s,
9H), 1.10 (t, J ) 7.3 Hz, 3H), 0.92 (d, J ) 5.9 Hz, 6H); 13C
NMR (CDCl3) δ 172.4, 82.8, 59.8, 48.5, 43.0, 41.9, 37.0, 35.3,
34.7, 29.0, 28.3, 27.7, 25.3, 24.0, 20.8, 12.3; MS (FAB): m/z
369.26 [M + 1]+; HRMS calcd for C19H36N2O4 [M]+ 368.2675;
found 368.2663.
(4S)-Isobutyl-(5S)-((1R,S)-methoxycarbonylmethyl-al-
lyl)-2,2-dimethyl-oxazolidine-3-carboxylic Acid tert-Bu-
tyl Ester (10 and 11). To a suspension of cuprous iodide (1.1
g, 6 mmol) in THF (25 mL) was added vinylmagnesium
bromide (1 M in THF, 12 mL, 12 mmol), followed by addition
of 3 (683 mg, 2 mmol) in THF (2 mL) at -78 °C, and the
mixture was stirred for 2 h. The reaction mixture was treated
with concentrated NH4OH and saturated NH4Cl (v/v, 1:1) at
-78 °C, then warmed to room temperature. The aqueous layer
was extracted with Et2O (3 × 30 mL). The combined organic
layers were dried (Na2SO4), filtered, and concentrated in vacuo.
The residue was purified by column chromatography (10%
EtOAc in hexanes) to afford 10 and 11 as light yellow oil (0.6
g, 80%, inseparable diastereomers: 10, 11; 9:1); 1H NMR
(CDCl3) δ 5.56 (m, 1H), 5.17 (m, 2H), 3.86 (m, 1H), 3.63 (m,
4H), 2.85 (m, 1H), 2.69 (m, 1H), 2.31 (m, 1H), 1.80-1.20 (m,
9H), 1.46 (S, 9H), 0.90 (m, 6H); 13C NMR (CDCl3) δ 173.2,
152.0, 137.8, 119.1, 94.9, 94.3, 82.7, 80.0, 59.1, 51.8, 46.3, 44.0,
43.0, 36.3, 31.9, 28.9, 28.5, 25.7, 24.4, 23.0, 21.4, 14.4; MS
(FAB): m/z 370.2 [M + 1]+; HRMS calcd for C20H36NO5 [M +
1]+ 370.2593; found 370.2586.
(5S)-((1R,S)-Formyl-2-methoxycarbonyl-ethyl)-(4S)-
isobutyl-2,2-dimethyl-oxazolidine-3-carboxylic Acid tert-
Butyl Ester (12). To a solution of 10 and 11 (0.55 g, 1.49
mmol) in CH2Cl2 (30 mL) was bubbled ozone (0.8 mL/s) at -78
°C until a permanent blue color was obtained. The reaction
mixture was stirred for 15 min at -78 °C, and then the flask
was charged with argon to remove excess ozone. The mixture
was quenched by addition of PPh3 (1.95 g, 9.5 mmol) at -78
°C and stirred for 12 h at room temperature, and the solvent
was evaporated under reduced pressure. The residue was
purified by column chromatography (66% EtOAc in hexanes)
to afford 12 as a colorless solid (534 mg, 99%); 1H NMR (CDCl3)
δ 9.73 (s, 1 H), 4.08 (br, 1 H), 3.85 (br, 1 H), 3.62 (s, 3 H), 2.99
(m, 1 H), 2.74 (d, J ) 7.2 Hz, 1 H), 1.80-1.25 (m, 10 H), 1.42
(s, 9 H), 0.87 (d, J ) 6.0 Hz, 6 H); 13C NMR (CDCl3) δ 201.8,
200.9, 172.1, 151.3, 106.7, 94.2, 79.9, 76.9, 76.6, 59.2, 58.9, 56.8,
51.8, 51.5, 43.6, 42.4, 41.8, 29.9, 28.2, 27.4, 25.2, 25.0, 24.8,
23.9, 21.0; MS (FAB): m/z 372.4 [M + 1]+; HRMS calcd for
C19H34NO6 [M + 1]+ 372.2386; found 372.2380.
(5S)-(1-Ethyl-(4R)-methoxycarbonyl-5-oxo-pyrrolidin-
(3R)-yl)-(4S)-isobutyl-2,2-dimethyl-oxazolidine-3-carbox-
ylic Acid tert-Butyl Ester (14). To the solution of 13a (300
mg, 0.81 mmol) in THF (9 mL) was added LDA (3.66 mL, 0.5
M, 1.83 mmol) at -78 °C. The reaction mixture was stirred
for 10 min at -78 °C, then methyl chloroformate (0.54 mL,
3.31 mmol) was added and stirred for 4 h at -78 °C. The
reaction was quenched by addition of saturated NH4Cl (10 mL)
and extracted with CH2Cl2 (3 × 30 mL). The combined organic
layers were dried (Na2SO4), filtered, and concentrated. The
residue was purified by column chromatography (50% EtOAc
in hexanes) to afford 14 as a colorless oil (336 mg, 97%); [R]D
1
+12.4 (c 0.7, CHCl3); H NMR (CDCl3) δ 3.85-3.45 (m, 2H),
3.37 (s 3H), 2.90 (m, 3H), 2.76 (br, 1H), 2.40 (br, 1H), 1.86-
1.10 (m, 8H), 1.37 (s, 9H), 0.89 (br, 3H), 0.81 (d, J ) 6.6 Hz,
3H), 0.66 (t, J ) 7.2 Hz, 3H); 13C NMR (CDCl3) δ 168.2, 165.5,
148.7, 79.1, 76.9, 56.6, 49.3, 44.1, 37.8, 34.5, 25.6, 22.8, 21.4,
18.3, 9.5; MS (FAB): m/z 427.3 [M + 1]+.
