Methyl penicillanate has been synthesized by condensation ofD-penicillamine with three different variations of the synthon RO2CCH2CHO, followed by esterification of the resulting 4-carboxythiazolidine-2-acetic acid ester, removal of R, and Mukaiyama ring closure. The conditions for the synthesis of an analogous 4-carboxyoxazolidine-2-acetic acid ester, containing anN-benzoyl protecting group, have been worked out. The condensation of threonine with diethinylketone does not lead directly to a vinylogous oxapenam.