Asymmetric Synthesis of 2′‐Trifluoromethylated Spiro‐pyrrolidine‐3,3′‐oxindoles via Squaramide‐Catalyzed Umpolung and 1,3‐Dipolar Cycloaddition

The introduction of a trifluoromethyl group into the 2′‐position of spiro‐pyrrolidine‐3,3′‐oxindoles is described. By using 1 mol% of a quinine‐derived squaramide as catalyst, the 2,2,2‐trfluoroethylamine (CF₃CH₂NH₂)‐derived ketimine is transformed initially into a trifluoromethylimine through an umpolung reaction. The subsequent 1,3‐dipolar cycloaddition gives the pharmaceutical important target compounds in excellent yields, enantioselectivities and diastereoselectivies.

     

Copper(I)‐Catalyzed Asymmetric 1,3‐Dipolar [3+4] Cycloaddition of Nitrones with Azoalkenes

The first copper(I)‐catalyzed asymmetric 1,3‐dipolar [3+4] cycloaddition of nitrones with azoalkenes has been developed, affording a variety of biologically important 1,2,4,5‐oxatriazepane derivatives in good yields with exclusive regioselectivities and excellent enantioselectivities.

     

Diastereo‐ and Enantioselective Synthesis of Chiral Pyrrolidine‐Fused Spirooxindoles via Organocatalytic [3+2] 1,3‐Dipolar Cycloaddition of Azomethine Ylides with Maleimides

In the presence of a Cinchona alkaloid‐based squaramide organocatalyst, the [3+2] cycloaddition of isatin‐derived azomethine ylides with maleimides proceeded readily, thus delivering the desired pyrrolidine‐fused spirooxindoles in 61–89% yields with >20:1 dr and 12 to >99 % ee. The absolute configuration of 5‐chloro‐1,5′‐dimethyl‐3′‐phenyl‐3′,3a′‐dihydro‐2′H‐spiro[indoline‐3,1′‐pyrrolo[3,4‐c]pyrrole]‐2,4′,6′(5′H,6a′H)‐trione was unambiguously determined by means of X‐ray single crystal structure analysis. The reaction mechanism was hypothesized to account for the enantioselective formation of 5‐chloro‐1,5′‐dimethyl‐3′‐phenyl‐3′,3a′‐dihydro‐2′H‐spiro[indoline‐3,1′‐pyrrolo[3,4‐c]pyrrole]‐2,4′,6′(5′H,6a′H)‐trione.

     

Copper(II)‐Catalyzed Asymmetric 1,3‐Dipolar [3+4] Cycloaddition and Kinetic Resolution of Azomethine Imines with Azoalkenes

An unprecedented copper(II)‐catalyzed enantioselective 1,3‐dipolar [3+4] cycloaddition of azomethine imines with in situ formed azoalkenes has been realized. This strategy provides a facile access to biologically important 1,2,4,5‐tetrazepine derivatives in high yield with exclusive regioselectivity and high stereoselectivity. Moreover, enantioenriched azomethine imines could be obtained via an efficient kinetic resolution using the same approach.

     

Enantioselective Synthesis of 4‐Isoxazolines by 1,3‐Dipolar Cycloadditions of Nitrones to Alkynals Catalyzed by Fluorodiphenylmethylpyrrolidines

The first organocatalytic enantioselective 1,3‐dipolar reaction between nitrones and alkynals catalyzed by (S)‐2‐(fluorodiphenylmethyl)pyrrolidine to give 4‐isoxazolines (2,3‐dihydroisoxazoles) with high enantiomeric excess, excellent yields and low catalyst loading (1–5 mol%) is presented. The catalytic loading could be reduced to 1 mol% with only slight increases in reaction times.     

Efficient Diastereo‐ and Enantioselective Synthesis of exo‐Nitroprolinates by 1,3‐Dipolar Cycloadditions Catalyzed by Chiral Phosphoramidite⋅Silver(I) Complexes

Chiral complexes formed by privileged phosphoramidites and silver triflate or silver benzoate are excellent catalysts for the general 1,3‐dipolar cycloaddition between azomethine ylides generated from α‐amino acid‐derived imino esters and nitroalkenes affording with high dr the exo‐cycloadducts 4,5‐trans‐2,5‐cis‐4‐nitroprolinates in high ee at room temperature. In general, better results are obtained using silver rather than copper(II) complexes. In many cases the exo‐cycloadducts can be obtained in enantiomerically pure form just after simple recrystallization. The mechanism and the justification of the experimentally observed stereodiscrimination of the process are supported by DFT calculations. These enantiomerically enriched exo‐nitroprolinates can be used as reagents for the synthesis of nitropiperidines, by ester reduction and ring expansion, which are inhibitors of farnesyltransferase.

