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

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–NaBH4 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–NaBH4 in MeOH, carried out on 16, 19 and 22, led to the formation of the open chain derivatives at the glycal site.