Base-Catalyzed [3 + 2] Cycloaddition of N-Benzyl Ketimines to Arylacetylenes Followed by Oxidation: A One-Pot Access to Polyarylated 2H-Pyrroles via Intermediate Pyrrolines

Regioselective Synthesis of Highly Functionalized 2H-Pyrroles via Dearomative Chlorination of 1H-Pyrroles

An efficient protocol was developed for the synthesis of highly functionalized 2H-pyrroles. This synthetic approach involves the in situ generation of highly reactive 2,5-dichloro-substituted 2H-pyrroles through dearomative chlorination of the corresponding 1H-pyrroles. The resulting reaction mixture is then treated with various amines, leading to the formation of 2,5-diaminated 2H-pyrroles. Subsequent nucleophilic substitution of fluorine with different N-, O-, and S-nucleophiles allows us to introduce additional functionality into a 2H-pyrrole core. The overall outcome of this reaction sequence is the triple nucleophilic modification of pyrroles. All steps of the sequence were found to be highly efficient, regioselective in the preparation of desired di- and trisubstituted derivatives in up to 96% overall yield. In addition, the computational study of this reaction sequence was carried out using density functional theory (DFT). The results of calculations are in perfect agreement with experimental observations.

Hypervalent Iodine Promoted Selective [2 + 2 + 1] Cycloaddition of Aromatic Ketones and Methylamines: A One-Pot Access to 1-Pyrrolines

Herein, a versatile highly regioselective three-component annulation of simple aromatic ketones and methylamines using a hypervalent iodine reagent for polyarylated 1-pyrrolines has been described in good to excellent yields. Meanwhile, unsymmetrical 1-pyrroline isomers could be realized and synthesized. Such an intriguing one-pot two-step tandem assembly strategy with green conditions and high regioselectivity shows predictable inspiration in related annulation reactions.

Tetraaryl-3H-Azepines: KOBut/DMSO-Catalyzed Assembly from Diaryldiynes and N-Benzylaldimines

Di(het)aryldiynes smoothly react with N-benzylaldimines in a [4 + 3] cycloaddition manner under the action of the KOBut/DMSO system (60 °C, 30 min) to afford pharmaceutically relevant tetra(het)arylsubstituted 3H-azepines in up to 71% yield. The process involves the addition of azaallyl anions to one of the triple bonds of diynes followed by prototropic isomerization and cyclization of anionic intermediates with participation of the second triple bond. The cascade mechanism is consistent with quantum-chemical analysis (B2PLYP-D3/6-311+G**//B3LYP-D3/6-31+G* + PCM).

Stable 2-Azaallyl Salts as a Bridge between 2H-Azirines and Densely Functionalized 2H-Pyrroles

An efficient protocol for the synthesis of stable 2-azaallyl anion salts by the reaction of alkyl 2-bromo-2H-azirine-2-carboxylates with trimethylsilyl cyanide/Bu4NF has been developed. The domino reaction proceeds in four steps via the cleavage of the azirine C-C bond to provide the tetrabutylammonium salts of stereochemically pure 2-azaallyl anions having U-configuration relative to the cyano groups. The anions with an ortho-substituted aryl group or styryl group exist as a mixture of two geometrical isomers across the N2-C3 bond. 2-Azaallyl anion salts have been shown to be convenient substrates for the one-pot synthesis of densely functionalized 2H-pyrroles by the alkylation-cyclization sequence.

Cu-Catalyzed Synthesis of 2-Silyl-1,3-butadienes from Allenols and Applications to One-Pot Synthesis of Tetrasubstituted Arylsilanes

A copper-catalyzed chemo- and stereoselective method for the synthesis of (E)-2-silyl-1,3-butadienes from a broad range of allenols using mild Si-B reagents is reported in this study. Our protocol required a short reaction time at ambient temperature to produce the desired dienes in high yields. Synthetic applications are highlighted by the one-pot synthesis of tetrasubstituted arylsilanes from allenols as well as the further functionalization of C-Si bonds.

Quantum-Chemical Study of the Assembly Mechanism of 1-Pyrrolines from N-Benzylaldimines and Arylacetylenes in KOtBu/DMSO Superbasic Medium

By using a quantum-chemical approach, B2PLYP-D2/6-311+G**//B3LYP/6-31+G*, we have carried out a detailed study of the assembly of 1-pyrrolines from N-benzyl-1-phenylmethanimine and phenylacetylene in the superbasic medium KO^tBu/dimethyl sulfoxide (DMSO). In this way, we have considered, both theoretically and experimentally, the mechanisms of the assembly through a concerted and stepwise nucleophilic cycloaddition and have addressed the side processes accompanying the assembly. It is found that the assembly via the concerted cycloaddition is kinetically more favorable than that via the stepwise cycloaddition. At the same time, the reaction of C-vinylation of aldimine with phenylacetylene occurs with a similar activation energy as the concerted cycloaddition and leads to the formation of 2-aza-1,4-pentadiene. The anion of 2-aza-1,4-pentadiene is an intermediate for the side processes leading to the formation of triarylpyridines and 1,3-diarylpropan-1-ones. Triarylpyridines are formed through the concerted cycloaddition of the next phenylacetylene molecule to 2-aza-1,4-pentadiene, while 1,3-diarylpropan-1-ones are formed as a result of the hydrolysis of 2-aza-1,4-pentadienes. It is found out that the mild conditions for the assembly of 1-pyrrolines (60 °C, 15 min) relate to the formation of complexes in the KO^tBu/DMSO superbasic medium, where the anion is readily accessible for the nucleophilic attack by the phenylacetylene molecule.

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.

Regio- and stereoselective base-catalyzed assembly of 6-methylene-5-oxaspiro[2.4]heptanones from alkynyl cyclopropyl ketones

6-Methylene-5-oxaspiro[2.4]heptanones have been synthesized via base-catalyzed dimerization of available alkynyl cyclopropyl ketones. The reaction proceeds effectively in the presence of the t-BuOK/t-BuOH/THF catalytic system at room temperature to afford the desired spirocycles in a regio- and stereoselective manner. A wider synthetic utility of alkynyl cyclopropyl ketones as novel building blocks was demonstrated by the synthesis of diverse spirocyclopropanes.

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.

Modular approach to non-aromatic and aromatic pyrroles through gold-catalyzed [3 + 2] cycloaddition of 2H-azirines and ynamides

The developed modular approach to hard-to-reach non-aromatic 3H- and 2H-pyrroles is based on the integration of 2H-azirines and ynamides.