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