Conformational study and enantioselective, regiospecific syntheses of novel aminoxy trans-proline analogues derived from an acylnitroso Diels-Alder cycloaddition

Article abstract of DOI:10.1021/jo010284l

The cis/trans isomerization of the proline amide bond has many implications in biological processes. The conformations of representative acylnitroso-derived proline analogues derived from cyclopentadiene were shown to exist exclusively as the E or trans conformation in CD₂Cl₂. The energetically favored conformations were determined using COSMO self-consistent reaction field calculations at the B3LYP/6-31G* level of theory in addition to low temperature ¹H NMR studies. The syntheses of the acylnitroso-derived peptides utilized two methods to selectively functionalize either of two chemically similar esters in the acylnitroso-derived amino acids. A novel transpeptidation of the amino acid that controlled the absolute stereochemistry in the acylnitroso Diels-Alder cycloaddition took advantage of an activated aminoxy amide linkage to control regiochemistry. Alternatively, an enantioselective and regiospecific enzymatic resolution of a racemic dimethyl ester provided a novel aminoxy acid.

Full text of DOI:10.1021/jo010284l

6046
J . Org. Chem. 2001, 66, 6046-6056
Con for m a tion a l Stu d y a n d En a n tioselective, Regiosp ecific
Syn th eses of Novel Am in oxy tr a n s-P r olin e An a logu es Der ived
fr om a n Acyln itr oso Diels-Ald er Cycloa d d ition
Brock T. Shireman, Marvin J . Miller,* Marco J onas, and Olaf Wiest‡
Department of Chemistry & Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall,
Notre Dame, Indiana 46556-5670
Marvin.J .Miller.2@nd.edu
Received March 14, 2001
The cis/trans isomerization of the proline amide bond has many implications in biological processes.
The conformations of representative acylnitroso-derived proline analogues derived from cyclopen-
tadiene were shown to exist exclusively as the E or trans conformation in CD2Cl2. The energetically
favored conformations were determined using COSMO self-consistent reaction field calculations
at the B3LYP/6-31G* level of theory in addition to low temperature 1H NMR studies. The syntheses
of the acylnitroso-derived peptides utilized two methods to selectively functionalize either of two
chemically similar esters in the acylnitroso-derived amino acids. A novel transpeptidation of the
amino acid that controlled the absolute stereochemistry in the acylnitroso Diels-Alder cycloaddition
took advantage of an activated aminoxy amide linkage to control regiochemistry. Alternatively, an
enantioselective and regiospecific enzymatic resolution of a racemic dimethyl ester provided a novel
aminoxy acid.
In tr od u ction
The cis/trans isomerization of the Xaa-proline amide
bond has many implications in biological systems.6-10
Prolines and 4-hydroxy prolines are important consti-
tutents of collagen.6 In addition, proline is integral to pro-
tein folding,7 regulatory switches,8 peptide antigens,9 and
peptide hormones.10 Since N-acylated prolines are ter-
tiary amides, they are unique among naturally occurring
amino acids in that they exist as mixtures of cis and trans
isomers about the N-CO bond.11 In large proteins, ap-
proximately 10% of the Xaa-pro bonds are in the cis con-
formation.11a Nature has chosen peptidyl-prolyl isomer-
ases12 such as the FK506 binding protein and the cyclo-
philins to catalyze cis/trans isomerization processes intra-
cellularly. The design and syntheses of small conforma-
tionally restricted mimetics that exist preferentially as
cis- or trans-proline analogues is, therefore, of interest.13
Conformationally restricted molecules are used to
represent bioactive conformations of substrates for a
particular biological targets. The design and syntheses
of peptide mimetics that are conformationally restricted
through covalent bonds1 is a rapidly expanding area.
Likewise, geometrically favored intramolecular hydrogen
bonds2-5 can bias a substrate into a given conformation.
Medicinally, this approach has great potential for the
design of new therapeutic agents.1
‡ To whom correspondence regarding the computational studies
should be addressed.
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10.1021/jo010284l CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/08/2001

Novel Aminoxy trans-Proline Analogues
J . Org. Chem., Vol. 66, No. 18, 2001 6047
Previously, we have reported the synthesis of a novel
mation of the aminoxy amide would presumably deter-
mine whether noncovalent interactions, such as hydrogen
bonding, could occur with functionalities attached at C-3
or C-5. Intuition would favor predominance of E-3 (w
≈180°) due to the presence of the repelling dipoles of the
aminoxy amide, therefore allowing it to represent a trans-
proline analogue.19b In fact, Ac-5-oxa-Pro has been shown
to exist exclusively in the trans conformation.20
cyclic disubstituted aminoxy dipeptide 214 (eq 1, n ) 1)
However, when heteroatom containing substitutents,
such as OMe, are in the R-position of aminoxy amides,
the Z-configurations are more stable (eq 3).21 For ex-
utilizing the cycloadduct obtained from the Diels-Alder
reaction15 between cyclopentadiene and acylnitroso spe-
cies 1, generated transiently from the corresponding
L-alanine-derived hydroxamic acid.14a,16 The cyclic ami-
noxy acid17 obtained from the acylnitroso Diels-Alder
cycloaddition possesses three sites for functionalization,
which include (1) the disubstituted hydroxylamine ni-
trogen, which is already functionalized in 2 with Boc-L-
ala, (2) the C-3 ester, and (3) the C-5 ester. The relative
stereochemistry of newly generated syn-stereocenters of
2 are controlled in the acylnitroso cycloaddition. More-
over, the use of larger dienes18 allows control of the ring
size (n ) 1-3) that makes up the disubstituted acylated
hydroxylamine.
ample, the relative energies of the Z and E conformations
of aminoxy amides 4 and 5 have been determined using
density functional calculations.21c The calculations indi-
cated that E-4 was more stable than Z-4 by 0.8 kcal/mol
in CHCl3 due to electron/electron repulsions between the
oxygen atoms of the aminoxy amide in Z-4, as would be
expected. However, in 5 the R-methoxy group renders Z-5
more stable than E-5 by 2.8 kcal/mol in CHCl3. The
electrostatic repulsion between the methoxy groups in
E-5 is apparently more destabilizing than the repulsion
between the oxygen atoms of the aminoxy amide in Z-5.
Herein, we report reproducible, regiospecific, and
enantioselective methodology to synthesize peptides re-
lated to 2 or 3. Regioselective functionalization of either
of the methyl esters in an acylnitroso peptide such as 3
would be necessary before rationally designed trans- or
cis-proline-like systems could be obtained. In addition,
E,Z-conformational analyses of representative N-acylated
acylnitroso-derived cyclic aminoxy amides utilizing den-
Acylnitroso-derived peptides, represented in a general
sense as 3, may exist as E-3 (w ≈180°) or Z-3 (w ≈ 0°)
with respect to the aminoxy amide (eq 2).19 The confor-
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1
sity functional calculations and variable temperature H
NMR are described.19
(14) (a) Ritter, A. R.; Miller, M. J . Tetrahedron Lett. 1994, 35, 9379.
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R.; Miller, M. J . J . Org. Chem. 1994, 59, 4602. (c) Shireman, B. T.;
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Resu lts a n d Discu ssion
Syn th esis. We envisioned two methods to synthesize
the desired peptides in a regio- and stereocontrolled
fashion (Scheme 1). Both of these methods rely on an
acylnitroso-derived diester represented as 6.14a If diester
6 was formed utilizing an L-Ala-derived acylnitroso
Diels-Alder cycloaddition, deprotection, and cyclization
should lead regiospecifically to diketopiperazine 7. Then,
upon methanolysis of the activated aminoxy amide, the
intermediate, substituted hydroxylamine 8, should be
(18) Shireman, B. T.; Miller, M. J . J . Org. Chem. 2001, 66, 4809.
(19) (a) Kolasa, T. Tetrahedron 1982, 39, 1753. (b) Price, B. J .;
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7667.

6048 J . Org. Chem., Vol. 66, No. 18, 2001
Shireman et al.
Sch em e 1
Sch em e 2
Sch em e 3
obtained. The net reaction would constitute a novel
transpeptidation of the amino acid initially utilized to
control both newly generated stereogenic centers.18 An-
other route would be to employ an enzyme-mediated
simultaneous enantio- and regioselective hydrolysis of
(()-6 (R ) OR′) to provide an acid/ester such as 9. The
carboxylic acid in 9 would be available for coupling
reactions to provide an ester/amide such as 10.
Implementation of the nonenzymatic, transpeptidation
route is shown in Scheme 2. Oxidation of hydroxamic acid
11 under Swern conditions22 in the presence of cyclopen-
tadiene provided the previously reported cycloadduct16b
12 in 70% combined yield as a 4.1:1 to 5.9:1 mixture of
diastereomers, as determined by HPLC analyses.16c Re-
crystallization provided 12 as a single diastereomer.
Oxidation of 12 with RuO4, generated in situ from NaIO4/
RuCl3‚H2O,14a,23 provided dimethyl ester 13 in 67% yield
after treatment with ethereal CH2N2. Removal of the Cbz
protecting group with Pd/C under a H2 atmosphere
provided the corresponding free amine that was dissolved
in CH3CN to provide diketopiperazine 7 in 74% yield for
the deprotection-cyclization sequence. It should be pointed
out that diketopiperazine 7 represents a conformationally
restricted cis-γ-substituted proline mimetic. Upon treat-
ing 7 with premixed SOCl2/MeOH, opening of the dike-
topiperazine occurred to provide 8 in 82% yield after
neutralization with aqueous Na2CO3. The transpeptida-
tion of the amino acid to the C-3 ester proceeded in 61%
combined yield for four steps beginning with dimethyl
ester 13. As a representative example, disubstituted
hydroxylamine 8 was coupled with Cbz-Gly, utilizing
1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydro-
chloride (EDC‚HCl) in CH3CN in the presence of Et3N
and 1-hydroxy-7-azabenzotriazole (HOAt),24 to give 14 in
68% yield.