(5S),((4S)-Butylcarbamoyl-1-ethyl-5-oxo-pyrrolidin-
(3R)-yl)-(4S)-isobutyl-2,2-dimethyl-oxazolidine-3-carbox-
ylic Acid tert-Butyl Ester (15). To the solution of 14 (280
mg, 0.66 mmol) in 5 mL of ethanol and 3 mL of water was
added Ba(OH)2‚8H2O (0.19 g, 0.6 mmol). The reaction mixture
was stirred for 3 h at 65 °C, then overnight at room temper-
ature. Sulfuric acid (0.1 N, 12 mL, 6 mmol) was added, and
the suspension was stirred for 0.5 h at 0 °C. The solvent was
evaporated, and the residue was directly used in the following
reaction without further purification.
To the solution of the above residue in CH2Cl2 (15 mL) was
added EDC (132 mg, 0.69 mmol) at 0 °C, followed by HOBt
(93.2 mg, 0.69 mmol), and then butylamine (66 µL, 0.69 mmol)
and DMAP (84.3 mg, 0.11 mmol). The reaction mixture was
stirred for 0.5 h at 0 °C and then overnight at 0 °C, quenched
by addition of saturated NaHCO3 (10 mL), and extracted with
CH2Cl2 (3 × 15 mL). The combined organic layers were dried
(Na2SO4), filtered, and concentrated. The residue was purified
by column chromatography (50% EtOAc in hexanes) to afford
15 as a colorless oil (228 mg, 74%); [R]D +20.3 (c 1.2, CHCl3);
1H NMR (CDCl3) δ 7.14 (br, 1H), 3.63 (m, 2H), 3.54 (t, J ) 8.9
Hz, 1H), 3.40-3.00 (m, 6H), 2.83 (d, J ) 9.8 Hz, 1H), 1.70-
1.28 (m, 9H), 1.25 (m, 2H), 1.02 (t, J ) 7.1 Hz, 3H), 0.91 (d, J
) 5.8 Hz, 3H), 0.87 (d, J ) 6.0 Hz, 3H), 0.81 (t, J ) 7.2 Hz,
3H); 13C NMR (CDCl3) δ 171.3, 167.6, 151.8, 94.5, 81.4, 80.3,
59.8, 51.1, 48.6, 48.5, 44.1, 42.8, 39.9, 37.8, 37.3, 31.8, 28.8,
28.0, 25.8, 24.5, 21.6, 20.3, 14.0, 12.6; MS (FAB): m/z 467.3
[M + 1]+; HRMS calcd for C25H45N3O5 [M + 1]+ 467.3359;
found 467.3367.
(4S)-Isobutyl-(5S)-(2-methoxycarbonyl-(1R)-nitro-
methyl-ethyl)-2,2-dimethyl-oxazolidine-3-carboxylic Acid
tert-Butyl Ester (16). A solution of 3 (340 mg, 1 mmol) and
1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (TBD) (55
mg, 0.4 mmol) in nitromethane (1 mL) was stirred at 0 °C for
48 h. The reaction mixture was directly purified by column
chromatography (16% EtOAc in hexanes) to afford 16 as a
colorless oil (398 mg, 99%); [R]D +19.4 (c 3.5, CHCl3); 1H NMR
(CDCl3) δ: 4.70 (dd, J ) 13.2 Hz, J ) 3.6 Hz, 1H), 4.62 (dd, J
) 13.2 Hz, J ) 7.6 Hz, 1H), 3.90 (d, J ) 8.2 Hz, 1H), 3.75 (br,
1H), 3.68 (s, 3H), 2.73 (br, 1H), 2.50 (d, J ) 6.0 Hz, 2H), 1.80-
1.25 (m, 7H), 1.46 (d, J ) 5.0 Hz, 9H), 0.92 (d, J ) 6.4 Hz,
6H); 13C NMR (CDCl3) δ: 172.5, 171.6, 151.8, 95.2, 80.7, 75.4,
59.0, 58.5, 52.4, 44.2, 42.9, 38.5, 33.7, 28.8, 28.1, 25.6, 24.5,
(5S)-(1-Ethyl-5-oxo-pyrrolidin-(3R)-yl)-(4S)-isobutyl-
2,2-dimethyl-oxazolidine-3-carboxylic Acid tert-Butyl
Ester (13a) and (5S)-(1-Ethyl-5-oxo-pyrrolidin-(3S)-yl)-
(4S)-isobutyl-2,2-dimethyl-oxazolidine-3-carboxylic Acid
tert-Butyl Ester (13b). To the solution of 12 (387 mg, 1.04
mmol) in 1,2-dichloroethane (30 mL) was added anhydrous
Na2SO4 powder (1.0 g, 7.5 mmol). The reaction mixture was
cooled to 0 °C, ethylamine (84 µL, 1.113 mmol) was added,
and the solution was stirred for 12 h at 0 °C. Sodium
triacetoxyborohydride (300 mg, 1.225 mmol) was added, and
the whole mixture was stirred for 72 h at room temperature.
The mixture was treated with saturated NaHCO3 (15 mL), and
the aqueous layer was extracted with CH2Cl2 (3 × 20 mL).
