N-Heterocyclic Carbene-Catalyzed Enantioselective Synthesis of Spiro-glutarimides via α,β-Unsaturated Acylazoliums

Pd/C-Catalyzed Carbonylative Amidation for the Synthesis of 2-Carboxamidocyclohexane-1,3-diones

Herein, a first-ever heterogeneous Pd/C-catalyzed single-step tandem approach for the synthesis of 2-carboxamidocyclohexane-1,3-diones via direct carbonylative C-H amidation of cyclohexane-1,3-diones is reported. The reaction progressed under base-, oxidant-, and ligand-free conditions employing oxalic acid as a CO surrogate and sodium azide as a nitrogen precursor in a double-layer vial system.

Annulations involving 1-indanones to access fused- and spiro frameworks

Indanones are prominent motifs found in number of natural products and pharmaceuticals. Particularly, 1-indanones occupy important niche in chemical landscape due to their easy accessibility and versatile reactivity. In the past few years, significant advancement has been achieved regarding cyclization of 1-indanone core. The present review focuses on recent (2016-2022) annulations involving 1-indanones for the construction of fused- and spirocyclic frameworks. In this context, new strategies for synthesis of various carbocyclic as well as heterocyclic skeletons are demonstrated. Mechanistic aspects of representative reactions are illustrated for better understanding of reaction pathways. A large number of transformations described in this review offer stereoselective formation of desired polycyclic compounds. Importantly, several reactions provide biologically relevant compounds and natural products, such as, plecarpenene/plecarpenone, swinhoeisterol A, cephanolides A-D, diptoindonesin G and atlanticone C.

Recent advances of N-heterocyclic carbenes in the applications of constructing carbo- and heterocyclic frameworks with potential biological activity

In recent years, N-heterocyclic carbenes (NHCs) have established themselves as a masterful and promising type of organocatalyst for the speedy construction of medicinally and biologically significant molecules from common and accessible small molecules. In particular, various cyclic scaffolds, including carbocycles and heterocycles, have been synthesized using NHCs via cycloaddition reaction. An exhaustive review focused on the chemistry of NHCs in these cyclic molecules has yet to be reported. In this contribution, a general picture of the utilization of NHCs in constructing twelve kinds of bioactive cyclic skeletons is firstly presented. We provide a systematic and comprehensive overview from the perspective of cycloaddition reactions; moreover, the limitations, challenges, and future prospects were discussed.

Enantioselective Synthesis of 5,6-Dihydroindolizines by N-Heterocyclic Carbene (NHC)-Catalyzed Core-Structure-Inspired Strategy of Azolium-Enolate Cascade

The core-structure motivated design has allowed the enantioselective synthesis of 5,6-dihydroindolizines via N-heterocyclic carbene (NHC) catalysis. The NHC-catalyzed reaction of α,β-unsaturated aldehydes with the suitably substituted pyrrole derivatives proceed via the initial generation of α,β-unsaturated acylazoliums from enals, and enolates from pyrroles and the reaction culminated in an efficient cascade process involving the Michael-aldol-lactonization-decarboxylation sequence to afford the products in reasonable yields and high selectivities. The method is further extended to the construction of spirocyclic 5,6-dihydroindolizines.

Revealing the Similarities of α,β-Unsaturated Iminiums and Acylazoliums in Organocatalysis

The secondary amine-catalyzed reactions proceeding via α,β-unsaturated iminiums and the N-heterocyclic carbene (NHC)-catalyzed transformations taking place via α,β-unsaturated acylazoliums are the two widely used electrophilic intermediates in organocatalysis. Over the last two decades, these two intermediates are extensively utilized for the enantioselective construction of valuable molecules. Both intermediates are generated by the covalent binding of catalysts to the substrates leading to LUMO activation of α,β-unsaturated carbonyls. A variety of soft nucleophiles are known to add to the α,β-unsaturated iminiums and acylazoliums in a conjugate fashion, and in many cases, striking similarity in reactivity has been observed. Having said this, there are few cases where these intermediates exhibit difference in reactivity. This Minireview is aimed at highlighting the resemblances in reactivity between α,β-unsaturated iminiums and acylazoliums thereby shedding light on the unnoticed parallels of the two intermediates in organocatalysis.

