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Preparation of 4a: To a stirred mixture of 3b (660 mg, 1.87 mmol), urea hydrogen peroxide (790 mg, 8.40 mmol), and Na2HPO4 (1.12 g, 7.89 mmol) in CH2Cl2 (10 mL) was added (CF3CO)2O (784 mg, 3.73 mmol) at 0 °C. The mixture was allowed to warm to r.t. and stirred
for 4 h. The reaction mixture was washed with a sat. aq solution of NaHCO3, a 5% solution of NaHSO3, and finally H2O. The separated organic layer was concentrated and MTBE (15 mL) was added to the
residue. The mixture was stirred for 5 h. The solid formed was filtered and washed
with MTBE (2 mL). The cake was dried over a stream of nitrogen to give 4a (615 mg, 86.1%) as a white solid; 4a t
R 31.4 min, ent
-4a t
R 40.8 min (Chiralpak® AD-H; 35 °C; 10% i-PrOH-hexane; 1 mL/min; 250 nm); ee of isolated 4a ³99.9%, while the ee of the filtrate was 50%, which reflects highly selective recrystallization.
1H NMR (CDCl3, 400 MHz): δ = 7.38-7.18 (10 H, m), 5.12 (1 H, d, J = 12.4 Hz), 5.09 (1 H, d, J = 12.4 Hz), 4.95 (1 H, br), 3.71 (1 H, m), 3.67 (3 H, s), 3.32 (1 H, m), 3.07 (1
H, m), 3.01 (1 H, dd, J = 8.0, 4.4 Hz), 2.85 (1 H, dd, J = 13.2, 8.0 Hz), 2.38 (1 H, dd, J = 16.8, 7.6 Hz), 2.03 (1 H, dd, J = 17.2, 4.8 Hz); 13C NMR (CDCl3, 125 MHz): δ = 170.8, 155.8, 136.6, 136.5, 129.6, 128.7, 128.6, 128.2, 128.1, 127.0,
67.0, 57.6, 53.6, 52.0, 51.8, 39.5, 33.3.
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<A NAME="RU28005ST-25">25</A>
Benzyl (1
S
)-1-[(2′
S
)-5′-oxotetrahydrofuran-2′-yl]-2-phenylethylcarbamate (
6): [α]D
25 = -9.6 (CHCl3, c 1); 1H NMR (CDCl3, 500 MHz): δ = 7.34-7.25 (10 H, m), 5.09 (1 H, d, J = 12.4 Hz), 5.05 (1 H, d, J = 12.4 Hz), 4.87 (d, 1 H, J = 10.0Hz), 4.48 (1 H, t, J = 7.8 Hz), 4.07 (dd, 1 H, J = 17.5, 8.7 Hz), 2.98 (1 H, dd, J = 13.3, 6.9 Hz), 2.90 (1 H, dd, J = 13.3, 8.7 Hz), 2.48 (dd, 2 H, J = 9.7, 7.4 Hz), 2.16-2.03 (2 H, m); 13C NMR (CDCl3,125 MHz): δ = 177.2, 156.7, 137.1, 136.4, 129.4, 128.8, 128.6, 128.2, 128.0, 126.9,
79.9, 54.9, 39.3, 28.7, 24.1; MS: m/z = 340 [M + H].
<A NAME="RU28005ST-26">26</A>
Casado-Bellver FJ.
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Amino-ester side-product (Figure
[1]
): 1H NMR (CDCl3, 400 MHz): δ = 7.34-7.28 (5 H, m), 3.57 (3 H, s), 3.51 (1 H, m), 3.24 (1 H, dd, J = 13.6, 5.6 Hz), 2.95 (1 H, dd, J = 13.6, 8.8 Hz), 2.32 (2 H, m), 1.95-1.69 (4 H, m); MS: m/z = 222 [M + H].
<A NAME="RU28005ST-28">28</A>
Lactam side-product (Figure
[2]
): 1H NMR (CDCl3, 400 MHz): δ = 7.35-7.17 (5 H, m), 5.65 (1 H, br), 3.61 (1 H, m), 2.88 (1 H, dd,
J = 13.6, 5.2 Hz), 2.62 (1 H, dd, J = 13.6, 9.2 Hz), 2.43-2.29 (2 H, m), 1.94 (2 H, m), 1.68 (1 H, m), 1.47 (1 H, m);
MS: m/z = 190 [M + H] .
<A NAME="RU28005ST-29">29</A>
Benzyl (1
S
)-1-[(2′
R
)-5′-oxotetrahydrofuran-2′-yl]-2-phenylethylcarbamate (
10): [α]D
25 = -8.6 (CHCl3, c 0.02); 1H NMR (CDCl3, 400 MHz): δ = 7.37-7.18 (10 H, m), 5.04 (2 H, s), 4.66 (1 H, br), 4.40 (1 H, m),
4.05 (1 H, m), 3.03 (1 H, dd, J = 14.4, 4.4 Hz), 2.90 (1 H, m), 2.52 (2 H, m), 2.25 (1 H, m), 2.11 (1 H, m); 13C NMR (CDCl3, 100 MHz): δ = 176.6, 165.9, 136.1, 132.7, 129.5, 128.8, 128.6, 128.3, 128.0, 127.0,
80.4, 60.4, 50.0, 30.9, 28.2, 24.7; MS: m/z = 340 [M + H] .