Isoxazoline-4-carboxylic acids esters

Saponification of the isoxazoline ring and subsequent decarboxylation of compound (243) afforded the 5-cyano-l,4-dihydropyridine-3-carboxylic acid ester (244) <89JOC5585>. If the acid (243) was first reesterified with ( — )-menthol, the resulting diastereomeric esters were separable. Saponification of the individual enantiomers followed by decarboxylation afforded a method to prepare 3-cyano-l,4-dihydropyridines (244) stereospecifically. 

The methyl and benzyl esters of proline were also used as chiral auxiliaries in respective acrylamides, but the isoxazoline cycloadducts were obtained with only poor to modest stereoselectivity (189,190). The related indoline-2-carboxylic acid derivative 33, however, showed excellent ability to direct nitrile oxide attack, favoring one rotamer (Scheme 6.37), and thereby leading to 3-phenylisoxazoline-5-carboxamide... 

Isoxazoline-4-carboxylic acid, N-methyl-5-oxo ethyl ester... 

Various 2-isoxazolines were synthesized from resin-bound nitrile oxide (Scheme 11.35) or alkene (Scheme 11.36). ° Oxidative treatment of aldoximes with bleach generated nitrile oxides that reacted with various alkenes. The ester-linked products were cleaved from the Wang resin as carboxylic acids using trifluoroacetic acid. Cycloaddition of terminal alkenes... 

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