During the course of these studies, an interesting N-O
bond cleavage was discovered (Scheme 3). In an attempt
to cyclize Boc amide 15 directly to the corresponding
diketopiperazine by deprotonation with NaH in THF,
dehydroalanine-containing dipeptide 1725 was isolated in
74% yield. NMR and mass spectral data were consistent
with the structure of 17. The NMR also matched litera-
ture data25b and was similar to spectra of related dehy-
droalanine derivatives previously prepared in our labora-
tory.25d Although this pathway is speculative, this unex-
pected product might arise by initial deprotonation of the
Boc-carbamate to give anion 16. Proton transfer with
simultaneous N-O bond cleavage would then give imine
(24) Carpino, L. A. J . Am. Chem. Soc. 1993, 115, 4397.
(25) (a) Dunn, M. J .; Gomez, S.; J acksom, R. F. W. J . Chem Soc.,
Perkin Trans. 1 1995, 1639. (b) Graham, D. W.; Ashton, W. T.; Barash,
L.; Brown, J . E.; Brown, R. D.; Canning, L. F.; Chen, A.; Springer, J .
P.; Rogers, E. F. J . Med. Chem. 1987, 30, 1074. (c) Nomoto, S.; Sano,
A.; Shiba, T. Tetrahedron Lett. 1979, 6, 521. (d) Miller, M. J . J . Org.
Chem. 1980, 45, 5, 3131.
(22) (a) Miller, A.; Paterson, T. M.; Procter, G. Synlett 1989, 1, 32-
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Org. Chem. 1981, 46, 3936.

Novel Aminoxy trans-Proline Analogues
J . Org. Chem., Vol. 66, No. 18, 2001 6049
Sch em e 4
Sch em e 5
18,26 and subsequent loss of methyl glyoxylate would give
17 after workup. We have observed similar anion-induced
N-O bond cleavage reactions in our earlier work on
substituted N-hyroxy â-lactams.26a
Sch em e 6
To increase the size of the ring containing the disub-
stituted hydroxylamine, a larger diene such as cyclo-
hexadiene was used. As shown in Scheme 4, oxidation of
hydroxamic acid 11 under Swern conditions in the
presence of 1,3-cyclohexadiene provided cycloadduct 19
in 93% combined yield as a 5.1:1 diastereomeric mixture,
as determined by 1H NMR analysis of the crude reaction
mixture. The diastereomeric ratio was enhanced to 10:1
by a single recrystallization. While further manipulations
were performed on the 10:1 diastereomeric mixture,
subsequent recrystallizations were required to provide
cycloadduct 19 as a single diastereomer, at which point
an X-ray crystal structure determination confirmed the
expected stereochemistry. The RuO4 protocol utilized
previously afforded low yields of 20. However, oxidation
of the 10:1 diastereomeric mixture with KMnO4 in the
presence of Na2CO3, followed by treatment with CH2N2,
provided 20 in a 70% combined yield as a mixture of
diastereomers. Without separation, the diastereomeric
mixture was subjected to Pd/C under a H2 atmosphere,
yielding the diastereomeric free amines. After dissolving
the diastereomeric amines in CH3CN, diketopiperazine
21 was isolated as a single diastereomer in 89% combined
yield for both steps. The deprotected amine derived from
the minor diastereomer did not cyclize under the reaction
conditions. Thus, intramolecular diketopiperazine forma-
tion readily differentiates the two methyl esters of the
elaborated acylnitroso Diels-Alder cycloadducts 6 and
homologues.
To pursue the proposed enzymatic methyl ester dif-
ferentiation, racemic dimethyl ester 24 was utilized
(Scheme 5).14b Oxidation of BocNHOH (22) with NaIO4
in MeOH/H2O at 0 °C in the presence of cyclopentadiene
provided (()-23 in 74% yield.27 Oxidative cleavage of
alkene (()-23 with RuO4 in EtOAc/CH3CN/H2O, gener-
ated in situ from NaIO4/RuCl3‚H2O, provided (()-24 in
84% combined yield after treatment with ethereal CH2N2.
Although dimethyl ester 24 was subjected to reactions
with a variety of enzymes, the best results were obtained
with Chirazyme L-228 in toluene and 0.1 M phosphate
buffer at pH ) 7.5 (Scheme 6). Under these conditions
acid 25 was obtained in 33% yield (64% ee) along with
diester (-)-24 in 45% yield (84% ee) at 46% conversion.
If the reaction was quenched at 25% conversion, an 18%
yield of acid 25 was obtained in 68% ee. To purify and
increase the enantiomeric purity of 25, dicyclohexylamine
salt 26 was generated in Et2O and precipitated in 77%
yield. After a single recrystallization from PhCH3/CH2-
Cl2 (64% recovery), followed by regeneration of the
corresponding free acid utilizing 5% aqueous KHSO4, 25
was obtained in >98% ee. From (()-24, 25 was obtained
in an overall 16% yield (32% based on only (+)-24 in (()-
24) in >98% ee as a single regioisomer. Analysis of the
X-ray crystal structure (CH2Cl2/PhCH3) of 26 confirmed
the regiochemistry as shown. To determine enantiomeric
excesses, (+)-24 was first obtained by treatment of 25
with CH2N2. Then, the Boc protecting groups of dimethyl
esters (-)-24 and (+)-24 were individually removed with
HCl(g)/EtOAc. Formation of the corresponding Mosher’s
amides then allowed % ee determinations by 19F NMR.
The absolute configuration of acid 25 was determined
indirectly by determination of the absolute configuration
of (-)-24 as shown in Scheme 7. Conversion of optically
enriched (-)-24 to the corresponding free amine occurred
in quantitative yield by removal of the Boc protecting
group with HCl(g) in EtOAc, followed by partitioning of
the resulting HCl salt between 5% Na2CO3 (aq) and
EtOAc. Coupling of the free amine, derived from the
recovered starting diester, with Cbz-L-Ala, mediated by
O-(7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyluronium
hexafluorophosphate (HATU)24 in CH3CN in the presence
of Et3N, provided diester 13 and the minor diastereomer
(26) (a) Lee, B. H.; Biswas, A.; Miller, M. J . J . Org. Chem. 1986, 51,
106. (b) Endo, Y.; Uchida, T.; Hizatate, S.; Shudo, K. Synthesis 1994,
1096. (c) Wu, M.; Begley, T. P. Org. Lett. 2000, 2, 1345. (d) Desai, M.
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(28) Purchased from Boehringer Mannheim which is now Roche
Molecular Biochemicals. The active enzyme in Chirazyme L-2, lyo. is
Candida antarctica F-B.

6050 J . Org. Chem., Vol. 66, No. 18, 2001
Shireman et al.
Sch em e 7
Sch em e 8
F igu r e 1. Computational and NMR Studies of 24.
of L-Phe-OBn and Et3N in CH3CN provided differentially
protected diester 28 in 89% yield. Hydrogenation, utiliz-
ing Pd/C in MeOH under a H2 atmosphere provided, in
quantitative yield, the corresponding acid that was used
without further purification. Treatment of the acid with
PyBOP in CH3CN in the presence of HCl‚L-Ala-OC(CH3)3
and Et3N provided peptide 29 in 86% combined yield for
the two steps. Saponification of 29 using 4.0 equiv30 of
0.1 N LiOH (aq) in THF at -10 °C provided the corre-
sponding acid in 99% crude mass recovery. Coupling of
the acid derived from 29 with L-Phe-OBn using EDC‚
HCl in CH3CN in the presence of HOAt and Et3N
provided peptide 30 in 60% overall yield. Selective
deprotection of the Boc aminoxy carbamate in preference
to the tert-butyl ester of 30, with HCl(g) in EtOAc, gave
the corresponding free amine in 81-92% yield. The free
amine was then coupled with Cbz-L-Val-Gly-OH medi-
ated by HATU in the presence of Et3N in CH3CN, to give
the target peptide 31 in 80% yield.
Con for m a tion a l Stu d ies. Determination of the E to
Z conformation of representative acylated acylnitroso-
derived amino acid derivatives in solution was deter-
1
mined using variable temperature H NMR19 and quan-
tum mechanical calculations. For example, at -90 °C in
CD2Cl2, the ratio of conformers of diester 24 was dem-
onstrated to be 2.6:1 by H NMR (Figure 1). A ratio of
2.6:1 would correspond to an energy difference of 0.35
kcal/mol at -90 °C. One might expect only a small
difference in stability between the two conformers since
27. The major diastereomer 13 was identical to the
1
diester obtained from the amino acid-derived acylnitroso
1
Diels-Alder cycloaddition in Scheme 2, by TLC and H
NMR analyses both of which readily distinguished dia-
stereomers 13 and 27.
With enantiomerically pure 25 in hand, an arbitrary
target peptide 31 was chosen to further demonstrate the
methodology (Scheme 8). Activation of acid 25 with
benzotriazol-1-yl-oxytripyrrolidine phosphonium hexaflu-
orophosphate (PyBOP)29 in the presence of the free base
(29) Coste, J .; Le-Nguyen, D.; Castro, D. Tetrahedron Lett. 1990,
31, 205.
(30) When 1.0 equiv of LiOH was utilized a 3:1 mixture of product
to an unidentified unseparable impurity was obtained. We found that
the equivalency of LiOH had an effect. Therefore, to avoid this
impurity, 4.0 equiv of LiOH must be utilized.

