Efficient synthesis of enantiomerically pure β2-amino acids via chiral isoxazolidinones

Article abstract of DOI:10.1021/jo026738b

We report a practical and scalable synthetic route for the preparation of α-substituted β-amino acids (β²-amino acids). Michael addition of a chiral hydroxylamine, derived from α-methylbenzylamine, to an α-alkylacrylate followed by cyclization gives a diastereomeric mixture of α-substituted isoxazolidinones. These diastereomers are separable by column chromatography. Subsequent hydrogenation of the purified isoxazolidinones followed by Fmoc protection affords enantiomerically pure Fmoc-β²-amino acids, which are useful for β-peptide synthesis. This route provides access to both enantiomers of a protected β²-amino acid.

Full text of DOI:10.1021/jo026738b

amino acids (â2-amino acids) tend to adopt the 14 helix,
which is defined by 14-membered ring CdO(i)- -HsN(i
- 2) hydrogen bonds. Oligomers containing both â2- and
â3-amino acid residues can form the 10/12 helix, a very
different conformation that contains two distinct types
of backbone hydrogen bonds.6 Di-â-peptide segments
containing a â3-residue followed by a â2-residue, with the
appropriate relative configurations, adopt a reverse turn
conformation. Seebach et al. have developed somatostatin
mimics based on this turn motif.7
Efficien t Syn th esis of En a n tiom er ica lly
P u r e â2-Am in o Acid s via Ch ir a l
Isoxa zolid in on es
Hee-Seung Lee,† J in-Seong Park,‡
Byeong Moon Kim,‡ and Samuel H. Gellman*,†
Department of Chemistry, University of Wisconsin,
Madison, Wisconsin 53706-1396, and School of Chemistry,
College of Natural Sciences, Seoul National University,
Seoul 151-742, Korea
Our group’s â-peptide studies began with conforma-
tionally preorganized residues, e.g., â-amino acids rigidi-
fied by small rings.8 We have shown that these residues
confer unique properties on â-peptides, including high
conformational stability in aqueous solution and folding
patterns that are inaccessible without backbone con-
straints.9 â-Peptides containing both cyclically con-
strained residues and acyclic â3-residues combine the
conformational stability provided by the former with the
ease of the side-chain introduction provided by the
latter.10 This synergy should extend to mixtures of
constrained and â2-residues. Furthermore, combining â2-
residues, â3-residues, and constrained residues in a single
sequence should provide great versatility in orienting
specific sets of side chains along a folded â-peptide
scaffold. Exploration of these latter two possibilities has
been hampered, however, because â2-amino acids are
more difficult to prepare than are â3-amino acids.
â3-Amino acids are readily available from the corre-
sponding R-amino acids via Arndt-Eistert methodology.11
This simple and enantiospecific route provides access to
building blocks with a wide range of side chains. Several
routes to â2-amino acids have been reported,12 but none
is as efficient or general as the Arndt-Eistert route to
gellman@chem.wisc.edu
Received November 19, 2002
Abstr a ct: We report a practical and scalable synthetic route
for the preparation of R-substituted â-amino acids (â2-amino
acids). Michael addition of a chiral hydroxylamine, derived
from R-methylbenzylamine, to an R-alkylacrylate followed
by cyclization gives a diastereomeric mixture of R-substi-
tuted isoxazolidinones. These diastereomers are separable
by column chromatography. Subsequent hydrogenation of
the purified isoxazolidinones followed by Fmoc protection
affords enantiomerically pure Fmoc-â2-amino acids, which
are useful for â-peptide synthesis. This route provides access
to both enantiomers of a protected â2-amino acid.
â-Amino acids have long been employed as precursors
for â-lactams and other medicinally important mol-
ecules.1,2 More recently, â-amino acids have gained
attention as building blocks for oligomers with well-
defined folding behavior (foldamers).3 Short â-amino acid
oligomers (â-peptides) can take on a variety of secondary
structures; the substitution patterns of the individual
residues within a â-peptide are critical determinants of
the folding pattern.4
Seebach et al. have pioneered the use of monosubsti-
tuted â-amino acids to create â-peptide foldamers.5
Oligomers containing exclusively â-substituted â-amino
acids (â3-amino acids) or exclusively R-substituted â-
(6) (a) Seebach, D.; Schreiber, J . V.; Abele, S. Helv. Chim. Acta 2000,
83, 34. (b) Schreiber, J . V.; Seebach, D. Helv. Chim. Acta 2000, 83,
3139.
