anti-Selective Asymmetric Michael Reactions of Aldehydes and Nitroolefins Catalyzed by a Primary Amine/Thiourea

Mannich-type Reactions of Cyclic Nitrones: Effective Methods for the Enantioselective Synthesis of Piperidine-containing Alkaloids

Even though there are dozens of biologically active 2-substituted and 2,6-disubstituted piperidines, only a limited number of approaches exist for their synthesis. Herein is described two Mannich-type additions to nitrones, one using β-ketoacids under catalyst-free conditions and another using methyl ketones in the presence of chiral thioureas, which can generate a broad array of such 2-substituted materials, as well as other ring variants, in the form of β-N-hydroxy-aminoketones. Both processes have broad scope, with the latter providing products with high enantioselectivity (up to 98 %). The combination of these methods, along with other critical steps, has enabled 8-step total syntheses of the 2,6-disubstituted piperidine alkaloids (-)-lobeline and (-)-sedinone.

Enantioselective synthesis of chiral heterocycles containing both chroman and pyrazolone derivatives catalysed by a chiral squaramide

An efficient chiral squaramide-catalysed enantioselective Michael addition of pyrazolin-5-ones to 3-nitro-2H-chromenes afforded chiral heterocycles containing both chroman and pyrazolone derivatives in high to excellent yields (up to 98%) with high enantioselectivities (up to 96%) under very low catalyst loading (0.2 mol%).

Asymmetric supramolecular catalysis: a bio-inspired tool for the high asymmetric induction in the enamine-based Michael reactions

King size: Utilization of large-size supramolecular rings in the pre-transition state (pre-TS) of enamine-based Michael reactions for high asymmetric induction is described. Enantiomerically pure, druglike hexahydroxanthenes with three contiguous stereocenters were synthesized through supramolecular catalysis by D-proline and quinine-NH-thiourea followed by reductive etherification from simple precursors under mild conditions (see scheme).

Adapting to substrate challenges: peptides as catalysts for conjugate addition reactions of aldehydes to α,β-disubstituted nitroolefins

Conjugate addition reactions of aldehydes to α,β-disubstituted nitroolefins are important because they provide synthetically useful γ-nitroaldehydes bearing three consecutive stereogenic centers. Such reactions are challenging due to the drastically lower reactivity of α,β-disubstituted nitroolefins compared to, for example, β-monosubstituted nitroolefins. The testing of a small collection of peptides of the type Pro-Pro-Xaa (Xaa=acidic amino acid) led to the identification of H-Pro-Pro-D-Gln-OH and H-Pro-Pro-Asn-OH as excellent stereoselective catalysts for this transformation. In the presence of 5 mol% of these peptides different combinations of aldehydes and α,β-disubstituted nitroolefins react readily with each other providing γ-nitroaldehydes in good yields and diastereoselectivities as well as excellent enantioselectivities. Chiral pyrrolidines as well as fully substituted γ-butyrolactams and γ-amino acids are easily accessible from the γ-nitroaldehydes. Mechanistic studies demonstrate that the configuration at all three stereogenic centers is induced by the peptidic catalysts. Only a minimal amount of products from homo-aldol reactions is observed demonstrating the high chemoselectivity of the peptidic catalysts.

Pyrrolidinyl-camphor derivatives as a new class of organocatalyst for direct asymmetric Michael addition of aldehydes and ketones to beta-nitroalkenes

Practical and convenient synthetic routes have been developed for the synthesis of a new class of pyrrolidinyl-camphor derivatives (7 a-h). These novel compounds were screened as catalysts for the direct Michael addition of symmetrical alpha,alpha-disubstituted aldehydes to beta-nitroalkenes. When this asymmetric transformation was catalyzed by organocatalyst 7 f, the desired Michael adducts were obtained in high chemical yields, with high to excellent stereoselectivities (up to 98:2 diastereomeric ratio (d.r.) and 99 % enantiomeric excess (ee)). The scope of the catalytic system was expanded to encompass various aldehydes and ketones as the donor sources. The synthetic application was demonstrated by the synthesis of a tetrasubstituted-cyclohexane derivative from (S)-citronellal, with high stereoselectivity.