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A comparative analysis carried out on 19 methyl cyanurates and thiocyanurates (some of which reported for the first time) allowed the following general conclusions to be drawn about their reactivity in nucleophilic substitution and thermal isomerisation reactions. The nucleophilic substitution of the methylthio-groups by methoxide anions is always possible, but requires different experimental conditions which are specific for the molecular region in which the methylthio is located ; the thermal isomerisation reaction of the methoxy-derivatives into the corresponding S-or N-methyl isomers requires temperature conditions which are also largely dependent upon the molecular site of the methoxy-group. In both reactions the reactivity is enhanced by local asymmetry of substitution at the ring carbon atom to which the XMe (X = S or 0) group is bound.WE have recently reported a synthetic procedure which by means of methanolysis and isomerisation allowed four trimethyl dithiocyanurates and four trimethyl monothiocyanurates to be obtained from (l)? and (9).t A preliminary study of the two reactions allowed us (i) to point out the selectivity of nucleophilic attack in methanolysis and (ii) to develop some rules for the methyl group migration in the thermal isomerisation reactions.

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