Solvent Effect Observed in Nucleophilic Substitution of 4'-(Benzoyloxy)cordycepin with AIMe_3: Stereochemical Evidence for S_Ni Mechanism

摘要

Nucleophilic substitution between the 4'-benzoyloxy derivative of cordycepin (3'-deoxyadenosine) andAIMe_3proceeds mostly with retention of configuration at the 4'-position: benzoyloxy substratehaving the β-D-configuration (8a) gave the 4'-methylated β-D-nucleoside (9a) with a high diastereomericexcess, while the substrate 8b having the opposite 4'-configuration gave the corresponding a-L-isomer(9b) with a comparatively lower stereoselectivity. The SNi mechanism is proposed for this reaction (from8 to 9). The polarity of the solvent has a significant influence on the stereoselectivity, especially for theformation of 9b.