Stereoselective (2-naphthyl)methylation of sugar hydroxyls by the hydrogenolysis of diastereoisomeric dioxolane-type (2-naphthyl)methylene acetals Original Research Article

The cis axial/equatorial OH groups of methyl α-l- and ethyl 1-thio-α-l-rhamnopyranoside, 1,6-anhydro-β-d-mannopyranose, and 1,6-anhydro-β-d-galactopyranose were reacted with 2-naphthaldehyde dimethyl acetal to diastereomeric dioxolane-type 2,3-O-(2-naphthyl)methylene or 3,4-O-(2-naphthyl)methylene acetals. The glycosides yielded the exo- and endo-isomers in nearly 1:1 ratio, 1,6-anhydro-β-d-mannopyranose gave predominantly the endo-, and 1,6-anhydro-β-d-galactopyranose exclusively endo-isomer. The acetals and some of their fully protected derivatives bearing benzyl or tert-butyldimethylsilyl groups were hydrogenolised with AlH3 (3LiAlH4-AlCl3) or with Me3N·BH3–AlCl3 reagents. The endo-isomers were cleaved by both reagents to give axial NAP ethers, the exo-isomers of pyranosides furnished equatorial NAP ethers. However, the exo-isomers of pyranoses gave irregular axial ethers with a >30-fold enhancement of the reaction rates with respect to the endo-isomer.