Zn-catalyzed enantio- and diastereoselective formal [4 + 2] cycloaddition involving two electron-deficient partners: asymmetric synthesis of piperidines from 1-azadienes and nitro-alkenes

Regio- and Diastereoselective Synthesis of Polysubstituted Piperidines Enabled by Boronyl Radical-Catalyzed (4+2) Cycloaddition

Piperidines are widely present in small molecule drugs and natural products. Despite many methods have been developed for their synthesis, new approaches to polysubstituted piperidines are highly desirable. This work presents a radical (4+2) cycloaddition reaction for synthesis of piperidines featuring dense substituents at 3,4,5-positions that are not readily accessible by known methods. Using commercially available diboron(4) compounds and 4-phenylpyridine as the catalyst precursors, the boronyl radical-catalyzed cycloaddition between 3-aroyl azetidines and various alkenes, including previously unreactive 1,2-di-, tri-, and tetrasubstituted alkenes, has delivered the polysubstituted piperidines in generally high yield and diastereoselectivity. The reaction also features high modularity, atom economy, broad substrate scope, metal-free conditions, simple catalysts and operation. The utilization of the products has been demonstrated by selective transformations. A plausible mechanism, with the ring-opening of azetidine as the rate-limiting step, has been proposed based on the experimental and computational results.

Tri-coordinated zinc alkyl complexes with N^S/Se coordination of imino-phosphanamidinate chalcogenide ligands as precursors for efficient hydroboration of nitriles and esters

A series of tri-coordinated zinc alkyl complexes with the general molecular formula [κ2NE-{NHIRP(Ph)(E)N-Dipp}ZnEt] [R = Dipp (2,6-diisopropylphenyl), E = S (3a), Se (3b) and R = tBu (tert-butyl), E = S (4a), Se (4b)] bearing imino-phosphanamidinate chalcogenide ligands were prepared in good yields from the reaction between the protic imino-phosphanamidinate chalcogenide ligand [NHIRP(Ph)(E)NH-Dipp] [R = Dipp, E = S (1a), Se (1b) and R = tBu, E = S (2a), Se (2b)] and diethylzinc at room temperature. The molecular structures of all the zinc complexes were established by single-crystal X-ray diffraction analysis. In the solid state, all complexes exhibited a distorted trigonal planar geometry around the zinc ion. Metal-chalcogenide (Zn-S/Se) interactions were observed in the coordination sphere. These zinc alkyl complexes were employed as pre-catalysts in the hydroboration reaction of nitriles and esters to obtain the corresponding N,N-diborylamines and boronate esters, respectively, under ambient conditions. A wide substrate scope of nitriles and esters is presented.

Rational design and identification of novel thiosemicarbazide derivatives as laccase inhibitors

BACKGROUND

Laccase is a key enzyme in the fungal 1,8-dihydroxynaphthalene (DHN) melanin biosynthesis pathway, which is a potential target for the control of pathogenic fungi. In our previous work, compound a2 was found with higher inhibition activity against laccase and antifungal activity than laccase inhibitor PMDD-5Y. The introduction of hydrogen-bonded receptors in the amino part was found to be beneficial in improving laccase inhibitory activity by target-based-biological rational design. In this work, the hydrogen-bonded receptors morpholine and piperazine were introduced for structure optimization to enhancing biological activity.

RESULTS

Enzyme activity tests indicated that all target compounds had inhibitory activity against laccase, and some compounds exhibited better activity against laccase than a2, it was further verified that the introduction of hydrogen-bonded receptors in the amino portion could enhance the laccase inhibitory activity of target compounds. Most compounds showed excellent antifungal activities in vitro. Compound m14 displayed good activity against Magnaporthe oryzae both in vitro and in vivo. The scanning electron microscopy (SEM) analysis showed that the mycelium of M. oryzae treated with m14 were destroyed. Molecular docking revealed the binding mode between laccase and target compounds.

CONCLUSION

Thirty-eight compounds were synthesized and showed good inhibitory activity against laccase, the introduction of morpholine and piperazine in the amino part was beneficial to improve antifungal activity and laccase activity. Further validation of laccase as a potential target for rice blast control, while m14 can be used as a candidate compound for the control of rice blast. © 2023 Society of Chemical Industry.

Copper-Catalyzed Intermolecular Cross-dehydrogenative C-N Coupling at Room Temperature via Remote Activating Group Enabled Radical Relay Strategy

Employing N-fluorobenzenesulfonimide (NFSI) as a nitrogen-centered radical (NCR) precursor, an intermolecular C(sp²)-N coupling on heteroarenes or substituted benzenes with remote activated aniline derivatives via copper catalyzed N-N radical relay strategy at room temperature is developed. Good to excellent yields are acquired, and no ligand or additive is required. Reaction scope investigation and preliminary mechanistic studies demonstrate that the remote activating strategy and delicate control on the reactivities of active NCR species are essential to guarantee satisfactory chemo- and site-selectivity.

