The Isomerization of Purine Nucleosides in a Dilute Alkaline Solution

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Abstract
A further study of the isomerization of inosine (1a) in a dilute alkaline solution has revealed the formation of α-pyranbsyl isomer (4a) and its β-isomer (3a). Adenosine (1b) was isomerized at 130 °C for 12 hr at pH 10, giving α-furanosyl (2b) (2% yield), β-pyranosyl (3b) (11%), and the α-pyranosyl isomer (4b) (0.8%), respectively; 65% of the 1b also remained. 9-β-d-Ribopyranosyl-6-mercaptopurine (3c) was prepared through this reaction from the corresponding β-furanoside (1c). Arabinosyl or xylosyl purine nucleosides were also isomerized, whereas neither 2′-deoxy nor 2′-substituted nucleoside showed any clear evidence of isomerization. In conclusion, the purine base and the 2′-hydroxyl anion are essential to the alkaline isomerization of nucleosides. By means of NMR, both 4a and 4b were found to have a 1C conformation, while 3a and 3b have a Cl conformation.