(7) (a) Seebach, D.; Abele, S.; Gademann, K.; J aun, B. Angew. Chem.,
Int. Ed. 1999, 38, 1595. (b) Gademann, K.; Kimmerlin, T.; Hoyer, D.;
Seebach, D. J . Med. Chem. 2001, 44, 2460.
(8) (a) Appella, D. H.; Christianson, L. A.; Karle, I. L.; Powell, D.
R.; Gellman, S. H. J . Am. Chem. Soc. 1996, 118, 13071. (b) Appella,
D. H.; Christianson, L. A.; Klein, D. A.; Powell, D. R.; Huang, X.;
Barchi, J . J .; Gellman, S. H. Nature 1997, 387, 381. (c) Wang, X.;
Espinosa, J . F.; Gellman, S. H. J . Am. Chem. Soc. 2000, 122, 4821. (d)
Lee, H.-S.; LePlae, P. R.; Porter, E. A.; Gellman, S. H. J . Org. Chem.
2001, 66, 3597. (e) LePlae, P. R.; Umezawa, N.; Lee, H.-S.; Gellman,
S. H. J . Org. Chem. 2001, 66, 5629.
† University of Wisconsin.
‡ Seoul National University.
(1) For reviews on the synthesis of â-amino acids, see: (a) Enanti-
oselective Synthesis of â-amino acids; J uaristi, E., Ed.; Wiley-VCH:
New York, 1997. (b) J uaristi, E.; Lopez-Ruiz, H. Curr. Med. Chem.
1999, 6, 983.
(2) (a) Barrow, R. A.; Hemscheidt, T.; Liang, J .; Paik, S.; Moore, R.
E.; Tius, M. A. J . Am. Chem. Soc. 1995, 117, 2479. (b) Floreancig, P.
E.; Swalley, S. E.; Trauger, J . W.; Dervan, P. B. J . Am. Chem. Soc.
2000, 122, 6342. (c) Reinelt, S.; Merce, M.; Dedier, S.; Reitinger, T.;
Folkers, G.; Lopez de Castro, J . A.; Rognan, D. J . Biol. Chem. 2001,
276, 24525. (d) For biologically active â-peptides, see: Werder, M.;
Hausre, H.; Abele, S.; Seebach, D. Helv. Chim. Acta 1999, 82, 1774.
Hamuro, Y.; Schneider, J . P.; DeGrado, W. F. J . Am. Chem. Soc. 1999,
121, 12200. Porter, E. A.; Wang, X.; Lee, H.-S.; Weisblum, B.; Gellman,
S. H. Nature 2000, 404, 565.
(3) For reviews, see: (a) Gellman, S. H. Acc. Chem. Res. 1998, 31,
173. (b) Gademann, K.; Hintermann, T.; Schreiber, J . V. Curr. Med.
Chem. 1999, 6, 905. (c) Cheng, R. P.; Gellman, S. H.; DeGrado, W. F.
Chem. Rev. 2001, 101, 3219. (d) Hill, D. J .; Mio, M. J .; Prince, R. B.;
Hughes, T. S.; Moore, J . S. Chem. Rev. 2001, 101, 3893.
(4) DeGrado, W. F.; Schneider, J . P.; Hamuro, Y. J . Pept. Res. 1999,
54, 206.
(9) Lee, H.-S.; Syud, F. A.; Wang, X.; Gellman, S. H. J . Am. Chem.
Soc. 2001, 123, 7721.
(10) LePlae, P. R.; Fisk, J . D.; Porter, E. A.; Weisblum, B.; Gellman,
S. H. J . Am. Chem. Soc. 2002, 124, 6820.
(11) Guichard, G.; Abele, S.; Seebach, D. Helv. Chim. Acta 1998,
81, 187.