Pd-catalyzed amidation of 1,3-diketones with CO and azides via a nitrene intermediate

An efficient Pd-catalyzed amidation of 1,3-diketones has been developed using carbon monoxide and organic azides. This reaction provides a step-economic approach to produce β-ketoamides from readily available compounds under mild ligand-, oxidant-, and base-free conditions. The mechanistic studies showed that the reaction occurred through an in situ generated isocyanate intermediate.

Imines as acceptors and donors in N-heterocyclic carbene (NHC) organocatalysis

The synthetic potential of imines as electrophiles or as a source of nucleophilic coupling partner in N-heterocyclic carbene (NHC) catalysis for the synthesis of various nitrogen heterocycles and functionalized amines is highlighted in this Feature Article. Electrophilic imines are suitable candidates for intercepting the NHC-derived acyl anions, homoenolate equivalents, and (di)enolates for the synthesis of α-amino ketones and a variety of lactam derivatives. Moreover, enamines generated from imines bearing α-hydrogen could be trapped with α,β-unsaturated acylazoliums for the synthesis of functionalized dihydropyridinones. NHCs are also useful for the umpolung of imines for the generation of aza-Breslow intermediates thus leading to the synthesis of indoles, quinolines, dihydroquinoxalines etc. A concise account of the diverse reactivity of imines in NHC catalysis has been presented.

NHC-catalyzed β-specific addition of N-based nucleophiles to allenoates

N-heterocyclic carbene (NHC) catalyzed reactions of nitrogenous heterocycles or trifluoromethylated acylhydrazone with allenoates gave regiospecific β-adducts. DFT calculations rationalized the origination of regio- and E/Z selectivities.

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.

Reliable Target Prediction of Bioactive Molecules Based on Chemical Similarity Without Employing Statistical Methods

The prediction of biological targets of bioactive molecules from machine-readable materials can be routinely performed by computational target prediction tools (CTPTs). However, the prediction of biological targets of bioactive molecules from non-digital materials (e.g., printed or handwritten documents) has not been possible due to the complex nature of bioactive molecules and impossibility of employing computations. Improving the target prediction accuracy is the most important challenge for computational target prediction. A minimum structure is identified for each group of neighbor molecules in the proposed method. Each group of neighbor molecules represents a distinct structural class of molecules with the same function in relation to the target. The minimum structure is employed as a query to search for molecules that perfectly satisfy the minimum structure of what is guessed crucial for the targeted activity. The proposed method is based on chemical similarity, but only molecules that perfectly satisfy the minimum structure are considered. Structurally related bioactive molecules found with the same minimum structure were considered as neighbor molecules of the query molecule. The known target of the neighbor molecule is used as a reference for predicting the target of the neighbor molecule with an unknown target. A lot of information is needed to identify the minimum structure, because it is necessary to know which part(s) of the bioactive molecule determines the precise target or targets responsible for the observed phenotype. Therefore, the predicted target based on the minimum structure without employing the statistical significance is considered as a reliable prediction. Since only molecules that perfectly (and not partly) satisfy the minimum structure are considered, the minimum structure can be used without similarity calculations in non-digital materials and with similarity calculations (perfect similarity) in machine-readable materials. Nine tools (PASS online, PPB, SEA, TargetHunter, PharmMapper, ChemProt, HitPick, SuperPred, and SPiDER), which can be used for computational target prediction, are compared with the proposed method for 550 target predictions. The proposed method, SEA, PPB, and PASS online, showed the best quality and quantity for the accurate predictions.

Highly Enantioselective Synthesis of Functionalized Glutarimide Using Oxidative N-Heterocyclic Carbene Catalysis: A Formal Synthesis of (-)-Paroxetine

A simple yet highly effective approach toward enantioselective synthesis of trans-3,4-disubstituted glutarimides from readily available starting materials is developed using oxidative N-heterocyclic carbene catalysis. The catalytic reaction involves a formal [3 + 3] annulation between enals and substituted malonamides enabling the production of glutarimide derivatives in a single chemical operation via concomitant formation of C-C and C-N bonds. The reaction offers easy access to a broad range of functionalized glutarimides with excellent enantioselectivity and good yield. Synthetic application of the method is demonstrated via formal synthesis of (-)-paroxetine and other bioactive molecules.

