Asymmetric catalyzed intramolecular aza-Michael reaction mediated by quinine-derived primary amines

Synthesis and Elimination Pathways of 1-Methanesulfonyl-1,2-dihydroquinoline Sulfonamides

A series of new Morita–Baylis–Hillman acetates were prepared and reacted with methanesulfonamide (K2CO3, DMF, 23 °C) to produce tertiary dihydroquinoline sulfonamides in high yields. Subsequent efforts to eliminate the methylsulfonyl group from these derivatives (K2CO3, DMF, 90 °C) as a route to quinolines were met with mixed results. Although dihydroquinoline sulfonamides prepared from ethyl acrylate and acrylonitrile generally underwent elimination to give excellent yields of quinolines, those generated from 3-buten-2-one failed to undergo elimination and instead decomposed. The failure of these ketone substrates to aromatize presumably derives from the enolizable methyl ketone at C-3. Finally, the attempted aromatization of the acrylate-derived 6,7-difluoro-1,2-dihydroquinoline sulfonamide demonstrated that other interesting processes could occur in preference to the desired elimination.

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.

Asymmetric hydrogenation for the synthesis of 2-substituted chiral morpholines

Asymmetric hydrogenation of unsaturated morpholines has been developed by using a bisphosphine-rhodium catalyst bearing a large bite angle. With this approach, a variety of 2-substituted chiral morpholines could be obtained in quantitative yields and with excellent enantioselectivities (up to 99% ee). The hydrogenated products could be transformed into key intermediates for bioactive compounds.

2-Substituted chiral morpholines were synthesized via a newly developed asymmetric hydrogenation of dehydromorpholines catalyzed by a bisphosphine–rhodium complex bearing a large bite angle.

Recent advances in organocatalytic asymmetric aza-Michael reactions of amines and amides

Nitrogen-containing scaffolds are ubiquitous in nature and constitute an important class of building blocks in organic synthesis. The asymmetric aza-Michael reaction (aza-MR) alone or in tandem with other organic reaction(s) is an important synthetic tool to form new C-N bond(s) leading to developing new libraries of diverse types of bioactive nitrogen compounds. The synthesis and application of a variety of organocatalysts for accomplishing highly useful organic syntheses without causing environmental pollution in compliance with 'Green Chemistry" has been a landmark development in the recent past. Application of many of these organocatalysts has been extended to asymmetric aza-MR during the last two decades. The present article overviews the literature published during the last 10 years concerning the asymmetric aza-MR of amines and amides catalysed by organocatalysts. Both types of the organocatalysts, i.e., those acting through non-covalent interactions and those working through covalent bond formation have been applied for the asymmetric aza-MR. Thus, the review includes the examples wherein cinchona alkaloids, squaramides, chiral amines, phase-transfer catalysts and chiral bifunctional thioureas have been used, which activate the substrates through hydrogen bond formation. Most of these reactions are accompanied by high yields and enantiomeric excesses. On the other hand, N-heterocyclic carbenes and chiral pyrrolidine derivatives acting through covalent bond formation such as the iminium ions with the substrates have also been included. Wherever possible, a comparison has been made between the efficacies of various organocatalysts in asymmetric aza-MR.

Two Decades of Progress in the Asymmetric Intramolecular aza-Michael Reaction

The asymmetric intramolecular aza-Michael reaction (IMAMR) is a very convenient strategy for the generation of heterocycles bearing nitrogen-substituted stereocenters. Due to the ubiquitous presence of these skeletons in natural products, the IMAMR has found widespread applications in the total synthesis of alkaloids and biologically relevant compounds. The development of asymmetric versions of the IMAMR are quite recent, most of them reported in this century. The fundamental advances in this field involve the use of organocatalysts. Chiral imidazolidinones, diaryl prolinol derivatives, Cinchone-derived primary amines and quaternary ammonium salts, and BINOL-derived phosphoric acids account for the success of those methodologies. Moreover, the use of N-sulfinyl imines with a dual role, as nitrogen nucleophiles and as chiral auxiliaries, appeared as a versatile mode of performing the asymmetric IMAMR.

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.

Asymmetric synthesis of (-)-sedacryptine through a diastereoselective Mannich reaction of N,O-acetals with ketones

An efficient diastereoselective approach to access the 3-hydroxy-2,6-disubstituted piperidine scaffold 1 has been developed through the Mannich process involving N,O-acetal (2S,3R)-6 and ketones in excellent yield with high diastereoselectivity (dr > 99 : 1). In addition, the utility of this convenient one-pot process is demonstrated by the asymmetric synthesis of (-)-sedacryptine 3.