(12) (a) For use of Evans’ chiral auxiliary, see: Evans, D. A.; Urpi,
F.; Somers, T. C.; Clark, J . S.; Bilodeau, M. T. J . Am. Chem. Soc. 1990,
112, 8215. Hintermann, T.; Seebach, D. Helv. Chim. Acta 1998, 81,
2093. Abele, S.; Guichard, G.; Seebach, D. Helv. Chim. Acta 1998, 81,
2141. Micuch, P.; Seebach, D. Helv. Chim. Acta 2002, 85, 1567. Sibi,
M. P.; Deshpande, P. K. J . Chem. Soc., Perkin Trans. 1 2000, 1461.
(b) For use of Oppolzer’s sultam, see: Posinet, R.; Chassaing, G.;
Vaissermann, J .; Lavielle, S. Eur. J . Org. Chem. 2000, 83. (c) For use
of pseudoephedrin, see: Myers, A.; Schnider, P.; Kwon, S.; Kung, D.
J . Org. Chem. 1999, 64, 3322. Nagula, G.; Huber, V. J .; Lum, C.;
Goodman, B. A. Org. Lett. 2000, 2, 3527. (d) For use of chiral
pyrimidinone, see: J uaristi, E.; Quintana, D. Tetrahedron: Asymmetry
1992, 3, 723. (e) For C-H activation, see: Davies, H. M. L.; Venkat-
aramani, C. Angew. Chem., Int. Ed. 2002, 41, 2197.
(5) (a) Seebach, D.; Overhand, M.; Kuhnle, F. N. M.; Martinoni, B.;
Oberer, L.; Hommel, U.; Widmer, H. Helv. Chim. Acta 1996, 79, 913.
(b) Hintermann, T.; Seebach, D. Synlett 1997, 437.
10.1021/jo026738b CCC: $25.00 © 2003 American Chemical Society
Published on Web 01/11/2003
J . Org. Chem. 2003, 68, 1575-1578
1575

SCHEME 1. Retr osyn th etic An a lysis
SCHEME 2. P r ep a r a tion of r-Alk yla cr yla tes
â3-amino acids. Most reported routes to â2-amino acids
involve chiral auxiliaries, the removal of which requires
basic or acidic conditions. Epimerization at the R-position
can arise under basic conditions. The acidic removal
conditions preclude the use of acid-labile protecting
groups for side-chain functionality, like Boc, which are
convenient when Fmoc-based oligomer synthesis is
planned.
We present a new route to â2-amino acids that offers
significant advantages relative to the existing methods.
Our approach is outlined in Scheme 1. The enantiomeri-
cally pure Fmoc-â2-amino acid is envisioned to arise from
the corresponding isoxazolidinone via hydrogenolysis,
which removes the chiral auxiliary and cleaves the N-O
bond without any possibility of racemization, followed by
N-protection. The R-substituted isoxazolidinone inter-
mediate should be available from the appropriate R-alkyl-
acrylate ester and enantiomerically pure R-methylben-
zylhydroxylamine. Related routes have been used for the
stereoselective preparation of â3- and â2,3-amino acids,
although in some cases the chiral auxiliary has been
linked to the acrylate, via esterification, rather than to
the hydroxylamine.13 Baldwin and Aube´ were the first
to employ a chiral hydroxylamine for isoxazolidinone
synthesis, but these workers examined only â-substituted
acrylate substrates.13b We explored the reaction of a
chiral hydroxylamine that is readily available in enan-
tiomerically pure form with achiral R-substituted acry-
lates because we anticipated that this approach would
provide access to protected â2-amino acids with protein-
like side chains.
meric mixture of the isoxazolidinones 5a and 6a (17%).
Hydroxylamine adducts analogous to 4a with â-substit-
uents rather than R-substituents are known to cyclize
to the corresponding isoxazolidinones in some cases,13b
but the in situ cyclization of 4a was sluggish. We could
obtain the R-substituted isoxazolidinone mixture 5a /6a
efficiently via the reaction of 4a with a base (LHMDS)
or a Lewis acid (n-Bu2SnO). The treatment of 4a with
LHMDS in THF at -78 °C for 10 min gave mixture 5a /
6a in excellent yield (92%) but with little diastereoselec-
tivity (5a /6a ) 1.5:1). Alternatively, refluxing 4a in
benzene with n-Bu2SnO provided the mixture 5a /6a in
excellent yield (99%) but again with little diastereose-
lectivity (5a /6a ) 1.5:1). Fortunately, the isoxazolidinones
5a and 6a (and the other isoxazolidinone mixtures we
examined, 5b-d /6b-d ) proved to be very easy to sepa-
1
rate preparatively via column chromatography. The H
NMR analysis of the R-substituted isoxazolidinones such
as 5a and 6a was hampered by nitrogen inversion, which
is slow on the NMR time scale at rt.13a Enantiomeric
excesses of these and the other isoxazolidinones discussed
below were readily determined via chiral HPLC (Chiral-
cel OD column) after diastereomer separation by pre-
parative chromatography (99% de). The isoxazolidinones
5a and 6a are precursors to the Fmoc-â2-phenylalanine
enantiomers 7a and 8a , and it is valuable to have access
to both â2-amino acid enantiomers from a single prepara-
tion.