Novel Aminoxy trans-Proline Analogues
J . Org. Chem., Vol. 66, No. 18, 2001 6051
both E-24 and Z-24 have an oxygen of the carbamate
(either the carbonyl O or the O-C(CH3)3) that can
interact with the oxygen lone pairs of electrons of the
N-O bond. In accordance with the literature,19b Ha was
shifted downfield in the E conformation.
The assignment of the proton labeled as Ha as the
downfield absorption was made by analogy to methyl/
allyl ester 32 (eq 4). Treatment of acid 25 with O-allyl-
N,N′-diisopropyl isourea31 followed by refluxing in CH3-
CN provided allyl/methyl ester 32 in 81% yield. Long-
range HMBC NMR experiments using 32 in CD2Cl2
confirmed Ha as the downfield absorption by correlation
of Ha with the carbonyl carbon of the methyl ester.
Moreover, proton Ha showed a cross-peak with the
carbamate carbonyl carbon of the Boc protecting group.
Additionally, a cross-peak was present between Hb and
the carbonyl carbon of the allyl ester functionality.
To gain additional insights into the conformational
assignment, density functional calculations were per-
formed using the G98 series of programs.32 The geom-
etries of the E and Z conformers of 24, 33, and 34 were
fully optimized at the B3LYP/6-31G* level of theory.
Energies were corrected for zero-point energy, obtained
from normal-mode analysis at the same level of theory.
Relative energies in solution were evaluated by self-
consistent reaction field single-point calculations using
the gas-phase geometries. The B3LYP/6-31+G** level of
theory and the conductor-like screening solvation model
(COSMO)33 were used to account for the electrostatic
effects of solvation. The dipole moment of the solvent
continuum was chosen to reproduce the dielectric con-
stant of CH2Cl2. The calculations confirmed that E-24
was more stable. The energy differences were 1.8 kcal/
mol in gas phase and 1.1 kcal/mol in CH2Cl2. This would
correspond to a 6.4:1 ratio of conformers at room tem-
perature, thus slightly overestimating the experimentally
observed ratio.
F igu r e 2. Computational and NMR Studies of 33.
amino)pyridine (DMAP) provided aminoxy acetamide 33
in 61% combined yield. Only a single conformer was
observed at -85 °C in CD2Cl2. Density functional calcu-
lations were in agreement with these results. The
calculations indicated that E-33 was 6.0 kcal/mol more
stable than the Z-33 in the gas phase. When the dielectric
constant of the continuum was chosen to represent CH2-
Cl2 as a solvent, the electrostatic repulsion between the
carbonyl and the ring oxygens gets screened by the
dielectricum. E-33 was therefore calculated to be only 3.7
kcal/mol more stable than the Z-33.
Since carbamates are known to exist as mixtures of
rotamers,34 the carbamate was replaced with an acet-
amide (Figure 2). Treatment of 24, with HCl(g) in EtOAc
provided the corresponding HCl salt. Acetylation with
AcCl in CH2Cl2 in the presence of Et3N and (dimethyl-
A question that still remained was whether a NH at
the R-position of acetamide 34 would stabilize either of
the conformers. As previously discussed, literature pre-
cedent indicates that the Z-conformer is more stable
when an R-OMe is incorporated into aminoxy amides.21a-c
Deprotection of 24 with HCl(g) in EtOAc followed by
neutralization provided the corresponding free disubsti-
tuted hydroxylamine (Figure 3). The Gly derived peptide
34 was obtained in 51% yield after the HOSu active ester
of Ac-Gly was treated with the free amine obtained from
24 in the presence of HOAt. Density functional calcula-
tions indicated that E-34 was 9.1 kcal/mol more stable
than the Z-34 in the gas phase. When the dielectric was
chosen to represent CH2Cl2 as solvent, E-34 was 7.0 kcal/
mol more stable than the Z-34. At least a portion of the
(31) Mathias, L. J . Synthesis 1979, 561.
(32) Gaussian98, Revision A9: Frisch, M. J .; Trucks, G. W.; Schlegel,
H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V.
G.; Montgomery, J r., J . A.; Stratmann, R. E.; Burant, J . C.; Dapprich,
S.; Millam, J . M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas,
O.; Tomasi, J .; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli,
C.; Adamo, C.; Clifford, S.; Ochterski, J .; Petersson, G. A.; Ayala, P.
Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J . B.; Cioslowski, J .; Ortiz, J . V.; Baboul, A. G.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.;
Martin, R. L.; Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; J ohnson, B.; Chen,
W.; Wong, M. W.; Andres, J . L.; Gonzalez, C.; Head-Gordon, M.;
Replogle, E. S.; Pople, J . A. Gaussian, Inc., Pittsburgh, PA, 1998.
(33) a) Barone, V.; Cossi, M. J . Phys. Chem. A 1998, 102, 1995. For
recent studies of the performance of the model, compare: (b) Rablen,
P. R. J . Org. Chem. 2000, 65, 7930. (c) Nielsen, P. A.; Norrby, P. O.;
Liljefors, T.; Rega, N.; Barone, V. J . Am. Chem. Soc. 2000, 122, 3151.
(34) (a) Kost, D.; Kornberg, N. Tetrahedron Lett. 1978, 35, 3275.
(b) Marcovici-Mizrahi, D.; Gottlieb, H. E.; Marks, V.; Nudelman, A. J .
Org. Chem. 1996, 61, 8402. (c) Moraczewski, A. L.; Banaszynski, L.
A.; From, A. M.; White, C. E.; Smith, B. D. J . Org. Chem. 1998, 63,
7258.

6052 J . Org. Chem., Vol. 66, No. 18, 2001
Shireman et al.
transpeptidation of the amino acid that controlled the
absolute stereochemistry in the subsequently prepared
and described peptides was utilized taking advantage of
the activated aminoxy amide linkage. Alternatively, an
enantioselective regiospecific, enzymatic resolution of a
racemic dimethyl ester provided a novel amino acid. The
conformations of representative acylnitroso-derived amino
acid derivatives were shown to exist exclusively as the
E or trans conformers. These types of derivatives may
then be considered trans-proline mimetics. With the
conformation of the aminoxy linkage determined in CD2-
Cl2 and the regioselective functionalization of the acylni-
troso-derived peptide demonstrated, access to rationally
designed conformationally restricted molecules is now
possible. Further conformational studies in more physi-
ologically relevant aqueous solvents will provide more
insight on the potential of these new proline analogues
for the design of conformationally controlled peptides.
The utility of these analogues for use as scaffolds in
combinatorial chemistry for development of biologically
active compounds also is under active consideration.
Exp er im en ta l Section
Gen er a l Meth od s. DMF and DMSO were distilled from
MgSO4 or 3 Å molecular sieves onto 3 Å molecular sieves and
stored under Ar. Et3N, CH3CN, and CH2Cl2 were distilled
directly before use from CaH2 under Ar. THF was distilled
from Na/benzophenone ketyl under Ar. Melting points were
performed on a Thomas-Hoover Capillary Melting Point Ap-
paratus and are uncorrected. 1H NMR were taken at 300, 500,
or 600 MHz as indicated on Varian Unity spectrometers. 13C
NMR were taken at 75, 125, or 150 MHz on Varian Unity
spectrometers. 1H NMR are reported in parts per million (ppm)
relative to tetramethylsilane (0.00 ppm), residual CHCl3 (7.26
ppm), or residual CD3SOCD2H (2.49 ppm). 13C NMR are
reported in ppm relative to CDCl3 (77.0 ppm), or C2D6SO (39.5
ppm). Coupling constants are reported in Hz. Optical rotations
were obtained utilizing a Rudolph Research Autopol III
polarimeter with a 1.0 dm cell length at ambient temperature.
TLC was performed utilizing silica gel 60 F254. Visualization
was typically performed by UV light, KMnO4, ninhydrin, or
PMA stain. Flash chromatography was performed utilizing
silica gel 60 (30-70 mm irregular particles). HPLC was
performed on a Alltech Econosil column (5µ, 4.6 × 250 mm)
with monitoring at 254 nm.
Gen er al P r ocedu r e for th e Am in o Acid-Der ived Acyln i-
tr oso Diels-Ald er Rea ction s.16b-c,18 A solution of N-pro-
tected hydroxamic acid (12.6 mmol) in CH2Cl2/DMSO (5.1:1,
0.25 M) was cooled to -78 °C, and 300 mol % cyclopentadiene
or 100 mol % cyclohexadiene was added. In a separate flask,
(COCl)2 (6.40 g, 4.40 mL, 50.4 mmol) in CH2Cl2 (57.0 mL) was
cooled to -78 °C and DMSO (5.91 g, 5.37 mL, 75.6 mmol) was
added. After 3 min, the (COCl)2/DMSO solution was trans-
ferred by cannula into the hydroxamic acid solution. After the
transfer was complete, the solution was allowed to stir at -78
°C for 15 min. Pyridine (9.98 g, 10.2 mL, 126.0 mmol) was
added dropwise over 10 min. The reaction was allowed to warm
slowly to room temperature, and Et2O (150 mL) was added
followed by 1 N HCl (150 mL). The layers were separated, and
the aqueous layer was extracted with Et2O (2 × 100 mL). The
combined organic layers were washed with saturated NaHCO3
and brine, dried (MgSO4), filtered, and concentrated under
reduced pressure to give a residue that was purified on silica
gel followed by recrystallization from EtOAc/hexanes.
F igu r e 3. Computational and NMR Studies of 34.
added stabilization compared to E-33 was attributed to
a hydrogen bond between the AcNH and the C-5 methyl
ester as indicated in Figure 3. The hydrogen bond
distance according to the calculations was 2.03 Å with
an NHO angle of 164°. However, experimentally this
1
hydrogen bond was not observed by H NMR titration
studies and FT-IR analysis35 in CH2Cl2. It is possible that
this hydrogen bond is an artifact of the geometry opti-
mization in the gas phase, where no alternative hydrogen
bond donors or acceptors are available. This leads in
many cases to the well-known globulization of polar
compounds in gas-phase calculations. Nevertheless, the
repulsion between the carbonyl and ring oxygens still
favors E-34 by several kcal/mol, as is evident from the
1H NMR spectrum shown in Figure 3.
