Highly Diastereoselective Synthesis of 2-(1-N-Boc-aminoalkyl)thiazole-5-carboxylates by Reduction of tert-Butylsulfinyl Ketimines.

Positional isomers of naturally occurring peptide subunits were synthesized via highly diastereoselective reduction of tert-butylsulfinyl ketimines as a key reaction. While NaBH4 reduction of ketimines derived from 2-thiazolyl ketones afforded the (RS,R)-isomer with moderate diastereoselectivity, L-Selectride® reduction afforded the (RS,S)-isomer as the sole product. In contrast, ketimines derived from tert-butyl 2-thiazolyl ketone afforded the (RS,R)-isomer with low diastereoselectivity by both NaBH4 and L-Selectride® reduction. Stereochemistry of the reaction was discussed based on calculation of the conformational energies for ketimines.