Organocatalytic enantioselective conjugate addition of aldehydes to maleimides

Curtin-Hammett paradigm for stereocontrol in organocatalysis by diarylprolinol ether catalysts

Detailed mechanistic study of two reactions catalyzed by diarylprolinol ether catalysts, the conjugate addition of aldehydes to nitro-olefins and the α-chlorination of aldehydes, leads to the proposal that the stereochemical outcome in these cases is not determined by the transition state of the step in which the stereogenic center is formed from enamine attack on the electrophile but instead is correlated with the relative stability and reactivity of diastereomeric intermediates downstream in the catalytic cycle. This combination of kinetic and thermodynamic factors illustrates a remarkable Curtin-Hammett scenario that can result in either an enhancement or an erosion of the selectivity that would be predicted by the transition state for enamine attack on the electrophile. Evidence is offered to suggest that this concept may represent a general phenomenon for pyrrolidine-based catalysts lacking an acidic directing proton. Implications for catalyst and reaction design are discussed.

Organocatalytic approach for the syntheses of corynantheidol, dihydrocorynantheol, protoemetinol, protoemetine, and mitragynine

O-Trimethylsilyl (TMS)-protected diphenylprolinol-catalyzed Michael addition of a functionalized alkylidene malonate and n-butanal affords an aldehyde. This adduct can serve as the common intermediate for the assembly of secologanin tryptamine and dopamine alkaloids; this is demonstrated by the total syntheses of corynantheidol, dihydrocorynantheol, protoemetinol, and protoemetine, and the formal synthesis of mitragynine. The key steps include reductive amination of this aldehyde with tryptamine, condensation of this aldehyde with 4-methoxytryptamine, condensation of dimethoxyphenethylamine with a lactone derived from this aldehyde, and subsequent Bischler-Napieralski cyclization and reduction.

Alpha,alpha-diarylprolinol ethers: new tools for functionalization of carbonyl compounds

Alpha,alpha-diaryl(dialkyl)prolinol ethers constitute a potent organocatalyst family which has been shown to be very general for a broad range of reactions involving enamine and iminium ion activation or a combination of both. The reactions are characterized by an efficient steric control approach and can lead to a variety of alpha-, beta-, gamma-, and alpha,beta-functionalized carbonyl compounds with excellent stereocontrol. As a full expression of their catalytic activity, these compounds are also excellent promoters of elegant cascade processes and valuable catalysts in water-compatible systems.

Efficient synthesis of gamma-lactams by a tandem reductive amination/lactamization sequence

A three-component method for the synthesis of highly substituted gamma-lactams from readily available maleimides, aldehydes, and amines is described. A new reductive amination/intramolecular lactamization sequence provides a straightforward route to the lactam products in a single manipulation. The general utility of this method is demonstrated by the parallel synthesis of a gamma-lactam library.