Oxaphosphoranylation of Methyl α-d-Glucopyranoside with Diethoxytriphenylphosphorane: Highly Stereoselective Approach to Anhydropyranosides

Diethoxytriphenylphosphorane (DTPP) efficiently oxaphosphoranylates methyl α-d -glucopyranoside to afford two unstable isomeric 1,3,2λ⁵-dioxaphospholanes. Thermolysis (60–90°C) of these 1,3,2λ⁵-dioxaphospholanes affords only two of four possible isomeric anhydropyranosides. Two distinct intermediate betaines and their unique “pre-transition state” chair or twist-boat structures are used to rationalize the regiochemistry-attending cyclodehydration. The decomposition of these rapidly interconverting 1,3,2λ⁵-dioxaphospholanes is accelerated in the presence of lithium bromide (LiBr) to afford a single anhydropyranoside.