Highly Enantioselective Organocatalytic Sulfa-Michael Addition to α,β-Unsaturated Ketones

Sequential Ring Opening/In Situ SO2-Capture/Alkynylation of Cyclopropanols with Alkynyl Triflones Initiated by Energy Transfer

A visible-light-triggered ring opening/in situ SO2-capture/alkynylation sequence of cyclopropyl alcohols with alkynyl triflones using 4CzIPN as a triplet energy transfer photocatalyst is herein described. This metal-free protocol provides a straightforward and atom-economical approach to alkynyl-substituted γ-keto sulfones with a broad scope of substituents. In this transformation, alkynyl triflones could be used as both radical acceptors and SO2 donors. Preliminary experimental mechanistic studies and synthetic utility are also demonstrated.

One-pot synthesis of thioethers from indoles and p-quinone methides using thiourea as a sulfur source

Thiourea is an inexpensive and user friendly sulfur reagent that acts as a sulfur source. A simple and efficient protocol has been developed to access thioethers by reacting indoles with p-quinone methides using thiourea as the sulfur source. In our experiments, the reaction apparently proceeded through an S-(3-indolyl)isothiuronium iodide intermediate and subsequent generation of indolethiol that attacked the 1,6 position of p-quinone methides to give desired thioethers in good to excellent yields.

Metal-free thiolation of sulfonyl hydrazone with thiophenol: synthesis of 4-thio-chroman and diarylmethyl thioethers

A simple, efficient, and transition metal-free approach was developed for accessing 4-thio-substituted chroman and diarylmethyl thioethers from sulfonyl hydrazones. This protocol provides straightforward access to a class of diarylmethane derivatives with good to excellent yields. This operationally simple protocol exhibited good tolerance for labile functional groups, providing biologically relevant chemical libraries. This safe and feasible route is applicable to the large-scale synthesis of 4-thio-substituted chromans, which are of great synthetic interest because of their further reaction potential.

NbCl5-catalyzed sulfa-Michael addition for constructing quaternary centers in enones

A novel NbCl5-catalyzed sulfa-conjugate addition has been developed to construct quaternary centers in various enones. This new method enables a range of functionalized thiols to access different β-sulfido carbonyl compounds bearing a quaternary center. 27 novel β-sulfido ketones have been obtained with moderate to excellent yields. The preparative scale reactions also proceed well, showing no decrease in yield. We further studied the mechanism by DFT calculations. This methodology is significant in sulfur chemistry, especially in sulfa-conjugate addition, giving a new pathway to add thiols to tri-substituted enones.

NBS-mediated elimination of β-keto sulfides to access enones and dienones

A novel transformation of β-keto sulfides into enones has been developed. The new method facilitates an NBS-mediated elimination of sulfides to access both enones and dienones. 22 enone products were obtained in moderate to high yields. 4 different dienones were also prepared in 73%-93% yields. 7 different alkylthio motifs have been removed efficiently from β-keto sulfides. We also found that our transformation proceeds well in gram-scale reactions showing no decrease in yield. This methodology is significant in the research and application of sulfides, giving a new pathway to transform β-keto sulfides into enones.

Unravelling the Development of Non-Covalent Organocatalysis in India

This review is devoted to underpinning the contributions of Indian researchers towards asymmetric organocatalysis. More specifically, a comprehensive compilation of reactions mediated by a wide range of non-covalent catalysis is illustrated. A detailed overview of vividly catalogued asymmetric organic transformations promoted by hydrogen bonding and Brønsted acid catalysis, alongside an assortment of catalysts is provided. Although asymmetric organocatalysis has etched itself in history, we aim to showcase the scientific metamorphosis of Indian research from baby steps to large strides within this field.

1 Introduction

2 Non-Covalent Catalysis and Its Various Activation Modes

3 Hydrogen-Bonding Catalysis

3.1 Urea- and Thiourea-Derived Organocatalysts

3.1.1 Thiourea-Derived Organocatalysts

3.1.2 Urea-Derived Organocatalysts

3.2 Squaramide-Derived Organocatalysts

3.2.1 Michael Reactions

3.2.2 C-Alkylation Reactions

3.2.3 Mannich Reactions

3.2.4 [3+2] Cycloaddition Reactions

3.3 Cinchona-Alkaloid-Derived Organocatalysts

3.3.1 Michael Reactions

3.3.2 Aldol Reactions

3.3.3 Friedel–Crafts Reactions

3.3.4 Vinylogous Alkylation of 4-Methylcoumarins

3.3.5 C-Sulfenylation Reactions

3.3.6 Peroxyhemiacetalisation of Isochromans

3.3.7 Diels–Alder Reactions

3.3.8 Cycloaddition Reactions

3.3.9 Morita–Baylis–Hilman Reactions

4 Brønsted Acid Derived Organocatalysts

4.1 Chiral Phosphoric Acid Catalysis

4.1.1 Diels–Alder Reactions

4.1.2 Addition of Ketimines

4.1.3 Annulation of Acyclic Enecarbamates

5 Conclusion

Oxa-Michael Addition Reactions of 3-hydroxy-2-azetidinones: Synthesis of 1, 3, 4-Trisubstituted-2-Azetidinones