1
When the H NMR of 34 was measured in CD2Cl2 at 0
°C the spectrum shown in Figure 3 was obtained. Upon
cooling of the sample to -90 °C, the chemical shifts of
the protons labeled Ha and Hb were essentially the same.
This indicates the presence of a single conformer.
Con clu sion s
1(R),4(S)-N-[N-(Ca r b ob en zyloxy)-L-a la n yl]-2,3-oxa za -
bicyclo[2.2.1]h ep t-5-en e (12).16b Following the general pro-
cedure, 11 (5.94 g, 25.0 mmol) was oxidized in the presence of
freshly generated cyclopentadiene (300 mol %) to give 5.25 g
(70%) of a 4.1:1-5.9:1 diastereomeric mixture of 12 as a white
solid after chromatography on silica utilizing 65% EtOAc/
hexanes. The major diastereomer was further purified by
In conclusion, we have demonstrated two methods for
selective functionalization of chemically similar methyl
esters of an acylnitroso-derived amino acid. A novel
(35) Blank, J . T.; Guerin, D. J .; Miller, S. J . Org. Lett. 2000, 2, 1247.

Novel Aminoxy trans-Proline Analogues
J . Org. Chem., Vol. 66, No. 18, 2001 6053
recrystallization from EtOAc/hexanes to give a single diaste-
reomer whose spectral characteristics matched those previ-
ously reported.16b HPLC (10% iPrOH/hexanes, 1.5 mL/min) tR
12.3 (major) and 9.7 (minor) min; mp 119-120 °C; Rf 0.44 (70%
Dik etop ip er a zin e 7. Following the general procedure, 13
(0.047 g, 0.119 mmol) was deprotected and cyclized to give a
residue which was purified on silica gel utilizing 5% MeOH/
CH2Cl2 to give 0.020 g (74%) of 7 as a white solid. mp 239.5-
241 °C, Rf 0.22 (5% MeOH in EtOAc); IR (KBr) 3269, 3001,
1743, 1682, 1660, 1405 cm-1; 1H NMR (DMSO-d6, 300 MHz) δ
1.22 (d, J ) 6.9 Hz, 3H), 2.66 (ddd, J ) 3.8, 5.8, 9.4 Hz, 1H),
2.88 (m, 1H), 3.60 (s, 3H), 4.18 (q, J ) 6.3 Hz, 1H), 4.63 (dd,
J ) 5.7, 9.9 Hz, 1H), 4.90 (dd, J ) 3.6, 8.4 Hz, 1H), 8.38 (s,
1H); 13C NMR (DMSO-d6, 75 MHz) δ 15.4, 34.1, 48.9, 52.2,
58.0, 75.4, 161.8, 166.6, 169.7; HRMS-FAB (m/z) [M + H]+
calcd for C9H13N2O5, 229.0824; found, 229.0820.
1
EtOAc/hexanes); H NMR (CDCl3, 300 MHz) δ 1.12 (m, 3H),
1.87 (d, J ) 8.7 Hz, 1H), 2.02 (d, J ) 8.4 Hz, 1H), 4.57 (dq, J
) 6.9, 7.5 Hz, 1H), 5.09 (s, 2H), 5.36 (m, 2H), 5.52 (d, J ) 7.8
Hz, 1H), 6.40 (m, 1H), 6.55 (m, 1H), 7.34 (s, 5H); 13C NMR
(DMSO-d6, 75 MHz) δ 16.0, 48.3, 48.6, 61.8, 66.6, 84.6, 128.0,
128.4, 133.0, 1361, 136.4, 155.5, 173.3.
1(R),4(S)-N-[N-(Ca r b ob en zyloxy)-L-a la n yl]-2,3-oxa za -
bicyclo[2.2.2]oct-5-en e (19). Following the general proce-
dure, 11 (3.00 g, 12.6 mmol) was oxidized in the presence of
cyclohexadiene (100 mol %) to give 3.69 g (93%) of a 5:1
diastereomeric mixture of 19 as a white solid after chroma-
tography on silica gel utilizing 65% EtOAc/hexanes. The major
diastereomer was further purified by recrystallization from
EtOAc/hexanes to give a 10:1 diastereomeric ratio. An analyti-
cal sample was obtained by an additional recrystallization to
provide a single diastereomer for characterization. HPLC (10%
iPrOH/hexanes, 1.0 mL/min) tR 12.3 (major) and 13.1 (minor)
min; major diasteromer: mp 103-104 °C; Rf 0.29 (60% EtOAc
in hexanes); IR (KBr) 3278, 3046, 1720, 1643, 1539, 1458, 1252
Dik etop ip er a zin e 21. Following the general procedure, 20
(0.117 g, 0.285 mmol) was hydrogenated and cyclized to give
a yellow solid that was chromatographed on silica gel utilizing
10% i-PrOH/CH2Cl2 to give 0.062 g (89%) of 21 as a white solid.
mp 148-150 °C; Rf 0.36 (10% i-PrOH/CH2Cl2); IR (KBr) 3292,
2939, 1752, 1687, 1439, 993; 1H NMR (CDCl3, 300 MHz) δ 1.54
(d, J ) 6.5 Hz, 3H), 1.91 (m, 1H), 2.16 (m, 1H), 2.35 (m, 2H),
3.77 (s, 3H), 4.13 (m, 1H), 4.27 (m, 1H), 4.76 (bd, J ) 5.1 Hz,
1H), 6.67 (bs, 1H);13C NMR (CDCl3, 75 MHz) δ 21.7, 24.0, 24.8,
50.4, 52.4, 59.1, 76.3, 162.8, 165.8, 169.2; HRMS-FAB (m/z)
[M + H]+ calcd for C10H15N2O5, 243.0981; found, 243.0995.
2-[N-(Ca r b oben zyloxy)-glycyl]-3(S)-[ca r b oxy-N-L-a la -
n yl-O-m eth yl]-5(R)-car bom eth oxy-2-oxazole (14). To MeOH
(1.0 mL) at 0 °C was added SOCl2 (0.082 g, 0.050 mL, 0.689
mmol) dropwise. Diketopiperazine 7 (0.030 g, 0.132 mmol) was
added, and the heterogeneous mixture was allowed to stir at
0 °C for 15 min and then allowed to warm to room tempera-
ture. The reaction was refluxed for 30 min and cooled to room
temperature. The reaction was partitioned between EtOAc (5.0
mL) and 5% Na2CO3 (aq) until the aqueous layer remained
basic. The layers were separated, and the aqueous layer was
saturated with NaCl and extracted with EtOAc (4×). The
combined organics were washed with brine, dried (MgSO4),
filtered, and concentrated to give an oil. Chromatography on
silica gel utilizing 100% EtOAc provided 0.028 g (82%) of 8 as
a clear residue that was utilized immediately in the next step.
Rf 0.18 (100% EtOAc); HRMS-FAB (m/z) [M + H]+ calcd for
1
cm-1; H NMR (CDCl3, 300 MHz) δ 1.17 (d, J ) 6.9 Hz, 3H),
1.50 (m, 1H) 1.65 (m, 1H), 2.19 (m, 2H), 4.64 (m, 1H), 4.80 (m,
1H), 5.09 (s, 2H), 5.23 (m, 1H), 5.62 (d, J ) 7.8 Hz), 7.35 (m,
5H); 13C NMR (DMSO-d6, 300 MHz) δ 14.7, 20.5, 22.9, 46.8,
47.1, 65.2, 71.3, 127.7, 127.7, 128.3, 131.7, 132.6, 137.0, 155.6,
172.4; HRMS-FAB (m/z) [M + H]+ calcd for C17H21N2O4,
317.1501; found, 317.1514.
2-[N-(Ca r b ob en zyloxy)-L-a la n yl]-3(S),5(R)-[d ica r b o-
m eth oxy]oxa zole (13). To cycloadduct 12 (0.056 g, 0.187
mmol) in EtOAc/CH3CN (1:1, 3.0 mL) was added NaIO4 (0.179
g, 0.840 mmol) in H2O (1.0 mL), followed by RuCl3‚H2O (0.090
g, 0.434 mmol). The reaction was allowed to stir at room-
temperature open to the air for 2 h. The reaction was
partitioned between EtOAc and brine. The layers were sepa-
rated and the aqueous layer was extracted with EtOAc (4×).
The combined organic extracts were dried (Na2SO4), filtered,
and concentrated under reduced pressure to give the crude
diacid as a brown oil.
C
10H17N2O6, 261.1087; found, 261.1066.
To disubstituted hydroxyamine 8 (0.028, 0.108 mmol) in
The brown oil was dissolved in THF (5.0 mL) and treated
with excess CH2N2 in Et2O at 0 °C. After 1 h, excess CH2N2
was quenched with 5% AcOH in Et2O. The reaction mixture
was washed with saturated NaHCO3. The aqueous layer was
extracted with Et2O (5.0 mL). The combined organic layers
were dried (MgSO4), filtered, and concentrated under reduced
pressure to give a clear oil. The residue was chromatographed
on silica gel utilizing 60% EtOAc/hexanes to give 0.080 g (66%)
of 13 as a clear oil whose spectral characteristics matched
those previously reported.14a Rf 0.52 (70% EtOAc/hexanes);
CH3CN (1.0 mL) was added Cbz-Gly-OH (0.025 g, 0.118 mmol)
followed by Et3N (0.012 g, 0.016 mL, 0.118 mmol), HOAt (0.016
g, 0.118 mmol), and EDC‚HCl (0.024 g, 0.118 mmol). After 6
h, the reaction was partitioned between Et2O and 1 N HCl.
