The Stereochemistry of the Reformatsky Reaction of Methyl 4-Bromo-3-methylbut-2-enoate with β-Cyclocitral and Related Compounds

Both methyl Z- and E-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 an E to Z inversion during the Reformatsky reaction. Similar results were obtained in the Reformatsky reactions of the Z- and E-bromo-esters with benzaldehyde and cyclohexenecarboxaldehyde. Here hydrolysis of the Reformatsky product gave, in each case, the corresponding Z-2,E-4-acids as the main products, indicating the formation of the δ-lactone as an intermediate. The synthesis of Z- and E-β-ionylideneacetic acid and the corresponding ring demethyl analogs using a Wittig reaction is also described.