The highly enantioselective Michael addition of ketones to nitrodienes catalyzed by the efficient organocatalyst system of pyrrolidinyl-thioimidazole and chiral thioureido acid

Synergistic Strategies in Aminocatalysis

Synergistic catalysis offers the unique possibility of simultaneous activation of both the nucleophile and the electrophile in a reaction. A requirement for this strategy is the stability of the active species towards the reaction conditions and the two concerted catalytic cycles. Since the beginning of the century, aminocatalysis has been established as a platform for the stereoselective activation of carbonyl compounds through HOMO-raising or LUMO-lowering. The burgeoning era of aminocatalysis has been driven by a deep understanding of these activation and stereoinduction modes, thanks to the introduction of versatile and privileged chiral amines. The aim of this review is to cover recent developments in synergistic strategies involving aminocatalysis in combination with organo-, metal-, photo-, and electro-catalysis, focusing on the evolution of privileged aminocatalysts architectures.

Asymmetric Brønsted acid catalysis with chiral carboxylic acids

This review provides an overview of various catalytic enantioselective transformations that utilize chiral carboxylic acids as Brønsted acid catalysts.

Using mutability landscapes of a promiscuous tautomerase to guide the engineering of enantioselective Michaelases

The Michael-type addition reaction is widely used in organic synthesis for carbon–carbon bond formation. However, biocatalytic methodologies for this type of reaction are scarce, which is related to the fact that enzymes naturally catalysing carbon–carbon bond-forming Michael-type additions are rare. A promising template to develop new biocatalysts for carbon–carbon bond formation is the enzyme 4-oxalocrotonate tautomerase, which exhibits promiscuous Michael-type addition activity. Here we present mutability landscapes for the expression, tautomerase and Michael-type addition activities, and enantioselectivity of 4-oxalocrotonate tautomerase. These maps of neutral, beneficial and detrimental amino acids for each residue position and enzyme property provide detailed insight into sequence–function relationships. This offers exciting opportunities for enzyme engineering, which is illustrated by the redesign of 4-oxalocrotonate tautomerase into two enantiocomplementary ‘Michaelases'. These ‘Michaelases' catalyse the asymmetric addition of acetaldehyde to various nitroolefins, providing access to both enantiomers of γ-nitroaldehydes, which are important precursors for pharmaceutically active γ-aminobutyric acid derivatives.

The Michael-type addition reaction is used for carbon-carbon bond formation; however biocatalytic methods for this reaction are rare. Here, the authors generate and exploit mutability landscapes of 4-oxalocrotonate tautomerase to direct the redesign of this promiscuous enzyme into enantio-complementary Michaelases.

Highly enantioselective Michael reaction employing cycloheptanone and cyclooctanone as nucleophiles

An organocatalytic Michael reaction of cycloheptanone and cyclooctanone with nitrodienes and nitroolefins catalyzed by primary amine catalysts has been accomplished.

Beyond classical reactivity patterns: shifting from 1,4- to 1,6-additions in regio- and enantioselective organocatalyzed vinylogous reactions of olefinic lactones with enals and 2,4-dienals

Organocatalysis is shown to expand the classical reactivity pattern for conjugate addition reactions. It is demonstrated that the site selectivity can be extended from 1,4- to 1,6-additions for the enantioselective vinylogous additions of methyl-substituted vinylogous lactones to enals and 2,4-dienals. This novel reactivity is demonstrated for methyl-substituted olefinic azlactones and butyrolactones. Their synthetic potential is first highlighted by the development of the organocatalytic regioselective vinylogous 1,4-addition to enals which proceeds with a very high level of double-bond geometry control and excellent enantioselectivity. The concept is developed further for the unprecedented intermolecular enantioselective organocatalyzed vinylogous 1,6-addition to linear 2,4-dienals, by which the site selectivity of the process is extended from the β-position to the remote δ-position of the 2,4-dienal. The organocatalyst controls the newly generated stereocenter six bonds away from the stereocenter of the catalyst with a high level of enantiocontrol, and the products are obtained with full control of double-bonds configuration. The scope of these new reaction concepts is demonstrated for a series of aliphatic and aryl-substituted enals and 2,4-dienals undergoing enantioselective vinylogous reactions with methyl-substituted olefinic azlactones and butyrolactones. Furthermore, mechanistic considerations are presented which can account for the change from 1,4- to 1,6-selectivity. Finally, a number of different transformations of the optically active 1,4- and 1,6-addition products are demonstrated.

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).

Practical access to highly enantioenriched quaternary carbon Michael adducts using simple organocatalysts

A three component catalyst system entailing an amino acid (O(t)Bu-L-threonine), a hydrogen bond donor (sulfamide), and an amine base (DMAP) allows α-branched aldehyde addition to nitroalkenes in good to high yield and excellent ee. Importantly, the lowest reported catalyst loading (5.0 mol%) and aldehyde stoichiometry (1.2-2.0 equiv) is demonstrated and in most instances the best current product profile is observed.