Mass spectral characteristics of some 3,5-disubstituted 4-isoxazolecarboxylic acids
Mass spectrometry has been applied for the structural study of some new unsymmetrically 3,5-disubstituted 4-isoxazolecarboxylic acids prepared by a modification of the general procedure of Quilico and Fusco. The new glucoside, 3-[2-(β-D-glucopyranosiloxy)-5-chlorophenyl]-5-methyl-4-isoxazolecarboxylic acid (1) and 3-(2-hydroxy-3,5-dichlrophenyl)-5-methyl-4-isoxazolecarboxylic acid (2) have been prepared starting from the glucoside helicin, 2-(β-D-glucopyranosiloxy) benzaldehyde or the corresponding aglucon, the salicylaldehyde, respectively. Fragmentation patterns of 2 and its analogue 3-(2-hydroxy-5-chlorophenyl)-5-methyl-4-isoxazolecarboxylic acid (3), obtained by the cleavage of the glycosidic linkage of 1 are discussed. The principal fragmentation routes of these compounds are formulated to occur via azirine intermediates.