Both methylZ- andE-4-bromo-3-methylbut-2-enoate react with β-cyclocitral in the presence of zinc to give the δ-lactone of 5-hydroxy-3-methyl-5-(2,6,6-trimethyl-1-cyclohexen-1-yl)-2-pentenoic acid as the main product, indicating anEtoZinversion during the Reformatsky reaction. Similar results were obtained in the Reformatsky reactions of theZ- andE-bromo-esters with benzaldehyde and cyclohexenecarboxaldehyde. Here hydrolysis of the Reformatsky product gave, in each case, the correspondingZ-2,E-4-acids as the main products, indicating the formation of the δ-lactone as an intermediate. The synthesis ofZ- andE-β-ionylideneacetic acid and the corresponding ring demethyl analogs using a Wittig reaction is also described.