Synthesis and properties of 2,3-anhydro-D-mannose and 3,4-anhydro-D-altrose

Treatment of benzyl 3,4,6-tri-O-acetyl-2-O-p-tolylsulphonyl-ß-D-glucopyranoside (5) with sodium methoxide in methanol gave benzyl 2,3-anhydro-ß-D-mannopyranoside (7) (65%) and benzyl 3,4-anhydro-ß-D-altropyranoside (8) (29%). Hydrogenolysis of the glycoside (7) in tetrahydrofuran over palladium-charcoal yielded crystalline 2,3-anhydro-ß-D-mannopyranose (9) which, in aqueous solution, rapidly underwent mutarotation to give an equilibrium mixture containing the a-pyranose (10) (23%), the ß-pyranose (9) (7%), the a-furanose (12) (65%). and the ß-furanose (11) (5%), whose composition was determined by g.l.c., g.l.c.-mass spectrometry, and n.m.r. spectroscopy; the assignment of anomeric configuration to the furanose isomers rests on conformational arguments. A similar mixture was obtained by hydrogenolysis of benzyl 2,3-anhydro-a-D-mannopyranoside (13). Hydrogenolysis of the glycoside (8) in methanol gave an equilibrium mixture of the a- and ß-forms [(14) and (15)] of 3,4-anhydro-D-altropyranose (32.5 and 67.5%, respectively).