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 were prepared by acid-catalyzed transacetalization reactions. The acetals were cleaved using either AlH3 (LiAlH4:AlCl3=3:1), NaCNBH3–HCl or BH3 Me3N–AlCl3 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 regioselectivity. In dioxolane-type acetals the direction of the cleavage with all three reagents is determined by the stereochemistry of the acetal center; equatorial ONAP/axial-hydroxy derivatives are obtained from the exo-naphthyl isomers; endo-naphthyl acetals, on the other hand, give rise to the formation of axial ONAP/equatorial-hydroxy compounds. The NAP ether and the (2-naphthyl)methylene acetal protecting groups can both be readily removed by treatment with DDQ.