Salicylaldehyde and some of its derivatives react with urea (or thiourea) and ethyl acetoacetate (the Biginelli reaction) to give products that depend on the type of the substituent in the ortho-position to the hydroxyl group. Aldehydes having oxygen-bearing substituents ortho to the hydroxyl group (e.g., OCH_3, NO_2, COOH) undergo chelation with the hydroxyl proton and lead to 4-aryl-6-methyl-4-aryl-2-oxo-1,2,3,4-tetrahydropyrimidine-5-ethyl carboxylate (the classical Biginelli product). Compounds not having such a property give oxacyclic products formed by addition of the hydroxyl proton to the C5-C6 double bond of the pyrimidine ring. This study also includes compounds having an amino group instead of the hydroxyl one.
ABBAS, EMAN M. H.; ABDALLAH, SHADIA M.; ABDOH, MERVAT H.; TAWFIK, HANAA A.; and EL-HAMOULY, WAGEEH S. (2008) "Behaviour of Salicylaldehyde and Some of Its Derivatives in the Biginelli Reaction for the Preparation of Aryl Tetrahydropyrimidines," Turkish Journal of Chemistry: Vol. 32: No. 3, Article 3. Available at: https://journals.tubitak.gov.tr/chem/vol32/iss3/3