Organocatalysis for the Acid‐Free <i>O</i>‐Arylidenation of Carbohydrates

Abstract

Abstract Methyl glycopyranosides of glucose, galactose, and mannose, their 2,3‐di‐ O ‐benzyl‐protected derivatives, as well as the unprotected sugars react with p ‐methoxybenzaldehyde dimethyl acetal ( 3 ) and with benzaldehyde dimethyl acetal ( 7 ) as reagents in the presence of thiourea 1 or squaramide 2 as the organocatalyst to afford regioselectively 4,6‐ O ‐arylidenated derivatives 5 and 8 . With an excess amount of 3 or 7 , diarylidenated derivatives are also obtained. In situ formation of acetals of type 3 and 7 from corresponding aldehydes 10 and 13 in the presence of an orthoester and organocatalyst 1 or 2 can be used to generate 5 and 8 directly from the aldehydes. Some substrates also lead to mixed orthoesters with this procedure. The reaction courses are discussed.