N-carbamylamino alcohols as the precursors of oxazolidinones via nitrosation-deamination reaction

Article abstract of DOI:10.1055/s-2000-6508

Oxazolidinones were effectively prepared from N-carbamylamino alcohols by treatment with nitrous acid, via N-nitroso compound as the intermediate. A new route to (R)-4-benzyloxazolidinone was developed starting from DL- phenylalanine, utilizing D-hydantoinase-catalyzed enantioselective hydrolysis of 5-benzylhydantoin under the dynamic kinetic resolution conditions, and the subsequent reduction to the precursor for the above-mentioned cyclization reaction, by taking advantage of the intermediates bearing an N-carbamylamino functionality.

Full text of DOI:10.1055/s-2000-6508

LETTER
189
N-Carbamylamino Alcohols as the Precursors of Oxazolidinones via
Nitrosation-Deamination Reaction
Masumi Suzuki, Takahiro Yamazaki, Hiromichi Ohta, Kyoko Shima, Katsuhide Ohi, Shigeru Nishiyama,
Takeshi Sugai*
Department of Chemistry, Keio University, 3-14-1 Hiyoshi, Yokohama 223-8522, Japan
Fax +81 45 563 5967; E-mail: sugai@chem.keio.ac.jp
Received 29 November 1999
the corresponding N-nitroso compounds 7,11 which would
eventually eliminate a nitrogen molecule and water under
milder conditions.
Abstract: Oxazolidinones were effectively prepared from N-car-
bamylamino alcohols by treatment with nitrous acid, via N-nitroso
compound as the intermediate. A new route to (R)-4-benzyloxaz-
olidinone was developed starting from DL-phenylalanine, utilizing
D-hydantoinase-catalyzed enantioselective hydrolysis of 5-benzyl-
hydantoin under the dynamic kinetic resolution conditions, and the
subsequent reduction to the precursor for the above-mentioned cy-
clization reaction, by taking advantage of the intermediates bearing
an N-carbamylamino functionality.
Toward this end, N-carbamylamino alcohol 6a was treat-
ed with sodium nitrite in the presence of hydrochloric acid
at room temperature. The initially formed intermediate 7a
immediately cyclized to the corresponding oxazolidinone
1a in 88% yield. The major byproduct was the further N-
nitrosated oxazolidinone.12 To avoid this undesirable re-
action, a two-phase method was attempted, by the addi-
tion of an organic co-solvent. For this purpose, ethyl
acetate was effective and the yield was slightly improved
(90%).
Key words: N-carbamylamino alcohols, N-nitrosourea, enzymes,
hydrolysis, oxazolidinones
The importance of oxazolidinones 1 has been addressed in
both pharmaceutical1 and synthetic organic chemistry,2
especially in asymmetric syntheses of natural- and non-
natural products.3,4 Needless to say, the starting materials
of oxazolidinones are the corresponding amino alcohols 2.
A variety of synthetic intermediates have been elaborated,
such as alkyl or aryl carbamates 3 which served for cy-
clization under either acidic5 or basic6 conditions, trichlo-
romethyl carbamates 4 derived from the action of
diphosgene7 or triphosgene,8 and triphenylphosphonium
salts 59 (Scheme 1).
The results were summarized in the Table. The efficiency
of the reaction was, indeed, affected by both the steric hin-
drance of the products and the solubility of the starting
material. For example, acetic acid was essential for the
dissolution of a crystalline substrate 6c. An N-carbamyl-
amino alcohol with a tertiary alcohol moiety 6d, which
was prepared by the addition of Grignard reagent on ami-
no acid ester,7c,13 was also effective. It was concluded that
the elaboration of the reaction conditions, especially
the equivalence of the nitrosation reagent as well as
the choice of the co-solvent was important.14 The above
O
O
O
NH2
Table
NHCX
HN
OH
R3
R1
OH
R3
R1
R1
O
O
O
R3
R2
R2
R2
NHCNH2
HN
NaNO2, HCl
OH
R3
2
3: X = OR
4: = OCCl3
1
R1
R1
/ H2O, Co-solvent
r.t.
