Co-Authors:
Hart, H., Department of Chemistry, Michigan State University, East Lansing, MI 48824, United States
Dunkelblum, E., Department of Chemistry, Michigan State University, East Lansing, MI 48824, United States
Abstract:
The stereochemistry of the photoinduced addition of methanol to 2-cycloheptenone (1), 2-cyclooctenone (11), and 2,3-benzo-2,6-cycloheptadienone (4) was studied using CH 3OD. In each case, the methoxyl and deuterium in the resulting 3-methoxycycloalkahones (3d, 12d, and 6d, respectively) were trans. A two-step mechanism is proposed to explain the observed results: (a) photoisomerization of the cis cycloalkenone to a trans cycloalkenone, and (b) regiospecific and stereospecific nucleophilic syn addition of methanol to the highly polarized, strained trans double bond. The addition exhibits a large positive isotope effect (4.3 for 1, and 5.7 for 11, at 29°C). The base-catalyzed (0.01 N CH 3ONa) Michael addition of CH 3OD to the cis enones 1, 4, and 11 proceeds in a stereospecific trans manner. Base-catalyzed deuterium exchange in the 3-methoxycycloalkanones 3, 6, and 12 at the methylene carbon between the carbonyl group and the methoxyl-bearing carbon is also highly stereoselective, exchange being much faster for the proton trans to the methoxyl than for the corresponding cis proton. These results may be explained by a reversible base-catalyzed elimination and addition of methanol, via a transition state in which the methoxyl and hydrogen undergoing elimination or addition are in an antiperiplanar arrangement, In contrast, the acid-catalyzed addition of CH 3OD to 2-cycloheptenone was found to be stereorandom, the enol intermediate being protonated approximately equally at each face. © 1978 American Chemical Society.
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