Concerted Enantioselective [2+2] Cycloaddition Reaction of Imines Mediated by a Magnesium Catalyst

Enantioselective [2 + 2] cyclization between an imine and a carbon-carbon double bond is a versatile strategy to build chiral azetidines. However, α-branched allenoates have never been successfully applied in [2 + 2] cyclization reactions with imines, as they always undergo Kwon's [4 + 2] annulation in previous catalytic methods. Herein, a simple in situ generated magnesium catalyst was employed to successfully achieve the enantioselective [2 + 2] cyclization reaction of DPP-imines and α-branched allenoates for the first time. Insightful experiments including KIE experiments, controlled experiments, Hammett plot analysis, and ³¹P NMR studies of initial intermediates indicate that the current [2 + 2] cyclization of imine most likely involves an asynchronous concerted transition state. Further mechanistic investigations by combining kinetic studies, ESI experiments, ³¹P NMR studies of coordination complexes, and controlled experiments on reaction rates under different catalyst loading amounts provided the coordination details for this [2 + 2] cyclization reaction between DPP-imines and α-branched allenoates. This new approach was applied to the synthesis of various chiral aza-heterocycles, including the enantioselective synthesis of the key intermediate of a lipid-lowering agent Ezetimibe.

Design, Synthesis, and Biological Activity of Novel Laccase Inhibitors as Fungicides against Rice Blast

Laccase is a potential target for novel agricultural fungicide discovery. PMDD-5Y was the first agent reported with high activity against laccase to control phytopathogenic fungi. Thirty-two novel agents containing cinnamaldehyde thiosemicarbazide were synthesized with PMDD-5Y as the lead compound, with most of the target compounds exhibiting excellent activity in vitro. Compound a2 (EC₅₀ = 9.71 μg/mL) exhibited greater potency against Magnaporthe oryzae than the commercial fungicide isoprothiolane (EC₅₀ = 18.62 μg/mL). The curative and protective effects of a2 against M. oryzae on rice were more than those of PMDD-5Y. Scanning electron microscopy indicated that a2 could cause mycelial growth atrophy and malformation. Furthermore, a2 (IC₅₀ = 0.18 mmol/L) showed higher activity against laccase than PMDD-5Y (IC₅₀ = 0.33 mmol/L) and cysteine (IC₅₀ = 0.30 mmol/L). Molecular docking analysis revealed the nature of interaction between these compounds and laccase. This research identified a novel laccase inhibitor a2 as a fungicide candidate to control rice blast in agriculture.

An efficient catalytic method for the borohydride reaction of esters using diethylzinc as precatalyst

A cheap and easily available ZnEt2 is an effective precatalyst, which can be used for the hydroboration reaction of various organic carbonates and esters with HBpin.

Direct para-Selective C-H Amination of Iodobenzenes: Highly Efficient Approach for the Synthesis of Diarylamines

Iodine(III)-mediated synthesis of 4-iodo-N-phenylaniline from iodobenzene has been achieved, and the reaction can proceed under mild conditions. A variety of functional groups were well tolerated, providing the corresponding products in moderate to good yields. The remaining iodine group provides an effective platform for converting the products into several valuable asymmetric diphenylamines. Most importantly, this reaction can be easily scaled up to the ten-gram scale, highlighting its synthetic utility. The mechanistic study revealed that the in situ generated aryl hypervalent iodine intermediate is the key factor to realize this para-selective C-H amination reaction.

Intermolecular [3+3] ring expansion of aziridines to dehydropiperi-dines through the intermediacy of aziridinium ylides

The importance of N-heterocycles in drugs has stimulated diverse methods for their efficient syntheses. Methods that introduce significant stereochemical complexity are attractive for identifying new bioactive amine chemical space. Here, we report a [3 + 3] ring expansion of bicyclic aziridines and rhodium-bound vinyl carbenes to form complex dehydropiperidines in a highly stereocontrolled rearrangement. Mechanistic studies and DFT computations indicate that the reaction proceeds through formation of a vinyl aziridinium ylide; this reactive intermediate undergoes a pseudo-[1,4]-sigmatropic rearrangement to directly furnish heterocyclic products with net retention at the new C-C bond. In combination with asymmetric silver-catalyzed aziridination, enantioenriched scaffolds with up to three contiguous stereocenters are rapidly delivered. The mild reaction conditions, functional group tolerance, and high stereospecificity of this method are well-suited for appending piperidine motifs to natural product and complex molecules. Ultimately, our work establishes the value of underutilized aziridinium ylides as key intermediates for converting small, strained rings to larger N-heterocycles.

Copper-catalyzed enantioselective arylalkynylation of alkenes

Enantioselective aryl and alkynylation of activated/nonactivated alkenes.