The combined organic layers were dried (Na2SO4), filtered, and
concentrated. The residue was purified by column chromatog-
raphy (50% EtOAc in hexanes) to afford 13a (312 mg, 82%)
and 13b (35 mg, 9%) as colorless syrups; For 13a [R]D -5.9 (c
1
1.1, CHCl3); H NMR (CDCl3) δ 3.82-3.50 (br, 1H) 3.68 (q, J
) 8.6 Hz, J ) 1.8 Hz, 1H), 3.43 (q, J ) 9.4 Hz, J ) 8.1 Hz,
1H), 3.30 (m, 2H), 2.98 (m, 1H), 2.56 (m, 1H), 2.46 (m, 2H),
1.75-1.30 (m, 8H), 1.46 (s, 9H), 1.09 (t, J ) 7.2 Hz, 3H), 0.93
(d, J ) 6.0 Hz, 6H); 13C NMR (CDCl3) δ 173.2, 81.5, 80.0, 59.5,
J. Org. Chem, Vol. 70, No. 17, 2005 6753

Hanessian et al.
21.6, 21.5; MS (FAB): m/z 425.2 [M + 23]+; HRMS calcd for
C19H34N2O7Na [M + 23]+ 425.2264; found 425.2258.
was dried (Na2SO4), filtered, and concentrated in vacuo. The
residue was purified by column chromatography (66% EtOAc
in hexanes) to afford 20 as a white solid (99 mg, 86%); [R]D
(4S)-Isobutyl-2,2-dimethyl-(5S)-(5-oxo-pyrrolidin-(3S)-
yl)-oxazolidine-3-carboxylic Acid tert-Butyl Ester (17).
A solution of 16 (398 mg, 0.99 mmol), Raney-Ni (200 mg), and
H2PtCl6 (20 mg) in methanol was hydrogenated under 60 psi
H2 at room temperature overnight. The solvent was removed
under reduced pressure, and the residue was partitioned
between CH2Cl2 (10 mL) and NH4OH-NH4Cl (10 mL, 1:1, v/v).
The aqueous layer was extracted with CH2Cl2 (3 × 10 mL).
The combined organic layers were dried (Na2SO4), filtered, and
evaporated in vacuo. The residue was purified by column
chromatography (14% EtOAc in hexanes) to afford 17 as a
1
-56.5 (c 4.0, CHCl3); H NMR (MeOD) δ 8.38 (d, J ) 8.3 Hz,
1H), 4.33 (br, 1H), 3.92 (d, J ) 5.2 Hz, 1H), 3.76 (d, J ) 5.6
Hz, 1H), 3.58 (t, J ) 8.3 Hz, 1H), 3.52-3.27 (m, 4H), 3.20 (m,
3H), 2.38 (br, 1H), 1.68-1.41 (m, 6H), 1.66 (s, 3H), 1.49 (d, J
) 7.0 Hz, 9H), 1.34 (m, 2H), 1.17 (t, J ) 7.2 Hz, 3H), 1.02 (d,
J ) 7.0 Hz, 3H), 1.00-0.90 (m, 11H); 13C NMR (MeOD) δ 172.3,
170.9, 169.6, 152.2, 94.2, 83.1, 80.4, 59.6, 59.8, 59.5, 52.2, 41.5,
39.3, 37.9, 37.4, 31.4, 29.8, 27.8, 27.5, 27.1, 25.7, 23.6, 21.5,
20.1, 19.2, 16.6, 13.2, 11.6; MS (FAB): m/z 567.3 [M + 1]+;
HRMS calcd for C30H55N4O6 [M + 1]+ 567.4122; found 567.4117.
1
white solid (240 mg, 72%); [R]D +5.5 (c 0.6, CHCl3); H NMR
Compound 21. To the solution of 20 (91 mg, 0.16 mmol)
in 2.5 mL of CH2Cl2 was added 0.5 mL of TFA, and the solution
was stirred for 0.5 h at room temperature. The solvent was
removed under reduced pressure. To the residue and Boc-Met-
OH (80 mg, 0.32 mmol) dissolved in 4 mL of CH2Cl2 and cooled
to 0 °C were added PyBOP (170 mg, 0.32 mmol) and i-Pr2NEt
(111 µL, 0.64 mmol). The reaction mixture was stirred for 4 h
at 0 °C, EtOAc (30 mL) was added, and the solution was
washed with 1 N HCl (3 × 15 mL), followed by saturated
NaHCO3 (3 × 15 mL) and brine (15 mL). The organic layer
was dried (Na2SO4), filtered, and concentrated. The residue
was purified by column chromatography (80% EtOAc in
hexanes) to afford a white solid (67 mg, 63%); [R]D -83.7 (c
1.3, MeOH); 1H NMR (CDCl3) δ 7.50 (br, 1H), 5.34 (d, J ) 5.9
Hz, 1H), 4.26 (m, 2H), 3.63-3.11 (m, 10H), 2.98 (m, 1H), 2.62
(br, 2H), 2.35 (m, 1H), 2.25-2.00 (br, 1H), 2.08 (s, 3H), 1.92
(m, 1H), 1.80-1.39 (m, 4H), 1.41 (s, 9H), 1.38-1.19 (m, 4H),
1.10 (t, J ) 7.2 Hz, 3H), 1.00-0.75 (m, 15 H); 13C NMR (CDCl3)
δ 173.1, 171.4, 170.4, 168.6, 155.7, 80.2, 74.1, 59.7, 54.1, 52.4,
51.5, 45.8, 40.0, 39.2, 38.3, 37.8, 31.1, 30.0, 29.2, 28.1, 24.6,
23.6, 23.2, 21.5, 19.9, 19.4, 17.8, 15.1, 13.5, 12.2; MS (FAB):
m/z 658.2 [M + 1]+; HRMS calcd for C32H60N5O7S [M + 1]+
658.4213; found 658.4220.
