STEREOCHEMISTRY OF THE PHOTOINDUCED AND MICHAEL ADDITION OF METHANOL TO 7-MEMBERED AND 8-MEMBERED 2-CYCLOALKENONES - EFFECT OF METHYL SUBSTITUENTS

Irradiation of 2-methyl-2-cycloheptenone (4) or 2-methyl-2-cyclooctenone (6) in methanol gave only the corresponding cis-3-methoxy-2-methylcycloalkanones 5c and 7c, respectively, rationalized as a consequence of syn methanol addition to the trans enones. The isotope effect (CH3OH/CH3OD) for 4 was 2.0, appreciably less than the previously observed value for 2-cycloheptenone, suggesting that the 2-methyl substituent destabilizes the trans intermediate. Photoinduced methanol addition also occurred when the double bond had a methyl substitutent at C3, but 2, 3-dimethyl-2-cyclooctenone failed to add methanol photochemically. The base-catalyzed Michael addition of methanol to 4 and 6 gave a mixture of cis-and trans-3-methoxy-2-methylcycloalkanones, in contrast to the previously reported stereospecific additions to the unsubstituted 2-cycloalkenones. The acid-catalyzed addition of methanol to 4 gave a different cis-trans ratio than the base-catalyzed addition. © 1979, American Chemical Society. All rights reserved.