C-Glycosides by Aqueous Condensation of β-Dicarbonyl Compounds with Unprotected Sugars

Abstract

Condensation of 1,3-diketones with unprotected aldoses in water, under mildly alkaline conditions, provides a convenient and effective route to C-glycosidic ketones in high yields. Reactions usually proceed with high β`anomeric' stereoselectivity because product composition is determined by thermodynamic control. Mechanistically, the condensation follows a typical Knoevenagel scheme, after which an intramolecular oxa-Michael cyclization determines C-glycoside configuration by following an exo- or endo-trig pathway. With open-chain 1,3-diketones, an ensuing retro-Claisen fragmentation occurs to furnish simple glycosyl monoketones. The C-glycosides then slowly further dehydrate under the reaction conditions to give 2,5-disubstituted furans.