Compounds of formula <;FORM:0765506/IV(b)/1>; wherein R is hydrogen or an alkyl, aryl or thiol group or an amino group optionally substituted by alkyl, aryl, acyl or sulphonyl radicals, and R1 and R2 are hydrogen, alkyl or aryl groups, are prepared by reacting beta-aminovinyl carbonyl compounds of formula <;FORM:0765506/IV(b)/2>; wherein R3 and R4 are hydrogen, alkyl or aryl groups, or a saturated ring including the nitrogen atom, with compounds of formula R-C(=NH)-NH2 or salts thereof. The reaction is preferably conducted at raised temperature, with or without condensing agents and with or without solvents such as alcohols, water, benzene and acetic acid. Specified amidine reactants are guanidine and salts thereof, para - aminobenzene - sulphonylguanidine, para - acetaminobenzene - sulphonylguanidine, thiourea, benzamidine, acetamidine and formamidine. The examples describe the preparation of 2-amino-, 2-amino-4-methyl-, 2 - (para - aminobenzene - sulphonamido)-, 2-phenyl-, 2-thiol-, 2-amino-4-phenyl- and 2 - (para - aminobenzene - sulphonamido)-4 -methyl-pyrimidines. The beta-aminovinyl carbonyl compounds of formula <;FORM:0765506/IV(b)/3>; are prepared by adding ammonia or primary and secondary amines of formula R3NHR4 on to alpha-acetylenic aldehydes or ketones. Specified amines are dimethylamine, diethylamine, pyrrolidine, piperidine, N-methylaniline and N-ethylaniline. Specified acetylenic compounds are butine-(1)-one-(3), pentine-(2)-one-(4), 3 - phenyl - propine - (1) - one - (3), 1-phenyl - butine - (1) - one - (3), heptine - (2)-al-(1) and propargylaldehyde.;FORM:0765506/IV(b)/3>;FORM:0765506/IV(b)/2>;FORM:0765506/IV(b)/1>