Metal-mediated synthesis of pyrrolines

Copper-Catalyzed [3 + 2] Annulation of O-Acyl Oximes with 4-Sulfonamidophenols for the Synthesis of 5-Sulfonamidoindoles and 2-Amido-5-sulfonamidobenzofuran-3(2H)-ones

A copper-catalyzed [3 + 2] annulation of O-acyl oximes with 4-sulfonamidophenols is developed. The advantage of this method lies in the concurrent double activation of two substrates to form nucleophilic enamines and electrophilic quinone monoimines. The substituent on the α-carbon of O-acyl oxime determines two different reaction pathways, thereby leading to the selective generation of 5-sulfonamidoindoles and 2-amido-5-sulfonamidobenzofuran-3(2H)-ones.

Mechanistic Analysis Reveals Key Role of Interchalcogen Multicatalysis in Photo-Aerobic 3-Pyrroline Syntheses by Aza-Wacker Cyclizations

A light-driven dual and ternary catalytic aza-Wacker protocol for the construction of 3-pyrrolines by partially disulfide-assisted selenium-π-acid multicatalysis is reported. A structurally diverse array of sulfonamides possessing homopolar mono-, di- and trisubstituted olefinic double bonds is selectively converted to the corresponding 3-pyrrolines in up to 95 % isolated yield and with good functional group tolerance. Advanced electrochemical mechanistic investigations of the protocol suggest a dual role of the disulfide co-catalyst. On the one hand, the disulfide serves as an electron hole shuttle between the excited photoredox catalyst and the selenium co-catalyst. On the other hand, the sulfur species engages in the final, product releasing step of the catalytic cycle by accelerating the β-elimination of the selenium moiety, which was found in many cases to lead to considerably improved product yields.

Enantio- and Diastereoselective Variations on α-Iminonitriles: Harnessing Chiral Cyclopropenimine-Thiourea Organocatalysts

Chiral 1-pyrrolines containing a nitrile motif serve as crucial structural scaffolds in biologically active molecules and exhibit diversity as building blocks owing to their valuable functional groups; however, the asymmetric synthesis of such compounds remains largely unexplored. Herein, we present an enantio- and diastereoselective method for the synthesis of α-chiral nitrile-containing 1-pyrroline derivatives bearing vicinal stereocenters through the design and introduction of chiral cyclopropenimine-based bifunctional catalysts featuring a thiourea moiety. This synthesis entails a highly stereoselective conjugate addition of α-iminonitriles to a wide array of enones, followed by cyclocondensation, thereby affording a series of cyanopyrroline derivatives, some of which contain all-carbon quaternary centers. Moreover, we demonstrate the synthetic utility of this strategy by performing a gram-scale reaction with 1% catalyst loading, along with a variety of chemoselective transformations of the product, including the synthesis of a vildagliptin analogue. Finally, we showcase the selective synthesis of all four stereoisomers of the cyanopyrroline products through trans-to-cis isomerization, highlighting the versatility of our approach.

Asymmetric Photoaerobic Lactonization and Aza-Wacker Cyclization of Alkenes Enabled by Ternary Selenium-Sulfur Multicatalysis

An adaptable, sulfur-accelerated photoaerobic selenium-π-acid ternary catalyst system for the enantioselective allylic redox functionalization of simple, nondirecting alkenes is reported. In contrast to related photoredox catalytic methods, which largely depend on olefinic substrates with heteroatomic directing groups to unfold high degrees of stereoinduction, the current protocol relies on chiral, spirocyclic selenium-π-acids that covalently bind to the alkene moiety. The performance of this ternary catalytic method is demonstrated in the asymmetric, photoaerobic lactonization and cycloamination of enoic acids and unsaturated sulfonamides, respectively, leading to an averaged enantiomeric ratio (er) of 92:8. Notably, this protocol provides for the first time an asymmetric, catalytic entryway to pharmaceutically relevant 3-pyrroline motifs, which was used as a platform to access a 3,4-dihydroxyproline derivative in only seven steps with a 92:8 er.

Atom-Economical Syntheses of Dihydropyrroles Using Flavin-Iodine-Catalyzed Aerobic Multistep and Multicomponent Reactions

Herein, we report facile, atom-economical syntheses of multisubstituted 2,3-dihydropyrroles using flavin-iodine-catalyzed aerobic oxidative multistep transformations of chalcones with β-enamine ketones or 1,3-dicarbonyl compounds and amines. Exploiting coupled flavin-iodine catalysis, the multistep reaction, including C-C and C-N bond formation, is promoted only by the consumption of O₂ (1 atm), thus allowing aerobic oxidative synthesis that generates green H₂O as the only waste.

Copper-Catalyzed Annulation of O-Acyl Oximes with Cyclic 1,3-Diones for the Synthesis of 7,8-Dihydroindolizin-5(6H)-ones and Cyclohexanone-Fused Furans

A copper-catalyzed annulation of O-acyl oximes with cyclic 1,3-diones has been developed for the concise synthesis of 7,8-dihydroindolizin-5(6H)-ones and cyclohexanone-fused furans through the substituent-controlled selective radical coupling process. 2-Alkyl cyclic 1,3-diones undergo C-C radical coupling, while 2-unsubstituted cyclic 1,3-diones undergo C-O radical coupling.