The layers were separated and the aqueous layer was ex-
tracted with Et2O (1×). The combined organic layers were
washed with saturated NaHCO3 and brine, dried (MgSO4),
filtered, and concentrated under reduced pressure to give a
yellow solid. Chromatography on silica gel utilizing 100%
EtOAc afforded 0.033 g (68%) of 14 as a clear glass. Rf 0.36
(100% EtOAc); IR (thin film) 3337, 2955, 1744, 1717, 1684,
1
IR (thin film) 3334, 2956, 1748, 1714 cm-1; H NMR (CDCl3,
1
300 MHz) δ 1.48 (d, J ) 6.9 Hz, 3H), 2.79 (m,1H), 2.92 (m,
1H), 3.76 (s, 3H), 3.81 (s, 3H), 4.80 (m, 2H), 4.99 (dd, J ) 4.5,
9.9 Hz, 1H), 5.05 (d, J ) 12.0 Hz, 1H), 5.11 (d, J ) 12.3 Hz,
1H), 5.38 (d, J ) 7.5 Hz, 1H), 7.34 (s, 5H); 13C NMR (CDCl3,
75 MHz) δ 18.4, 34.9, 47.6, 52.7, 52.9, 56.6, 66.9, 77.8,
128.0, 128.1, 128.5, 136.3, 155.8, 168.2, 169.2, 173.6; HRMS-
FAB (m/z) [M + H]+ calcd for C18H23N2O8, 395.1454; found,
395.1453.
Gen er a l P r oced u r e for Hyd r ogen a tion of Cbz P r o-
tectin g Gr ou p s a n d Cycliza tion to Dik etop ip er a zin es.
To N-Cbz-protected dimethyl ester in MeOH (0.1 M), purged
with Ar, was added 10 wt % Pd/C. The flask was purged with
Ar followed by the addition of H2 by a balloon. The reaction
was monitored closely (TLC, 100% EtOAc) until all starting
material was consumed and an intense ninhydrin positive
baseline spot was present. The H2 balloon was then removed
and the reaction mixture was again purged with Ar. The
reaction mixture was filtered through a short pad of Celite
and concentrated under reduced pressure. The crude product
was dissolved in CH3CN (0.1 M) and allowed to stir at room
temperature for 1 h. The volatiles were removed under reduced
pressure to give a yellow solid that was purified by silica gel
flash chromatography.
1532, 1456 cm-1; H NMR (CDCl3, 500 MHz) δ 1.40 (d, J )
7.5 Hz, 3H), 2.81 (ddd, J ) 4.0, 13.0, 18.0 Hz, 1H), 3.05 (ddd,
J ) 4.2, 10.2, 13.0 Hz, 1H), 3.74 (s, 3H), 3.79 (s, 3H), 4.19 (dd,
J ) 4.5, 18.5 Hz, 1H), 4.35 (dd, J ) 6.0, 18.5, 1H), 4.51 (dq, J
) 8.4, 8.4 Hz, 1H), 4.72 (dd, J ) 6.5, 8.0 Hz, 1H), 4.91 (dd, J
) 4.0, 9.5 Hz, 1H), 5.13 (ABq, J ) 14.7 Hz, 2H), 5.42 (m, 1H),
7.20 (d, J ) 7.0 Hz, 1H), 7.30-7.37 (m, 5H); 13C NMR (CDCl3,
125 MHz) δ 18.0, 34.2, 42.4, 48.5, 52.5, 52.8, 58.8, 67.1, 77.7,
128.1, 128.2, 128.5, 136.2, 156.4, 167.0, 168.3, 171.3, 172.7;
HRMS-FAB (m/z) [M + H]+ calcd for C20H26N3O9, 452.1669;
found, 452.1687.
2-(2-ter t-Bu toxyca r bon yla m in o-pr op ion yla m in o)a cr yl-
ic Acid Meth yl Ester (17).25 To dimethyl ester 1514a (0.122
g, 0.34 mmol) in THF (3.4 mL) at 0 °C was added NaH (60%
in mineral oil, 0.014 g, 0.34 mmol). After 2 h at 0 °C followed
by 14 h at room temperature, the reaction was partitioned
between Et2O (10.0 mL) and 1/2 saturated NH4Cl (10.0 mL).
The layers were separated, and the aqueous layer was
extracted with Et2O (2 × 10.0 mL). The combined organics
were washed with brine, dried (MgSO4), filtered, and concen-
trated under reduced pressure to give a clear residue that was
chromatographed on silica gel utilizing 40% EtOAc/hexanes
to give 0.070 g (75%) of 17 as a clear oil whose spectral

6054 J . Org. Chem., Vol. 66, No. 18, 2001
Shireman et al.
characteristics were consistent with the known compound.25b
Rf 0.42 (40% EtOAc/hexanes); H NMR (CDCl3, 300 MHz) δ
1.40 (d, J ) 7.20 Hz, 3H), 1.45 (s, 9H), 3.84 (s, 3H), 4.25 (m,
1H), 4.97 (m, 1H), 5.90 (d, J ) 1.8 Hz, 1H), 6.59 (s, 1H), 8.44
(s, 1H); 13C NMR (CDCl3, 75 MHz) δ 18.0, 28.3, 51.0, 53.0, 80.6,
109.2, 130.9, 155.1, 164.3, 171.5; HRMS-FAB (m/z) [M + H]+
calcd for C12H20N2O5, 273.1450; found, 273.1453.
silica gel utilizing 50% EtOAc/hexanes to give 9.87 g (84%) of
(()-24 as a white solid. Rf 0.35 (50% EtOAc/hexanes); mp
77.5-79 °C; IR (KBr) 3040, 3006, 2978, 2952, 1752, 1730, 1716
cm-1; 1H NMR (CDCl3, 300 MHz) δ 1.49 (s, 9H), 2.81 (m, 2H),
3.78 (s, 3H), 3.79 (s, 3H), 4.60 (dd, J ) 6.9, 8.1 Hz, 1H), 4.81
(dd, J ) 8.1, 5.1 Hz, 1H); 13C NMR (DMSO-d6, 75 MHz) δ 27.7,
35.2, 52.3, 52.4, 59.7, 76.6, 82.2, 155.2, 168.5, 170.1; HRMS-
FAB (m/z) [M + H]+ calcd for C12H20NO7, 290.1240; found,
290.1265.
1
2-[N -(C a r b o b e n zy lo x y )-L -a la n y l]-3(S ),6(R )-[d ic a r -
bom eth oxy]oxa zin e (20). To cycloadduct 19 (0.100 g, 0.316
mmol) in acetone/t-BuOH (44:1, 4.5 mL) was added Na2CO3
(0.030 g, 0.284 mmol). The flask was cooled to -10 °C and
KMnO4 (0.140 g, 0.885 mmol) was added portionwise such that
the internal temperature of the reaction did not exceed 0 °C.
The reaction was kept at -10 °C for 1 h and then allowed to
warm to room temperature and stir an additional 18 h. EtOAc
(5.0 mL) was added followed by 10% aqueous Na2S2O5 (5.0
mL). The aqueous layer was acidified to pH ) 3 (pH paper)
with 1 N HCl. The layers were separated, and the aqueous
layer was saturated with NaCl and extracted with EtOAc (3
× 5 mL). The combined organic extracts were dried (MgSO4),
filtered, and concentrated to give a yellow residue that was
dissolved in THF (5.0 mL) and treated with excess CH2N2 in
Et2O at 0 °C. After 1 h, excess CH2N2 was removed with a
stream of Ar, that was passed through an AcOH trap. The
reaction mixture was concentrated under reduced pressure and
chromatographed on silica gel utilizing 55% EtOAc/hexanes
to give 0.088 g (68%) of 20 as a clear oil. Rf 0.28 (50% EtOAc/
hexanes); IR (thin film) 3331, 2956, 1732, 1682, 1520, 1455,
Repr esen tative P r ocedu r e for th e Resolu tion of 2-Car -
b o-ter t-b u t oxy-3(R)-ca r b om et h oxy-5(S)-ca r b oxy-2-ox-
a zole (25) Usin g Ch ir a zym e L-2.28 To 24 (2.11 g, 7.3 mmol)
in PhCH3/0.1 N phosphate pH ) 7.8 (1:3.3, 100 mL) equipped
with a pH probe was added Chirazyme L-2 (0.026 g). The
reaction was stirred rapidly, and the pH was maintained
between 7.6 and 7.8 with the slow addition of 0.2 N NaOH
(ca. 8 h). After 14.3 mL of the NaOH solution had been added,
the reaction was extracted with Et2O (3×).
The combined Et2O layers were washed with saturated
NaHCO3 and brine, dried (MgSO4), filtered, and concentrated
under reduced pressure to give a yellow solid that was
chromatographed on silica gel utilizing 50% EtOAc/hexanes
to give (-)-24 as a white solid whose spectral characteristics
matched that of (()-24 except for optical rotation, [R]20D -44.4°
(c ) 1.05 in CHCl3) after recrystallization from EtOAc/hexanes.
An optically enriched (ca. 80% ee) sample of 25 was treated
with CH2N2 to give the corresponding diester (+)-24. [R]20
+34.8 ° (c ) 0.52 in CHCl3).
D
1
1210, 1026 cm-1; H NMR (CDCl3, 300 MHz) δ 1.42 (d, J )
The aqueous layer from above was acidified to pH ) 1.5 with
6 N HCl under a blanket of EtOAc, saturated with NaCl, and
extracted with EtOAc (2×). The combined EtOAc layers were
washed with brine, dried (MgSO4), filtered, and concentrated
under reduced pressure to give 25 as a light yellow oil that
was further purified and characterized as the corresponding
dicyclohexyamine salt 26.
