Stereoselective synthesis of tetrahydrofurans and linear methyl enol-ethers from glycals

Abstract

The O-benzyl derivatives of 1,5-anhydro-2-deoxy- d- arabino-hex-1-enitol ( d-glucal, 1), 1,5-anhydro-2,6-dideoxy- l- arabino-hex-1-enitol ( l-rhamnal, 7), and 1,5-anhydro-2-deoxy- d- lyxo-hex-1-enitol ( d-galactal, 9), underwent stereoselectively a ring contraction by treatment with thallium(III) nitrate (TTN) in MeOH, giving respectively the dimethylacetal derivatives of 3,4,6-tri- O-benzyl-2,5-anhydro- d-mannose, 3,4-di- O-benzyl-6-deoxy-2,5-anhydro- l-mannose ( 8) and 3,4,6-tri- O-benzyl-2,5-anhydro- d-talose ( 10). Conversely, the protected glycals 1, 7 and 9, underwent the ring opening reaction by action of the TTN–NaBH 4 reagent in MeOH, providing the enantiomerically pure methyl enol-ethers 3,4,6-tri- O-benzyl-2-deoxy-1- O-methyl- d- arabino-hex-1-enitol, 3,4-di- O-benzyl-2,6-dideoxy-1- O-methyl- l- arabino-hex-1-enitol and 3,4,6-tri- O-benzyl-2-deoxy-1- O-methyl- d- lyxo-hex-1-enitol. The perbenzylated glycosyl-glycals, such as 3,6-di- O-benzyl-4- O-(2,3,4,6-tetra- O-benzyl- β- d-glucopyranosyl)-1,5-anhydro-2-deoxy- d- arabino-hex-1-enitol (cellobial) ( 16), 3,6-di- O-benzyl-4- O-(2,3,4,6-tetra- O-benzyl- β- d-galactopyranosyl)-1,5-anhydro-2-deoxy- d- arabino-hex-1-enitol (lactal) ( 19) and 3,4-di- O-benzyl-6- O-(2,3,4,6-tetra- O-benzyl- α- d-galactopyranosyl)-1,5-anhydro-2-deoxy- d- arabino-hex-1-enitol (melibial) ( 22), showed the same reactivity as the corresponding glycals by reaction with TTN in MeOH, resulting selectively in the ring contracted compounds at the glycal moiety. The reaction with TTN–NaBH 4 in MeOH, carried out on 16, 19 and 22, led to the formation of the open chain derivatives at the glycal site.