Scheme 2 summarizes the synthesis of R-alkylacrylate
esters in which the R-substituent will ultimately become
the side chain of the â2-amino acid. Monoalkylation of
acetoacetate esters with various alkyl halides (RX) gave
1a -d in moderate to good yields. Deprotonation followed
by the addition of paraformaldehyde afforded the desired
R-alkylacrylates 2a -d in good yields.14
Low yields were obtained when the route in Scheme 3
was pursued by starting with ethyl R-isopropylacrylate
(i.e., the ethyl ester analogue of 2c). Switching to the
benzyl ester 2c13d significantly improved the reaction
efficiency. We found that changing the solvent from
ethanol to DMF and running the reaction at rt gave the
intermediate 4c exclusively (no isoxazolidinone byprod-
ucts). Use of the benzyl ester in place of the ethyl ester
enhanced the subsequent cyclization reaction to produce
5c/6c, presumably because benzyloxy is a better leaving
group than ethoxy. Another benefit of the switch to the
benzyl ester is that the acrylate intermediate is less
volatile, which simplifies product isolation after the
reactions outlined in Scheme 2.
Scheme 3 outlines the preparation of four Fmoc-â2-
amino acids bearing side chains found on proteinogenic
R-amino acids. Ethyl R-benzylacrylate (2a ) was treated
with (S)-R-methylbenzylhydroxylamine oxalic acid salt
(3)15 and triethylamine in refluxing ethanol to give adduct
4a as the major product (72%) along with a diastereo-
(13) (a) Baldwin, J . E.; Harwood, L. M.; Lombard, M. J . Tetrahedron
1984, 40, 4363. (b) Baldwin, S. W.; Aube´, J . Tetrahedron Lett. 1987,
28, 179. (c) Ishikawa, T.; Nagai, K.; Kudoh, T.; Saito, S. Synlett 1995,
1171. (d) Ishikawa, T.; Nagai, K.; Kudoh, T.; Saito, S. Synlett 1998,
1291. (e) Niu, D.; Zhao, K. J . Am. Chem. Soc. 1999, 121, 2456. (f) Sibi,
M. P.; Liu, M. Org. Lett. 2000, 2, 3393.
(14) Ueno, Y.; Setoi, H.; Okawara, M. Tetrahedron Lett. 1978, 39,
3753.
(15) (a) Tokuyama, H.; Kuboyama, T.; Amano, A.; Yamashita, T.;
Fukuyama, T. Synthesis 2000, 9, 1299. (b) Traber, B.; J ung, Y.-G.;
Park, T. K.; Hong, J .-I. Bull. Korean Chem. Soc. 2001, 22, 547.
Th intermediates 4b-d , as diastereomeric mixtures,
were treated with LHMDS in THF at -78 °C to give the
1576 J . Org. Chem., Vol. 68, No. 4, 2003

SCHEME 3. Syn th etic Rou te for Eith er En a n tiom er of F m oc-â2-a m in o Acid s
SCHEME 4. Alter n a tive Rou te for F m oc-â2-h om ova lin e-OH
TABLE 1. Deter m in a tion of Absolu te Con figu r a tion
we used enantiomerically pure R-methylbenzylamine as
the nucleophile rather than the corresponding hydroxyl-
amine, and we used the tert-butyl ester form of the
acrylate. The Michael adducts were formed in an ap-
proximate 1:1 ratio, but as observed with the isoxazoli-
dinones, the diastereomeric products were readily sepa-
rated by preparative chromatography. Acid treatment
removed the tert-butyl esters of the isolated diastereo-
mers. Hydrogenolysis then removed the chiral auxiliary,
and N-protection provided the desired product. This route
may be a useful alternative to the one shown in Scheme
3 for â2-amino acid derivatives that do not bear acid-labile
protecting groups in their side chains.