6.9 Hz, 3H), 1.98 (m, 3H), 2.49 (bd, J ) 11.40 Hz, 1H), 3.76 (s,
3H), 3.79 (s, 3H), 4.52 (m, 1H), 4.81 (m, 1H), 5.06 (d, J ) 12.3
Hz, 1H), 5.13 (d, J ) 12.6 Hz, 1H), 5.21 (m, 1H), 5.45 (d, J )
7.8 Hz, 1H), 7.35 (s, 5H); 13C NMR (CDCl3, 75 MHz) δ 17.4,
23.0, 24.3, 47.5, 52.5, 52.6, 52.8, 66.8, 79.8, 128.0, 128.1, 128.5,
136.3, 155.8, 168.1, 168.9, 172.4; HRMS-FAB (m/z) [M + H]+
calcd for C19H25N2O8, 409.1611; found, 409.1637.
2-Ca r bo-ter t-bu toxy-3(R)-ca r bom eth oxy-5(S)-ca r boxy-
2-oxa zole Dicycloh exyla m m on iu m Sa lt (26). To acid 25
(0.985 g, 3.59 mmol) in Et2O (10.0 mL) at 0 °C was added
dicyclohexylamine (0.65 g, 0.71 mL, 3.59 mmol) in Et2O (3.0
mL) over 20 min. Then, without stirring, the solution was
allowed to stand at 0 °C for 1 h. The resulting solid was filtered
and washed with Et2O to give 1.26 g (77%) of dicyclohexy-
lamine salt 26. The solid was then recrystallized from PhCH3/
CH2Cl2 (5:1, 30.0 mL) to give 0.790 g (63% recovery) of a white
solid; mp 174-177 °C (dec); IR (KBr) 3432, 2931, 2860, 1761,
(()-N-(Ca r bo-ter t-bu toxy)-2,3-oxa za bicyclo[2.2.1]h ep t-
5-en e (23). To BocNHOH (15.0 g, 113.0 mmol) in MeOH (1100
mL) cooled to 0 °C equipped with an overhead stirrer was
added freshly generated cyclopentadiene (33.0 g, 500.0 mmol,
40.0 mL). Then, NaIO4 (26.6 g, 124.3 mmol) in H2O (300 mL)
was added dropwise over 20 min. After stirring at 0 °C for 30
min, the reaction mixture was partitioned between brine (1500
mL) and EtOAc (1500 mL). The layers were separated, and
the aqueous layer was saturated with NaCl and extracted with
EtOAc (3 × 300 mL). The combined organic layers were
washed with saturated NaHCO3 and brine, dried (MgSO4),
filtered, and concentrated under reduced pressure to a brown
oil that was chromatographed on silica gel utilizing 30%
EtOAc/hexanes to give 16.4 g (74% yield) of (()-2327 as a light
brown solid that could be further purified by recrystallization
from hexanes. mp 43-45 °C; Rf 0.5 (50% EtOAc/hexanes); 1H
NMR (CDCl3, 300 MHz) δ 1.47 (s, 9H), 1.73 (d, J ) 8.6 Hz, 1
H), 1.99 (ddd, J ) 1.8, 1.8, 8.6 Hz, 1H), 4.98 (m, 1H), 5.21 (m,
1H), 6.41 (m, 2H); 13C NMR (CDCl3, 75 MHz) δ 28.1, 48.1, 65.0,
81.9, 83.5, 132.9, 134.1, 158.5; HRMS-FAB (m/z) [M + H]+
calcd for C10H16NO3, 198.1130; found, 198.1137.
(()-2-Ca r bo-ter t-bu toxy-3,5-syn -d ica r bom eth oxy-2-ox-
a zole (24). To (()-23 (8.0 g, 40.6 mmol) in EtOAc/CH3CN (1:
1, 220.0 mL) at 0 °C was added NaIO4 (39.0 g, 182.0 mmol) in
H2O (440.0 mL) followed by RuCl3‚H2O (1.5 g, 7.2 mmol). The
mixture was allowed to stir at 0 °C open to the air for 15 min
and then room temperature for 2 h. The reaction mixture was
suction filtered through a short pad of Celite, and the aqueous
layer was saturated with NaCl. The layers were separated,
and the aqueous layer was extracted with EtOAc (3 × 100 mL).
The combined organic extracts were dried (MgSO4), filtered,
and concentrated to give the crude diacid as a brown oil.
The brown oil was dissolved in Et2O (80.0 mL) and treated
with excess CH2N2 in Et2O at 0 °C. After 1 h, the excess CH2N2
was quenched with 5% AcOH in Et2O. The reaction mixture
was washed with saturated NaHCO3. The aqueous layer was
extracted with Et2O (1×). The combined organic layers were
then dried (MgSO4), filtered, and concentrated under reduced
pressure to give a white solid that was chromatographed on
1713, 1613, 1451, 1333 cm-1 1H NMR (CDCl3, 300 MHz) δ
;
1.20-1.32 (m, 6H), 1.47 (s, 9H), 1.47-1.65 (m, 8H), 1.78-1.82
(m, 4H), 2.03-2.07 (m, 4H), 2.54 (ddd, J ) 6.5, 9.3, 12.6 Hz,
1H), 2.96 (ddd, J ) 7.2, 9.3, 12.6 Hz), 3.05-3.12 (m, 2H), 3.74
(s, 3H), 4.39 (dd, J ) 7.2, 9.6 Hz, 1H), 4.67 (dd, J ) 6.6, 9.3
Hz, 1H); 13C NMR (CDCl3, 75 MHz) δ 24.8, 25.1, 28.1, 29.2,
37.2, 52.4, 53.0, 60.3, 79.7, 82.5, 156.0, 170.9, 171.9.
A heterogeneous mixture of EtOAc and dicyclohexylamine
salt 26 was washed with 5% KHSO4 (2×). The combined
aqueous layers were then extracted with EtOAc and the
combined organic layers were washed with brine, dried
(MgSO4), filtered, and concentrated under reduced pressure
to give 25 as a clear residue in quantitative recovery. Rf 0.31
(77% CHCl3, 20% MeOH, 3% AcOH); IR (thin film) 3411-
(broad), 2982, 1746, 1716, 1370, 1160 cm-1; 1H NMR (DMSO-
d6, 300 MHz) δ 1.42 (s, 9H), 2.45 (ddd, J ) 5.1, 7.5, 12.6 Hz,
1H), 2.88 (ddd, J ) 8.1, 9.0, 12.3 Hz, 1H), 3.67 (s, 3H), 4.51
(dd, J ) 7.8, 8.1 Hz, 1H), 4.78 (dd, J ) 5.0, 9.3 Hz, 1H), 13.3
(bs, 1H); HRMS-FAB (m/z) [M + H]+ calcd for C11H18NO7,
276.1083; found, 276.1080.
Rep r esen ta tive Mosh er ’s Am id e An a lysis of (()-24. To
(()-24 (0.010 g, 0.035 mmol) in EtOAc (1.0 mL) at 0 °C was
bubbled HCl (g) until TLC analysis indicated starting material
was consumed. The reaction was concentrated under reduced
pressure, redissolved in CH2Cl2 (0.1 M), and cooled to 0 °C.
Then, Et3N (200 mol %) and DMAP (150 mol %) were added
followed by Mosher’s-Cl (150 mol %) in CH2Cl2 (0.100 g in 2.0
mL of CH2Cl2). After 1 h, the reaction was partitioned between
Et2O and 1 N HCl. The layers were separated, and the aqueous
layer was extracted with Et2O. The combined organic layers

Novel Aminoxy trans-Proline Analogues
J . Org. Chem., Vol. 66, No. 18, 2001 6055
were washed with saturated NaHCO3, brine, dried (MgSO4),
filtered, and concentrated under reduced pressure to give a
clear to slightly yellow oil that was analyzed by 19F NMR
without further purification.
(d, J ) 7.2 Hz, 1H), 1.39 (s, 9H), 1.41 (s, 9H), 2.13 (ddd, J )
5.4, 8.4, 12.4 Hz, 1H), 2.74-2.80 (m, 1H, overlapping with 2.84
ppm), 2.84 (dd, J ) 9.4, 13.6 Hz, 1H), 3.02 (dd, J ) 4.0, 13.6
Hz, 1H), 3.67 (s, 3H), 4.12 (dq, J ) 7.2, 7.2 Hz, 1H), 4.31 (dd,
J ) 8.1, 8.1 Hz, 1H), 4.58 (ddd, J ) 4.1, 9.0, 9.0 Hz, 1H), 4.75
(dd, J ) 5.4, 9.6 Hz, 1H), 7.16-7.24 (m, 5H), 7.98 (d, J ) 8.4
Hz, 1H), 8.50 (d, J ) 6.9 Hz, 1H); 13C NMR (CDCl3, 125 MHz)
δ 18.5, 27.9, 28.1, 36.1, 37.8, 48.7, 52.8, 54.3, 59.5, 77.8, 81.7,
82.0, 83.7, 127.0, 128.6, 129.4, 136.3, 155.4, 168.9, 169.2, 170.2,
171.4; MS m/z (MH+) calcd for C27H40N3O9 550.2765, found
550.2762.
2-ter t-Bu toxycar bon yl-3(R)-[car boxy-N-L-ph en ylalan yl-
O-ben zyl]-5(S)-[car boxy-N-L-ph en ylalan yl-L-alan yl-O-ter t-
bu tyl]-2-oxa zole (30). To ester 29 (0.088 g, 0.160 mmol) in
THF (1.6 mL) at -10 °C was added 0.1 N LiOH (3.0 mL). After
10 min, the solution had solidified and was allowed to warm
to room temperature. The reaction was partitioned between
EtOAc and 1 N HCl. The layers were separated, and the
aqueous layer was saturated with NaCl and extracted with
EtOAc. The combined organic layers were washed with brine,
dried (MgSO4), filtered, and concentrated under reduced
pressure to afford 0.085 g (99% mass recovery) of a clear
residue.
