A Theoretical Investigation of Enantioselectivity:Michael Reaction of Secondary Enamines with Enones

Transition states of amine-catalyzed aldol reactions involving enamine intermediates: theoretical studies of mechanism, reactivity, and stereoselectivity

The mechanisms, transition states, relative rates, and stereochemistries of amine-catalyzed aldol reactions involving enamine intermediates have been explored with density functional theory (B3LYP/6-31G*) and CPCM solvation models. Primary enamine-mediated aldol reactions involve half-chair transition states with hydrogen bonding leading to proton transfer. This leads to charge stabilization and low activation energies as compared to secondary enamine-mediated aldol reactions. Oxetane intermediates can be formed when C-C bond formation occurs without H-transfer in the transition state. The stereoselectivities of reactions of ketone enamines with aldehydes, including the facial stereoselectivity involving chiral aldehydes, were modeled and compared with experimental results. Transition states for the intramolecular aldol reactions leading to the formation of hydrindanone-beta-ketol and decalone-beta-ketol aldol products showed a preference for the formation of the cis-fused rings, in agreement with experimental results.

Reaction of cyclohexanone benzylimines with ethylidenemalonate diesters. Diphenyl 2-ethylidenemalonate: a highly electrophilic synthetic equivalent of crotonic esters

The diastereoselective Michael alkylation of alpha-substituted and alpha,alpha'-disubstituted cyclohexanone benzylimines with ethylidenemalonate diesters was carried out for mechanistic and synthetic purposes. In the first case, an inverse regioselectivity occurred in comparison with what is generally observed since the Michael adducts resulted from alkylation of the non substituted enamine tautomer. With alpha,alpha'-disubstituted imines, in all cases, the stereochemistry of the major diastereomer was the one anticipated from a mechanism including a chairlike complex approach with a preferred exo position for the beta-methyl group of the ethylidenemalonic acid diesters. Furthermore, diphenyl 2-ethylidenemalonate 4 was found to be a highly electrophilic synthetic equivalent of crotonic esters.

Enantioselective Michael reactions of chiral secondary enaminoesters with 2-substituted nitroethylenes. Syntheses of trans, trans-2,4-disubstituted pyrrolidine-3-carboxylates

The Michael reaction of chiral 3-substituted secondary enaminoesters with 2-substituted nitroethylenes leads to (Z)-adducts, with good to excellent diastereoselectivity. The nitro group of these adducts was catalytically reduced to give, after cyclization and chiral amine elimination, pyrrolines or pyrrolidines after further reduction. In particular, the syntheses of ethyl (2R,3S,4S)-2,4-dimethylpyrrolidine-3-carboxylate and ethyl (2R,3R,4S)-2-(4-methoxyphenyl)-4-(3,4-(methylenedioxy)phenyl)pyrrolidine-3-carboxylate are described.

Intramolecular Ene Reactions. Stereo- and Enantioselective Synthesis of Spirolactams through Thermolysis of Enamino Carboxamides

A new and facile access to spirolactams based on the thermal rearrangement of tertiary and secondary enamino carboxamides has been developed. The enamine group of an enamino carboxamide, in which no electron-withdrawing group is present in the enophile, can be involved in the ene reaction and the enamino carboxamide can be transformed into enamino or imino spirolactams. In the case of secondary carboxamido enamines, the diastereoselectivity is higher than 98%. If chiral nonracemic analogs are utilized, 50-54% enantiomeric excesses can be achieved in the final products.