yield (%)a [R]25 (observed)
[R]rt (literature)
D
D
7a
7b
8c
8d
82
80
84
78
+11.8
+10.8
-12.1
-4.6
+8.2 (7a , c 0.91, CHCl3)b
+10.8 (7b, c 0.6, CHCl3)b
+11.5 (7c, c 0.97, CHCl3)b
+4.8 (7d , c 0.54, CHCl3)c
a
b
Yield from 5 or 6. Reference 11. c Reference 7b.
isoxazolidinone mixtures 5b-d /6b-d in excellent yield
(>95%). Transfer hydrogenolysis of the isolated R-sub-
stituted isoxazolidinone isomers with 10% Pd/C and
ammonium formate in refluxing methanol led to efficient
N-O bond cleavage and removal of the R-methylbenzyl
group to give the corresponding â2-amino acids. N-pro-
tection then provided the desired Fmoc-â2-amino acids.
The absolute configurations of the Fmoc-â2-amino acids
were established by the comparison of the optical rota-
tions with the values reported in the literature (Table
1). In each case, the isoxazolidinone retrospectively
assigned to series 5 eluted more rapidly during prepara-
tive chromatography than did the diastereomer assigned
to series 6.
We have developed an efficient synthetic route that
provides both enantiomers of Fmoc-â2-amino acids. The
reaction sequence can be run on a reasonably large scale,
and this route can therefore conveniently support ex-
ploratory â-peptide studies. The ready availability of
R-acrylate esters such as 2a -d should allow one to
generate a wide range Fmoc-â2-amino acids, including
versions that contain functional groups with acid-labile
protection in the side chains [exemplified by Fmoc-â2-(ꢀ-
N-Boc)homolysine]. This route avoids expensive chiral
auxiliaries and potential epimerization problems. Our
method is complementary to an elegant approach recently
reported by Davies and Venkataramani that involves
asymmetric C-H insertion and gives access to â2-amino
acids bearing aryl and vinyl side chains.12e The broad
range of Fmoc-â2-amino acids available through our route
In the case of Fmoc-â2-homovaline, we examined an
alternate synthetic route (Scheme 4) that is based on
chemistry reported by Davies and Fenwick.16 In this case,
(16) Davies, S. G.; Fenwick, D. R. J . Chem. Soc., Chem. Commun.
1995, 1109.
J . Org. Chem, Vol. 68, No. 4, 2003 1577

mL) was added to the reaction mixture, and the aqueous layer
was extracted with ethyl acetate (3 × 100 mL). The combined
organic extracts were dried over MgSO4, filtered, and concen-
trated to give a crude diastereomeric mixture of isoxazolidinones
as a yellow oil. The mixture could be separated by column
chromatography on silica gel [5 (diameter) × 20 (height) cm
column, 1:10 EtOAc/hexanes] to afford 5a (3.76 g) and 6a (2.51
g), each in diastereomerically pure form and in 82% overall yield
from 2a . The diastereomeric excesses (de) were determined by
chiral HPLC (250 × 4.6 mm Chiralcel OD column, 1.0 mL/min,
2:98 2-propanol/hexanes); tR ) 15.69 min (5a ) and tR ) 20.53
min (6a ).
(4S)-Ben zyl-2-[(1S)-p h en yleth yl]isoxa zolid in -5-on e (5a ).
99% de. Rf ) 0.23 (1:10 EtOAc/hexanes). 1H NMR (DMSO-d6,
600 MHz, 54 °C) δ: 7.40-7.10 (10H), 3.95 (s, 1H), 3.50-3.10
(br, 3H), 3.05 (m, 1H), 2.80 (s, 1H), 1.40 (br s, 3H). ESI-MS
m/z: 304.1 (M + Na, calcd 304.1), 585.3 (2M + Na, calcd 585.3).
(4R )-B e n zy l-2-[(1S )-p h e n y le t h y l]isox a zo lid in -5-on e
(6a ). 99% de. Rf ) 0.17 (1:10 EtOAc/hexanes). 1H NMR (DMSO-
d6, 600 MHz, 54 °C) δ: 7.40-7.10 (10H), 3.95 (s, 1H), 3.42 (br,
1H), 3.20 (s, 1H), 3.10 (m, 1H), 2.95 (s, 1H), 2.80 (m, 1H), 1.30
(br s, 3H). ESI-MS m/z: 304.1 (M + Na, calcd 304.1), 585.3
(2M + Na, calcd 585.3).
