Peptide aldehydes 15a-c are protocol for the preparation and utilizaprepared without epimerization from tion of THF solutions of 11, the isolation enantiomerically pure (S)-a-amino acids of air-sensitive intermediates can be (Scheme 3). Reductive pinacol homocoupling of 15a-c, induced by vanadium complex 11 or niobium complex 16 in refluxing THF, yields C,-symmetrical (S,R,R,S)-configurated 6a, 6 b and 2, respectively, with moderate to high stereoselectivity (Scheme 4). In a novel
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