Pyrazolopyrimidine nucleosides. Part IV. Synthesis and chemical reactivity of the C-nucleoside selenoformycin B and derivatives

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Abstract
The synthesis of 7-selenoxo-3-(β-D-ribofuranosyl)pyrazolo[4,3-d]pyrimidine (2)(selenoformycin B) has been accomplished by a nucleophilic displacement of the chloro-group from 7-chloro-3-(β-D-ribofuranosyl)pyrazolo-[4,3-d]pyrimidine (1) with selenourea in ethanol at reflux temperature. Alkylation of (2) has furnished several 7-alkylseIeno-3-(β-D-ribofuranosyl)pyrazolo[4,3-d]pyrimidines. A study on the relative reactivity of certain 7-substituted 3-(β-D-ribofuranosyl)pyrazolo[4,3-d]pyrimidines and the corresponding 6-substituted 9-(β-D-ribofuranosyl)purines toward nucleophilic displacement is discussed. 7-Methoxy-3-(β-D-ribofuranosyl)pyrazolo[4,3-d]pyrimidine (5) and the nebularine derivative 3-(β-D-ribofuranosyl)pyrazolo[4,3-d]pyrimidine (3) have also been prepared. The stability of (2) towards basic and approximate physiological pH conditions is discussed.

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