BRANCHED-CHAIN SUGARS .13. STEREOSELECTIVITIES IN THE REACTION OF METHYL 4,6-O-BENZYLIDENE-ALPHA AND BETA-D-HEXOPYRANOSID-2-ULOSES WITH DIAZOMETHANE

The stereoselectivities in the reaction with diazomethane of methyl 4,6-O benzylidene-3-O-methyl-α-d-arabino-hexopyranosid-2-ulose (2), its 3-epimer (3), its β anomer (5), and the corresponding 3-O-benzoyl derivative (4) have been examined in comparison with those in the Grignard reaction and in reduction with sodium borohydride. The opposite stereoselectivity of the diazomethane reaction, with equatorial attack on α anomers (2 and 3) and axial attack on β anomers (4 and 5), to that of the other reactions, is explained by an attractive, electrostatic interaction in the transition state between the diazomethyl cation and either the axial O-1 of the methoxyl group or lone-pair electrons of O-5. © 1979.