AbstractAn asymmetric transformation of racemict‐butyl 2‐(3,4‐O‐carbonyldioxy‐phenyl)‐2‐(phthalimidooxy)acetate [(RS)‐2b] into one of its optically active forms was carried out by a combination of preferential crystallization of a desired enantiomer and the simultaneous racemization of the antipode. (R)‐2bwas easily racemized in diethylketone in the presence of a small amount of 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU). Under the conditions for racemization, the asymmetric transformation was achieved successfully to give (S)‐2bwith 84% optical purity in 80% yield. A potent antipseudomonal cephalosporin M‐14659 (1) was prepared from the pure (S)‐2bwhich was obtained by the recrystallization of the crude (