The lithium enolates of ethyl butyrate (7) and ethyl isovalerate (19) react with N-p-methoxyphenylcinnamaldimine (8) in tetrahydrofuran (THF) - hexamethylphosphoric triamide (HMPA) to afford predominantly trans β-lactams 9 (67%) and 20 (78%), respectively. β-Lactam 9 was converted to PS-5 (5) intermediate 18 in 21% overall yield (8 steps). β-Lactam 20 was converted to PS-6 (6) analog 28 in 22% overall yield using an eight step sequence.
