THIAZOLE-BASED SYNTHESIS OF FORMYL C-GLYCOSIDES

A method for the installation of the formyl group at the anomeric position of pyranoses and furanoses starting from the corresponding lactones has been developed. The strategy involves the addition of 2-lithiothiazole to the sugar lactone, followed by the silane reduction of the acetylated resultant ketol and the unmasking of the formyl group from the thiazole ring. All steps have been studied in some details to improve chemical efficiency and stereochemical control. Hence, reversed alpha:beta ratios of ketols were found in kinetic and thermodynamic mixtures, the former being consistent with a steric effect control of the substituents and the latter by the electronic effect of the ring oxygen. Seven sugar aldehydes with different D-pyranosidic (2,3,4,6-tetra-O-benzyl-gluco, -galacto, and -manno, 2-azido-3,4,6-tri-O-benzyl-2-deoxy-galacto) and D-furanosidic moieties (5-O-benzyl-2,3-isopropylidene-ribo; 2,3,5-tri-O-benzyl-ribo; 2,3:5,6-di-O-isopropylidene-manno) were prepared in 52-65% isolated overall yield from the corresponding lactone.