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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
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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Stereochemistry of the photoinduced and Michael addition of methanol to seven- and eight-membered 2-cycloalkenones
100
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
Stereochemistry of the photoinduced and Michael addition of methanol to seven- and eight-membered 2-cycloalkenones
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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