R3
R2
R2
5:
6:
7:
=
O
PPh3
6
1
= NH2
Substrate
Yield of
1 (%)
R1
R2
H
R3
H
Co-solvent
=
N
N O
6
H
a
88
74
PhCH2
Scheme 1
(i-Pr)2O
AcOEt
90
Among them, the use of urea derivatives 6 was developed
more than 50 years ago.1b,10 This is an excellent method,
in terms of the cheapness of the derivatizing reagents; the
intermediates can be prepared by the reaction of amino al-
cohols between either urea or potassium cyanate. The cy-
clization reaction of the urea intermediates 6, however,
requires an elevated temperature. Our plan was the treat-
ment of 6 with nitrous acid to convert the amino group to
b
H
H
92
i-Pr
AcOEt
AcOH
91
c
d
Ph
H
quant.
87
Ph
Me
Me
PhCH2
O
NHCNH2
e
30
OH
Synlett 2000, No. 2, 189–192 ISSN 0936-5214 © Thieme Stuttgart · New York

190
M. Suzuki et al.
LETTER
cyclization of N-carbamylamino phenol 6e, which is sus-
ceptible to nitrosation on aromatic ring, proceeded, al-
though the yield of 1e was as low as 30%. An attempt for
the application of this reaction on the related substrate,
N-thiocarbamylamino alcohol,15 resulted only in decom-
position of the precursor.
O
O
lit. 19
HN
D
-hydantoinase
NHCNH2
DL-Phe
NH
PhH2C
/ borate buffer
pH 9.0, 40 °C
11
PhH2C
CO2H
O
(73%)
12
10
The reaction intermediate after the elimination of leaving
groups from the precursors 3-6 had been supposed to be
isocyanates 8 via path a.1b,16 In the present case, however,
N-methyl derivative 6f also afforded efficiently (89%
yield) the corresponding oxazolidinone 9.17 This result
suggests another path, b, involving the participation of the
neighboring hydroxyl group, at the stage of the elimina-
tion.
O
O
Amberlyst-15
MeOH
NHCNH2
NHCNH2
OH
LiAlH4
PhH2C
CO2Me
/ THF, r.t.
(70%)
PhH2C
reflux
(quant.)
6a
13
Scheme 3
O
H
O
O
O
O
O
O
C
a
O
C
N
NH
Cl
(ClCH2CO)2O
X
X
HN
N=C=O
OH
R3
N
N
NH2
OH
R3
N
path a
b
OH
R3
120 °C
(97%)
R1
PhH2C
R1
R1
PhH2C
15a
(X = H)
R2
R2
R2
1a
6a-d: X = H
6f: X = Me
1 = R2 = R3 = H
7
8
O
O
O
O
path b
EtBr, NaBr, MS 3A
1) PPh3
R
Br
Ph3P
N
N
O
(X = Me)
/ N-methylpyrrolidin-
2-one, 65 °C
(99%)
O
2) NaOH aq.
(83%)
O
PhH2C
15b
PhH2C
14
O
O
Me
HN
N
O
R1
R3
R2
Scheme 4
9
1
Scheme 2
In conclusion, new route to oxazolidinones from amino
alcohols and amino acids was developed, and some routes
for the derivatization of the products became available.
Next, we turned our attention to the N-carbamylamino
functionality, which plays an important role in the cy-
clization reaction. The N-carbamylamino alcohols would
be obtained by the reduction of the corresponding N-car-
bamylamino acids, whose enantiomerically enriched
forms have been obtained by the enzyme-catalyzed
hydrolysis18 of the corresponding hydantoins.
Acknowledgement
The authors thank Professors Tadao Ishizuka of Kumamoto Univer-
sity, Yasuo Kato of Toyama Prefectural University, and Dr. Kenji
Miyamoto of Kaneka Co. for discussion, and the reviewers for va-
luable comments. We are indebted to Dr. Atsuhito Kuboki for NMR
measurement. This work was accomplished as "Science and Tech-
nology Program on Molecules, Supra-Molecules and Supra-Struc-
tured Materials" of an Academic Frontier Promotional Project, by
Japan’s Ministry of Education, the Monbusho, and acknowledged
with thanks. Also this work was supported by the “Research for the
Future” Program (JSPS-RFTF 97100302) from the Japan Society
for the Promotion of Science.
