Aldehydes isoxazoline intermediates

A regioselective method affording directly 3-phenyl-5-substituted isoxazoles 341, without isolation of isoxazoline intermediates, exploited reactions of NH2OH and a-benzotriazolyl-a,/3-unsaturated ketones 340, stereoselectively generated from benzotriazolylacetophenone and aldehydes in the presence of piperidine (Scheme 83) <2001JOC6787>. Treatment of /3-lithiated benzotriazolylvinyl ethyl ether with acid chlorides followed by cyclocondensation with hydroxylamine hydrochloride gave 4-benzotriazolyl-substituted isoxazoles <2005S245>. 

Analogous intermediates seem to be involved in at least some of the reported photoreactions of related dihydroheteroaromatic systems. On irradiation in benzene, the isoxazolines 181 are converted to benzonitrile (182) and the aldehydes 183 via fragmentation pathways (see Scheme 6)... 

The use of alkynes has been investigated in the context of intramolecular nitronate cycloadditions (130). In this case, the starting material is consumed in 24 h at room temperature, however, the corresponding isoxazoline is not isolated (Table 2.41). Instead, the intermediate cycloadduct undergoes a fragmentation which, following the loss of the nitroso moiety, leads to an a,p-unsaturated aldehyde. 

Isoxazolines have been prepared by the condensation of aldehydes and ketones with the di-lithium salts of oximes it was unnecessary to isolate the intermediate -hydroxyoximesX (Scheme 127). 

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