     

Intramolecular Azide‐Alkene 1,3‐Dipolar Cycloaddition/Enamine Addition(s) Cascade Reaction: Synthesis of Nitrogen‐Containing Heterocycles

A cascade intramolecular azide‐alkene 1,3‐dipolar cycloaddition/1,2 enamine and/or 1,4 enamine addition reaction sequence has been developed, and provides access to a variety of nitrogen containing heterocycles from readily available ω‐azido alkenes.     

Effect of Aqueous Polyethylene Glycol on 1,3‐Dipolar Cycloaddition of Benzoylnitromethane/Ethyl 2‐Nitroacetate with Dipolarophiles: Green Synthesis of Isoxazoles and Isoxazolines

A 1:1 mixture of water‐polyethylene glycol (PEG) facilitated the 1,3‐dipolar cycloaddition of benzoylnitromethane/ethyl 2‐nitroacetate with terminal alkynes or alkenes leading to isoxazoles or isoxazolines under green conditions. The methodology is free from the use of any base, catalyst, dehydrating agent or hazardous solvent.

     

Total Synthesis of (−)‐Doliculide,Structure–Activity Relationship Studies and Its Binding to F‐Actin

Actin, an abundant protein in most eukaryotic cells, is one of the targets in cancer research. Recently, a great deal of attention has been paid to the synthesis and function of actin‐targeting compounds and their use as effective molecular probes in chemical biology. In this study, we have developed an efficient synthesis of (?)‐doliculide, a very potent actin binder with a higher cell‐membrane permeability than phalloidin. Actin polymerization assays with (?)‐doliculide and two analogues on HeLa and BSC‐1 cells, together with a prediction of their binding mode to F‐actin by unbiased computational docking, show that doliculide stabilizes F‐actin in a similar way to jasplakinolide and chondramide C.     

Design,Synthesis, in vitro MAO‐B Inhibitory Evaluation,and Computational Studies of Some 6‐Nitrobenzothiazole‐Derived Semicarbazones

Monoamine oxidase B (MAO‐B) is an important drug target for the treatment of neurological disorders. A series of 6‐nitrobenzothiazole‐derived semicarbazones were designed, synthesized, and evaluated as inhibitors of the rat brain MAO‐B isoenzyme. Most of the compounds were found to be potent inhibitors of MAO‐B, with IC₅₀ values in the nanomolar to micromolar range. Molecular docking studies were performed with AutoDock 4.2 to deduce the affinity and binding mode of these inhibitors toward the MAO‐B active site. The free energies of binding (ΔG) and inhibition constants (K_i) of the docked compounds were calculated by the Lamarckian genetic algorithm (LGA) of AutoDock 4.2. Good correlations between the calculated and experimental results were obtained. 1‐[(4‐Chlorophenyl)(phenyl)methylene]‐4‐(6‐nitrobenzothiazol‐2‐yl)semicarbazide emerged as the lead MAO‐B inhibitor, with top ranking in both the experimental MAO‐B assay (IC₅₀: 0.004±0.001 μM ) and in computational docking studies (K_i: 1.08 μM ). Binding mode analysis of potent inhibitors suggests that these compounds are well accommodated by the MAO‐B active site through stable hydrophobic and hydrogen bonding interactions. Interestingly, the 6‐nitrobenzothiazole moiety is stabilized in the substrate cavity with the aryl or diaryl residues extending up into the entrance cavity of the active site. According to our results, docking experiments could be an interesting approach for predicting the activity and binding interactions of this class of semicarbazones against MAO‐B. Thus, a binding site model consisting of three essential pharmacophoric features is proposed, and this can be used for the design of future MAO‐B inhibitors.     

