ASYMMETRIC SYNTHESIS: A GLANCE AT VARIOUS METHODOLOGIES FOR DIFFERENT FRAMEWORKS

Abstract:

Asymmetric reactions have made a significant advancement over the past few decades and involved the production of enantiomerically pure molecules using enantioselective organocatalysis, chiral auxiliaries/substrates, and reagents via controlling the absolute stereochemistry. The laboratory synthesis from an enantiomerically impure starting material gives a combination of enantiomers which are difficult to separate for chemists in the fields of medicine, chromatography, pharmacology, asymmetric synthesis, studies of structure-function relationships of proteins, life sciences and mechanistic studies. This challenging step of separation can be avoided by the use of asymmetric synthesis. Using pharmacologically relevant scaffolds/pharmacophores, the drug designing can also be achieved using asymmetric synthesis to synthesize receptor specific pharmacologically active chiral molecules. This approach can be used to synthesize asymmetric molecules from wide variety of reactants using specific asymmetric conditions which is also beneficial for environment due to less usage and discharge of chemicals into the environment. So, in this review, we have focused on the inclusive collation of diverse mechanisms in this area, to encourage auxiliary studies of asymmetric reactions to develop selective, efficient, environment-friendly and high yielding advanced processes in asymmetric reactions.

Regioselective construction of pyridazine and tetrahydrocinnoline derivatives via [4 + 2] cycloaddition–elimination with α-halogeno hydrazones and enaminones

Herein, we report the de novo construction of pyridazine scaffolds via a [4 + 2] cycloaddition–elimination reaction with α-halogeno hydrazones and enaminones.

Recent advances in the asymmetric synthesis of pharmacology-relevant nitrogen heterocycles via stereoselective aza-Michael reactions

In this review, recent applications of a stereoselective aza-Michael reaction for asymmetric synthesis of naturally occurring N-containing heterocyclic scaffolds and their usefulness to pharmacology are summarized.

Enantioselective Alkylation of N-Arylhydrazones Derived from α-Keto Esters and Isatin Derivatives through Asymmetric Phase-Transfer Catalysis

The phase-transfer-catalyzed asymmetric alkylation reactions of N-arylhydrazones derived from α-keto-esters and isatin derivatives afford enantioenriched azo compounds that bear a tetra-substituted carbon stereocenter in good yields with high chemo- and enantioselectivity. The alkylation products can be readily converted into chiral amino esters, hydrazine derivatives, and aza-β-lactams without loss of enantiopurity.

Organocatalytic asymmetric synthesis of 2,4-disubstituted imidazolidines via domino addition-aza-Michael reaction

The first highly diastereo- and enantioselective synthesis of 2,4-disubstituted imidazolidines has been developed via a formal [3+2] cyclization reaction. Bidentate aminomethyl enones and N-tosyl imines were used as the reaction partners in the reaction. Bifunctional squaramide catalysts were found to be efficient for this reaction and few transformations of the products have been demonstrated.

N-Tosylhydrazones: versatile synthons in the construction of cyclic compounds

N-Tosylhydrazones have been extensively explored as versatile building blocks in the construction of various cyclic compounds.

Organocatalytic enantio- and diastereoselective synthesis of 3,5-disubstituted prolines

The asymmetric synthesis of substituted pyrrolidines has been accomplished using a novel organocatalytic cyclization reaction promoted by aCinchonaalkaloid base primary amine.

Core-Scaffold-Inspired Asymmetric Synthesis of Polysubstituted Chiral Hexahydropyridazines that Potently Inhibit Breast Cancer Cell Proliferation by Inducing Apoptosis

The highly enantioselective preparation of pharmacologically interesting hexahydropyridazine derivatives based on a multicomponent cascade reaction is described. This one-pot approach utilizes an organocatalytic Michael reaction followed by intermolecular α-amination and intramolecular hemiaminalization to yield a chiral pyridazine backbone with contiguous stereogenic centers and multiple functional groups in good yield and with high stereoselectivity. Compounds synthesized by this method potently inhibited proliferation of MCF-7 breast cancer cells. Mechanistic studies suggest that compound 5 c exerts these anticancer effects by inducing apoptosis through extracellular signal related kinase (ERK)- and poly(adenosine diphosphate ribose) polymerase (PARP)-regulated pathways, as well as mitochondrial pathways.

Fluorine effects in organocatalysis - asymmetric Brønsted acid assisted Lewis base catalysis for the synthesis of trifluoromethylated heterocycles exploiting the negative hyperconjugation of the CF3-group

An efficient Brønsted acid assisted Lewis base catalysis protocol for the synthesis of enantiomerically pure trifluoromethylated dihydropyridazines starting from readily available hydrazones and α,β-unsaturated aldehydes has been developed. The reaction exhibits high tolerance towards many functional groups and is applicable to various aliphatic, aromatic and hetero-aromatic α,β-unsaturated aldehydes, and provides the products in good yields and with excellent enantioselectivities.

4-Alkenyl-5H-1,2,3-oxathiazole 2,2-dioxides in catalytic and enantioselective [4 + 2] cycloaddition through iminium activation. Straightforward access to the trans-decaline framework and to densely functionalized cyclohexanes

4-Alkenyl-5H-1,2,3-oxathiazole 2,2-dioxides undergo Michael/Michael cascade reaction with enals through iminium/enamine activation.

Squaramide-catalysed asymmetric cascade aza-Michael/Michael addition reaction for the synthesis of chiral trisubstituted pyrrolidines

A squaramide catalysed aza-Michael/Michael cascade reaction between nitroalkenes and tosylaminomethyl enones or enoates afforded highly functionalized chiral pyrrolidines in good yields (up to 99%) with good diastereoselectivities (up to 91 : 9 dr) and excellent enantioselectivities (up to >99% ee) under mild conditions.

Organocatalytic cascade aza-Michael/hemiacetal reaction between disubstituted hydrazines and α,β-unsaturated aldehydes: Highly diastereo- and enantioselective synthesis of pyrazolidine derivatives

The catalytic synthesis of nitrogen-containing heterocycles is of great importance to medicinal and synthetic chemists, and also a challenge for modern chemical methodology. In this paper, we report the synthesis of pyrazolidine derivatives through a domino aza-Michael/hemiacetal sequence with chiral or achiral secondary amines as organocatalysts. Thus, a series of achiral pyrazolidine derivatives were obtained with good yields (up to 90%) and high diastereoselectivities (>20:1) with pyrrolidine as an organocatalyst, and enantioenriched pyrazolidines are also achieved with good results (up to 86% yield, >10/1 regioselectivity, >20:1 dr, 99% ee) in the presence of (S)-diphenylprolinol trimethylsilyl ether catalyst.