This pathway is exemplified in Scheme 3. A hydantoin
10,19 prepared from DL-phenylalanine (11) was hydro-
lyzed with commercially available Adzuki bean D-hydan-
toinase (Sigma).20 The enantioselective hydrolysis
proceeded with concomitant racemization of the
substrate18 to give (R)-12. This was converted to N-car-
bamylamino alcohol 6a via methyl ester 13.21
The increased availability of (R)-1a encouraged us to de-
velop an expeditious route to the Wittig phosphorane 14,
whose advantage in natural product synthesis has recently
been demonstrated.22 The intermediate 15a could effec-
tively be prepared by the reaction of 1a with excess
amount of chloroacetic anhydride1b in 97% yield.23 Chlo-
roacetamide 15a was readily converted to the correspond-
ing bromide 15b,24,25 which is the direct precursor of 14,26
and this total path could avoid the use of lachrymator bro-
moacetyl bromide for the acylation step (Scheme 4).
References and Notes
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Synlett 2000, No. 2, 189–192 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
N-Carbamylamino Alcohols as the Precursors of Oxazolidinones
191
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mL) was added a solution of NaNO2 (57 mg, 0.83 mmol) in
water (0.3 mL). After stirring at room temperature for 40 min,
the reaction mixture was extracted with AcOEt three times.
The combined organic layer was washed with brine and dried
over Na2SO4, concentrated in vacuo. The residue was purified
by silica gel column chromatography (14 g). Elution with
hexane-AcOEt (1:3) afforded (R)-1b (92% yield).
Recrystallization from hexane-AcOEt afforded colorless
20
needles. Mp 71.0 °C; [a]D22+19.3 (c 1.00, EtOH) [lit.6e [a]D
-20.0 (c 1, EtOH) for (S)-1b]; 1H NMR (400 MHz, CDCl3) d
6.77 (br s, 1H), 4.45 (t, 1H, J = 8.8 Hz), 4.10 (dd, 1H, J = 6.3,
8.8 Hz), 3.62 (m, 1H), 1.73 (m, 1H), 0.97 (d, 3H, J = 6.8 Hz),
0.90 (d, 3H, J = 6.3 Hz); IR (KBr) 3270, 2961, 1750, 1726,
1472, 1406, 1362, 1247, 1091, 1050, 1010 cm-1.
(S)-1d [from (S)-6d]: Mp 67.4-67.8 °C; [a]D22 -103.9 (c 1.00,
CHCl3) [lit.27 [a]D23 -103.5 (c 0.6, CHCl3)]; 1H NMR (400
MHz, CDCl3) d 7.19 (m, 5H), 5.02 (br s, 1H), 3.62 (dd, 1H,
J = 3.9, 10.5 Hz), 2.76 (dd, 1H, J = 3.9, 13.4 Hz), 2.61 (dd,
1H, J = 10.5, 13.4 Hz), 1.40 (s, 3H), 1.38 (s, 3H); IR (KBr)
3260, 2976, 1748, 1732, 1374, 1307, 1270, 1145, 1089, 997
cm-1.
1e: Mp 106.0-107.0 °C; 1H NMR (400 MHz, CDCl3) d 9.86
(br s, 1H), 7.37-7.22 (m, 4H); IR (KBr) 3231, 1775, 1736,
1481, 1343, 1147, 1009 cm-1.
(4) (a) Hintermann, T.; Seebach, D. Helv. Chim. Acta 1998, 81,
2093-2126. (b) Tomoyasu, T.; Tomooka, K.; Nakai, T. Synlett
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(5) (a) Matsunaga, H.; Ishizuka, T.; Kunieda, T. Tetrahedron
1997, 53, 1275-1294. (b) Rück-Braun, K.; Stamm, A.; Engel,
S.; Kunz, H. J. Org. Chem. 1997, 62, 967-975.
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(b) Gage, J. R.; Evans, D. A. Org. Synth. 1989, 68, 77-82.
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7407-.7410. (d) Maleski, R. J.; Osborne, C. E.; Cline, S. M. J.
Heterocyclic Chem. 1991, 28, 1937-1939. (e) Knölker, H.-J.;
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Org. Chem. 1989, 54, 3231-3233. (b) Tamura, O.; Hashimoto,
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1992, 33, 3487-3490. (c) O'Hagan, D.; Tavasli, M.
Tetrahedron: Asymmetry 1999, 10, 1189-1192.
