Chlorimine formation and deamination of the model nucleobase 1‐methylcytosine

Treatment of the model nucleobase 1‐methylcytosine with chlorine in aqueous solution gives, depending on the reaction conditions, 5‐chloro‐1‐methylcytosine ( 3 ) or its hemiprotonated form 2 , 5,5‐dichloro‐6‐hyd‐roxy‐5,6‐dihydro‐4‐chlorimino‐1‐methylpyrimidin‐2‐one ( 5 ), and 5,5‐dichloro‐6‐hydroxy‐5,6‐dihydro‐1‐methyl‐uracil ( 4 ). While substitution of H5 of 1‐methylcytosine by chlorine giving 2 and 3 , and subsequent addition of hypochlorous acid to the 5,6‐double bond is not unexpected, chlorination of the exocyclic amino group 5 and deamination to give 4 is novel. The X‐ray structures of 4 and 5 are reported.