β-Lactams derived from phenylalanine and homologues: effects of the distance between the aromatic rings and the α-stereogenic reactive center on the memory of chirality

The enantioselectivity in the base-promoted cyclization of N-chloroacetyl derivatives of Phe, Phg, and Hph is dependent on the side-chain length, with the best results for Phg analogues (up to 74% ee). In contrast, shortening of the N-substituent, from a (p-methoxy)benzyl group to a (p-methoxy)phenyl moiety, led to a decrease in selectivity.