Diphenylprolinol silyl ether catalyzed asymmetric Michael reaction of nitroalkanes and ß,ß-disubstituted α,ß-unsaturated aldehydes for the construction of all-carbon quaternary stereogenic centers.

The asymmetric Michael reaction of nitroalkanes and ß,ß-disubstituted α,ß-unsaturated aldehydes was catalyzed by diphenylprolinol silyl ether to afford 1,4-addition products with an all-carbon quaternary stereogenic center with excellent enantioselectivity. The reaction is general for ß-substituents such as ß-aryl and ß-alkyl groups, and both nitromethane and nitroethane can be employed. The addition of nitroethane is considered a synthetic equivalent of the asymmetric Michael reaction of ethyl and acetyl substituents by means of radical denitration and Nef reaction, respectively. The short asymmetric synthesis of (S)-ethosuximide with a quaternary carbon center was accomplished by using the present asymmetric Michael reaction as the key step. The reaction mechanism that involves the E/Z isomerization of α,ß-unsaturated aldehydes, the retro-Michael reaction, and the different reactivity between nitromethane and nitroethane is discussed.

A theoretical and experimental study of the effects of silyl substituents in enantioselective reactions catalyzed by diphenylprolinol silyl ether.

The effect of silyl substituents in diphenylprolinol silyl ether catalysts was investigated. Mechanistically, reactions catalyzed by diphenylprolinol silyl ether can be categorized into three types: two that involve an iminium ion intermediate, such as for the Michael-type reaction (type A) and the cycloaddition reaction (type B), and one that proceeds via an enamine intermediate (type C). In the Michael-type reaction via iminium ions (type A), excellent enantioselectivity is realized when the catalyst with a bulky silyl moiety is employed, in which efficient shielding of a diastereotopic face of the iminium ion is directed by the bulky silyl moiety. In the cycloaddition reaction of iminium ions (type B) and reactions via enamines (type C), excellent enantioselectivity is obtained even when the silyl group is less bulky and, in this case, too much bulk reduces the reaction rate. In other cases, the yield increases when diphenylprolinol silyl ethers with bulky substituents are employed, presumably by suppressing side reactions between the nucleophilic catalyst and the reagent. The conformational behaviors of the iminium and enamine species have been determined by theoretical calculations. These data explain the effect of the bulkiness of the silyl substituent on the enantioselectivity and reactivity of the catalysts.

One-Pot Synthesis of (S)-Baclofen via Aldol Condensation of Acetaldehyde with Diphenylprolinol Silyl Ether Mediated Asymmetric Michael Reaction as a Key Step.

An efficient asymmetric total synthesis of (S)-baclofen was accomplished via a one-pot operation from commercially available materials using sequential reactions, such as aldol condensation of acetaldehyde, diphenylprolinol silyl ether mediated asymmetric Michael reaction of nitromethane, Kraus-Pinnick oxidation, and Raney Ni reduction. Highly enantioenriched baclofen was obtained in one pot with a good yield over four reactions.

Diphenylprolinol silyl ether as a catalyst in an asymmetric, catalytic, and direct michael reaction of nitroethanol with alpha,beta-unsaturated aldehydes.

Diphenylprolinol silyl ether was found to be an effective organocatalyst in the enantioselective and direct Michael reaction of nitroethanol and alpha,beta-unsaturated aldehydes, affording the 1-hydroxy-trans-3,4-disubstituted tetrahydropyrans after isomerization. The generated Michael addition products are useful synthetic intermediates, which can be converted into chiral tetrahydropyran with a quaternary stereocenter, 3-substituted cis- and trans-prolines, and alpha-amino acid derivatives.

Diphenylprolinol silyl ether as a catalyst in an enantioselective, catalytic michael reaction for the formation of alpha,alpha-disubstituted alpha-amino acid derivatives.

Direct and enantioselective: Diphenylprolinol silyl ether was found to catalyze the direct, asymmetric Michael reaction of 4-substituted 2-aryl-2-oxazoline-5-one and alpha,beta-unsaturated aldehydes, affording the chiral alpha,alpha-disubstituted alpha-amino acid derivatives with excellent enantioselectivity.