Use of AcOEt as co-solvent. To a solution of (R)-6a (200 mg,
1.03 mmol) in 2 M HCl (1.5 mL, 3.0 mmol) and AcOEt (4.0
mL) was added a solution of NaNO2 (197 mg, 2.85 mmol) in
water (1.0 mL). The reaction mixture was stirred at room
temperature for 5 min. The reaction was worked up in the
same manner as above. (R)-1a: Mp 88.2-88.8 ¡C; [a]D23+63.5
(c 1.035, CHCl3) [lit.7a [[a]D25+64 (c 1.00, CHCl3)]; 1H NMR
(270 MHz, CDCl3) d 7.32 (m, 5H), 5.12 (br s, 1H), 4.52-4.04
(m, 3H), 2.95-2.81 (m, 2H); IR (KBr) 3282, 2925, 1753, 1709,
1475, 1405, 1245, 1096, 1021 cm-1. This reaction could
reproducibly be performed in more than 5 g scale.
Use of AcOH as co-solvent. To a suspension of (1S,2R)-6c (50
mg, 0.20 mmol) in 2 M HCl (2.5 mL, 5.0 mmol) and acetic
acid (2.5 mL) was added a solution of NaNO2 (38 mg, 0.55
mmol) in water (0.5 mL). After stirring at room temperature
for 5 min, a white solid was deposited. To the reaction was
added aqueous KOH until the pH of the mixture reached 10,
and worked up in the same manner as above. (4R,5S)-1c: Mp
226.5-226.7 °C; [a]D19+61.3 (c 1.00, MeOH) [lit.28
[a]D20+60.6 (c 0.858, MeOH)]; 1H NMR (270 MHz, CDCl3) d
7.03 (m, 5H), 6.89 (m, 5H), 5.88 (d, 1H, J = 8.1 Hz), 5.68 (br
s, 1H), 5.12 (d, 1H, J = 8.1 Hz); IR (KBr) 3271, 1748, 1711,
1455, 1352, 1236, 1095, 1072 cm-1. The spectral data of 1a-1e
and 9 were identical with those of commercially available
samples.
(8) (a) Eckert, H.; Forster, B. Angew. Chem. Int. Ed. Engl. 1987,
26, 894-895. (b) Sicker, D. Synthesis 1989, 875-876. (c)
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39, 6733-6736.
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Choukroun, H.; Commeyras, A. J. Mol. Evol. 1999, 48, 638-
645.
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Braxmeier, T. Synlett 1997, 925-928.
(17) 9:1H NMR (400 MHz, CDCl3) d 4.32 (t, 2H, J = 8.1 Hz), 3.58
(t, 2H, J = 8.1 Hz), 2.90 (s, 3H); IR (NaCl) 3504, 2922, 1746,
1498, 1442, 1407, 1271, 1125, 1045 cm-1.
(12) Newman, M. S.; Kutner, A. J. Am. Chem. Soc. 1951, 73, 4199-
4204.
(18) (a) Yamada, H.; Shimizu, S.; Yoneda, K. Hakko to Kogyo
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J. Chem. Soc., Perkin Trans. 1 1985, 2039-2044. (b) Davies,
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674.
(14) Typical cyclization procedure: To a solution of (R)-N-
carbamyl-valinol (6b, 100 mg, 0.68 mmol) in 2 M HCl (5.1
(b) Takahashi, S. Hakko Kogaku (Fermentation Technology)
1983, 61, 139-151. (c) Bommarius, A. S.; Schwarm, M.;
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(20) Morin, A. Enzyme Microb. Technol. 1993, 15, 208-214.
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M. Suzuki et al.
LETTER
(21) D-Hydantoinase (Sigma, H4028, 54.0 mg, 25.3 units) was
added to a solution of the hydantoin (10, 400 mg, 2.10 mmol)
in borate buffer (pH 9.0, 105 mL), and the mixture was stirred
for 2 days at 40 °C. After adjustment of its pH to 3.0 with
diluted H2SO4, the reaction mixture was lyophilized. The
residue was charged in a glass-made column and eluted with
EtOH. The extract was concentrated in vacuo, and the residue
was purified by silica gel column chromatography (8 g).
Elution with AcOEt afforded 12 (73% yield). Quantitative
determination of 12 was carried out colorimetrically
according to the reported procedure.29a 12: mp 178.0-178.4
°C; [a]D20 -40.2 (c 0.99, MeOH); 1H NMR (270 MHz,
CD3OD) d 7.17 (m, 5H), 4.46 (dd, 1H, J = 5.0, 7.3 Hz), 3.08
(dd, 1H, J = 5.0, 13.9 Hz), 2.91 (dd, 1H, J = 7.3, 13.9 Hz); IR
(KBr) 3455, 3303, 1696, 1637, 1561, 1303, 1259, 1157 cm-1.
