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

Methyl esters of methyl cis ‐ and trans ‐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% BF 3 −MeOH to produce a mixture of methyl 9‐ and 10‐methoxyoctadecanoates. 14% BF 3 −MeOH does not react with cyclopropane and olefinic esters under the reaction conditions employed.