Highly convergent stereoselective synthesis of chiral key intermediates in the synthesis of Palinavir from imines derived from l-glyceraldehyde

Imines derived from O-protected (S)-glyceraldehyde are valuable intermediates in the synthesis of different kinds of amino acids. We have developed a highly convergent and stereoselective method to obtain (2S,3S)-N-tert-butoxycarbonyl-1-phenyl-3,4-epoxy-2-butylamine and (2S,4R)-N-tert-butoxycarbonyl-4-hydroxypipecolic acid tert-butylamide, which are key intermediates in the synthesis of Palinavir, that consist in the treatment of the appropriate imine with benzylmagnesium bromide and Danishefskyʹs diene, respectively, and subsequent transformation of the obtained adducts into the desired compounds. The reaction of N-benzylimine derived from (S)-2,3-di-O-benzylglyceraldehyde with benzylmagnesium bromide is completely diastereoselective at low temperature. Hetero Diels–Alder reaction of imine derived from (S)-2,3-di-O-benzylglyceraldehyde and (R)-N-α-methylbenzylamine is completely diastereoselective at low temperature in the presence of ZnI2.