C-Glycosides from Palladium-Mediated Reactions of Pyranoid Glycals. Stereochemistry of Formation of Intermediate Organopalladium Adducts and Factors Affecting Their Stability and Decomposition

Article abstract of DOI:10.1021/jo00390a022

Palladium-mediated reactions of eight 3-O-substituted pyranoid glycals with (1,3-dimethyl-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-5-yl)mercuric acetate occurred regio- and stereospecifically to form, in each case, a single organopalladium adduct by attack of the organopalladium reagent on the glycal double bond from the face of the glycal opposite the allylic 3-O-substituent.Yields of C-glycosides formed by adduct decomposition were higher for glycals with pseudoequatorial 3-O-substituent than for glycals with this substituent pseudoaxial.Intermediate adduct stability and decomposition modes were sensitive to reaction medium composition.The presence of chloride ions resulted in relatively stable organopalladium adducts for which subsequent medium changes influenced decomposition modes.Reaction mixture containing only acetate anions did not lead to stabile adducts and C-glycoside products appeared early.Conformational constraints on glycals, achieved by incorporation of 4- and 6-oxygen atoms into a ring, resulted in highly selective adduct decomposition producing a single C-glycoside product; unfortunately, this constraint also lowered product yields