Asymmetric Synthesis of Enantioenriched 6-Hydroxyl Butyrolactams Promoted by N-Heterocyclic Carbene

Mechanism and Origin of Stereoselectivity of N-Heterocyclic Carbene (NHC)-Catalyzed Transformation Reaction of Benzaldehyde with o-QDM as Key Intermediate: A DFT Study

N-heterocyclic carbene (NHC)-bound ortho-quinodimethane, served as a nucleophile, has occupied an important position for constructing various all-carbon or heterocyclic compounds and attracted increasing attention for the functionalization of benzylic carbon of aromatic aldehydes, whereas the mechanistic studies on the generation and transformations of dienolate intermediate are rare. In the present study, the mechanism of activation/transformation of aldehyde catalyzed by NHC was theoretically studied using the density functional theory (DFT) method. Based on the calculations, the nucleophilic addition process is the stereoselectivity-determining step with RS-configured product being generated preferentially. Furthermore, non-covalent index (NCI) and atoms-in-molecules (AIM) analyses have been performed to disclose the origin of stereoselectivity, by which the larger number and stronger weak interactions are the key for stabilizing the low-energy transition state and thus leading to the stereoselectivity inducing.

NHC-Catalyzed Enantioselective Transformations Involving α-Bromoenals

α-Haloenals, especially, α-bromoenals considered as one of the important building blocks in organic synthesis. They can participate in various (3+2)-, (3+3)-, (3+4)-, and (2+4)-annulation reactions with other organic molecules in the presence of an NHC catalyst to produce enantioenriched carbo-, and heterocyclic compounds. Herein, we have described NHC-catalyzed enantioselective transformations of α-bromoenals in the synthesis of various heterocycles, and carbocycles, as well as acyclic organic compounds.

One-pot synthesis of γ-lactams from ketoaziridines

The remarkable biological activities of γ-lactams have stimulated the search for efficient synthetic methods to achieve these scaffolds. In this work, we have developed a simple one-pot diastereoselective synthesis of new γ-lactams from ketoaziridines with moderate to good yields via the Horner-Wadsworth-Emmons reaction, followed by an intramolecular ester-aziridine cyclization and its opening in situ. Preliminary efforts towards an enantioselective version of this method are also reported.

The preparation of optically active epineoclausenamide and enantiomeric separation of its racemate

We synthesized the optically active epineoclausenamide by utilizing chiral reagents, such as R-α-methylbenzylamine and S-α-methylbenzylamine, for the resolution of the intermediate (trans-3-phenyl-oxiranecarboxylic acid 12), followed by amide exchange, cyclization, and reduction, unlike previously reported methods. The Meerwein-Ponndorf-Verley reduction was used to asymmetrically reduce neoclausenamidone. A plausible reduction mechanism of this method was elucidated. Thereafter, high-performance liquid chromatography (HPLC) was investigated for the resolution of the epineoclausenamide enantiomers. HPLC was also used to determine the optical purity of these isomers. Two chiral stationary phases (CSPs) for separating the enantiomers were compared. Different mobile phase compositions were tested at 298.15 K. The results showed that the best separation was obtained when the mobile phase was composed of n-hexane and isopropanol (IPA) (75/25, v/v), the racemate was separated on a Chiralcel OJ-H column, and the flow rate was 1.0 mL/min at a wavelength of 210 nm and a temperature of 25°C. The enantiomeric ratio (e.r.) values of both the synthetic (-)-epineoclausenamide and (+)-epineoclausenamide were 1.3(+):98.7(-) and 99.3(+):0.7(-), respectively. In this study, a new synthetic route was designed with a yield of 12.3-14.1%, and a quick (8 min) effective separation method was obtained. This provides basis for pharmacological research and quality control of clausenamide analogues.

Brønsted Base-Catalyzed Formal Reductive [3+2] Annulation of 4,4,4-Trifluorocrotonate and α-Iminoketones

A formal reductive [3+2] annulation of 4,4,4-trifluorocrotonate and α-iminoketones was developed under Brønsted base catalysis. A single phosphazene base efficiently catalyzes the one-pot tandem reaction involving two mechanistically different elementary processes, namely the chemoselective reduction of an imine moiety of α-iminoketones with thiols as the reductant and the subsequent intermolecular Michael addition of an enolate of α-aminoketones concomitant with lactam formation. This operationally simple method provides β-trifluoromethyl-substituted γ-lactams with a tetrasubstituted carbon as a single diastereomer.

Asymmetric synthesis of γ-lactams under low-loading N-heterocyclic carbene catalysis

Low-loading N-heterocyclic carbene-catalyzed oxidative formal [3 + 2] annulation of enals with N-Ts diethyl aminomalonate has been successfully developed.

Facile Synthesis of Polysubstituted Indolizines via One-Pot Reaction of 1-Acetylaryl 2-Formylpyrroles and Enals

An efficient method for the synthesis of polysubstituted indolizines has been developed based on formal [4+2] annulation of 1-acetylaryl 2-formylpyrroles with enals, followed by oxidative aromatization. Pyridine-type six-membered rings were constructed in this transformation. This transition metal-free reaction features mild reaction conditions, a broad substrate scope, and excellent functional group tolerance. Notably, the formyl group is well tolerated under reaction conditions.

NHC-Catalyzed [3 + 3] Annulation of Thioamides and Modified Enals for the Enantioselective Synthesis of Functionalized Thiazinones

N-Heterocyclic carbene (NHC)-catalyzed [3 + 3] annulation of thioamides with modified enals allowing the enantioselective synthesis of functionalized 1,3-thiazin-4-ones is reported. The NHC generated from the chiral triazolium salt was optimal and the reaction is initiated by the thia-Michael addition to catalytically generated α,β-unsaturated acylazolium intermediates derived from 2-bromoenals, followed by intramolecular cyclization. This operationally simple procedure offers a straightforward and rapid access to target compounds in moderate to good yields and enantiomeric ratio values.