Cesium carbonate → 4-Methoxybenzyl 2-[(3S,4S)-4-t-Butyldimethylsilyloxymethyl-2-oxo-3-tritylaminoazetidin-1-yl]acetate

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Cesium carbonate (1.58 g) was added to a stirred solution of (3S,4S)-4-t-butyldimethylsilyloxymethyl-3-tritylaminoazetidin-2-one (1.45 g) (IV) (H. Mastalerz et al., J. Med. Chem., 1988, 31, 1190-1196) in a mixture of acetonitrile (44 ml) and dimethylformamide (35 ml). 4-Methoxybenzyl bromoacetate (1.44 g) was then added and the mixture stirred at room temperature for 48 h. The mixture was then partitioned between ethyl acetate and water, the organic phase was washed four times with water, and then with brine. The solution was dried over magnesium sulphate and evaporated. The title compound (2.29 g) was isolated by column chromatography of the residue (Silica gel, 3:1 hexane:ethyl acetate as eluent). [α]D21 -9.0°; νmax (CHCl3) 3352, and 1755 cm-1 ; δ(CDCl3) -0.04 (3H, s), 0.00 (3H, s), 0.88 (9H, s), 1.69 (1H, br s), 2.54 (1H, dd, J 3.33 and 11.68 Hz), 3.19 (1H, dd, J 2.05 and 11.68 Hz), 3.38 (1H, m), 3.51 (1H, d, J 17.98 Hz), 3.90 (3H, s), 4.46 (1H, d, J 17.98 Hz), 4.54 (1H, m), 5.06 (1H, d, J 11.79 Hz), 5.15 (1H, d, J 11.81 Hz), 6.94 (2H, d, J 8.68 Hz), 7.2-7.65 (17H, m).

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