Benzyl 6-benzyl-7-oxo-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylate (138) (0.420 g) was dissolved in dry dimethylformamide (5 ml) and treated with ethanethiol (0.12 ml) and potassium carbonate (0.087 g). It was stirred at ambient temperature for 1 hour and the solvent was then distilled off under high vacuum. The residue was partitioned between ethyl acetate and brine, and the ethyl acetate solution was dried over sodium sulphate and concentrated. The residue was carefully chromatographed on a column of silica gel 60 (<230 mesh) eluting with ethyl acetate/60°-80° petroleum ether 3:7 to give three diastereoisomers of benzyl 6-benzyl-3-ethylthio-7-oxo-1-azabicyclo[3.2.0]heptane-2-carboxylate as gums. The first compound in increasing order of polarity was the 2α, 3α-isomer (139) (0.054 g); νmax (CHCl3) 2950, 1765 and 1745 sh; τ(CDCl3) 2.6-2.9 (10H, m, phenyls), 4.86 (2H, s, OCH2), 5.59 (1H, d J 5 Hz, C2-H), 6.1-6.5 (2H, m, C3-Hand C5-H), 6.6-7.2 (3H, m, C6-H), 7.50 (2H, q J 7 Hz, SCH2), 7.55 (1H, dt J 14 and 7 Hz, C4-H), 8.41 (1H, dt J 14 and 7 Hz, C4-H) and 8.84 (3H, t J 7 Hz, CH3); (M+ at m/e 395.1554. C23H25NO3S requires 395.1553). Next was the 2α, 3β-isomer (98) (0.260 g); νmax (CHCl3) 2960, 1765 and 1750 sh cm-1 ; τ(CDCl3) 2.6-2.9 (10H, m, phenyls), 4.87 (2H, s, OCH2), 5.26 (1H, d, J 7 Hz, C2-H), 6.10 (1H, td, J 51/2 & 21/2 Hz, C5-H), 6.52 (1H, td J9 and 7 Hz, C3-H), 6.7-7.2 (3H, m, C6-H, C8-H2), 7.47 (2H, q, J7 Hz, SCH2), 7.89 (2H, dd J9 and 51/2 Hz, C4-H2) and 8.85 (3H, t J7 Hz, CH3); (M+ at m/e 395.1553. C23H25NO3S requires 395.1553). Lastly was obtained the 2β, 3α -isomer (140) (0.112 g); νmax (CHCl3) 2960, 1770 and 1745 cm-1 ; τ(CDCl3) 2.6-2.9 (10H, m, phenyls), 4.82 (2H, s, OCH2), 5.94 (1H, d J8 Hz, C2-H), 6.2-7.2 (5H, m, C3-H, C5-H, C6-H and C8-H2), 7.51 (2H, d J7 Hz, SCH2), 7.83 (1H, dt J12 and 6 Hz, C4-H), 8.10 (1H, td J12 and 9 Hz, C4-H) and 8.86 (3H, t J7 Hz, CH3); (M30 at m/e 394/1560. C23H25NO3S requires 395.1553).
