Stereoselective Synthesis of Tetrahydrofurans and Linear Methyl Enol‐Ethers from Glycals.

TLDR: The O -benzyl derivatives of 1,5-anhydro-2-deoxy-d - arabino -hex-1-enitol (d -glucal, 1,7 and 9) showed the same reactivity as the protected glycals by action of the TTN-NaBH 4 reagent in MeOH, resulting selectively in the ring contracted compounds at the glycal moiety as discussed by the authors.

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.