(CDCl3) δ 7.30 (br, 1H), 3.82-3.51 (m, 2H), 3.48 (m, 1H), 3.40
(dd, J ) 10.1 Hz, J ) 6.0 Hz, 1H), 2.45 (m, 1H), 2.43 (dd, J )
16.8 Hz, J ) 9.0 Hz, 1H), 1.96 (m, 1H), 1.70-1.25 (m, 9H),
1.47 (s, 9H), 0.94 (d, J ) 5.9 Hz, 6H); 13C NMR (CDCl3) δ:
178.5, 177.6, 151.9, 94.6, 93.9, 83.1, 82.4, 80.4, 60.3, 45.0, 43.6,
42.6, 39.2, 34.2, 28.9, 28.5, 28.2, 25.9, 24.5, 21.4; MS (FAB):
m/z 363.2 [M + 23]+; HRMS calcd for C18H32N2O4Na [M + 23]+
363.2260; found 363.2254.
(5S)-(1-Ethyl-5-oxo-pyrrolidin-(3S)-yl)-(4S)-isobutyl-
2,2-dimethyl-oxazolidine-3-carboxylic Acid tert-Butyl
Ester (18). To a solution of 17 (54 mg, 0.16 mmol) in DMF (1
mL) was added NaH (7 mg, 60% dispersion in mineral oil).
The reaction mixture was stirred at room temperature for 1.5
h until the evolution of gas had ceased, and then a solution of
EtI (25.4 µL, 0.32 mmol) was added. The mixture was poured
into saturated NH4Cl (10 mL) and extracted with EtOAc (5 ×
10 mL). The combined organic layers were dried (Na2SO4),
filtered, and evaporated. The residue was purified by column
chromatography (EtOAc/hexanes, 4:1) to afford 18 as a white
solid (47 mg, 80%); [R]D -32.2 (c 3.6, CHCl3); 1H NMR (CDCl3)
δ 3.83-3.55 (m, 2H), 3.41 (m, 2H), 3.32 (m, 2H), 2.53 (m, 2H),
1.98 (m, 1H), 1.70-1.35 (m, 9H), 1.46 (s, 9H), 1.10 (t, J ) 7.3
Hz, 3H), 0.92 (d, J ) 5.9 Hz, 6H); 13C NMR (CDCl3) δ 172.4,
82.8, 59.8, 48.5, 43.0, 41.9, 37.0, 35.3, 34.7, 29.0, 28.3, 27.7,
25.3, 24.0, 20.8, 12.3; MS (FAB): m/z 369.3 [M + 1]+.
(5S)-(1-Ethyl-(4S)-methoxycarbonyl-5-oxo-pyrrolidin-
(3S)-yl)-(4S)-isobutyl-2,2-dimethyl-oxazolidine-3-carbox-
ylic Acid tert-Butyl Ester (19). To the solution of 18 (63 mg,
0.17 mmol) in THF (1.9 mL) was added LDA (0.75 mL, 0.5 M,
0.38 mmol) at -78 °C. The reaction mixture was stirred for
10 min at -78 °C. Methyl chloroformate (114 µL, 0.684 mmol)
was added and stirred for 4 h at -78 °C. The reaction was
quenched by addition of saturated NH4Cl (10 mL) and ex-
tracted with CH2Cl2 (3 × 30 mL). The combined organic layers
were dried (Na2SO4), filtered, and concentrated. The residue
was purified by column chromatography (50% EtOAc in
hexanes) to afford 19 as a colorless oil (72 mg, 99%); 1H NMR
(CDCl3) δ 3.85-3.45 (m, 2H), 3.77 (s 3H), 3.67 (m, 2H), 3.50
(t, J ) 9.3 Hz, 1H), 3.38 (m, 3H), 3.13 (br, 1H), 2.92 (m, 1H),
1.75-1.25 (m, 8H), 1.44 (s, 9H), 1.11 (t, J ) 7.3 Hz, 3H), 0.91
(d, J ) 5.9 Hz, 6H); 13C NMR (CDCl3) δ 170.4, 168.2, 151.9,
94.6, 82.6, 80.4, 59.6, 53.2, 52.6, 47.6, 43.7, 42.7, 39.9, 38.1,
28.8, 28.4, 28.1, 25.9, 24.5, 21.4, 12.7; MS (FAB): m/z 427.2
[M + 1]+.
