Furanoside–pyranoside isomerization of tubercidin and its 2′-deoxy derivatives: influence of nucleobase and sugar structure on the proton-catalysed reaction
2′-Deoxy-2-methoxytubercidin (6a) which was prepared from the nucleobase (1a) with the halogenose (2)via phase-transfer glycosylation isomerizes rapidly under acidic conditions. Two pyranosides [(7a) and (8a)] and the anomeric furanoside (5a) are formed. The isomerization process was followed kinetically, demonstrating that furanoside formation is kinetically controlled whereas the β-pyranoside (7a) is the thermodynamically most stable product. From 2′-deoxytubercidin (6b) similar results were obtained but isomerization was slow, compared with (6a). The ribonucleoside tubercidin (6c) did isomerize only under vigorous acid treatment leading to the α-furanoside (5c) and the nucleobase (1c) by cleavage of the N-glycosylic bond.