Abstract:

The manuscript describes a facile and an efficient methodology for the synthesis of 1, 3, 4 trisubstituted-β-lactams by base mediated oxa-Michael addition reactions of 3-hydroxy-2- azetidinones with acetylenic esters under different reaction conditions. These functionalized 1, 3, 4-trisubstituted-azetidin-2-ones are useful organic synthons for the synthesis of various heterocyclic compounds having diverse pharmacological applications.

Organocatalytic Enantioselective Sulfa-Michael Additions to α,β-Unsaturated Diazoketones

Enantioselective sulfa-Michael additions to α,β unsaturated diazocarbonyl compounds have been developed. Quinine-derived squaramide was found to be the best catalyst to promote C-S bond formation in a highly stereoselective fashion for alkyl and aryl thiols. The easy-to-follow protocol allowed the preparation of 22 examples in enantiomeric ratios up to 97:3 and reaction yields up to 94%. The mechanism and origins of enantioselectivity were determined through density functional theory (DFT) calculations.

Non-Covalent Interactions in Enantioselective Organocatalysis: Theoretical and Mechanistic Studies of Reactions Mediated by Dual H-Bond Donors, Bifunctional Squaramides, Thioureas and Related Catalysts

Chiral bifunctional dual H-bond donor catalysts have become one of the pillars of organocatalysis. They include squaramide, thiosquaramide, thiourea, urea, and even selenourea-based catalysts combined with chiral amines, cinchona alkaloids, sulfides, phosphines and more. They can promote several types of reactions affording products in very high yields and excellent stereoselectivities in many cases: conjugate additions, cycloadditions, the aldol and Henry reactions, the Morita–Baylis–Hilman reaction, even cascade reactions, among others. The desire to understand mechanisms and the quest for the origins of stereoselectivity, in attempts to find guidelines for developing more efficient catalysts for new transformations, has promoted many mechanistic and theoretical studies. In this review, we survey the literature published in this area since 2015.

The synthesis of chiral β-naphthyl-β-sulfanyl ketones via enantioselective sulfa-Michael reaction in the presence of a bifunctional cinchona/sulfonamide organocatalyst

Cinchona alkaloid-derived organocatalysts are widely employed in various asymmetric transformations, yielding products with high enantiopurity. In this respect, a bifunctional quinine-derived sulfonamide organocatalyst was developed to catalyze the asymmetric sulfa-Michael reaction of naphthalene-1-thiol with trans-chalcone derivatives. The target sulfa-Michael adducts were obtained with up to 96% ee under mild conditions and with a low (1 mol %) catalyst loading. Selected enantiomerically enriched sulfa-Michael addition products were subjected to oxidation to obtain the corresponding sulfones.

Organocatalyzed Sulfa-Michael Addition of Thiophenols on Trisubstituted α-Fluoroacrylates, a Straightforward Access to Chiral Fluorinated Compounds

In this manuscript, a simple and efficient sulfa-Michael addition reaction of aryl thiols to trisubstituted α-fluoro-α,β-unsaturated esters both in racemic and, for the first time, in enantioselective version is reported. The commercially available dimer of cinchona derivatives (DHQ)₂PYR was used as a catalyst. This strategy showed a great tolerance for various substrates and substituents, providing fair to excellent yields, moderate to excellent diastereoselectivities (2:1 to >99:1), and low to good enantioselectivities (2 to 87%). The reaction has been applied to the synthesis of fluorinated analogues of diltiazem and tiazesim, both therapeutic agents.

A metal-free reaction of sulfur dioxide, cyclopropanols and electron-deficient olefins

The importance of γ-keto sulfones in medicinal chemistry and organic synthesis is known. An efficient route to γ-keto sulfones via a metal-free reaction of cyclopropanols, sulfur dioxide and electron-deficient olefins is achieved. This reaction proceeds smoothly under mild conditions without the need of catalyst, oxidant or additive. A plausible mechanism is proposed, which occurs through a γ-keto sulfinate intermediate generated in situ from the reaction of cyclopropanol with sulfur dioxide. The γ-keto sulfinate intermediate would be trapped by the electron-deficient olefin, resulting in the formation of γ-keto sulfones. Various functional groups in the cyclopropanols and electron-deficient olefins are compatible in this transformation.

