Enamides and dienamides in phosphoric acid-catalysed enantioselective cycloadditions for the synthesis of chiral amines

This feature article describes how enamides and dienamides can participate in chiral phosphoric acid catalyzed enantioselective cycloadditions to prepare a wide range of cyclic amines.

Organocatalytic synthesis of 5-hydroxyisoxazolidine catalyzed by camphor sulfonyl hydrazines through aza-Michael addition/cyclization

A series of chiral 5-hydroxy isoxazolidines has been successfully synthesized through camphor sulfonyl hydrazine-catalyzed asymmetric aza-Michael addition reaction between N,O-protected hydroxyamines and enals. Moderate yields with moderate to good enantioselectivities (up to 96% enantiomeric excess [ee]) were achieved. It provides an alternative asymmetric approach to preparing isoxazolidine derivatives.

Chiral Imidazolidin-4-one with catalytic amount of Dicationic ionic liquid act as a recoverable and reusable Organocatalyst for asymmetric Diels-Alder reaction

Imidazolidin-4-one is used as a recoverable organocatalyst for the asymmetric Diels-Alder reaction in the presence of catalytic amount of dicationic ionic liquid and trifluoroacetic acid as a co-catalyst. The Diels-Alder reaction between model substrate cyclopentadiene and crotonaldehyde gave the product in 95% conversion and 87% ee of the endo-product. The catalyst was shown better reusability when the 20 mol% of dicationic ionic liquid was used and catalyst was reused upto 5 cycles, conversion remains upto 3 recycles but ee of endo-9 was slightly droped.

Organocatalytic asymmetric Michael addition between 3-subsituted oxindoles and enals catalyzed by camphor sulfonyl hydrazine

3,3-Disubstituted oxindoles containing vicinal stereogenic carbon centers have been synthesized through organocatalytic asymmetric Michael addition between 3-substituted oxindoles and enals catalyzed by chiral camphor sulfonyl hydrazines (CaSHs).

Asymmetric Diels-Alder Reaction of α-Substituted and β,β-Disubstituted α,β-Enals via Diarylprolinol Silyl Ether for the Construction of All-Carbon Quaternary Stereocenters

The asymmetric Diels-Alder reaction of α-substituted acrolein proceeds in the presence of the trifluoroacetic acid salt of trifluoromethyl-substituted diarylprolinol silyl ether to afford the exo-isomer with both excellent diastereoselectivity and high enantioselectivity. In the Diels-Alder reaction of a β,β-disubstituted α,β-unsaturated aldehyde, good exo-selectivity and excellent enantioselectivity was obtained when the perchloric acid salt of the bulky triisopropyl silyl ether of trifluoromethyl substituted diarylprolinol was employed as an organocatalyst in the presence of water. In both cases, all-carbon quaternary stereocenters are constructed enantioselectively.

An Organocatalytic Cope Rearrangement

The first example of an organocatalytic Cope rearrangement is reported. Acyclic and cyclic acyl hydrazides catalyze the rearrangement of 1,5-hexadiene-2-carboxaldehydes via iminium ion formation. A correlation between ring size and catalyst activity was observed for the cyclic hydrazides, with seven- and eight-membered-ring catalysts being the most active. Diazepane carboxylate 5 c (10 mol %) catalyzed the rearrangement of a range of dienes at room temperature in acetonitrile using triflic acid as a co-catalyst. Preliminary proof of principle for asymmetric catalysis was provided by rearrangement of 3,3-dimethyl-7-phenyl-1,5-heptadiene-2-carboxaldehyde in the presence of a novel 7-substituted diazepane carboxylate.

Improving catalyst activity in secondary amine catalysed transformations

The effect on catalyst performance of altering substituents at the 2-position of the Macmillan imidazolidinone has been examined. Condensation of L-phenylalanine N-methyl amide with acetophenone derivatives results in a series of imidazolidinones whose salts can be used to accelerate the Diels-Alder cycloaddition. Electron withdrawing groups significantly increase the overall rate of cycloaddition without compromise in selectivity. The most effective catalyst was shown to be efficient for a variety of substrates and the applicability of this catalyst to alternative secondary amine catalysed transformations is also discussed.

New insights into the asymmetric Diels–Alder reaction: the endo- and S-selective retro-Diels–Alder reaction

The endo- and S-selective retro-Diels–Alder reactions have been proposed and verified in an imidazolethione-catalyzed asymmetric Diels–Alder reaction.

Synthesis of MacMillan catalyst modified with ionic liquid as a recoverable catalyst for asymmetric Diels–Alder reaction

MacMillan catalyst was modified with imidazolium ionic liquid by ester linkage and acts as recoverable and reusable catalyst for asymmetric Diels–Alder reactions.

A novel hydrazide type organocatalyst for enantioselective Diels-Alder reactions

The development of a new class of hydrazide type organocatalyst, (4R,5R)-1,3-bis(isopropylamino)-4,5-dihenylimidazolidin-2-one 2a, for enantioselective Diels-Alder reactions between cyclopentadiene and α,β-unsaturated aldehydes are presented. The new organocatalyst 2a promoted the reaction, affording Diels-Alder adducts in good yields with good levels of enantioselectivity.

Secondary and primary amine catalysts for iminium catalysis

Formation of iminium ions from the condensation of chiral secondary or primary amines with alpha,beta-unsaturated aldehydes or ketones can be used as an effective platform for the acceleration of a wide variety of catalytic asymmetric cycloaddition and conjugate addition reactions. The reversible formation of the active iminium ion species simulates the pi-electronics and equilibrium dynamics traditionally associated with Lewis acid activation of alpha,beta-unsaturated carbonyl compounds lowering the energy level of the LUMO associated with the pi-system and activating subsequent reaction. Importantly, these iminium ion catalysed processes offer the opportunity to conduct reactions in the presence of both moisture and air greatly adding to the practicality and general applicability of the chemistry described. Proposed catalytic cycles and transition state models for the induction of asymmetry provide reliable and robust predictive tools for the outcome of reactions and high functional group tolerance suggests this class of transformation will have broad application in the arena of synthetic organic chemistry as the area matures. This review describes the rapid expansion of iminium ion catalysis over recent years from its conceptual introduction to the development of a whole new arsenal of highly practical and effective methods with which to approach challenging and fundamental bond construction processes.

Organocatalytic reactions in water

Organocatalysts have emerged as a third major way of catalyzing a wide variety of reactions, besides metal catalysts and biocatalysts. They have gained tremendous importance because of their green chemistry perspective. The criteria for green chemistry would be largely fulfilled if the major component of the reaction mixture, i.e. the solvent, is water which is a suitable solvent in various biosynthetic reactions. In this feature article we have described reactions promoted by organocatalysts in a large excess of water, without any organic solvent or excess of any reactant. We have also explained the structural features required for organocatalysts to work well in aqueous media.

Camphor sulfonyl hydrazines (CaSH) as organocatalysts in enantioselective Diels-Alder reactions

Cyclic sulfonyl hydrazine was demonstrated for the first time as a new functionality for organocatalysis. A series of six-membered cyclic hydrazines derived from camphor sulfonic acid were synthesized. With trichloroacetic acid as cocatalyst, they are efficient organocatalysts for enantioselective Diels-Alder reactions with good chemical yields and up to 96% ee. The reactions took place in brine at 0 degrees C to room temperature.