NHC-Catalyzed Generation of α,β-Unsaturated Acylazoliums for the Enantioselective Synthesis of Heterocycles and Carbocycles

This Account is aimed at highlighting the recent developments in the N-heterocyclic carbene (NHC)-catalyzed generation of α,β-unsaturated acylazolium intermediates and their subsequent reactivity with (bis)nucleophiles thereby shedding light on the power of this NHC-bound intermediate in organocatalysis. This key intermediate can be generated by the addition of NHCs to α,β-unsaturated aldehyde or acid derivatives. A wide variety of bisnucleophiles can add across the α,β-unsaturated acylazoliums to form various five and six-membered heterocycles and carbocycles. Moreover, suitably substituted nucleophiles can add to this intermediate and result in valuable products following cascade processes. Employing chiral NHCs in the process can result in the enantioselective synthesis of valuable compounds. In 2013, we developed a unified strategy for the enantioselective synthesis of dihydropyranones and dihydropyridinones by the NHC-catalyzed formal [3 + 3] annulation of 2-bromoenals with readily available 1,3-dicarbonyl compounds or primary vinylogous amides. This reaction takes place under mild conditions with low catalyst loading. Interestingly, employing enolizable aldehydes as the bisnucleophiles in this annulation afforded chiral 4,5-disubstituted dihydropyranones in spite of the competing benzoin/Stetter pathways. Moreover, the use of cyclic 1,3-dicarbonyl compounds such as 4-hydroxy coumarin/pyrazolone afforded the coumarin/pyrazole-fused dihydropyranones. In addition, a [3 + 2] annulation for the synthesis of spiro γ-butyrolactones was demonstrated using 3-hydroxy oxindoles as the bisnucleophile. The interception of α,β-unsaturated acylazolium intermediates with malonic ester derivatives having a γ-benzoyl group resulted in the enantioselective synthesis of functionalized cyclopentenes via a cascade process involving a Michael-intramolecular aldol-β-lactonization-decarboxylation sequence. The use of cyclic β-ketoamides as the coupling partner for catalytically generated α,β-unsaturated acylazoliums resulted in the enantioselective synthesis of spiro-glutarimide and the reaction proceeds in a Michael addition-intramolecular amidation pathway. We have recently demonstrated the enantioselective synthesis of tricyclic δ-lactones with three contiguous stereocenters by the reaction of enals with dinitrotoluene derivatives bearing electron-withdrawing groups, under oxidative conditions. This atom-economic cascade reaction proceeds in a Michael/Michael/lactonization sequence tolerating a range of functional groups. This technique was also used for the N-H functionalization of indoles for the enantioselective synthesis of pyrroloquinolines following the aza-Michael/Michael/lactonization sequence. The use of α-arylidene pyrazolinones as the bisnucleophiles for the tandem generation of dienolate/enolates combined with the NHC-catalyzed generation of α,β-unsaturated acylazoliums resulted in the enantioselective synthesis of pyrazolone-fused spirocyclohexadienones. This formal [3 + 3] annulation proceeds via the vinylogous Michael/spiroannulation/dehydrogenation sequence to afford spirocyclic compounds with an all-carbon quaternary stereocenter. It is reasonable to believe that the chemistry of α,β-unsaturated acylazoliums, catalytically generated through NHCs, will continue to flourish and will lead to amazing results. Future challenges in this area include the applications of this key intermediate in the synthesis of biologically active natural products and drugs.

Facile Synthesis of 2,2-Diacyl Spirocyclohexanones via an N-Heterocyclic Carbene-Catalyzed Formal [3C + 3C] Annulation

A novel strategy for the construction of 2,2-diacyl spirocyclohexanones 3 has been demonstrated on the basis of an NHC-catalyzed [3C + 3C] annulation of potassium 2-oxo-3-enoates with 2-ethylidene 1,3-indandiones. Furthermore, enantioenriched 3 was obtained in good to excellent yields with good enantioselectivities when chiral N-heterocyclic carbene (NHC) was employed. Notably, ring opening of the resulting 2,2-diacyl spirocyclohexanones 3 with hydrazine led to the formation of phthalazinones in good to excellent yields.

Enantioselective Organocatalytic Four-Atom Ring Expansion of Cyclobutanones: Synthesis of Benzazocinones

An enantioselective Michael addition- four-atom ring expansion cascade reaction involving cyclobutanones activated by a N-aryl secondary amide group and ortho-amino nitrostyrenes has been developed for the preparation of functionalized eight-membered benzolactams using bifunctional aminocatalysts. Taking advantage of the secondary amide activating group, the eight-membered cyclic products could be further rearranged into their six-membered isomers having a glutarimide core under base catalysis conditions without erosion of optical purity, featuring an overall ring expansion- ring contraction strategy.