Its optical rotation was identical with an authentic sample
prepared from D-phenylalanine. Although a microbial D-
hydantoinase from Pseudomonas putidaIFO 1299629 was also
available, the effectiveness of the commercial Adzuki bean
enzyme was superior for this substrate, especially in the
laboratory-scale preparation.
prisms. Mp 75.0-75.3 °C; [a]D24 -72.8 (c 1.00, CHCl3); 1H
NMR (400 MHz, CDCl3) d 7.28 (m, 5H), 4.73 (s, 2H), 4.69
(m, 1H), 4.30-4.22 (m, 2H), 3.32 (dd, 1H, J = 3.4, 13.7 Hz),
2.82 (dd, 1H, J = 9.3, 13.7 Hz); IR (KBr) 3527, 3427, 3060,
3023, 2938, 1768, 1721, 1392, 1356 1238, 1212 cm-1. Anal.
Calcd. for C12H12ClNO3: C, 56.82; H, 4.77; N, 5.52. Found: C,
57.10; H, 4.64; N, 5.47.
(24) Willy, W. E.; McKean, D. R.; Garcia, B. A. Bull. Chem. Soc.
Jpn. 1976, 49, 1989-1995.
(25) A mixture of 15a (500 mg, 1.97 mmol), ethyl bromide (1.49
mL, 19.7 mmol), NaBr (41mg, 0.39 mmol), N-methyl-
pyrrolidin-2-one (1.96 mL) and MS 3A (1.20 g) was stirred
and heated at 65 °C for 20 hours. Conventional workup and
chromatographic separation [hexane-AcOEt (1:1)] afforded
15b (99% yield). Recrystallization from Et2O provided
21
colorless needles. Mp 36.2-36.8 °C [lit.30 mp 41-42 °C]; [a]D
-79.3 (c 1.01, CH2Cl2); 1H NMR (400 MHz, CDCl3) d 7.28
(m, 5H), 4.71 (m, 1H), 4.56 (d, 1H, J = 12.9 Hz), 4.52 (d, 1H,
J = 12.9 Hz), 4.30-4.21 (m, 2H), 3.33 (dd, 1H, J = 3.2, 13.4
Hz), 2.81 (dd, 1H, J = 9.5, 13.4 Hz); IR (KBr) 3530, 3381,
3063, 3030, 2986, 1781, 1698, 1386, 1359, 1328, 1200 cm-1.
Anal. Calcd. for C12H12BrNO3: C, 48.34; H, 4.06; N,4.70.
Found: C, 48.40; H, 4.15; N, 4.53. Its NMR spectrum was
identical with that reported previously.30
A stirring solution of (R)-N-carbamyl-phenylalanine (12, 1.50
g, 7.20 mmol) in MeOH (100 mL) was treated with
Amberlyst-15 (1.52 g) and refluxed for 70 min. After cooling
to room temperature, the reaction mixture was filtered. The
organic layer was concentrated in vacuo and the residue was
purified through silica gel (12 g). Elution with AcOEt afforded
(R)-N-carbamyl-phenylalanine methyl ester 13 (quant.).
Recrystallization from hexane-AcOEt provided colorless
needles. Mp 103.2-103.7 °C; [a]D21 -66.9 (c 1.00, CHCl3); 1H
NMR (400 MHz, CDCl3) d 7.19 (m, 5H), 5.78 (d, 1H, J = 8.3
Hz), 4.76-4.69 (m, 3H), 3.69 (s, 3H), 3.07 (dd, 1H, J = 5.6,
13.9 Hz), 2.97 (dd, 1H, J = 6.6, 13.9 Hz); IR (KBr) 3413,
3327, 3214, 3026, 2956, 1726, 1650, 1542, 1391, 1255 cm-1.
Anal. Calcd. for C11H14N2O3: C, 59.45; H, 6.35; N, 12.60.
Found: C, 59.49; H, 6.31; N, 12.54.
