Mass spectra of pyrrolidides of oxy, hydroxy and trimethylsilyloxy octadecanoic acids

Abstract

Pyrrolidides of 16 isomeric oxooctadecanoic acids and 17 isomeric hydroxyoctadecanoic acids and their trimethylsily (TMS) ethers have been prepared and their mass spectra measured. Comparison of spectra of all the isomers has shown the variation in amide- and substituent-directed fragmentation with position of the substitutent on the 18 carbon chain. Five diagnostic ions are present in spectra of most oxo pyrrolidides, and the most important one is formed by cleavage between the carbons α and β to the oxo group on the amide side. Mass spectra of hydroxy pyrrolidides have four diagnostic ions; the principal one in most spectra is formed by α-cleavage at the oxygenated carbon on the terminal methyl side. There are four characteristic ions in spectra of the trimethylsiloxy pyrrolidides, and the two formed by α-cleavage on one or the other side of the oxygenated carbon are the most important. Spectra of the 2-, 3- and 4-substituted isomers are considerably different from those of the other isomers. Mass spectra of pyrrolidides of three specifically deuterated oxo acids and of eight deuterated hydroxy acids and their trimethylsilyl ethers also were recorded and used to confirm the mode of formation of almost all the ions in the spectra. Two unusual hydrogen migrations, from the carbon α to the oxygenated carbon on the terminal methyl side, are involved in fragmentation of the trimethylsilyl ethers. Spectra have been compared with those of the corresponding oxygenated methyl esters.