Oxaphosphoranylation of methyl α-D-glucopyranoside with diethoxytriphenylphosphorane. A highly stereoselective route to anhydropyranosides
Abstract
Diethoxytriphenylphosphorane (DTPP) efficiently oxaphosphoranylates methyl α-D-glucopyranoside (1) to afford two isomeric 1,3,2λ5-dioxaphospholanes (2 and 3); thermolysis of the mixture of intermediates (2) and (3) affords methyl 2,3-anhydro-α-D-allopyranoside (4) and methyl 3,4-anhydro-α-D-galactopyranoside (5); however, in the presence of lithium bromide (LiBr), thermolysis of (2) and (3) is regiospecific, affording exclusively allopyranoside (4).