Unexpected shift to a 4-imino tautomer upon N4-acylation of 5-methylcytosin-1-yl nucleoside analogues

In contrast with the behaviour of 5-unsubstituted cytosine nucleoside analogues, 5-methylcytosine derivatives show upon N-4-benzoylation, commonly used as base protection in oligonucleotide synthesis, a tautomeric change of the base moiety from a 4-amino- into a 4-imino isomer. In the latter form, which is easily diagnosticized by C-13 NMR and confirmed by X-ray data, the compounds seem to be hydrolytically less stable.