Synthesis of an α‑Amylase Inhibitor: Highly Stereoselective Glycosidation and Regioselective Manipulations of Hydroxyl Groups in Carbohydrate Derivatives

Here, we describe the efficient synthesis of α-amylase inhibitor 1. To introduce the most expensive C ring unit at a late stage in the synthesis, we developed 1,2-cis-O-glycosidation of AB and C ring intermediates. Taking advantage of the effect of non-neighboring protecting groups, reaction solvents, and temperature for the glycosidation led to high stereoselectivity and high yield of the 1,2-cis-glycoside product bearing the API skeleton. We also explored protection and deprotection methods for regioselective manipulation of hydroxyl groups in A and B ring intermediates. One-pot benzylation of the 2,3-hydroxyl groups of d-glucose under phase-transfer conditions and regioselective anomeric deacetylation with N-methylpiperazine were developed for the syntheses of A, B, and AB ring intermediates. Thus, the efficiency of the process was dramatically improved. The raw material cost of API was reduced to approximately one-third that of the original route, and the total process was decreased by six steps.