The destruction of the cyclopropenoid ring system of methyl 9,10 methyleneoctadec‐9‐enoate (methyl sterculate) with boron trifluoride etherate has been shown to give a complex mixture of products, including methyl esters of C19 allenes (12%), a C18 alkyne (11%) and a variety of C19 and C20 conjugated dienes containing either a methyl or methylene branch. The methylene group lost from the methyl sterculate reactant in the formation of methyl octadec‐9‐ynoate is incorporated into a second molecule of reactant to yield a mixture of methyl 9‐methylene‐trans‐nonadec‐10‐enoate and the 11‐methylene‐trans‐9‐isomer.