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.

Bifunctional Brønsted Base Catalyzed [3 + 3] Annulations of Indolin-2-imines and α,β-Unsaturated Imides: An Enantioselective Approach to α-Carbolinones

Asymmetric construction of α-carbolinones with easily available starting materials has recently attracted considerable attention from the synthesis community, and the development of effective catalysis for this target is in great demand. Here, a bifunctional Brønsted base catalyzed asymmetric [3 + 3] cyclization of indolin-2-imines and α,β-unsaturated N-acylated succinimides was developed by using the strategy of noncovalent bonding catalysis. With this organocatalytic protocol, a variety of tetrahydro-α-carbolinones bearing different substituents were synthesized with up to 99% yield and up to 96:4 er.

Catalytic enantioselective one-pot approach to cis- and trans-2,3-diaryl substituted 1,5-benzothiazepines

The first enantioselective catalytic approach to cis- and trans-2,3-diaryl substituted 1,5-benzothiazepines has been conveniently developed in a one-pot fashion, starting from α,β-unsaturated acyl pyrazoles and 2-aminothiophenol. The organocatalytic two-step sulfa-Michael/lactamization sequence is promoted by a readily available bifunctional thiourea and p-toluenesulfonic acid, respectively. The protocol enables access to both N-unprotected cis- and trans-diastereoisomers in moderate to satisfactory overall yields (up to 84%) and good to excellent ee values (up to 99%). Mechanistic investigations helped to shed light on the regio- and stereoselective outcome of the process.

Mechanistic Insight into Asymmetric Hetero-Michael Addition of α,β-Unsaturated Carboxylic Acids Catalyzed by Multifunctional Thioureas

Carboxylic acids and their corresponding carboxylate anions are generally utilized as Brønsted acids/bases and oxygen nucleophiles in organic synthesis. However, a few asymmetric reactions have used carboxylic acids as electrophiles. Although chiral thioureas bearing both arylboronic acid and tertiary amine were found to promote the aza-Michael addition of BnONH₂ to α,β-unsaturated carboxylic acids with moderate to good enantioselectivities, the reaction mechanism remains to be clarified. Detailed investigation of the reaction using spectroscopic analysis and kinetic studies identified tetrahedral borate complexes, comprising two carboxylate anions, as reaction intermediates. We realized a dramatic improvement in product enantioselectivity with the addition of 1 equiv of benzoic acid. In this aza-Michael reaction, the boronic acid not only activates the carboxylate ligand as a Lewis acid, together with the thiourea NH-protons, but also functions as a Brønsted base through a benzoyloxy anion to activate the nucleophile. Moreover, molecular sieves were found to play an important role in generating the ternary borate complexes, which were crucial for obtaining high enantioselectivity as demonstrated by DFT calculations. We also designed a new thiourea catalyst for the intramolecular oxa-Michael addition to suppress another catalytic pathway via a binary borate complex using steric hindrance between the catalyst and substrate. Finally, to demonstrate the synthetic versatility of both hetero-Michael additions, we used them to accomplish the asymmetric synthesis of key intermediates in pharmaceutically important molecules, including sitagliptin and α-tocopherol.

Stereoselective organocatalytic sulfa-Michael reactions of aryl substituted α,β-unsaturated N-acyl pyrazoles

A variety of functionalised sulfides has been obtained in good to high stereoselectivity via sulfa-Michael reaction of α,β-unsaturated N-acyl pyrazoles with thiols or thioacetic acid using readily available organocatalysts and working under mild conditions.

Chiral squaramide-catalysed enantioselective Michael/cyclization cascade reaction of 3-hydroxyoxindoles with α,β-unsaturated N-acylated succinimides

A squaramide-catalysed asymmetric Michael/cyclization cascade reaction of 3-hydroxyoxindoles with α,β-unsaturated N-acylated succinimides afforded spirooxindole lactones in good yields with high stereoselectivities.

Organocatalytic enantioselective construction of isatin-derived N-alkoxycarbonyl 1,3-aminonaphthols via sterically encumbered hydrocarbon-substituted quinine-based squaramide

2-Adamantyl substituted quinine-squaramide catalyzed aza-Friedel–Crafts reaction of N-carbamate isatin ketimines with naphthol afforded chiral isatin-derived 1,3-aminonaphthols with excellent enantioselectivity up to greater than 99% and quantitative yield.