Reaction Steps
A solution of n-butyl lithium (1.64M in hexane, 0.73 ml) was added to a solution of diisopropylamine (0.17 ml) in dry tetrahydrofuran (THF) (8 ml) at -30° C. under dry argon. After stirring at -30° C. for 10 minutes the mixture was cooled to -76° C. and treated with a solution of 1-t-butyldimethylsilyl-4-tritylthioazetidin-2-one (1) (459 mg) (Bristol-Myers Patent GB No. 2042515 A) in dry THF (4 ml). After a further 15 minutes at -76° C. the stirred mixture was treated with a solution of 5-formylisothiazole (135 mg) (M. P. L. Caton et. al., J. Chem. Soc., 1964, 446) in dry THF (1 ml). After 20 minutes at -76° C. the stirred mixture was treated with saturated ammonium chloride solution (5 ml) and allowed to attain 0° C. The mixture was diluted with ethyl acetate (25 ml) and washed with brine (5 ml), dilute scdium bisulphite solution (5 ml) and brine (3×5 ml). The dried (MgSO4) organic layer was evaporated and the residue chromatographed on silica gel eluting with ethyl acetate/hexane mixtures to give two fractions. The less polar fraction contained a 3:1 mixture of a trans (Isomer A) and a cis-isomer (Isomer B) of the title azetidinone (2) (112 mg), a solid, νmax (CHCl3) 3600-3100, 1745 cm-1 ; δppm (CDCl3) 0.91 and 0.99 (9H, each s, ratio 3:1), 1.60-2.10 (1H, broad signal, exch. D2O), 3.35-3.56 (1.25H, m), 4.06 (0.75H, d, J3 Hz), 4.57 (0.75H, d, J2 Hz), 4.88 (0.25H, d, J5 Hz), 7.00-7.60 (16H, m), 8.29 (1H, d, J2 Hz), both Me3Si signals were obscured by the TMS signal. The more polar fraction contained a trans-isomer (Isomer C) of the title azetidinone (2) (144 mg), a solid, mp 183°-184° C. (prisms ex ethyl acetate/hexane); νmax (CHCl3) 3600-3100, 1735 cm-1 ; δppm (CDCl3) 0.81 (9H, s), 2.70-3.10 (1H, broad signal, exch. D2O), 3.70 (1H, dd, J7 and 2 Hz), 4.10 (1H, d, J2 Hz), 4.30-4.50br (1H, m, collapses to d, J7 Hz on exch. D2O), 6.81br (1H, s), 7.20-7.50 (15H, m), 8.22 (1H, d, J2 Hz), both Me3Si signals were obscured by the TMS signal. (Found: C, 67.1; H, 6.3; N, 4.9; S, 11.2. C32H36N2O2S2Si requires C, 67.1; H, 6.3; N, 4.9; S, 11.2%).
