Chiral 2-C-methylene glycosides and carbohydrate-derived pyrano[2,3-b][1]benzopyrans: synthesis via InCl3 catalyzed stereoselective Ferrier rearrangement of 2-C-acetoxymethyl glycal derivatives

2-C-Acetoxymethyl glycal derivatives react with aliphatic alcohols in the presence of InCl₃ (30 mol %) to furnish the corresponding 2-C-methylene glycosides in excellent yields and with exclusive α-selectivity except for the methyl 2-C-methylene glycosides, which are formed in ∼2:1 anomeric ratio in favour of the α-anomer. The reaction of 2-C-acetoxyglycals with phenols, however, produces the corresponding chiral carbohydrate-derived pyranobenzopyran derivatives via initial Ferrier rearrangement followed by tandem cyclization in excellent yields and moderate to high stereoselectivities in favour of the corresponding 10a-R-pyrano2,3-b]1]benzopyran derivatives.