2,2-Difluoro-1,3,2-oxazaborolidin-5-ones: novel approach for selective side-chain protection of serine and threonine

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 yields and in pure form using highly reactive reagents.