To a solution of (3aR*,7S*,7aS*)-7-(4-benzyloxycarbonylamino-3-fluoro-benzyl)-2,2-dimethyl-tetrahydro-1-oxa-5-thia-3-aza-indene-3-carboxylic acid tert-butyl ester (54.1 g, 102 mmol) in THF (500 mL) was added AcOH (200 mL) and H2O2 (27.8 g, 50% in H2O, 408 mmol) at 0-5° C. and the reaction mixture was stirred for 16 h at 25° C. The reaction mixture was slowly added to a cold 25% aq. Na2S2O3 solution, stirred for 3 h at 0-5° C. and extracted with EtOAc. Combined organic layers were washed with water, saturated K2CO3 solution and brine, dried over MgSO4, filtered and evaporated. The crude product was filtered through a short plug of ammonia deactivated silicagel and the title compound was obtained as a 3:2 mixture of the (3aR*,5R*,7S*,7aS*)- and (3aR*,5S*,7S*,7aS*)-diastereoisomer after crystallization from Et2O-hexane: TLC (EtOAc-MeOH 19:1) Rf=0.51; HPLC RtA=2.34 min and 2.38 min; ESIMS [M+H]+=547. Separation of the racemic 3:2 mixture on Chiralpak IC (heptene-EtOH 85:15) and on Chiralpak AD-I (hexane-EtOH-CHCl3 80:10:10) provided the diastereoisomers (3aR,5R,7S,7aS)-7-(4-benzyloxycarbonylamino-3-fluoro-benzyl)-2,2-dimethyl-5-oxo-hexa-hydro-1-oxa-5lambda*4*-thia-3-aza-indene-3-carboxylic acid tert-butyl ester (peak 1 on Chiralpak IC) in >98% ee: [α]D−21.5° (c 1, CHCl3); 1H NMR (600 MHz, DMSO-d6): δ 9.43 (s, 1H), 7.56 (s, 1H), 7.40 (m, 4H), 7.33 (t, 1H), 7.08 (d, 1H), 6.99 (d, 1H), 5.14 (s, 2H), 3.6-3.8 (m, 2H), 3.40 (t, 1H), 2.92 (m, 2H), 2.6-2.8 (m, 3H), 2.37 (t, 1H), 1.55 (s, 3H), 1.50 (s, 3H), 1.41 (s, 9H) and (3aR,5S,7S,7aS)-7-(4-benzyloxycarbonylamino-3-fluoro-benzyl)-2,2-di-methyl-5-oxo-hexahydro-1-oxa-5lambda*4*-thia-3-aza-indene-3-carboxylic acid tert-butyl ester (peak 2 on Chiralpak IC): 1H NMR (600 MHz, DMSO-d6): δ 9.44 (s, 1H), 7.57 (s, 1H), 7.40 (m, 4H), 7.35 (t, 1H), 7.08 (d, 1H), 6.99 (d, 1H), 5.14 (s, 2H), 3.9-4.2 (m, 1H), 3.65 (t, 1H), 2.5-3.5 (m, 6H), 2.25 (m, 1H), 1.51 (s, 6H), 1.42 (s, 9H) followed by the (3aS,5R,7R,7aR)- and (3aS,5S,7R,7aR)-diastereoisomers (peak 3 and peak 4).