Organocatalytic Allylic Alkylation of α-(Alkylideneamino)nitriles and Its Application in the Preparation of Multisubstituted 1-Pyrrolines

A synthetic approach for the construction of functionalized diverse 1-pyrrolines incorporating β-quaternary carbon centers under mild reaction conditions has been reported, in which α-allyl α-(alkylideneamino)nitriles generated from a Lewis base-catalyzed allylic alkylation reaction engaged in a Lewis base-mediated tandem intramolecular cyclization to deliver the targeted molecules in a catalytically atom-economic fashion.

One-Pot Synthesis of Multifunctionalized 1-Pyrrolines from 2-Alkyl-2H-azirines and Diazocarbonyl Compounds

A novel strategy for the synthesis of 1-pyrrolines based on formal [4 + 1] annulation of 2-alkyl-2H-azirines with diazocarbonyl compounds has been developed. This one-pot approach includes the Rh(II)-catalyzed formation of 4-alkyl-2-azabuta-1,3-dienes, followed by the DBU-promoted cyclization, and features a good substrate tolerance. The 1-pyrrolines containing an ester group at the C3 were prepared in a three-step one-pot procedure starting from 5-alkoxyisoxazoles. The cyclization of 2-azabutadienes to 1-pyrrolines most likely proceeds via the 6π electrocyclization of a conjugated NH-azomethine ylide.

Synthesis of Stable N-H Imines with a Benzo[7,8]indolizine Core and Benzo[7,8]indolizino[1,2-c]quinolines via Copper-Catalyzed Annulation of α,β-Unsaturated O-Acyl Ketoximes with Isoquinolinium N-Ylides

A copper-catalyzed annulation of α,β-unsaturated O-acyl ketoximes with isoquinolinium N-ylides has been developed for the concise synthesis of stable N-H imines with a benzo[7,8]indolizine core. When β-(2-bromoaryl)-α,β-unsaturated O-acyl ketoximes are used as the starting materials, a cascade cyclization occurs to afford the benzo[7,8]indolizino[1,2-c]quinolines.

Synthesis of 1-pyrroline derivatives via cyclization of terminal alkynes with 2-azaallyls

An efficient transition-metal-free cyclization reaction to prepare 1-pyrroline derivatives bearing various functional groups is described. In this method, a simple combination of a base and a solvent allows the cyclization reaction of terminal alkynes and 2-azaallyls to be carried out efficiently under mild and metal-free conditions. This cyclization reaction will provide an efficient method for the synthesis of medicinally relevant polysubstituted and multifunctionalized pyrrolines.

Copper-Catalyzed Bisannulations of Malonate-Tethered O-Acyl Oximes with Pyridine, Pyrazine, Pyridazine, and Quinoline Derivatives for the Construction of Dihydroindolizine-Fused Pyrrolidinones and Analogues

A copper-catalyzed bisannulation reaction of malonate-tethered O-acyl oximes with pyridine, pyrazine, pyridazine, and quinoline derivatives has been developed for the concise synthesis of structurally novel dihydroindolizine-fused pyrrolidinones and their analogues. The present reaction shows excellent regioselectivity and stereoselectivity. Theoretical calculations reveal that the coordination effect of the carbonyl group in the nucleophilic substrate determines the excellent regioselectivity. Further functionalization of the generated dihydroindolizine-fused pyrrolidinone could be easily realized through substitution, Michael addition, selective aminolysis, and hydrolysis reactions.

Synthesis and Toxicity Evaluation of New Pyrroles Obtained by the Reaction of Activated Alkynes with 1-Methyl-3-(cyanomethyl)benzimidazolium Bromide

A series of new pyrrole derivatives were designed as chemical analogs of the 1,4-dihydropyridines drugs in order to develop future new calcium channel blockers. The new tri- and tetra-substituted N-arylpyrroles were synthesized by the one-pot reaction of 1-methyl-3-cyanomethyl benzimidazolium bromide with substituted alkynes having at least one electron-withdrawing substituent, in 1,2-epoxybutane, acting both as the solvent and reagent to generate the corresponding benzimidazolium N3-ylide. The structural characterization of the new substituted pyrroles was based on IR, NMR spectroscopy as well as on single crystal X-ray analysis. The toxicity of the new compounds was assessed on the plant cell using Triticum aestivum L. species and on the animal cell using Artemia franciscana Kellogg and Daphnia magna Straus crustaceans. The compounds showed minimal phytotoxicity on Triticum rootlets and virtually no acute toxicity on Artemia nauplii, while on Daphnia magna, it induced moderate to high toxicity, similar to nifedipine. Our research indicates that the newly synthetized pyrrole derivatives are promising molecules with biological activity and low acute toxicity.

Copper-Catalyzed Cascade Annulation of Malonate-Tethered O-Acyl Oximes with Cyclic 1,3-Dicarbonyl Compounds for the Synthesis of Spiro-Pentacyclic Derivatives

A copper-catalyzed cascade annulation of malonate-tethered O-acyl oximes with cyclic 1,3-dicarbonyl compounds has been developed for the rapid synthesis of spiro-pentacyclic derivatives. This reaction allows the one-step formation of five C-C/N/O bonds and an angular tricyclic core under very mild conditions and shows excellent regioselectivity and stereoselectivity.

