Competition between cyclisation and bisimine formation in the reaction of 1,3-diaminopropanes with aromatic aldehydes

Condensation of 1,3-diamines with aldehydes or ketones gives rise to two major products, the hexahydropyrimidine and the bisimine. Experimental studies of the reaction between a range of aromatic aldehydes and 1,3-diaminopropane or 1,3-diamino-2-propanol establish that the hexahydropyrimidine is favoured by the less nucleophilic amine and by the presence of electron withdrawing groups on the aryl ring of the aldehyde. Calculations indicate that the electronic nature of this aryl ring substituent influences both the relative thermodynamic stability of the final products and the reactivity of the aldehyde as an electrophile.