The Biginelli reactions of salicylaldehyde and 2-hydroxy-l-naphthaldehyde with ethyl or methyl acetoacetate, ethyl benzoylacetate, and urea have been reinvestigated both in the structures of the reaction products and in the reaction conditions. Salicylaldehyde with ethyl and methyl acetoacetate resulted in oxygen-bridged tricyclic tetrahydro pyrimidines, whereas with ethyl benzoylacetate afforded the only normal 3,4-dihydropyrimidin-2-one. 2-Hydroxy-l-naphthaldehyde with ethyl and methyl acetoacetate formed the tricyclic compounds. Steric effect is likely to be the principal determinant in governing the formation of product dichotomy. Previous controversial results as to the structure of the Biginelli products have been discussed and settled. The molecular structures of the dihydropyrimidinones and bridged tricyclic products have been fully characterized and confirmed unambiguously by single crystal X-ray diffraction.
UV irradiation of N-methyl-2,3-naphthalenedicarboximide with phenylacetylene in benzene solution afforded a novel hexacyclic adduct 2 confirmed unambiguously by single crystal X-ray diffraction.