(2S)-Ben zyl-3-(9H-flu or en -9-ylm eth oxyca r bon yla m in o)-
p r op ion ic Acid (F m oc-(S)-â2-HP h e-OH; 7a ). Isoxazolidinone
5a (3.00 g, 10.7 mmol) was dissolved in methanol (50 mL).
Ammonium formate (6.7 g, 107 mmol) and 10% Pd/C (1.0 g) were
added to the solution. The mixture was stirred at 60 °C for 2 h.
After the reaction was complete (the disappearance of starting
material was monitored by TLC), the resulting mixture was
filtered through Celite, and the filtrate was concentrated to give
a white solid. The crude amino acid was characterized but not
purified. 1H NMR (CD3OD, 300 MHz) δ: 7.28-7.16 (m, 5H), 3.21
(m, 1H), 2.89-2.60 (m, 4H). ESI-MS m/z: 178.1 (M - H, calcd
178.1).
and that of Davies and Venkataramani should provide
access to new â-peptides with specific shapes and specific
functions.
Exp er im en ta l Section
Gen er a l P r oced u r es. Melting points were determined on a
capillary melting-point apparatus and are uncorrected. Optical
rotations were measured using sodium light (D line, 589.3 nm).
THF was distilled from sodium benzophenone ketyl under N2.
Unless otherwise noted, all other commercially available re-
agents and solvents were used without further purification.
Analytical thin-layer chromatography (TLC) was carried out on
Whatman TLC plates precoated with silica gel 60 (250-µm layer
thickness). Visualization was accomplished using either a UV
lamp, potassium permanganate stain (2 g of KMnO4, 13.3 g of
K2CO3, 3.3 mL of 5% (w/w) NaOH, and 200 mL of H2O), or
phosphomolybdic acid stain (10% phosphomolybdic acid in
ethanol). Column chromatography was performed on EM Science
silica gel 60 (230-400 mesh). The solvent mixtures used for TLC
and column chromatography are reported in v/v ratios. Diaster-
eomeric excesses of 5a -d and 6a -d were determined by chiral
HPLC (250 × 4.6 mm Chiralcel OD column, 1.0 mL/min).
A representative synthetic procedure is provided below. Other
procedures may be found in the Supporting Information.
2-Ben zyl-3-oxobu tyr ic Acid Eth yl Ester (1a ). To a sus-
pension of t-BuOK (8.8 g, 76.8 mmol) in THF (200 mL) were
added ethyl acetoacetate (9.6 mL, 75.3 mmol) and t-BuOH (7.0
mmol) at 0 °C. The resulting clear solution was stirred for 30
min, and then benzyl bromide (8.9 mL, 74.8 mmol) was added
to the solution. The solution was stirred at 70 °C for 12 h. The
reaction was quenched with water, and then a saturated aqueous
sodium bicarbonate solution was added. The aqueous layer was
extracted with diethyl ether (3 × 100 mL). The combined organic
extracts were dried over MgSO4, filtered, and concentrated to
give a yellow oil. The crude product was purified by silica gel
column chromatography (1:10 EtOAc/hexanes, Rf ) 0.18) to give
1a (10.9 g) in 66% yield. 1H NMR (CDCl3, 300 MHz) δ: 7.31-
7.16 (m, 5H), 4.05 (q, J HH ) 7.2 Hz, 2H), 3.78 (t, J HH ) 7.5 Hz,
1H), 3.16 (d, J HH ) 7.5 Hz, 2H), 2.18 (s, 3H), 1.20 (t, J HH ) 7.2
Hz, 3H). 13C NMR (CDCl3, 75.4 MHz) δ: 202.37, 169.09, 138.14,
128.76, 128.54, 126.64, 110.10, 61.43, 61.31, 33.97, 29.56, 13.99.
ESI-MS m/z: 243.0 (M + Na, calcd 243.1), 463.1 (2M + Na,
calcd 463.2).
