Preparation of (2-naphthyl)methylene acetals of glycosides and their hydrogenolytic transformation into 2-naphthylmethyl (NAP) ethers

Dioxane- and dioxolane-type (2-naphthyl)methylene acetals of glycosides wer e prepared by acid-catalyzed transacetalization reactions. The acetals were cleaved using either AlH3 (LiAlH4:AlCl3=3:1), NaCNBH3-HCl or BH3 . Me3N-Al Cl3 reagents. Reaction of the 4,6-O-acetals with AlH3 yielded 4-ONAP ethers whereas the other two reagents gave 6-ONAP derivatives with excellent regi oselectivity. In dioxolane-type acetals the direction of the cleavage with all three reagents is determined by the stereochemistry of the acetal cente r; equatorial ONAP/axial-hydroxy derivatives are obtained from the exo-naph thyl isomers; endo-naphthyl acetals, on the other hand, give rise to the fo rmation of axial ONAP/equatorial-hydroxy compounds. The NAP ether and the ( 2-naphthyl)methylene acetal protecting groups can both be readily removed b y treatment with DDQ. (C) 2000 Elsevier Science Ltd. All rights reserved.