Compound 20. A solution of 19 (146 mg, 0.34 mmol) and
NaOH (27 mg, 0.68 mmol) in MeOH (1.2 mL) and H2O (0.6
mL) was stirred at 65 °C for 1 h at room temperature
overnight. The solution was treated with 1 N HCl (4 mL),
extracted with EtOAc (3 × 10 mL), and washed with brine
(10 mL). The combined organic layers were dried (Na2SO4),
filtered, and concentrated. The crude acid was dissolved in
CH2Cl2 (2 mL) and treated with H-Val-NHBu (from Boc-Val-
NHBu 93 mg, 0.34 mmol), followed by addition of PyBOP (164
mg, 0.51 mmol) and i-Pr2NEt (141 µL, 1.03 mmol). The reaction
mixture was stirred for 0.5 h at 0 °C, then at room temperature
overnight, CH2Cl2 (30 mL) was added, and the solution was
washed with 5% NaHSO4 (3 × 20 mL), followed by saturated
NaHCO3 (3 × 20 mL) and brine (20 mL). The organic layer
To a solution of the above solid (16 mg, 0.02 mmol) in CH2-
Cl2 (0.3 mL) was added TMSI (12 µL) and stirred at room
temperature for 0.5 h. MeOH (0.5 mL) was added, and after
10 min the solvent was removed under reduced pressure. The
residue was dissolved in CH2Cl2 (0.5 mL) and H2O (0.5 mL)
and followed by addition of Ac-leucine (5 mg, 0.03 mmol), EDC
(6 mg, 0.03 mmol), and HOBt (4 mg, 0.03 mmol). The reaction
mixture was stirred at 0 °C for 24 h, and another portion of
EDC (6 mg, 0.03 mmol) was added. The solution was stirred
for at 0 °C for 24 h, then diluted by addition of EtOAc (30 mL),
NaHCO3 (3 × 10 mL), and brine (15 mL). The organic layer
was dried (Na2SO4), filtered, and concentrated, and the residue
was purified by column chromatography (10% MeOH in
EtOAc) to afford 21 as a white solid (14 mg, 83%); [R]D -83.7
1
(c 1.3, CHCl3); H NMR (CDCl3) δ 7.45 (d, J ) 9.4 Hz, 1H),
4.49 (q, J ) 4.8 Hz, 1H), 4.35 (t, J ) 7.5 Hz, 1H), 4.29 (d, J )
5.1 Hz, 1H), 3.98 (m, 1H), 3.57 (m, 4H), 3.33 (m, 4H), 3.19 (m,
2H), 2.98 (m, 1H), 2.48 (m, 1H), 2.41 (m, 1H), 2.22 (m, 1H),
2.15 (m, 1H), 2.11 (s, 3H), 2.00 (s, 3H), 1.96 (m, 1H), 1.70 (m,
1H), 1.59 (t, J ) 7.4 Hz, 3H), 1.51 (m, 3H), 1.34 (m, 4H), 1.17
(t, J ) 7.2 Hz, 3H), 1.03 (d, J ) 6.9 Hz, 3H), 0.95 (m, 18H);
13C NMR (CDCl3) δ 174.2, 172.9, 172.5, 171.4, 171.2, 75.3, 59.5,
53.3, 52.3, 52.1, 49.9, 41.4, 40.7, 39.3, 39.1, 37.7, 31.6, 31.3,
31.0, 30.3, 24.9, 24.7, 22.8, 22.5, 21.4, 21.3, 21.0, 20.2, 18.8,
17.0, 14.3, 13.1, 11.5; MS (FAB): m/z 735.4 [M + 23]+; HRMS
calcd for C35H64N6O7SNa [M + 23]+ 735.4455; found 735.4450.
(5R)-Formyl-(4S)-isobutyl-2,2-dimethyl-oxazolidine-3-
carboxylic Acid tert-Butyl Ester (22). Into a solution of 3
(0.341 g, 1.0 mmol) in CH2Cl2 (20 mL) was bubbled ozone (0.8
mL/s) at -78 °C until a permanent blue color was obtained.
The reaction mixture was stirred for 15 min at -78 °C, and
the flask was purged with argon to remove excess ozone. The
mixture was quenched by addition of PPh3 (0.79 g, 3.0 mmol)
at -78 °C and allowed to stir at room temperature overnight.
The solvent was evaporated under reduced pressure, and the
residue was purified by column chromatography (66% EtOAc
6754 J. Org. Chem., Vol. 70, No. 17, 2005

Synthesis of Azacyclic Hydroxyethylene Isosteres
in hexanes) to afford 22 as a colorless solid (284 mg, 99%);
[R]D +29.5 (c 1.2, CHCl3); 1H NMR (CDCl3) δ 9.85 (s, 1H), 4.15
(br, 2H), 1.80-1.49 (m, 9H), 1.48 (s, 9H), 0.97 (t, J ) 7.2 Hz,
6H); 13C NMR (CDCl3) δ 203.2, 151.6, 85.0, 80.7, 71.6, 57.2,
43.5, 28.8, 28.0, 27.1, 26.2, 25.9, 24.2, 21.6; MS (FAB): m/z
369.0 [M ]+.
(50% EtOAc in hexanes) to afford 25a and 25b (204 mg, 99%)
1
as an inseparable colorless oil; H NMR (CDCl3) δ 3.85-3.51
(m, 4H), 2.85 (m, 3H), 2.75-2.32 (m, 2H), 1.70-1.30 (m, 6H),
1.48 (s, 9H), 0.95 (m, 6H); 13C NMR (CDCl3) δ 174.3, 173.7,
172.2, 171.9, 151.9, 95.3, 81.8, 80.8, 65.0, 63.9, 60.0, 59.6, 53.1,
52.9, 43.8, 43.4, 40.6, 40.1, 33.6, 31.5, 29.3, 28.9, 28.2, 26.0,
25.8, 24.6, 21.6, 21.4; MS (FAB): m/z 435.2 [M + 23]+; HRMS
calcd for C21H36N2O6Na [M + 23]+ 435.2471; found 435.2468.
(5S)-((Z)-2-Bromo-2-methoxycarbonyl-vinyl)-(4S)-isobu-
tyl-2,2-dimethyl-oxazolidine-3-carboxylic Acid tert-Butyl
Ester (23). To a solution of 22 (414 mg, 1.4 mmol) in toluene
(3.8 mL) and CH2Cl2 (1.9 mL) was added Ph3PC(Br)CO2Me
(752 mg, 1.82 mmol). The reaction mixture was stirred at 80
°C for 4 h. After removal of the solvent under reduced pressure,
the residue was purified by column chromatography (14%
EtOAc in hexanes) to afford 23 as a colorless oil (524 mg, 89%)
and an isomer (as a colorless oil 50 mg, 9%); For 23; [R]D -2.5
Compounds 26 and 27. A solution of 25a and 25b (120
mg, 0.29 mmol) and NaOH (23.3 mg, 0.58 mmol) in MeOH
(4.0 mL) and H2O (2.0 mL) was stirred at 65 °C for 1 h at
room temperature overnight. The solution was treated with 1
N HCl (7 mL), extracted with EtOAc (3 × 10 mL), and washed
with brine (10 mL). The combined organic layers were dried
(Na2SO4), filtered, and concentrated in vacuo. The mixture of
crude acid was dissolved in CH2Cl2 (4.8 mL) and treated with
H-Val-NHBu (from Boc-Val-NHBu, 118 mg, 0.44 mmol),
followed by addition of PyBOP (233 mg, 0.44 mmol) and i-Pr2-
NEt (202 µL, 1.16 mmol). The reaction mixture was stirred
for 0.5 h at 0 °C and then at room temperature overnight.