Deter m in a tion of Absolu te Con figu r a tion (-)-24. To
optically enriched diester (-)-24 (0.200 g, 0.692 mmol) in
EtOAc (2.0 mL) cooled to 0 °C was bubbled HCl(g) until
starting material was consumed. The reaction was concen-
trated under reduced pressure and partitioned between EtOAc
and 5% Na2CO3 (aq). The aqueous layer was saturated with
NaCl, then extracted with EtOAc (3×). The combined organic
layers were washed with brine, dried (MgSO4), filtered, and
concentrated under reduced pressure to give a clear oil that
was dissolved in CH3CN (6.0 mL). Cbz-L-Ala (0.139 g, 0.624
mmol) was added followed by Et3N (0.063 g, 0.087 mL, 0.624
mmol) and HATU (0.237 g, 0.624 mmol). After 18 h, the
reaction was partitioned between EtOAc and 1 N HCl. The
layers were separated and the aqueous layer was extracted
with EtOAc (1×). The combined organic layers were washed
with saturated NaHCO3 and brine, dried (MgSO4), filtered,
and concentrated to a light yellow oil. Separation of the
diastereomers by chromatography on silica gel utilizing 70%
EtOAc/hexanes provided 13 as the major diastereomer whose
spectral characteristics matched 13 obtained in Scheme 2.
2-ter t-Bu t oxyca r b on yl-3(R)-ca r b om et h oxy-5(S)-[ca r -
boxy-N-L-p h en yla la n yl-O-ben zyl]-2-oxa zole (28). p-TsOH-
L-Phe-OBn (0.855 g, 2.0 mmol) was partitioned between
CH2Cl2 and 5% Na2CO3 (aq). The layers were separated and
the aqueous layer was extracted with CH2Cl2 (1×). The
combined organic layers were dried (MgSO4), filtered and
concentrated to give L-Phe-OBn as a clear oil that was
dissolved in CH3CN (15.0 mL) and added to 25 (0.428 g, 1.56
mmol). Then Et3N (0.174 g, 1.72 mol, 0.240 mL) was added
followed by PyBOP reagent (0.895 g, 1.72 mmol). After 10 h,
the reaction was partitioned between Et2O (20.0 mL) and 1 N
HCl (10.0 mL). The layers were separated, and the aqueous
layer was extracted with Et2O (20.0 mL). The combined organic
layers were washed with saturated NaHCO3 and brine, dried
(MgSO4), filtered, and concentrated under reduced pressure
affording a faint yellow residue. Chromatography on silica gel
utilizing 60% EtOAc/hexanes provided 0.714 g (89%) of 28 as
To the residue from above was added L-Phe-OBn (0.045 g,
0.176 mmol) in CH3CN (2.0 mL) followed by Et3N (0.016 g,
0.022 mL, 0.155 mmol) and PyBOP (0.081 g, 0.156 mmol).
After 3 h, the reaction was partitioned between EtOAc and 1
N HCl. The layers were separated and the aqueous layer was
extracted with EtOAc (1×). The combined organic layers were
washed with brine, dried (MgSO4), filtered, and concentrated
under reduced pressure to afford a clear residue. Chromatog-
raphy on silica gel utilizing 70% EtOAc/hexanes provided 0.066
g (60%) of 30 as a clear residue. Rf 0.30 (70% EtOAc/hexanes);
IR (thin film) 3325, 2980, 1738, 1668, 1524 cm-1 1H NMR
;
(CDCl3, 300 MHz) δ 1.32 (d, J ) 7.2 Hz, 3H), 1.42 (s, 9H),
1.50 (s, 9H), 2.59 (ddd, J ) 3.9, 7.3, 13.2 Hz, 1H), 2.78 (ddd, J
) 9.3, 9.3, 13.2 Hz, 1H), 2.97 (dd, J ) 7.5, 13.5 Hz, 1H), 3.08
(dd, J ) 6.6, 13.2 Hz, 1H), 3.13 (dd, J ) 3.4, 5.8 Hz, 2H), 4.33
(dq, J ) 7.3, 7.3 Hz, 1H), 4.46 (dd, J ) 7.0, 9.1 Hz, 1H), 4.52
(dd, J ) 7.5, 15.3 Hz, 1H), 4.69 (dd, J ) 3.7, 9.4, 1H), 4.92
(ddd, J ) 5.7, 5.7, 8.7 Hz, 1H), 5.10 (d, J ) 12.1 Hz, 1H), 5.22
(d, J ) 12.1 Hz, 1H), 6.26 (d, J ) 6.9 Hz, 1H), 6.81 (d, J ) 8.4
Hz, 1H), 7.0-7.35 (overlapping NH and aromatic, m, 16 H);
13C NMR (DMSO-d6, 75 MHz) δ 16.9, 27.5, 27.6, 27.6, 36.4,
36.9, 37.8, 48.3, 53.0, 53.6, 61.0, 66.1, 78.2, 80.4, 82.0, 126.3,
126.5, 127.9, 128.2, 128.4, 129.1, 129.3, 135.6, 137.0, 137.3,
156.0, 166.6, 169.5, 170.3, 170.9, 171.5; HRMS-FAB (m/z) [M
+ H]+ calcd for C42H53N4O10, 773.3762; found, 773.3734.
2-[N-(Ca r b ob en zyloxy)-L-va lyl-glycyl]-3(R)-[ca r b oxy-
N -L -p h en yla la n yl-O-ben zyl]-5(S)-[ca r boxy-N -L -p h en yl-
a la n yl-L-a la n yl-O-ter t-bu tyl]-2-oxa zole (31). To 30 (0.066
g, 0.085 mmol) at 0 °C was added EtOAc saturated with HCl-
(g) until TLC analysis indicated no starting material was
present. The reaction mixture was concentrated under reduced
pressure, and the resulting residue was partitioned between
EtOAc and 5% Na2CO3 (aq). The aqueous layer was saturated
with NaCl and extracted with EtOAc (2×). The combined
organic layers were washed with saturated NaHCO3 and brine,
dried (MgSO4), filtered, and concentrated under reduced
pressure to afford 0.052 g (91%) of a clear residue that was
chromatographed on silica utilizing 100% EtOAc (Rf 0.19) and
utilized in the next reaction.
a clear oil. [R]20 +18.4° (c ) 0.97 in CHCl3); Rf 0.40 (60%
D
EtOAc/hexanes); IR (thin film) 3407, 3355, 1746, 1686, 1523
cm-1; 1H NMR (DMSO-d6, 300 MHz) δ 1.41 (s, 9H), 2.15 (ddd,
J ) 5.8, 8.4, 13.2 Hz, 1H), 2.78 (m, 1H), 2.99-3.10 (m,2H),
3.66 (s, 3H), 4.37 (dd, J ) 7.8, 7.8 Hz, 1H), 4.59 (dd, J ) 8.1,
14.2 Hz, 1H), 4.69 (dd, J ) 5.5, 9.1 Hz, 1H), 5.10 (s, 2H), 7.17-
7.36 (m, 10H), 8.44 (d, J ) 7.8 Hz, 1H); 13C NMR (DMSO-d6,
75 MHz) δ 27.7, 36.1, 52.4, 53.2, 59.9, 66.1, 78.0, 82.2, 126.6,
127.9, 128.0, 128.2, 128.4, 129.1, 135.7, 137.0, 155.4, 167.1,
170.2, 170.7; HRMS-FAB (m/z) [M + H]+ calcd for C27H33N2O8,
513.2237; found, 513.2249.
2-ter t-Bu t oxyca r b on yl-3(R)-ca r b om et h oxy-5(S)-[ca r -
b oxy-N-L-p h en yla la n yl-L-a la n yl-O-ter t-b u t yl]-2-oxa zole
(29). To benzyl ester 28 (0.110 g, 0.214 mmol) in MeOH (2.0
mL) purged with Ar was added 10 mol % Pd/C. The flask was
purged with Ar followed by addition of a H2 balloon. After all
starting material was consumed, the H2 balloon was then
removed and the reaction mixture was purged with Ar. The
reaction mixture was filtered through a short pad of Celite
and concentrated under reduced pressure to give 0.088 g of a
clear oil (98%) that was utilized without further purification.
To the acid (0.079 g, 0.187 mmol) from above in CH3CN (2.0
mL) was added HCl‚L-Ala-OC(CH3)3 (0.037 g, 0.206 mmol),
followed by Et3N (0.040 g, 0.054 mL 0.393 mmol) and PyBOP
reagent (0.107 g, 0.206 mmol). After 7 h, the reaction mixture
was partitioned between EtOAc and 1 N HCl. The layers were
separated and the aqueous layer was extracted with EtOAc
(1×). The combined organic layers were washed with saturated
NaHCO3 and brine, dried (MgSO4), filtered, and concentrated
under reduced pressure to afford a clear residue. Chromatog-
raphy on silica gel using 70% EtOAc/hexanes provided 0.088
g (86%) of 29 as a clear residue. Rf 0.32 (70% EtOAc/hexanes);
To the disubstituted hydroxylamine (0.009 g, 0.013 mmol)
in CH3CN (0.2 mL) was added Cbz-L-Val-Gly-OH (0.006 g,
0.019 mmol) followed by Et3N (0.002 mL, 0.015 mmol) and
HATU (0.007 g, 0.019 mmol). After 24 h, the reaction was
partitioned between EtOAc and 1 N HCl. The layers were
separated, and the aqueous layer was extracted with EtOAc
(1×). The combined organic layers were washed with saturated
NaHCO3 and brine, dried (MgSO4), filtered, and concentrated
under reduced pressure to give a clear residue that was
chromatographed on silica utilizing 10% iPrOH/CH2Cl2 to give
0.010 g of 31 (80% yield) as an amorphous white solid. Rf 0.45
[R]20 ) +5.8° (c ) 1.12 in CHCl3); IR (thin film) 3380, 3317,
(10% PrOH/CH2Cl2); 1H NMR (DMSO-d6, 600 MHz) d 0.86
i
D
1737, 1664, 1154 cm-1; H NMR (DMSO-d6, 300 MHz) d 1.26
(d, J ) 6.6 Hz, 3H), 0.90 (d, J ) 7.2 Hz, 3H), 1.27 (d, J ) 7.8
1

6056 J . Org. Chem., Vol. 66, No. 18, 2001
Shireman et al.
