On the synthesis of vinyl and phenyl C-furanosides by stereospecific debenzylative cycloetherification

Open-chain benzyl-ether-protected polyols in which one of the alcohols is either allylic or benzylic are synthesised by addition of organometallic vinyl or phenyl reagents to benzyl-ether-protected carbohydrate hemiacetals. The diastereoselectivity of addition is dependent on whether a Grignard reagent or a trialkylzincate reagent is used. The open-chain compounds undergo a stereospecific cyclisation reaction on treatment with catalytic strong Brønsted acid with heating to form tetrahydrofurans with inversion of configuration at the allylic or benzylic carbon (C-1) and loss of hydroxyl from this position. A short synthesis of the starting material, 1,2,4,6-tetra-O-benzyl-d-galactose, from lactose is described.