EtOAc (30 mL) was added, and the solution was washed with
1 N HCl (3 × 15 mL), followed by saturated NaHCO3 (3 × 15
mL) and brine (15 mL). The organic layer was dried (Na2SO4),
filtered, and concentrated in vacuo. The residue was purified
by column chromatography (EtOAc) to afford 26 (41 mg, 27%)
and 27 (35 mg, 23%); For 26; [R]D -6.4 (c 2.0, MeOH); 1H NMR
(MeOD) δ 4.28 (d, J ) 3.3 Hz, 1H), 4.19 (d, J ) 8.4 Hz, 1H),
3.79 (m, 2H), 3.34 (d, J ) 15.5 Hz, 1H), 3.33 (m, 1H), 2.82-
2.65 (m, 1H), 2.78 (s, 3H), 2.60 (m, 1H), 2.04 (m, 2H), 1.63-
1.54 (m, 11H), 1.49 (s, 9H), 1.40 (m, 3H), 0.96 (m, 15H); 13C
NMR (MeOD) δ 175.2, 171.5, 171.4, 82.8, 80.4, 66.1, 59.8, 59.6,
43.8, 40.1, 39.2, 39.1, 33.7, 31.4, 31.3, 27.8, 27.7, 25.5, 23.6,
20.6, 20.2, 18.7, 18.2, 13.1; MS (FAB): m/z. 553.3 [M + 1]+;
1
(c 1.0, CHCl3); H NMR (CDCl3) δ 7.32 (d, J ) 8.4 Hz, 1H),
4.69 (dd, J ) 8.4 Hz, J ) 1.7 Hz, 1H), 4.00-3.60 (br, 1H), 3.79
(s, 3H), 1.75-1.49 (m, 9H), 1.42 (s, 9H), 0.88 (d, J ) 7.3 Hz,
3H), 0.84 (d, J ) 8.6 Hz, 3H); 13C NMR (CDCl3) δ 162.8, 151.7,
143.8, 140.3, 117.6, 96.3, 80.5, 79.6, 61.4, 53.9, 53.7, 43.5, 42.6,
28.8, 28.0, 25.9, 24.2, 21.8; MS (FAB): m/z 442.1 [M + 23]+;
HRMS calcd for C18H30BrNO5Na [M + 23]+ 442.1205; found
442.1199. For isomer; [R]D -36.6 (c 0.7, CHCl3); 1H NMR
(CDCl3) δ 7.38 (d, J ) 6.8 Hz, 1H), 5.16 (dd, J ) 9.0 Hz, J )
1.5 Hz, 1H), 4.00-3.60 (m, 1H), 3.84 (s, 3H), 1.75-1.30 (m,
9H), 1.50 (s, 9H), 0.94 (t, J ) 6.7 Hz, 6H); 13C NMR (CDCl3) δ
163.1, 146.4, 114.5, 80.6, 61.7, 53.9, 53.7, 28.9, 25.7, 25.6, 24.2,
22.0; MS (FAB): m/z 442.1 [M + 23]+; HRMS calcd for C18H30-
BrNO5Na [M + 23]+ 442.1205; found 442.1199.
(4S)-Isobutyl-(5S)-[(2S)-methoxycarbonyl-1-methyl-5-
oxo-(4S)-(toluene-4-sulfonyl)-pyrrolidin-(3S)-yl]-2,2-di-
methyl-oxazolidine-3-carboxylic Acid tert-Butyl Ester
(24a) and (4S)-Isobutyl-(5S)-[(2R)-methoxycarbonyl-1-
methyl-5-oxo-(4R)-(toluene-4-sulfonyl)-pyrrolidin-(3R)-
yl]-2,2-dimethyl-oxazolidine-3-carboxylic Acid tert-Butyl
Ester (24b). To a suspension of NaH (104 mg, 2.6 mmol,
dispersed in mineral oil and washed three times with dry
pentane) in THF (8 mL) was added 2-tolylsulfonyl N-methyl-
acetamide (196 mg, 0.86 mmol) in portions. After being stirred
at room temperature for 20 min, a solution of 23 (363 mmol,
0.86 mmol) was added to the suspension mixture during a
period of 30 min. The reaction mixture was stirred overnight
at room temperature and quenched by addition of saturated
NH4Cl (10 mL) and 1 N HCl (2 mL). The aqueous layer was
extracted with EtOAc (5 × 15 mL), and the combined organic
layers were dried (Na2SO4), filtered, and evaporated. The
residue was purified by column chromatography (50% EtOAc
in hexanes) to afford a mixture of 24a and 24b (397 mg, 81%)
as an inseparable colorless oil; 1H NMR (CDCl3) δ 7.81 (d, J )
8.1 Hz, 2H), 7.65 (d, J ) 8.3 Hz, 2H), 4.14 (m, 1H), 4.03 (m,
1H), 4.01-3.75 (m, 4H), 3.61 (m, 1H), 3.36 (m, 1H), 3.00 (d,
3H), 2.47 (d, 3H), 1.68-1.32 (m, 18H), 0.96 (m, 6H); 13C NMR
(CDCl3) δ 171.1, 170.8, 152.6, 152.5, 146.5, 146.5, 134.2, 133.2,
130.1, 130.0, 129.7, 82.2, 80.6, 69.0, 68.1, 67.0, 63.7, 62.0, 61.2,
59.5, 58.3, 53.2, 53.1, 42.3, 30.9, 30.8, 28.9, 28.8, 27.5, 26.1,
26.0, 25.9, 24.7, 22.1, 21.7, 21.1; MS (FAB): m/z 589.3 [M +
23]+; HRMS calcd for C28H42N2O8SNa [M + 23]+ 589.2560;
found 589.2556.