Hz, 3H), 1.39 (s, 9H), 1.95-2.00 (m, 1H), 2.67-2.71 (m, 1H),
2.80 (dd, J ) 9.9, 13.5 Hz, 1H), 2.95 (dd, J ) 9.0, 13.8 Hz,
1H), 2.93-3.07 (m, 4H), 3.97 (dd, J ) 7.2, 8.4 Hz, 1H), 4.02-
4.09 (m, 2H), 4.13 (dq, J ) 7.2, 7.2 Hz, 1H), 4.43 (dd, J ) 6.6,
9.6 Hz, 1H), 4.52 (ddd, J ) 6.2, 8.4, 8.4 Hz, 1H), 4.58 (ddd, J
) 4.0, 9.3, 9.3 Hz, 1H), 4.68 (dd, J ) 8.0, 8.0 Hz), 5.03 (dd, J
) 12.3, 17.7 Hz, 2H), 5.08 (dd, J ) 12.6, 14.4 Hz, 2H), 7.12-
7.36 (m, 21H, overlapping aromatic and NH), 8.08 (t, J ) 5.1
Hz, 1H), 8.25 (d, J ) 8.4 Hz, 1H), 8.54 (m, 2H). 13C NMR
(DMSO-d6, 150 MHz) δ 18.8, 18.1, 19.2, 27.6, 30.4, 36.1, 36.6,
37.7, 48.4, 53.3, 53.7, 58.5, 60.1, 65.4, 66.1, 79.2, 80.4, 126.4,
126.6, 127.7, 127.8, 127.9, 128.0, 128.3, 128.4, 128.4, 129.2,
129.4, 135.7, 136.9, 137.1, 137.4, 165.7, 168.9, 170.5, 170.9,
171.6, 171.7; HRMS-FAB (m/z) [M + H]+ calcd for C52H62N6O12,
963.4504; found, 963.4498.
2.23 (s, 3H), 2.78-2.96 (m, overlapping methylenes, 2H), 3.77
(s, 3H), 3.82 (s, 3H), 4.64 (dd, J ) 6.1, 7.9 Hz, 1H), 4.93 (dd, J
) 4.5, 9.3 Hz, 1H); 13C NMR (CDCl3, 75 MHz) δ 20.4, 35.6,
52.8, 52.9, 56.2, 77.3, 168.5, 169.6, 171.0; HRMS-FAB (m/z)
[M + H]+ calcd for C9H14NO6, 232.0821; found, 232.0840.
(()-2-[N-(Acetyl)-glycyl]-3,5-syn -[d ica r bom eth oxy]ox-
a zole (34). The Boc group of (()-24 (0.150 g, 0.520 mmol) in
EtOAc (2.5 mL) at 0 °C was removed as in the case of 33.
To Ac-Gly (0.073 g, 0.620 mmol) in CH2Cl2 (3.0 mL) was
added HOSu (0.071 g, 0.620 mmol) followed by EDC‚HCl
(0.119 g, 0.620 mmol). After 3.5 h, Boc-deprotected 34 in CH2-
Cl2 (2.0 mL) was added followed by HOAt (0.036 g, 0.265
mmol). After 15 h, the reaction was partitioned between EtOAc
and 1 N HCl. The layers were separated, and the aqueous layer
was saturated with NaCl and extracted with CH2Cl2 (4×). The
combined organics were washed with saturated NaHCO3 and
brine, dried (MgSO4), filtered, and concentrated to give a
residue that was chromatographed on silica, utilizing 7%
MeOH in CH2Cl2 to give 0.076 mg (51%) of 34 as a white solid
that could be further purified by recrystallization from EtOAc.
Rf 0.24 (7% MeOH/CH2Cl2); mp 121-123 °C; IR (thin film)
3364 (br), 2958, 1744, 1659, 1435, 1283 cm-1; 1H NMR (CDCl3,
500 MHz) δ 2.04 (s, 3H), 2.84 (ddd, J ) 4.0, 5.5, 13.0 Hz, 1H),
2.93 (ddd, J ) 8.2, 9.7, 13.0 Hz, 1H), 3.77 (s, 3H), 3.81 (s, 3H),
4.19 (dd, J ) 4.2, 18.7 Hz, 1H), 4.41 (dd, J ) 5.5, 18.5 Hz,
1H), 4.87 (dd, J ) 5.5, 8.0 Hz, 1H), 4.87 (dd, J ) 3.5, 9.5 Hz,
1H), 6.21 (bs, 1H); 13C NMR (CDCl3, 125 MHz) δ 23.0, 35.4,
41.0, 53.0, 53.1, 56.6, 77.4, 168.3, 168.6, 169.0, 170.3; HRMS-
FAB (m/z) [M + H]+ calcd for C11H17N2O7, 289.1036; found,
289.1029.
Va r ia ble Tem p er a tu r e NMR Exp er im en ts. Variable
temperature NMRs were measured using 0.005-0.010 g of
sample in CD2Cl2 (0.75 mL) at intervals of 15 °C beginning at
25 °C and cooling stepwise to -90 °C.
2-Ca r bo-ter t-bu toxy-3(R)-ca r bom eth oxy-5(S)-ca r boa l-
lyloxy-2-oxa zole (32). To acid 25 (0.54 g, 1.96 mmol) was
added O-allyl-N,N′-diisopropylisourea (0.379 g, 0.437 mL, 2.06
mmol).31 The mixture was allowed to stir at room temp for 6
h. Then CH3CN (1.0 mL) was added, and the reaction was
brought to reflux for 1h. The reaction was cooled to room
temperature, Et2O was added, and the reaction was filtered
through a short pad of silica gel. The solution was concentrated
under reduced pressure to give a red oil that was chromato-
graphed utilizing 40% EtOAc/hexanes to give 0.509 g (82%)
of 32 as a clear oil. Rf 0.38 (40% EtOAc/hexanes); IR (thin film)
2981, 1744, 1370, 1207, 1160 cm-1; 1H NMR (CDCl3, 500 MHz)
δ 1.49 (s, 9H), 2.81 (m, 2H), 3.77 (s, 3H), 4.60 (dd, J ) 7.5, 7.5
Hz, 1H), 4.67 (ddd, J ) 1.3, 1.3, 5.8 Hz, 2H), 4.74 (dd, J ) 5.8,
7.0, 1H), 5.27 (m, 1H), 5.34 (ddd, J ) 1.26, 3.0, 17.4 Hz, 1H),
5.92 (ddt, J ) 5.8, 10.5, 17.1 Hz, 1H); 13C NMR (CDCl3, 125
MHz) δ 28.1, 35.7, 52.9, 59.9, 66.3, 83.4, 119.2, 131.2, 156.0,
Ack n ow led gm en t. The authors thank the NIH
(AI30988) for support of this research. We acknowl-
edgeDr. Maoyu Shang for X-ray crystal structure de-
termination, Don Schifferl and Dr. J aroslav Zajicek for
NMR assistance, Dr. William Boggess and Nonka
Sevova for mass spectometry, and the Office of Informa-
tion Technologies for computational resources. B.T.S.
also gratefully acknowledges financial support in the
form of Lubrizol and J . Peter Grace fellowships at the
University of Notre Dame. Special thanks are extended
to Maureen Metcalf for her assistance with this manu-
script.
168.1, 170.1; HRMS-FAB (m/z) [M + H]+ calcd for C14H22
NO7, 316.1396; found, 316.1416.
-
(()-2-Acetyl-3,5-d ica r bom eth oxy-2-oxa zole (33). To (()-
24 (0.250 g, 0.865 mmol) in EtOAc (4.0 mL) at 0 °C was
bubbled HCl (g) until TLC indicated starting material was
consumed. The reaction was concentrated under reduced
pressure and redissolved in CH2Cl2 (4.0 mL) and cooled to 0
°C. Then Et3N (0.087 g, 0.726 mmol) was added dropwise over
5 min followed by DMAP (0.127 g, 1.04 mmol) and acetyl
chloride (0.075 g, 0.072 mL, 0.952 mmol) dropwise over 5 min.
After 30 min, the reaction was partitioned between EtOAc and
1 N HCl. The layers were separated, and the aqueous layer
was extracted with EtOAc (1×). The combined organic layers
were washed with brine, dried (MgSO4), filtered, and concen-
trated under reduced pressure to give a residue that was
purified, utilizing 70% EtOAc/hexanes to give 0.127 (64%) of
33 as a clear oil. Rf 0.47 (100% EtOAc); IR (thin film) 2958,
Su p p or t in g In for m a t ion Ava ila b le: 1H NMR and 13C
NMR for compounds 7, 12-14, 17, 19-21, 23, 24, 26, and 28-
34 as well as coordinates and energies of the calculated
structures of E- and Z-24, 33, and 34 are included. This
material is available free of charge via the Internet at
http://pubs.acs.org.
1749, 1670, 1438, 1219 cm-1
;
1H NMR (CDCl3, 300 MHz) δ
J O010284L