Spectral and structural correlations of 1,4- and 3,4-dihydroquinazolines

Alkylation of the nucleophilic addition product of 2,4-diphenylquinazoline and organometallic derivatives produced a mixture of 1,4- and 3,4-dihydroquinazoline dreivatives. The structures were established through pmr correlations of chemical shifts of the N-CH₃ groups produced when the alkylating agent was methyl iodide.

Spectra (uv, pmr) of known open-chain amidines were used to confirm the structures of the isomeric dimethyl 2,4-diphenyldihydroquinazolines. The uv spectra of 2-phenyl-3,4-dihydroquinazolines possessed an absorption maximum at 320 nm while the 1,4-dihydroanalogues had a maximum at 290 nm. Characteristic absorptions in the 1550–1650 cm⁻¹ region of the infrared spectra also differentiated these two groups of compounds. Removal of the 2-phenyl substituent renders these correlations less reliable.