(4S)-Isobutyl-(5S)-((2S)-methoxycarbonyl-1-methyl-5-
oxo-pyrrolidin-(3S)-yl)-2,2-dimethyl-oxazolidine-3-car-
boxylic Acid tert-Butyl Ester (25a) and (4S)-Isobutyl-
(5S)-((2R)-methoxycarbonyl-1-methyl-5-oxo-pyrrolidin-
(3R)-yl)-2,2-dimethyl-oxazolidine-3-carboxylic Acid tert-
Butyl Ester (25b). A solution of 24a and 24b (283 mg, 0.5
mmol), dibasic sodium phosphate (235 mg, 2 mmol), and
sodium amalgam (10%, 450 mg) in methanol (12 mL) was
vigorously stirred at 0 °C for 1 h. To the reaction mixture was
added saturated NH4Cl (20 mL), the solvent was evaporated
under reduced pressure, and the residue was dissolved in H2O
(20 mL) and extracted with EtOAc (5 × 15 mL). The combined
organic layers were dried (Na2SO4), filtered, and evaporated
in vacuo. The residue was purified by column chromatography
1
For 27; [R]D -31.8 (c 1.6, CHCl3); H NMR (CDCl3) δ 6.83 (d,
J ) 8.6 Hz, 1H), 6.26 (br, 1H), 4.24 (t, J ) 8.0 Hz, 1H), 3.90
(br, 1H), 3.81 (br, 1H), 3.68 (br, 1H), 3.30 (m, 1H), 3.19 (m,
1H), 2.80 (s, 3H), 2.48 (m, 3H), 2.06 (m, 1H), 1.60-1.44 (m,
13H), 1.46 (s, 9H), 1.33 (m, 3H), 0.96 (m, 18H); 13C NMR
(CDCl3) δ 171.1, 170.5, 167.6, 152.0, 94.4, 82.5, 80.4, 59.4, 58.8,
51.6, 47.3, 43.8, 39.6, 38.3, 36.4, 31.7, 29.8, 28.8, 28.2, 27.6,
25.8, 24.6, 21.6, 20.4, 20.1, 17.2, 14.1, 12.6; MS (FAB): m/z
553.3 [M + 1]+.
Compound 28. To the solution of 26 (40 mg, 0.07 mmol)
in 0.4 mL of CH2Cl2 was added 0.1 mL of TFA, and the solution
was stirred for 1 h at room temperature. The solvent was
evaporated under reduced pressure. The residue and Boc-Met-
OH (36 mg, 0.15 mmol) were dissolved in 2 mL of CH2Cl2 and
cooled to 0 °C. PyBOP (77 mg, 0.15 mmol) and i-Pr2NEt (50
µL, 0.29 mmol) were added. The reaction mixture was stirred
for 0.5 h at 0 °C overnight. EtOAc (20 mL) was added, and
the solution was washed with 1 N HCl (3 × 10 mL), followed
by saturated NaHCO3 (3 × 10 mL) and brine (10 mL). The
organic layer was dried (Na2SO4), filtered, and concentrated.
The residue was purified by column chromatography (10%
MeOH in EtOAc) to afford a white solid (34 mg, 73%); [R]D
-16.2 (c 1.7, CHCl3). To a solution of the solid (32 mg, 0.05
mmol) in CH2Cl2 (1.5 mL) was added TMSI (50 µL) and stirred
at room temperature for 0.5 h. MeOH (0.5 mL) was added and
stirred for 10 min. The solvent was removed under reduced
pressure, and the residue was dissolved in CH2Cl2 (0.9 mL)
and H2O (0.6 mL), followed by addition of Ac-leucine (10.4 mg,
0.06 mmol), EDC (13 mg, 0.07 mmol), and HOBt (9 mg, 0.07
mmol). The reaction mixture was stirred at 0 °C for 24 h,
another portion of EDC (13 mg, 0.07 mmol) was added, and
the solution was stirred for at 0 °C for 24 h, then diluted by
addition of EtOAc (30 mL), NaHCO3 (3 × 10 mL), and brine
(15 mL). The organic layer was dried (Na2SO4), filtered, and
concentrated, and the residue was purified by column chro-
matography (20% MeOH in EtOAc) to afford 28 as a white
solid (26 mg, 75%); [R]D -49.3 (c 1.4, MeOH); 1H NMR (MeOD)
δ 8.05 (t, J ) 5.6 Hz, 1H), 7.54 (d, J ) 9.5 Hz, 1H), 4.48 (dd,
J ) 8.9 Hz, J ) 5.4 Hz, 1H), 4.37 (t, J ) 7.6 Hz, 1H), 4.27 (d,
J. Org. Chem, Vol. 70, No. 17, 2005 6755

Hanessian et al.
