Ring location in cyclopropane fatty acid esters by boron trifluoride-catalyzed methoxylation followed by mass spectroscopy

Methyl esters of methylcis- andtrans-9,10-methyleneoctadecanoic acids react with 50% boron trifluoride-methanol to produce unsaturated and methoxy-esters; both products are shown by gas chromatography to be a mixture of several isomers. Mass spectra of the methoxylated esters are characterized by intense peaks due to cleavage adjacent to methoxy-functions which allow the position of the ring in the original cyclopropane ester to be easily assigned. Methyl oleate is also partially attacked by 50% BF3−MeOH to produce a mixture of methyl 9- and 10-methoxyoctadecanoates. 14% BF3−MeOH does not react with cyclopropane and olefinic esters under the reaction conditions employed.