The crude solid was dissolved in 1:1 THF/H2O (100 mL) and
cooled to 0 °C, and then Fmoc-OSu (3.6 g, 10.7 mmol) and
NaHCO3 (9.0 g, 107 mmol) were added. The reaction mixture
was stirred at rt for 2 h, and THF was removed under reduced
pressure. The aqueous residue was washed with diethyl ether
(2 × 50 mL), and the organic layers were discarded. The aqueous
layer was acidified with a NaHSO4 solution and extracted with
methylene chloride (3 × 50 mL). The combined organic extracts
were dried over MgSO4, filtered, and concentrated. The residue
was purified by column chromatography on silica gel (1:50
MeOH/CH2Cl2) to afford the desired product 7a as a white solid
(3.52 g, 82.0% yield for two steps). This solid was recrystallized
2-Ben zyla cr ylic Acid Eth yl Ester (2a ). To a stirred solution
of compound 1a (6.60 g, 30.0 mmol) in THF (200 mL) was added
LHMDS (33.0 mL, 33.0 mmol, and 1.0 M solution in THF) at
-78 °C. The solution was stirred for 30 min, and then paraform-
aldehyde (4.2 g, excess) was added as a solid in one portion. The
resulting suspension was stirred at rt for 6 h and then filtered
through Celite to remove the excess paraformaldehyde. The
filtrate was concentrated, and the residue was purified by
column chromatography (1:19 EtOAc/hexanes, Rf ) 0.37) to give
2a (5.3 g) in 93% yield. 1H NMR (CDCl3, 300 MHz) δ: 7.34-
7.20 (m, 5H), 6.25 (t, J HH ) 0.6 Hz, 1H), 5.46 (m, 1H), 4.20 (q,
J HH ) 7.2 Hz, 2H), 3.65 (s, 2H), 1.28 (t, J HH ) 7.2 Hz, 3H). 13C
NMR (CDCl3, 75.4 MHz) δ: 166.87, 140.38, 138.77, 129.02,
128.35, 126.26, 125.90, 60.69, 38.05, 14.11. ESI-MS m/z: 213.1
(M + Na, calcd 213.1).
from chloroform/hexane: mp 148-152 °C; [R]25 +11.8° (c 0.91,
D
CHCl3), [R]rt +8.2° (7a ,11 c 0.91, CHCl3). This compound exists
D
as a mixture of slowly interconverting rotamers according to
NMR spectroscopy. 1H NMR (CD3OD, 300 MHz) δ: 7.76 (d, J HH
) 7.2 Hz, 2H), 7.63 (d, J HH ) 7.5 Hz, 2H), 7.39-7.16 (m, 9H),
4.31 (d, J HH ) 6.2 Hz, 2H), 4.18 (t, J HH ) 6.6 Hz, 1H), 3.40-
3.05 (m, 2H), 2.98-2.60 (m, 3H). 13C NMR (CD3OD, 75.4 MHz)
δ: 177.45, 158.70, 145.22, 142.50, 140.13, 129.93, 129.36, 128.70,
128.09, 127.37, 126.15, 120.87, 67.72, 48.36, 43.38, 36.78. ESI-
MS m/z: 178.1 (M - C15H11O2 (Fmoc), calcd 178.1), 400.1 (M -
H, calcd 400.2), 801.3 (2M - H, calcd 801.3).
4-Ben zyl-2-[(1S)-p h en yleth yl]isoxa zolid in -5-on e (5a /6a ).
To a stirred solution of 2a (4.95 g, 26.0 mmol) in DMF (100 mL)
were added (1S)-phenylethylhydroxylamine oxalate (3;15a 11.8
g, 52.0 mmol) and triethylamine (36.2 mL, 260 mmol) at rt. The
resulting clear solution was stirred at rt for 24 h. The resulting
cloudy suspension was concentrated via vacuum-rotary evapora-
tion and then purified by column chromatography on silica gel
(1:10 EtOAc/hexanes, Rf ) 0.12) to afford 4a (7.31 g) as a
colorless oil in 86% yield. This material was dissolved in THF
(100 mL), the solution was cooled to -78 °C, and then LHMDS
(24.5 mL, 24.5 mmol, 1.0 M solution in THF) was added. The
yellow solution was stirred at -78 °C for 30 min. Water (100
Ack n ow led gm en t. This research was supported by
NIH (GM56414). J .-S.P. was supported by the Division
of Chemistry and Molecular Engineering, Brain Korea
21 Program.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
1
cedures for 1b-d , 2b-d , 5b-d /6b-d , 7b, 8c, and 8d and H
NMR and 13C NMR spectra of compounds 1-8. This material
is available free of charge via the Internet at http://pubs.acs.org.
J O026738B
1578 J . Org. Chem., Vol. 68, No. 4, 2003