Synthesis and in vitro Evaluation of 3H‐Pyrrolo[3,2‐f]quinolin‐9‐one Derivatives That Show Potent and Selective Anti‐leukemic Activity

A series of new substituted 7‐phenyl‐3H‐pyrrolo[3,2‐f]quinolin‐9‐ones were synthesized and evaluated for their antiproliferative activity. The most active derivatives showed high selectivity against human leukemia cell lines and potently inhibited their growth, with GI₅₀ values in the nanomolar range. The active compounds strongly blocked tubulin assembly and colchicine binding to tubulin. Their activities were equal to or greater than that of the reference compound combretastatin A‐4. Flow cytometry studies showed that the two most active compounds arrested Jurkat cells in the G₂/M cell‐cycle phase in a concentration‐dependent manner. This effect was associated with apoptosis, mitochondrial depolarization, generation of reactive oxygen species, activation of caspase‐3, and cleavage of the enzyme poly(ADP‐ribose) polymerase.     

Design,Synthesis, and Evaluation of 2‐Amino‐6‐nitrobenzothiazole‐Derived Hydrazones as MAO Inhibitors: Role of the Methylene Spacer Group

A series of 2‐amino‐6‐nitrobenzothiazole‐derived extended hydrazones were designed, synthesized, and investigated for their ability to inhibit monoamine oxidase A and B (MAO‐A/MAO‐B). The compounds were found to exhibit inhibitory activities in the nanomolar to micromolar range. Some of the compounds showed excellent potency and selectivity against the MAO‐B isoform. N′‐(5‐Chloro‐2‐oxoindolin‐3‐ylidene)‐2‐(6‐nitrobenzothiazol‐2‐ylamino)acetohydrazide (compound 31 ) showed the highest MAO‐B inhibitory activity (IC₅₀=1.8±0.3 nm , selectivity index [SI]=766.67), whereas compound 6 [N′‐(1‐(4‐bromophenyl)ethylidene)‐2‐(6‐nitrobenzothiazol‐2‐ylamino)acetohydrazide] was found to be the most active MAO‐A inhibitor (IC₅₀=0.42±0.003 μm ). Kinetic studies revealed that compounds 6 and 31 exhibit competitive‐type reversible inhibition against both MAO‐A and MAO‐B, respectively. Structure–activity relationship (SAR) studies disclosed several structural aspects significant for potency and the contribution of the methylene spacer toward MAO‐B inhibitory potency, with minimal or no neurotoxicity. Molecular modeling studies yielded a good correlation between experimental and theoretical inhibitory data. Binding pose analysis revealed the significance of cumulative effects of π–π stacking and hydrogen bond interactions for effective stabilization of virtual ligand–protein complexes. Further optimization studies of compound 31 , including co‐crystallization of inhibitor–MAO‐B complexes, are essential to develop these compounds as potential therapeutic agents for MAO‐B‐associated neurodegenerative diseases.     

Gold(I)‐Catalyzed Regio‐ and Stereoselective 1,3‐Dipolar Cycloaddition Reactions of 1‐(1‐Alkynyl)cyclopropyl Oximes with Nitrones: A Modular Entry to Highly Substituted Pyrrolo[3,4‐d][1,2]oxazepines

An efficient approach for the synthesis of highly substituted pyrrolo[3,4‐d][1,2]oxazepines has been achieved by gold(I)‐catalyzed 1,3‐dipolar cycloaddition reactions of 1‐(1‐alkynyl)cyclopropyl oximes with nitrones in good to excellent yields as a single diastereomer. A complete chirality transfer was observed in this transformation.     

Catalytic Asymmetric Construction of Spirocycles Containing Pyrrolidine Motifs and Spiro Quaternary Stereogenic Centers via 1,3‐Dipolar Cycloaddition of Azomethine Ylides with 2‐Alkylidene‐Cycloketones

The first catalytic asymmetric 1,3‐dipolar cycloaddition of azomethine ylides with various sterically hindered α,α,β‐trisubstituted 2‐alkylidene‐cycloketones has been developed successfully with silver acetate/TF‐BiphamPhos complex for the construction of spiro heterocyclic compounds containing pyrrolidine motifs and a spiro quaternary stereogenic carbon center. The highly efficient catalytic system exhibited high reactivity, excellent diastereoselectivity, good enantioselectivity and broad substrate scope under mild conditions. Subsequent transformations led to the expedient preparation of synthetically useful spiro[pyrrolidine‐tetrahydropyranone] and spiro[pyrrolidine‐isochroman‐1‐one] without loss of the diastereo‐ and enantiomeric excesses.     

