STRUCTURES AND MECHANISMS OF FORMATION OF THE MAJOR SIDE PRODUCTS FROM THE PREPARATION OF 4,4-DIMETHYLCYCLOHEX-2-ENONE BY THE POTASSIUM HYDROXIDE PROMOTED ANNELATION OF METHYL VINYL KETONE AND ISOBUTYRALDEHYDE

Reaction of equimolar quantities of methyl vinyl ketone and isobutyraldehyde in aqueous methanol in the presence of potassium hydroxide produced 4, 4-dimethylcyclohex-2-enone (1, 38%), 3-methoxy-4, 4-dimethyl-cyclohexanone (2, 2%), (E)-4, 4-dimethyl-6-(2-methylpropylidene)cyclohex-2-enone (3, 25%), 4, 4-dimethyl-2-(l-hydroxy-2-methylpropyl)cyclohex-2-enone (4, 14%), and 6, 6-dimethyl-4, 4a, 5, 6-tetrahydro-2(3ff)-naphthalenone (5, 3%). The structures of compounds 2-5 were elucidated from spectral data. Increasing the molar ratio of methyl vinyl ketone to isobutyraldehyde from 2:2 to 2:1.5 to 2:1 resulted in modest improvements in the yield of 1 (42 and 47%, respectively). The mechanisms for formation of the major side products 2-5 are discussed. It was found that 3, 4, 4-trimethylcyclohex-2-enone and 4, 4, 5-trimethylcyclohex-2-enone do not interconvert when treated with potassium carbonate in aqueous methanol; similarly, no crossover was observed between 2, 4, 4-tri-methylcyclohex-2-enone and 4, 4, 6-trimethylcyclohex-2-enone. These results suggested that hydroxy ketone 4 was not formed via a pathway involving a [1, 5] sigmatropic shift of hydrogen but rather by an aldol reaction between 3-hydroxy-4, 4-dimethylcyclohex-2-enone (or 2) and isobutyraldehyde followed by the loss of water (or methanol). © 1979, American Chemical Society. All rights reserved.