Mechanistic study of the thermal acid-catalysed rearrangement of trans-methyl chrysanthemate to lavandulyl derivatives

trans-Methyl chrysanthemate (17) under acid conditions at room temperature rearranged to the lavandulyl esters: methyl trans-5-methyl-2-propen-2-ylhex-3- enoate (18) and methyl trans-5-methyl-2-(2-hydroxypropan-2-yl)hex-3-enoate (19) which was formed by a cyclopropylcarbinylhomoallyl rearrangement. Both (18) and (19) further isomerise and dehydrate to the stable methyl trans-5-methyl-2- propan-2-ylidenehex-3-enoate (20). Under similar acidic conditions, (18) and (19) each gave a mixture of lavandulyl esters and (17); minor amounts of side products, methyl trans-5-methylhex-3-enoate (21), methyl trans-5-methyl-2-(2- methoxypropan-2-yl)hex-3-enoate (22), methyl cis-5-methyl-2-propen-2-ylhex-2- enoate (23) were detected after some time. Acid methanolysis of (17) gave a substantial amount of (22) and its isomer, methyl 2,2-dimethyl-3-(2- methoxypropan-2-yl)cyclopropanecarboxylate (26). There was also found in low concentration, methyl trans-5-methyl-2-propan-2-ylhex-3-enoate (24) and methyl 5-methyl-2-propan-2-ylidenehexanoate (25) possibly the result of a hydride transfer from the solvent. Deuterium exchange was observed in the isobutenyl sidechain of (17) and the isobutenyl moiety of (18) and (19). At 130 °C under similar acidic conditions a mixture of unsaturated γ- and δ-lactones was obtained from (17). The principal lactone was dihydro-5-propan-2-yl-3-propan-2-ylidenefuran-2(3H)-one (29). Mechanisms for the formation of these products are discussed. The fact that lavandulyl derivatives were obtained may shed new light on the biogenesis of chrysanthemic acid in plants.