J ) 5.0 Hz, 1H), 4.25 (d, J ) 6.0 Hz, 1H), 4.03 (m, 1H), 3.53
(d, J ) 8.8 Hz, 1H), 3.30 (m, 1H), 3.19 (m, 1H), 2.79 (s, 3H),
2.66 (m, 2H), 2.49 (m, 2H), 2.25 (m, 2H), 2.15-1.85 (m, 2H),
2.08 (s, 3H), 2.00 (s, 3H), 1.65 (m, 7H), 1.48-1.20 (m, 3H), 0.96
(m, 21H); 13C NMR (MeOD) δ 175.8, 174.1, 172.8, 172.7, 172.6,
172.6, 172.5, 172.1, 172.0, 75.8, 66.0, 59.3, 59.2, 52.5, 40.7, 30.9,
30.3, 28.0, 24.9, 24.7, 22.8, 22.4, 21.4, 21.3, 21.1, 20.2, 18.9,
17.2, 14.4, 13.2; MS (FAB): m/z 721.4 [M + 23]+; HRMS calcd
for C34H62N6O7SNa [M + 23]+ 721.4299; found 721.4292.
Compound 29. To the solution of 27 (34 mg, 0.06 mmol)
in 0.4 mL of CH2Cl2 was added 0.1 mL of TFA, the solution
was stirred for 1 h at room temperature, and the solvent was
evaporated under reduced pressure. The residue and Boc-Met-
OH (31 mg, 0.12 mmol) were dissolved in 1.8 mL of CH2Cl2
and cooled to 0 °C. PyBOP (65 mg, 0.12 mmol) and i-Pr2NEt
(43 µL, 0.25 mmol) were added. The reaction mixture was
stirred for 0.5 h at 0 °C overnight, EtOAc (20 mL) was added,
and the solution was washed with 1 N HCl (3 × 10 mL),
followed by saturated NaHCO3 (3 × 10 mL) and brine (10 mL).
The organic layer was dried (Na2SO4), filtered, and concen-
trated, and the residue was purified by column chromatogra-
phy (10% MeOH in EtOAc) to afford a white solid (29 mg, 73%);
[R]D -40.0 (c 1.5, CHCl3). To a solution of the white solid (27
mg, 0.04 mmol) in CH2Cl2 (1.2 mL) was added TMSI (40 µL)
and stirred at room temperature for 0.5 h. MeOH (0.5 mL)
was added and stirred for 10 min. The solvent was removed
under reduced pressure, the residue was dissolved in CH2Cl2
(0.9 mL) and H2O (0.5 mL), followed by addition of Ac-leucine
(9 mg, 0.05 mmol), EDC (11 mg, 0.05 mmol), and HOBt (7 mg,
0.05 mmol). The reaction mixture was stirred at 0 °C for 24 h,
another portion of EDC (11 mg, 0.05 mmol) was added, and
the solution was stirred for at 0 °C for 24 h, then diluted by
addition of EtOAc (30 mL), NaHCO3 (3 × 10 mL), and brine
(15 mL). The organic layer was dried (Na2SO4), filtered, and
concentrated. The residue was purified by column chromatog-
raphy (20% MeOH in EtOAc) to afford 29 as a white solid (20
(t, J ) 5.2 Hz, 1H), 7.54 (d, J ) 9.3 Hz, 1H), 4.49 (dd, J ) 9.6
Hz, J ) 4.6 Hz, 1H), 4.36 (t, J ) 7.5 Hz, 1H), 4.18 (d, J ) 3.3
Hz, 1H), 4.14 (d, J ) 11.7 Hz, 2H), 3.47 (dd, J ) 6.3 Hz, J )
2.6 Hz, 1H), 3.31 (m, 1H), 3.24 (m, 1H), 2.75 (s, 3H), 2.73-
2.50 (m, 4H), 2.38 (m, 1H), 2.26-1.92 (m, 3H), 2.10 (s, 3H),
2.01 (s, 3H), 1.75 (m, 1H), 1.65 (m, 4H), 1.60 (m, 2H), 1.42-
1.24 (m, 3H), 0.96 (m, 21H); 13C NMR (MeOD) δ 176.6, 174.3,
172.6, 172.2, 172.1, 171.5, 72.5, 65.9, 60.0, 52.7, 31.5, 31.0, 30.4,
28.0, 24.9, 24.8, 22.9, 22.4, 21.5, 21.2, 21.1, 20.1, 18.7, 18.5,
14.3, 13.1; MS (FAB): m/z 721.4 [M + 23]+; HRMS calcd for
C34H62N6O7SNa [M + 23]+ 721.4299; found 721.4292.
Modeling of Compounds in BACE1. A 10 Å shell around
the inhibitor in the BACE1 OM99-2 cocrystal structure (PDB
ref 1FKN) was used for calculations. In this binding site model,
the Monte Carlo docking/energy minimization protocol of the
MCDOCK routine in the QXP program19 (within the Flow96
package) was applied. Depending on the size and flexibility of
the ligands, 1000 or 2000 search and energy minimization
cycles were performed to ensure an in-depth conformational
search and the exploration of different possible binding modes.
Acknowledgment. We thank the National Science
and Engineering Council of Canada (NSERC) and
Novartis Pharma for generous financial assistance, Dr.
Michel Simard (Universite´ de Montre´al) for X-ray
analyses, and Eric Therrien for technical assistance.
Supporting Information Available: NMR spectra for the
synthetic molecules and CIF files of X-ray structures 6 and
15. X-ray crystallographic data have been deposited in the
Cambridge Crystallographic Database. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
JO050740W
(19) McMartin, C.; Bohacek, R. S. J. Comput.-Aided Mol. Des. 1997,
11, 333.
1
mg, 68%); [R]D -75.0 (c 1.0, MeOH); H NMR (MeOD) δ 8.18
6756 J. Org. Chem., Vol. 70, No. 17, 2005