The addition of dichlorocarbene to the double bond of I -alkoxy-dihydrophosphole oxides and subsequent thermolysis of the adduct so obtained affords mixtures of the two double bond isomers of alkoxy-dihydrophosphinine oxides, i f the latter step is carried out in the presence of triethylamine. Experimental data support the involvement of a cationic intermediate during the opening of the cyclopropane ring. A simplified procedure for the preparation of the starting dihydrophospholes is also presented.

NMR studies showed that, in addition to the expected N(1) protonation, 2,4,6-pyrimidinetriamine, N,N,N',N',N",N"-hexamethyl- (1) could also be protonated at the C(5) position in water, leading to an equilibrium between the C(5) and N(1) protonated forms. Analysis of the NMR titration data gives 6.87 and 6.89 for the pK(a) of the C(5) and N(1) protonation equilibria. Moreover, the reaction of 1 with chloroacetyl chloride leads to a novel 1,1-bis(pyrimidin-5-yl)-2-chloroethene type derivative (4) that is, peculiarly, fully monoprotonated at the C(5) position in either of the pyrimidine rings, forming a stable cationic sigma-complex.

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