Recent Advances in Synthesis of Chiral Thioethers

Chiral thioethers is an important class of organosulfur molecules with extensive applications, especially in the field of medicine and organic synthesis. This review discusses the recent progress of synthesis of enantioenriched chiral thioethers and hopes to be helpful for related research in the future. It is summarized from organosulfur compounds-participating organic reaction types, including nucleophilic substitution, cross coupling, sulfa-Michael addition, sulfenylation, asymmetric allylic reaction, asymmetric Doyle-Kirmse reaction, Pummerer-type rearrangement, Smiles rearrangement,^[2,3] Stevens and Sommelet-Hauser rearrangement.

Zinc complexes supported by pyridine-N-oxide ligands: synthesis, structures and catalytic Michael addition reactions

Zinc complexes supported by pyridine-N-oxide ligands show highly efficient catalysis for Michael addition reaction. The NMR and MS experiments analysis displays intermediate [Zn(SR)₂(HSR)₄] has real catalytic activity for the addition reactions.

A one-pot metal-free protocol for the synthesis of chalcogenated furans from 1,4-enediones and thiols

Transition-metal-free synthesis of chalcogenated furans through the sequential thiol-Michael/Paal–Knorr reaction of 1,4-enediones in the presence of a catalytic amount of p-toluene sulfonic acid has been developed.

Conjugate addition from the excited state

Conjugate addition occurs efficiently from excited hydrazide based acrylanilides under both UV and metal free visible light irradiations. The reaction proceeds via an excited state encounter complex that bifurcates either via an electron or energy transfer pathway. The generality of excited state conjugate addition is demonstrated using chloromethylation and by thiol addition.

Synthesis of γ-keto sulfones by copper-catalyzed oxidative sulfonylation of tertiary cyclopropanols

Preparation of γ-keto sulfones via ring-opening sulfonylation of cyclopropanols.

Enantioselective bifunctional iminophosphorane catalyzed sulfa-Michael addition of alkyl thiols to unactivated β-substituted-α,β-unsaturated esters

The highly enantioselective sulfa-Michael addition of alkyl thiols to unactivated β-substituted-α,β-unsaturated esters catalyzed by a bifunctional iminophosphorane (BIMP) organocatalyst is described.

The highly enantioselective sulfa-Michael addition of alkyl thiols to unactivated β-substituted-α,β-unsaturated esters catalyzed by a bifunctional iminophosphorane (BIMP) organocatalyst is described. The low acidity of the alkyl thiol pro-nucleophiles is overcome by the high Brønsted basicity of the catalyst and the chiral scaffold/thiourea hydrogen-bond donor moiety provides the required enantiofacial discrimination in the addition step. The reaction is broad in scope with respect to the alkyl thiol and β-substituent of the α,β-unsaturated ester, affords sulfa-Michael adducts in excellent yields (up to >99%) and enantioselectivity (up to 97 : 3 er) and can operate down to 1 mol% catalyst loading.

Chiral Brønsted acid-catalysed enantioselective synthesis of isoindolinone-derived N(acyl),S-acetals

Chiral Brønsted acid catalyzed synthesis of N(acyl),S-acetals from in situ generated ketimines proceeds in high yields and enantioselectivities.

Enantioselective Synthesis of Highly Substituted Chromans via the Oxa-Michael-Michael Cascade Reaction with a Bifunctional Organocatalyst

A highly enantioselective synthesis of chiral chroman derivatives via an oxa-Michael-Michael cascade reaction has been developed using a bifunctional thiourea organocatalyst. The products were obtained with excellent enantioselectivities (up to >99%), good yields (up to 95%), and diastereoselectivities (up to 5:1).

Organocatalytic, Enantioselective Synthesis of 1- and 3-Substituted Isochromans via Intramolecular Oxa-Michael Reaction of Alkoxyboronate: Synthesis of (+)-Sonepiprazole

The enantioselective oxa-Michael reaction of alkoxyboronate strategy was demonstrated to provide a new and practical route to enantioriched 1- and 3-substituted isochromans using a chiral bifunctional organocatalyst. Furthermore, this methodology was extended to the enantioselective synthesis of (+)-sonepiprazole, a dopamine receptor antagonist.

Highly enantioselective sulfa-Michael addition reactions using N-heterocyclic carbene as a non-covalent organocatalyst

We report the first asymmetric sulfa-Michael addition (SMA) reactions using a chiral N-heterocyclic carbene (NHC) as a non-covalent organocatalyst. We demonstrate that a triazolium salt derived NHC functions as an excellent Brønsted base to promote enantioselective carbon-sulfur bond formation. The reaction is applicable to a wide range of thiols and electrophilic olefins. Notably, quaternary chiral centers bearing both an S atom and a CF3 group were synthesized with excellent asymmetric control. Mechanistic studies suggest that the facial discrimination is likely to be guided by non-covalent interactions: hydrogen bonding and π-π stacking.