... 3, 2c, C 6 H 4 -p-OCH 3, 3c, 77. ... D (Scheme 2). A final irreversible lactamization occurs most rapidly from oxazolidine D 1 through a chairlike six-membered transition state in which the propionate chain avoids repulsive interactions with R 1 (A 1,2 strain) and C 6 H 5 groups.(12). ...