A copper-catalyzed enantioselective arylalkynylation of alkenes with diaryliodonium salt and a monosubstituted alkyne is reported. The three-component coupling reactions proceed under mild reaction conditions with a broad substrate scope, leading to synthetically valuable 1,2-diaryl-3-butynes. The key to the success of this chemistry is the employment of the chiral bisoxazoline-phenylaniline (BOPA) ligand. A novel reaction pathway involving the phenyl radical generation under thermal copper catalysis is proposed according to mechanistic studies.

Construction of Dispiro-Indenone Scaffolds via Domino Cycloaddition Reactions of α,β-Unsaturated Aldimines with 2-Arylidene-1,3-indenediones and 2,2'-(Arylmethylene)bis(1,3-indenediones)

The catalyst-free domino reaction of α,β-unsaturated N-alkyl or N-arylaldimines with two molecules of 2-arylidene-1,3-indanediones in dry acetonitrile resulted in polysubstituted spiro[indene-2,3'-indeno[2',1':5,6]pyrano[2,3-b]pyridines] in moderate to good yields and with high diastereoselectivity. The reaction mechanism included sequential aza/oxa-Diels-Alder reactions via both endo-transition states. On the other hand, the catalyst-free domino reaction of α,β-unsaturated N-arylaldimines with 2,2'-(arylmethylene)bis(1,3-indenediones) afforded the mixed diastereoisomeric dispiro[indene-2,1'-cyclohexane-3',2″-indene] derivatives in satisfactory yields. The reaction mechanism of this formal [3 + 3] cycloaddition was believed to proceed with sequential nucleophilic 1,4-/1,2-additions.

Rh(II)-Catalyzed Nitrene-Transfer [5 + 1] Cycloadditions of Aryl-Substituted Vinylcyclopropanes

Formal [5 + 1] cycloadditions between aryl-substituted vinylcyclopropanes and nitrenoid precursors are reported. The method, which employs Rh₂(esp)₂ as a catalyst, leads to the highly regioselective formation of substituted tetrahydropyridines. Preliminary mechanistic studies support a stepwise, polar mechanism enabled by the previously observed Lewis acidity of Rh-nitrenoids. Overall, this work expands the application of nitrene-transfer cycloaddition, a relatively underexplored approach to heterocycle synthesis, to the formation of six-membered rings.

Diastereo- and Enantioselective Dearomative [3 + 2] Cycloaddition Reaction of 2-Nitrobenzofurans with 3-Isothiocyanato Oxindoles

Enantioselective dearomative [3 + 2] cycloaddition reaction of 2-nitrobenzofurans with 3-isothiocyanato oxindoles was developed. The reaction employs a chiral bis(oxazoline)/Zn(OTf)₂ catalyst, allowing a practical, straightforward access to structurally diverse spirooxindoles containing a 2,3-dihydrobenzofuran motif and three contiguous stereocenters with excellent diastereo- and enantioselectivities. The synthetic potentials of the method have been demonstrated by the scale-up experiment and transformations of the products. The preliminary mechanism was investigated with experimental observations, nonlinear effects studies, and MS experiments.

Domino aza/oxa-hetero-Diels–Alder reaction for construction of novel spiro[pyrido[3′,2′:5,6]pyrano[2,3-d]pyrimidine-7,5′-pyrimidine]

The unprecedented reaction of α,β-unsaturated N-arylaldimines with two molecular 5-arylidene-1,3-dimethylbarbituric acids in methylene dichloride at room temperature afforded unique spiro[pyrido[3′,2′:5,6]pyrano[2,3-d]pyrimidine-7,5′-pyrimidines] in moderate to good yields and with high diastereoselectivity.

Oxazolines as Dual-Function Traceless Chromophores and Chiral Auxiliaries: Enantioselective Photoassisted Synthesis of Polyheterocyclic Ketones

2-(o-Amidophenyl)oxa- and -thiazolines undergo excited-state intramolecular proton transfer (ESIPT), generating aza-o-xylylenes capable of intramolecular [4+2] and [4+4] cycloadditions with tethered unsaturated pendants. Facile hydrolysis of the primary photoproducts, spiro-oxazolidines and thiazolidines, under mild conditions unmasks a phenone functionality. Variations in linkers allow for access to diverse core scaffolds in the primary photoproducts, rendering the approach compatible with the philosophy of diversity-oriented synthesis. Chiral oxazolines, readily available from the corresponding amino alcohols, yield enantioenriched keto-polyheterocycles of complex topologies with enantiomeric excess values up to 90%.

Dual C–H functionalization of N-aryl tetrahydroisoquinolines: a highly diastereoselective synthesis of dibenzo[a,f]quinolizines via visible-light induced oxidation and inverse electron-demand aza-Diels–Alder reaction

Dual C–H functionalization of amines with high diastereoselectivity is developedviavisible-light photocatalysis and inverse electron-demand aza-Diels–Alder reaction.