To a vigorously stirred suspension of LiAlH4 (23 mg, 0.61
mmol) in dry THF (1.5 mL) was added, a solution of 13 (100
mg, 0.45 mmol) in dry THF (2.5 mL) dropwise. After stirring
for 40 min, the reaction was quenched with H2O, added
Na2SO4 (5 g) and lyophilized. The residue was charged in a
glass-made column and eluted with MeOH. The extract was
concentrated in vacuo to give 6a (70% yield). 6a:1H NMR
(270 MHz, D2O) d 7.31 (m, 5H), 3.88 (br s, 1H), 3.63 (dd, 1H,
J = 4.9, 11.5 Hz), 3.51 (dd, 1H, J = 6.6, 11.5 Hz), 2.89 (dd,
1H, J = 4.9, 13.7 Hz), 2.64 (dd, 1H, J = 9.0, 13.7 Hz). This
was further derived to 1a: mp 88.0-88.5 °C; [a]D24+64.1 (c
1.00, CHCl3).
(26) PPh3 (97 mg, 0.37 mmol) was added to a solution of the
bromoacetamide (15b, 109 mg, 0.37 mmol) in CH2Cl2 (4.0
mL), and the mixture was stirred at room temperature
overnight. The reaction mixture was concentrated in vacuo
and the residue was dissolved in hot-water (150 mL). After the
addition of 4M NaOH (120 mL), the mixture was immediately
extracted with AcOEt and the organic layer was washed with
brine. The organic layer was dried over Na2SO4 and
concentrated in vacuo afforded 14 (83% yield in 2 steps) as
colorless solid. Mp 168.2-170.2 °C; [a]D20 -41.3 (c 1.09,
CHCl3); 1H NMR (400 MHz, CDCl3) d 7.70 (m, 6H), 7.52 (m,
9H), 7.25 (m, 5H), 4.80-4.74 (m, 2H), 4.10 (m, 1H), 4.02 (dd,
1H, J = 2.7, 8.8 Hz), 3.28 (dd, 1H, J = 3.3, 13.2 Hz), 2.83 (dd,
1H, J = 9.3, 13.2 Hz); IR (KBr) 3059, 1753, 1595, 1438, 1362,
1343, 1192, 1106 cm-1.
(27) Bull, S. D.; Davies, S. G.; Jones, S.; Polywka, M. E. C.;
Prasad, R. S.; Sanganee, H. J. Synlett 1998, 519-521.
(28) Akiba, T.; Tamura, O.; Hashimoto, M.; Kobayashi, Y.; Katoh,
T.; Nakatani, K.; Kamada, M.; Hayakawa, I.; Terashima, S.
Tetrahedron 1994, 50, 3905-3914.
(29) (a) Yamada, H.; Takahashi, S.; Kii, Y.; Kumagai, H. J.
Ferment. Technol. 1978, 56, 484-491. (b) Takahashi, S.;
Ohashi, T.; Kii, Y.; Kumagai, H.; Yamada, H. J. Ferment.
Technol. 1979, 57, 328-332.
(22) Ohi, K.; Shima, K.; Hamada, K.; Saito, Y.; Yamada, N.; Ohba,
S.; Nishiyama, S. Bull. Chem. Soc. Jpn. 1998, 71, 2433-2440.
(23) A mixture of 1a (1.00 g, 5.64 mmol) and chloroacetic
anhydride (4.82 g, 28.2 mmol) was heated at 120 °C and
stirred overnight. After cooling to room temperature, the
reaction mixture was poured into a mixture of pyridine (4.11
mL, 50.8 mmol) and catalytic amounts of DMAP in ice, and
stirred for the decomposition of excess chloroacetic
(30) Evans, D. A.; Weber, A. E. J. Am. Chem. Soc. 1987, 109,
7151-7157. They reported the sign of rotation to be plus for
(R)-15b. We confirmed its minus sign of the rotation of (R)-
15b, by the convertion to (R)-(+)-1a by hydrolysis.31
(31) Evans, D. A.; Britton, T. C.; Ellman, J. A. Tetrahedron Lett.
1987, 28, 6141-6144.
anhydride. Conventional workup and chromatographic
separation [hexane-AcOEt (4:1)] afforded 15a (97% yield).
Recrystallization from Et2O-CHCl3 provided colorless
Article Identifier:
1437-2096,E;2000,0,02,0189,0192,ftx,en;Y21099ST.pdf
Synlett 2000, No. 2, 189–192 ISSN 0936-5214 © Thieme Stuttgart · New York