From Expected Cyclization to Observed Solvolysis: The Case of a Conjugated Oxazolone

Abstract

This investigation delineates the solvent- and temperature-mediated divergent reactivity of 2-phenyl
4-(3-phenylallylidene)oxazol-5(4H)-one (1) with a series of amines. The mechanistic pathway is
directed toward cyclization under specific conditions, as evidenced by the reaction with o
phenylenediamine, para-aminohippuric acid, 4-amino-2,3-dimethyl-1-phenyl-3-pyrazolin-5-one, and
2-chloroaniline in glacial acetic acid at 110 °C, affording imidazolone and bis-amide derivatives (I, II,
IV, Va, Vb, VI). A bis-amide product (III) was similarly obtained from 2-aminobenzothiazole in
toluene at 230 °C. Conversely, a ring-opening mechanism predominated with p-chloroaniline, 2,4
dichloroaniline, or 2-aminobenzothiazole in glacial acetic acid at 110 °C, yielding the amide-acid (VII),
and with o-chloroaniline, 2-aminobenzothiazole, or para-aminohippuric acid in absolute ethanol at 230
°C, producing the amide-ester (VIII). This work conclusively establishes solvent and temperature as
critical parameters for steering the reaction mechanism, enabling the targeted synthesis of distinct
molecular architectures via straightforward procedures. All novel compounds were isolated in
satisfactory yields and unequivocally characterized by comprehensive spectroscopic analysis (¹H
NMR, ¹³C NMR, IR, and LC-MS).