Stereoselective Pd-catalyzed synthesis of quaternary α-d-C-mannosyl- (S)-amino acids

摘要

In this paper, we report the stereoselective synthesis of α-d-C-mannosyl-(S)-amino acids exploiting, as a key step, an allylic alkylation of glycal-derived π-allyl Pd(II) intermediates, prepared by oxidative addition of Pd(0) species to 2,3-unsaturated pyranosides (pseudoglycals). The reaction of 4,6-di-O-acetyl α-pseudoglucal carbonate 10a with racemic alanine-, valine-, and phenylalanine-derived azlactones gave the corresponding (4S)-4-α-d-C-mannosyl-2-phenyloxazol-5(4H)-ones as the major diastereoisomers in high yields. The final α-d-C-mannosyl-(S)-amino acids were obtained in a few steps comprising highly diastereoselective dihydroxylation of the glucal derivative double bond followed by the one-pot hydrolysis of the benzamido and acetate protecting groups. Main features of this method are the conciseness of the synthetic sequence, the high diastereoselection of the allylic alkylation step, the use of racemic α-amino acids as starting material, and the good overall yields.