Nucleophilic assistance in the cyclisation of aldose acetals

Abstract

The dimethyl acetals of D-glucose, D-galactose, and L-arabinose undergo ring closure to furanosides concurrent with their hydrolyses in dilute aqueous hydrochloric acid. The rates of the ring closure reactions are substantially greater than the rates of ionisation of a hexanal dimethyl acetal as calculated by the use of the Taft ρ*σ* relationship and of the rate of hydrolysis of D-glyceraldehyde dimethyl acetal. It is therefore concluded that the ring closures are concerted processes with the internal hydroxy-groups providing nucleophilic assistance for the acid-catalysed rupture of acetal bonds. The acid-catalysed ring closures of the acetals to furanosides in methanolic solution were also investigated and thought to be concerted processes.