2, 2-Difluoro-1, 3, 2-oxazaborolidin-5-ones 1, which are synthesized from BF3 and salts of amino acids, are highly effective, convenient and, moreover, inexpensive intermediates for the simultaneous protection of both α-amino and α-carboxy groups in α-amino acids. The new method streamlines the hitherto tedious procedures for side-chain protection of Ser and Thr. Ser (But), Thr (But), Ser (Bzl) and Thr (Bzl) are obtained by this procedure in high ...