964
N. Al Shaye et al.
LETTER
n-Butyllithium (0.36 mL, 2.5 M in hexane, 0.90 mmol) was
added to a stirred solution of 4-phenyloxazolidin-2-thione
(S)-4 (0.15 g, 0.84 mmol) in THF (5 mL) at –78 °C. After
stirring for 1 h, a solution of pentafluorophenyl 2-phenyl-
propionate [(rac)-2, 0.26 g, 0.84 mmol] in THF (1 mL) was
added. The resulting mixture was stirred for 2 h at –78 °C.
The reaction was quenched with H2O (10 mL). The organic
layer was extracted with CH2Cl2 (2 × 10 mL), dried
(MgSO4), and evaporated under reduced pressure to give a
mixture of diastereomeric oxazolidin-2-thiones syn-5 and
anti-5 (ratio 94:6 syn/anti). The crude residue was purified
by flash chromatography on SiO2 eluting with light PE (bp
40–60 °C)–Et2O (7:3) to give the oxazolidin-2-thione (R,S)-
syn-5 (77 mg, 30%) as a white solid and the pentafluoro-
phenyl 2-phenylpropionate (S)-2 (0.123 g, 47%) as a
colorless liquid.
Oxazolidin-2-thione (R,S)-syn-5: Rf = 0.67 [light PE (bp
40–60 °C)–Et2O, 1:1]; mp 87–89 °C [lit.6 (S,R) 84–86 °C];
[a]D20 +66.1 (c 3.6, CHCl3) [lit. (S,R) [a]D20 –58.3 (c 4.0,
CHCl3)]. IR (CHCl3): nmax = 1708 (C=O), 1216 (C=S) cm–1.
1H NMR (400 MHz, CDCl3): d = 7.20–7.08 (6 H, m, 6 × CH,
PhA and PhB), 6.94 (2 H, dt, J = 6.9, 1.8 Hz, 2 × CH, PhA),
6.88 (2 H, dt, J = 7.0, 1.8 Hz, 2 × CH, PhB), 5.98 (1 H, q, J =
6.9 Hz, PhCHCH3), 5.62 (1 H, dd, J = 9.2, 6.1 Hz, PhCHN),
4.68 (1 H, t, J = 9.2 Hz, CHAHBO), 4.20 (1 H, dd, J = 9.2, 6.1
Hz, CHAHBO), 1.35 (3 H, d, J = 6.9 Hz, PhCHCH3).
13C NMR (100 MHz, CDCl3): 185.2 (C=S), 174.8 (C=O),
139.1 and 136.9 (2 × i-C; 2 × Ph), 128.8,2 128.7,1 128.5,2
128.3,2 127.11 and 126.42 (10 × CH, 2 × Ph), 73.6 (CH2O),
62.6 (PhCHN), 43.9 (PhCHCH3), 18.7 (PhCHCH3). HRMS:
m/z calcd for C18H18NO2S [MH+]: 312.1053; found:
312.1054.
Pentafluorophenyl 2-phenylpropionate (S)-2: Rf = 0.63 [light
PE (40–60 °C)–Et2O, 9:1]; [a]D20 +40.8 (c 4.6, CHCl3) {ca.
55% ee based on lit.10 (S) [a]D20 +74.5 (c 4.9, CHCl3); lit.11
(R) [a]D20 –75.0 (c 3.3, CHCl3)}. IR (film): nmax = 1784
(C=O) cm–1. 1H NMR (400 MHz, CDCl3): d = 7.41–7.28 (5
H, m, 5 × CH, Ph), 4.07 (1 H, q, J = 7.2 Hz, CH3CH), 1.64
(3 H, d, J = 7.2 Hz, CH3CH). 13C NMR (100 MHz, CDCl3):
d = 170.6 (OC=O), 141.1 [142.40 and 139.90, 2 C, ddt,
1JC,F = 251.3 Hz, 2JC,F = 12.2 Hz, 3JC,F = 3.8 Hz, C(2)-F],
139.4 [140.70 and 138.18, 1 C, dtt, 1JC,F = 253.2 Hz,
2JC,F = 13.4 Hz, 3JC,F = 4.2 Hz, C(4)-F], 138.7 (i-C, Ph),
137.8 [139.05 and 136.58, 2 C, dtdd, 1JC,F = 249.1 Hz,
2JC,F = 14.5 Hz, 3JC,F = 5.7 Hz, 4JC,F = 3.1 Hz, C(3)-F],
128.9, 127.8, 127.5 (3 × CH, Ar), 125.2 (1 C, tdt, 2JC,F = 14.2
Hz, 4JC,F = 4.2 Hz, 3JC,F = 2.0 Hz, i-CO, OC6F5), 45.1
(PhCH), 18.5 (CH3CH). 19F NMR (378 MHz, CDCl3):
d = –152.6 (2 F, d, 3JF,F = 20.9 Hz, Fortho), –157.9 (1 F, t,
3JF,F = 20.9 Hz, Fpara), –162.3 (2 F, t, 3JF,F = 20.9 Hz, Fmeta).
HRMS: m/z calcd for C15H9F5O2 [M+]: 316.0517; found:
316.0514.
Synlett 2009, No. 6, 960–964 © Thieme Stuttgart · New York