Synthesis and Reactivity of Sydnone Derived 1,3,4-Oxadiazol-2(3H)-ones

5-(Bromomethyl)- and 5-(trichloromethyl)-3-phenyl-1,3,4-oxadiazol-2(3H)-ones were synthesized from bromocarbonyl hydrazine intermediate salts and acid halides to provide further evidence for our proposed mechanism for the formation of oxadiazolone ring systems from sydnones. 5-(Bromomethyl)-3-phenyl-1,3,4-oxadiazol-2(3H)-one was a compound of interest due to its SN2 possibilities and three potential sites of diversification. The procedure to synthesize 5-(bromomethyl)-3-phenyl-1,3,4-oxadiazol-2(3H)-one was developed originally by Madaram in the Turnbull lab, but was further optimized in the present work by altering the time, temperature and stoichiometric equivalents of the acid halide to provide high purity and excellent yields of the product. While this compound of interest had been isolated and characterized previously it had yet to be probed for chemical reactivity and, accordingly, it was subjected to a variety of nucleophilic and electrophilic reagents. For hard nucleophiles, an attack at the hard, carbamate carbonyl group was observed, resulting in undesirable ring-opening of the oxadiazolinone ring system. Accordingly, hard, but bulky, substrates were utilized in order to impede an attack at the carbamate carbonyl group and favor one at the alkyl halide functionality but, unfortunately, this resulted in no reaction. Soft substrates preferred the SN2 possibility at the methylene group attached to the C5-position and, in a few cases, viz. with sodium azide and ammonium thiocyanate, successful displacement of the halogen was observed. Electrophilic aromatic substitution (EAS) of 5-methyl-3-phenyl-1,3,4-oxadiazol-2(3H)-one was explored by Gerritsen, Klaasen and Conde in the Turnbull lab, with the result that nitration, bromination and iodination were shown to occur at the para position of the phenyl ring attached to the oxadiazolinone. In the present work, nitration was extended to the 5-bromomethyl oxadiazolinone and its chloromethyl congener, however, under the previously employed conditions, inseparable mixtures were obtained. Accordingly, the amount of nitrating agent was increased to provide the corresponding dinitro compounds where substitution was both para and ortho to the oxadiazolinone ring. All oxadiazolinones synthesized were characterized by GC/MS, 1H and 13C NMR and IR spectroscopies. The second focus of this work considered the optimization of a route to chlorinate 3-arylsydnones utilizing ICl in DCM. Imad Nashashibi of the Turnbull lab, had previously attempted this reaction and observed the formation of two products, viz. the 4-chloro- and 4-iodo-3-arylsydnones. In the present work, the time and the molar equivalents of ICl were manipulated, providing satisfactory yields and purity of the 4-chloro congener alone. A series of 4-chloro-3-arylsydnones were prepared and characterized by melting point analyses along with 1H NMR and 13C NMR spectra. With the 4-chloro sydnones in hand, their conversion to the corresponding oxadiazolinones was explored. Previously, 3-arylsydnones had been converted to oxadiazolinones using bromine in acetic anhydride as both solvent and reagent, however, in the present work, it was elected to react the 4-chloro-3-arylsydnones with acetic anhydride in acetic acid as solvent. The corresponding 5-methyl-3-aryl-1,3,4-oxadiazol-2(3H)-ones were obtained by this novel process in high yields and purity and were characterized by their melting points and their 1H NMR and 13C NMR spectra