1.3.4- Oxadiazoline ring opening

A -Pyrazolines and A -isoxazolines (302 Z = NH, O) are cyclic hydrazones and oximes, respectively. 2-Pyrazolines are quaternized at the 2-position (306 307) (64AHC(. ll). 1,3,4-Oxadiazolines (e.g. 308) are very easily ring-opened (66AHCi7 183). 

A development in the synthesis of macromolecular scintillators is the radical co-polymerization of alkenes with the fluorescent co-monomer 2-t-butyl-5-(4 -vinyl-4-biphenylyl)-l,3,4-oxadiazole <89MI 406-04). Basic initiators cause ring opening of 2-phenyl-l,3,4-oxadiazolin-5-one with subsequent anionic polymerisation to form A-benzamido-1-nylon [-N(NHCOPh)CO-] <90PB583>. 

Ring opening by photolysis also occurs in the oxadiazoline ring. Srivastava and Clapp109 noted that a simple rearrangement [Eq. (44)] of 4-/3-ethoxyethyl-3-methyl-5-p-nitrophenyl-l,2,4-oxadiazoline to a substituted amidine (50) took place on a bench top in ordinary daylight or... 

Ring opening is the most common reaction of A2-oxadiazolines, either thermally or on attack by nucleophiles. A rare example of oxidation to a 1,3,4-oxadiazole is the conversion of 4-acetyl-2,5-diphenyl-A2-l,3,4-oxadiazoline into 2,5-diphenyl-l,3,4-oxadiazole on treatment with LTA (67JOC3318). 

Oxadiazoline (59), on irradiation in benzene, forms an equilibrium mixture with its synthetic precursors diphenylketene and diphenyldiazomethane. Ring opening occurs on irradiation in the presence of alcohols (66AHC(7)183). Photolysis of oxadiazolinones (60a) proceeds similarly to thermolysis and a ketone R COR2 is produced concurrently with loss of nitrogen and carbon dioxide. In the presence of cyclohexene the major product is the hydrazone (63) which is believed to arise from a bimolecular photoreaction of the alkene with the oxadiazolinone. Similar photoreactions occur with 2-phenylpropene and diethyl ether (78S535). 

Hydrogenation of oxadiazoline (59) over Raney nickel and of oxadiazolinimine (60c) over palladium charcoal yields hydrazones, probably by reduction to oxadiazolidines followed by ring opening. A3-l,3,4-Oxadiazolin-2-imines may be hydrolyzed to the corresponding oxadiazolinones in cold acid. 

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