Abstract: | 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. |