Heterocycles Structurally Influenced by a Side Chain. I. 3-Phenacyl-2(1H)-quinoxalinones and 3-Phenacyl-2H-1,4-benzoxazin-2-ones

Seven phenacyl derivatives of 2(1H)-quinoxalinone and 2H-1,4-benzoxazin-2-one (IV and V) have been synthesized to introduce a common structural element –CH2–CO– into their 3-positions. Their structures are compared with those of the related compounds described previously. The main purpose of the present study was to see whether or not their ring double bonds ((Remark: Graphics omitted.)) have ever been displaced onto the side chains, constituting an enamine so as to form =CH–CO–, the carbonyl of which is hydrogen bonded with the secondary amino group of the rings thereby formed. Consequently, existence as the enamine form, facilitated by an intramolecular hydrogen bonding, has been evidenced by IR and NMR spectral data. It is, therefore, fitting to call them by the more practical expression “3-phenacylidene-3,4-dihydro-2(1H)-quinoxalinone” (IVa), and so on. Hydrolysis of IV gave the corresponding 3-methyl-2-(1H)-quinoxalinones (VI) and benzoic acid (VII), while that of V afforded the starting substances, o-aminophenols (III) and benzoylpyruvic acid (VIII) which had been partially decomposed into pyruvic acid (IX) and VII.