In this study, a highly stereoselective preparation of (3S,4S)-3-[(1R)-1-tert-butyldimethylsilyloxyethyl]-4-[(1R)-1-carboxy-ethyl]-azetidin-2-one (2) is reported. It involves a Lewis acid mediated condensation of (3S,4R)-4-acetoxy-3-[(1R)-1-tert-butyldimethylsilyloxyethyl]azetidin-2-one (3) with simple O-silylenol ethers of thiopropionates. From all the examples reported in this paper the 2-picolyl thiopropionate or similar arrangements were found to be essential for this stereoselection. Finally, a mechanism involving chelation control seems to be operative.
The 4-chloroazetidinone 10, a very reactive chiral intermediate for 1-β-methylcarbapenems (1b), was easily prepared and isolated as a solid crystalline compound from the corresponding sulfide 5. Another chiral intermediate (16d) bearing the 1-β-methyl moiety was prepared by stereoselective aldol type condensation of either azetidinone 4 or 10 with metal enolates 14 (R = S—Bu′, M = ZrCp2Cl, [Formula: see text], and SnBr). The β/α ratios were 9:1, 3:1, 2:1 respectively.