Abstrakt
Contrary to a number of reports, alkylations of the privileged 3,4-dihydroquinazoline scaffold provide N3-alkylated products, and not 4-alkoxyquinazolines. To correctly assign the structure, C-13 NMR shifts of the -Z-CHn- (Z=O, N) fragment are necessary; resonances in the 45-55 ppm range are indicative of N3-allcylation.
Treatment of 3,4-dihydroquinazoline-4-one with p-TsCl afforded the N3-tosylated compound, whose reaction with an amine yielded the corresponding N3-alkyl derivative. A mechanism corroborated by N-15-labeling involving pyrimidine ring opening and recyclisation is proposed.
Finally, the unambiguous preparation of 4-alkoxyquinazolines is described via treatment of 3,4-dihydroquinazoline-4-ones with PCl5 followed by an alkoxide.