(E)-9-Chloro-4-methyl-8-oxo-4-nonenoic acid, a useful intermediate in the synthesis of the potent antigestational agent ORF-13811 (a 3,8-dioxabicyclo[3.2.1]octane derivative), is efficiently prepared from 4-Choro-2-methyl-2-butene by using the ortho ester Claisen rearrangement.