MAO Inhibitory Activity of 2‐Arylbenzofurans versus 3‐Arylcoumarins: Synthesis,in vitro Study,and Docking Calculations

Monoamine oxidase (MAO) is an important drug target for the treatment of neurological disorders. Several 3‐arylcoumarin derivatives were previously described as interesting selective MAO‐B inhibitors. Preserving the trans‐stilbene structure, a series of 2‐arylbenzofuran and corresponding 3‐arylcoumarin derivatives were synthesized and evaluated as inhibitors of both MAO isoforms, MAO‐A and MAO‐B. In general, both types of derivatives were found to be selective MAO‐B inhibitors, with IC₅₀ values in the nano‐ to micromolar range. 5‐Nitro‐2‐(4‐methoxyphenyl)benzofuran ( 8 ) is the most active compound of the benzofuran series, presenting MAO‐B selectivity and reversible inhibition (IC₅₀=140 nM ). 3‐(4′‐Methoxyphenyl)‐6‐nitrocoumarin ( 15 ), with the same substitution pattern as that of compound 8 , was found to be the most active MAO‐B inhibitor of the coumarin series (IC₅₀=3 nM ). However, 3‐phenylcoumarin 14 showed activity in the same range (IC₅₀=6 nM ), is reversible, and also severalfold more selective than compound 15 . Docking experiments for the most active compounds into the MAO‐B and MAO‐A binding pockets highlighted different interactions between the derivative classes (2‐arylbenzofurans and 3‐arylcoumarins), and provided new information about the enzyme–inhibitor interaction and the potential therapeutic application of these scaffolds.     

Stereoselective Synthesis of 3‐Hydroxy‐4‐arylcyclopentanones and 4‐Arylcyclopentenones through a Heck–Matsuda Desymmetrization of meso cis‐4‐Cyclopentene‐1,3‐diol

A new palladium‐catalyzed route to 3‐hydroxy‐4‐arylcyclopentanones and 4‐arylcyclopentenones in a diastereo‐ and enantioselective manner by a Heck–Matsuda desymmetrization was achieved from the commercially available meso cis‐4‐cyclopentene‐1,3‐diol. This method is highly practical, mild, high yielding and is carried out under “open vessel” conditions. Protected and unprotected substrates provide distinct products bearing considerable value as synthetic scaffolds for the synthesis of natural and unnatural bioactive compounds containing a five‐membered ring.

     

Copper‐Catalyzed C‐Arylation and Denitrogenation of Tetrazoles: Domino Synthesis of 1,3‐Diaminoisoquinoline Derivatives

An unexpected copper‐catalyzed sequential C‐arylation and denitrogenation of tetrazoles leading to 1,3‐diaminoisoquinoline derivatives has been developed, and the corresponding 1,3‐diaminoisoquinoline derivatives were obtained in moderate to good yields. The method provides a novel strategy for the synthesis of isoquinoline derivatives containing various functional groups.     

Design,Synthesis, and Biological Evaluation of Tetrahydro‐β‐carboline Derivatives as Selective Sub‐Nanomolar Gelatinase Inhibitors

Targeting matrix metalloproteinases (MMPs) is a pursued strategy for treating several pathological conditions, such as multiple sclerosis and cancer. Herein, a series of novel tetrahydro‐β‐carboline derivatives with outstanding inhibitory activity toward MMPs are present. In particular, compounds 9 f , 9 g , 9 h and 9 i show sub‐nanomolar IC₅₀ values. Interestingly, compounds 9 g and 9 i also provide remarkable selectivity toward gelatinases; IC₅₀=0.15 nm for both toward MMP‐2 and IC₅₀=0.63 and 0.58 nm , respectively, toward MMP‐9. Molecular docking simulations, performed by employing quantum mechanics based partial charges, shed light on the rationale behind binding involving specific interactions with key residues of S1′ and S3′ domains. Taken together, these studies indicate that tetrahydro‐β‐carboline represents a promising scaffold for the design of novel inhibitors able to target MMPs and selectively bias gelatinases, over the desirable range of the pharmacokinetics spectrum.