2.5- Dimethyl-1,3,4-oxadiazole, formation

Muchall et al. (98CC238) have recently investigated the gas-phase thermolysis of 2,5-dihydro-2,2-dimethoxy-2,5,5-trimethyl-l//-l,2,4-oxadiazole (75) by PE spectroscopy. Decomposition of 75 was induced by means of a continuous wave (CW) C02 laser as directed heat source at 26 W, which corresponds to a temperature of 500 50°C. When the PE spectra of acetone, tetramethoxyethene, and dimethyl oxalate were subtracted from the pyrolysis spectrum, a sim-ple spectrum remained that could be identified as that of dimethoxycarbene. Thermolysis in solution (94JA1161) had shown formation of tetramethoxyethene, and FVP experiments (92JA8751) gave dimethyl oxalate, both of which arise from the common precursor, dimethoxycarbene. Thermolysis of oxadiazolines similar to 75 in solution affords dialkoxycarbenes via an intermediate carbonyl ylide (94JOC5071). 

An interesting but not synthetically useful reaction has been reported by Mloston and co-workers <2002CEJ2184>. The treatment of 2,2,4,4-tetramethylcyclobutan-l,3-dithione with dimethoxycarbene, generated by thermolysis of 2,2-dimethoxy-5,5-dimethyl-2,5-dihydro-[l,3,4]oxadiazole, led to the formation of 4-isopropyl-idene-3,3-dimethyl-thietane-2-thione in trace amounts (2% yield), together with other products (Equation 23). 

The polycondensation of terephthalic acid, dimethyl terephthalate, and hydrazine sulfate results in the formation of p-phenylene oxadiazole/ iV-methyl hydrazide copolymers. The process is conducted in fuming sulfuric acid. 

Addition of 2,5-dihydro-2,2-dimethyl-5,5-bis(propylthio)-l,3,4-oxadiazole in refluxing benzene and an aryl isocyanate releases the bis(propylthio)carbene in situ which then adds easily to the aryl isocyanate to yield a substituted isatin with the ketone functionality protected as a thioacetal (eq 4)7 Ring closure also occurred when the 2,5-dihydro-2,2-dimethyl-5,5-bis(propylthio)-1,3,4-oxadiazole carbene precursor was added to 1-naphthyl isocyanate in refluxing acetonitrile (eq 5)7 Formation of thioacetal protected isatin products are unique to the bis(propylthio)carbene as other nucleophilic carbenes added to aryl isocyanates afforded only modest yields of hydantoin products. ... 

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