Nicotinylidene derivatives for the structural elucidation of glycerol mono-ethers and mono-esters by gas chromatography/mass spectrometry

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Abstract
Nicotinylidene and methylnicotinylidene derivatives of representative glycerol ethers and glycerol esters were prepared by their reaction with pyridine-3-carboxaldehyde and 3-acetyl pyridine respectively. Each compound gave two sets of diastereoisomers which could be separated by gas-liquid chromatography. The extent of formation of the derivatives was nearly quantitative and chromatographic peak shape was good. The compounds producing both peaks gave similar spectra but with differences that were characteristic of the isomer. The derivatives fragmented in an analogous manner to that reported earlier for nicotinate derivatives of alcohols and picolinyl derivatives of carboxylic acids, with the production of a series of diagnostic ions formed by radical-induced cleavage of the chain following random hydrogen abstraction. Branched chain compounds, exemplified by glycerol ethers, gave spectra in which one of the ions diagnostic of the branch-point was missing. Unsaturated compounds, exemplified by the glycerol esters, gave characteristic mass spectra in which separation between ions representing formal cleavage at each side of the double bond enabled the position of this bond to be determined. In addition the spectra of unsaturated compounds contained two abundant ions whose formation could be rationalized by abstraction of the allylic hydrogen atoms. The nicotinylidene derivatives gave more diagnostic spectra than the methylnicotinylidene derivatives, whose spectra were complicated by the appearance of ions produced by loss of the methyl and pyridyl groups from the derivative. Both derivatives were superior to the nicotinate derivatives of these dihydroxy compounds in that only one pyridine residue was introduced on derivatization; this gave a lower molecular weight increment and allowed compounds with longer chains to be examined. In addition, the relative abundance of the diagnostic ions was considerably higher.

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