5-bromo- and 5-iodo-N-1-sulfonylated cytosine derivatives. Exclusive formation of keto-imino tautomers

N-1-Sulfonylated cytosine 5 and 5-halogeno-N-1-sulfonylated cytosine deriva tives 1 and 2 were synthesized by condensation of silylated cytosine or 5-h alogenocytosine bases with tosylchloride in acetonitrile, or by the reactio n of cytosine or 5-halogenocytosine bases with tosylchloride in pyridine. T he NMR evidences are presented, showing that 1 and 2 form exclusively rare keto-imino tautomers in DMSO-d(6) solution, while N-1-sulfonylated cytosine 5 appears as a common keto-amino tautomer.