Cyclic fatty esters: Stereochemistry of monounsaturated products from the hydrogenation and reduction of 9-(6-propyl-3-cyclohenxenyl)-8-nonenoic acid or its ester

Abstract

Diunsaturated, C-18 cyclic fatty acid methyl esters (CFAME) were previously synthesized as model derivatives for characterization and biological evaluation of cyclic fatty acids (CFA) formed in heat-abused vegetable oils. The propyl substituted, diunsaturated CFMAE (I) was selectively reduced to prepare two monounsaturated, positional isomers with the double bond located either in the ester substituent (alkene isomer II) or in the ring (cyclohexene isomer III). The stereochemistry of these monounsaturated products was investigated by capillary GLC and NMR. Capillary GLC showed that each positional isomer was a mixture of two ‘ring’ isomers (i.e. a mixture of two isomers with side chains either cis or trans). The ring double bond in diene I was readily hydrogenated with various metal catalysts, and no cyclohexene isomer III was detected in the product. Platinum oxide poisoned with Ph 3P was the most selective catalyst examined to convert diene I to monoene II. Diimide reduction was the only method foud to reduce selectively the double bond in the ester side chain of diene I. This diimide reduction was facilitated when the Z-double bond in the side chain was isomerized to E-double bond with p-toluenesulfinic acid. Cyclohexene isomer III and alkene isomer II were separated by argentation HPLC. These two isomeric monoenes were characterized by GC-MS, capillary GLC, micro-ozonolysis, IR and NMR. Catalytic hydrogenation with Ph 3P-poisoned PtO 2 and diimide reduction of the diunsaturated cyclic ester may provide useful methods to synthesize and label monounsaturated cyclic fatty esters.