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.