The reaction may proceed through acylimine formation between an aldehyde and urea. Subsequent
addition of the enolate of the b-keto-ester to the acylimine followed by cyclodehydration would afford dihydropyrimidinone-(1H)-one 4. The b-ketoester enolate31 can be formed by coordinating the aldehyde with TPP which promotes deprotonation of the b-ketoester (Scheme 2).