L-DOPA Dioxygenase Activity on 6-Substituted Dopamine Analogues

Dioxygenase enzymes are essential protein catalysts for the breakdown of catecholic rings, structural components of plant woody tissue. This powerful chemistry is used in nature to make antibiotics and other bioactive materials or degrade plant material, but we have a limited understanding of the breadth and depth of substrate space for these potent catalysts. Here we report steady-state and pre-steady-state kinetic analysis of dopamine derivatives substituted at the 6-position as substrates of L-DOPA dioxygenase, and an analysis of that activity as a function of the electron-withdrawing nature of the substituent. Steady-state and pre-steady-state kinetic data demonstrate the dopamines are impaired in binding and catalysis with respect to the cosubstrate molecular oxygen, which likely afforded spectroscopic observation of an early reaction intermediate, the semiquinone of dopamine. The reaction pathway of dopamine in the pre-steady state is consistent with a nonproductive mode of binding of oxygen at the active site. Despite these limitations, L-DOPA dioxygenase is capable of binding all of the dopamine derivatives and catalyzing multiple turnovers of ring cleavage for dopamine, 6-bromodopamine, 6-carboxydopamine, and 6-cyanodopamine. 6-Nitrodopamine was a single-turnover substrate. The variety of substrates accepted by the enzyme is consistent with an interplay of factors, including the capacity of the active site to bind large, negatively charged groups at the 6-position and the overall oxidizability of each catecholamine, and is indicative of the utility of extradiol cleavage in semisynthetic and bioremediation applications.

Nitrone and Alkyne Cascade Reactions for Regio- and Diastereoselective 1-Pyrroline Synthesis

The synthesis of 1-pyrrolines from N-alkenylnitrones and alkynes has been explored as a retrosynthetic alternative to traditional approaches. These cascade reactions are formal [4+1] cycloadditions that proceed through a proposed dipolar cycloaddition and N-alkenylisoxazoline [3,3']-sigmatropic rearrangement. A variety of cyclic alkynes and terminal alkynes have been shown to undergo the transformation with N-alkenylnitrones under mild conditions to provide the corresponding spirocyclic and densely substituted 1-pyrrolines with high regio- and diastereoselectivity. Mechanistic studies provide insight into the balance of steric and electronic effects that promote the cascade process and control the diastereo- and regioisomeric preferences of the 1-pyrroline products. Diastereoselective derivatization of the 1-pyrrolines prepared by the cascade reaction demonstrate the divergent synthetic utility of the new method.

Tin(iv) chloride mediated (3 + 2) annulation of trans-2-aroyl-3-styrylcyclopropane-1,1-dicarboxylates with nitriles: diastereoselective access to 5-vinyl-1-pyrroline derivatives

A tin(iv) chloride promoted (3 + 2) annulation of trans-2-aroyl-3-styrylcyclopropane-1,1-dicarboxylates with nitriles is reported. The transformation involves the Lewis acid assisted formation of 1,5-dipolar intermediates from the cyclopropane dicarboxylates and nitriles followed by cyclization. The reactions proceed in a highly diastereoselective manner and afford 5-vinyl-1-pyrroline derivatives in 60-88% yields.

Brønsted Acid Catalyzed Cyclization of Aminodiazoesters with Aldehydes to 3-Carboxylate-N-Heterocycles

A Brønsted acid catalyzed cyclization of aminodiazoesters with aldehydes is described. This reaction features broad substrate generality and functional group compatibility, affording a wide range of 5-7-membered 3-carboxylate-N-heterocycles containing different functional groups. The title products are able to be further elaborated through simple functional group transformations to produce synthetically useful N-heterocycles.

Synthesis of 3-[(4-Nitrophenyl)thio]-Substituted 4-Methylene-1-pyrrolines from N-Propargylic β-Enaminones

A facile and efficient method for the synthesis of 3-[(4-nitrophenyl)thio]-substituted 4-methylene-1-pyrrolines is described. When treated with 4-nitrobenzenesulfenyl chloride in refluxing acetonitrile, N-propargylic β-enaminones produced α-sulfenylated N-propargylic β-enaminones, which, in the presence of sodium hydride or cesium carbonate, underwent nucleophilic cyclization to afford 4-methylene-3-[(4-nitrophenyl)thio]-1-pyrrolines in good to high yields. It was shown for the first time that on N-propargylic β-enaminone systems, α-sulfenylation dominates over the formation of thiirenium ion. This one-pot two-step process was found to be general for a variety of N-propargylic β-enaminones and demonstrated good tolerance to a diversity of aromatic and heteroaromatic groups with electron-withdrawing and electron-donating substituents. This process is also applicable to the cyclization of internal alkyne-tethered N-propargylic β-enaminones. The enrichment of 1-pyrroline core with an aryl sulfide moiety might exhibit potential for the synthesis of molecules of pharmacological interest.