Metal Free C–H Functionalization Enabled Diastereoselective Multicomponent Reaction of N-Heterocycles to Fused Heteropolycycles

A two-step access to fused-/spiro-polycyclic frameworks via double Heck cascade and acid-driven processes

This report illustrates the rapid construction of two divergent classes of polycyclic frameworks, benzo[a]fluorenones and spiro-chromenone indenes, via a double Heck cascade and an acid-driven cyclization from easily accessible precursors, alkyl 2-bromocinnamate esters and diphenylacetylenes. The present strategy has surveyed a broad substrate scope and delivered an array of products with interesting structural features. Besides, fluorescence studies were performed for the synthesized benzo[a]fluorenones.

Condensation-Based Methods for the C-H Bond Functionalization of Amines

This review aims to provide a comprehensive overview of condensation-based methods for the C-H bond functionalization of amines that feature azomethine ylides as key intermediates. These transformations are typically redox-neutral and share common attributes with classic name reactions such as the Strecker, Mannich, Friedel-Crafts, Pictet-Spengler, and Kabachnik-Fields reaction, while incorporating a redox-isomerization step. This approach provides an ideal platform to rapidly transform simple starting materials into complex amines.

Diastereoselective and Reversed Regioselective Annulations of N-Alkyl Anilines to Julolidines and Lilolidines

A three-component annulation reaction of N-alkyl anilines, cyclic 1,3-dicarbonyl compounds, and aryl aldehydes to julolidines and lilolidines is reported. The 6π-electrocyclization enabled the annulation to proceed with reversed regioselectivity as compared with the annulation that occurs in the Povarov reaction. Both cyclic and acyclic N-alkyl anilines participated in the reaction to provide a wide range of julolidines and lilolidines as the single regio- and diastereoisomers in good to excellent yields.

Acid mediated coupling of aliphatic amines and nitrosoarenes to indoles

Traditionally, amines react with nitrosoarenes to provide the corresponding imines or azo compounds. Herein, we report an acid mediated annulation reaction of aliphatic amines and nitrosoarenes to provide indole derivatives. The elusive direct annulation of aliphatic amines and nitrosoarenes via simultaneous C-C and C-N bond formation was achieved under metal free conditions. This conceptually novel method for indole synthesis does not require pre-functionalization steps for the new C-C and C-N bond formation. The method has been applied for an elegant synthesis of nor-neocryptolepine and neocryptolepine.

Metal free biomimetic deaminative direct C-C coupling of unprotected primary amines with active methylene compounds

An unprecedented direct C-C coupling reaction of unprotected primary amines with active methylene compounds is reported. The reaction involves a biomimetic deamination of amines which was achieved under conditions free of metallic reagents and strong oxidizing agents. A wide range of primary amines was reacted with different active methylene compounds to provide structurally diverse trisubstituted alkenes and dihydropyridines. A kinetic study revealed an activation barrier of 10.1 kcal mol^-1 for the conversion of a key intermediate of the reaction.

Synthesis of 7'-Arylidenespiro[indoline-3,1'-pyrrolizines] and 7'-Arylidenespiro[indene-2,1'-pyrrolizines] via [3 + 2] Cycloaddition and β-C-H Functionalized Pyrrolidine

The acetic acid catalyzed three-component reaction of pyrrolidine, aromatic aldehydes, and 3-arylideneoxindolin-2-ones in refluxing toluene afforded functionalized 7'-arylidenespiro[indoline-3,1'-pyrrolizines] in good yields and with high diastereoselectivity. The similar three-component reaction with 2-arylidene-1,3-indanediones also gave 7'-arylidenespiro[indene-2,1'-pyrrolizines] in good yields. However, the reaction with 3-phenacylideneoxindoles resulted in a mixture of spiro[indoline-3,1'-pyrrolizines] and 7'-arylidene-substituted spirooxindoles in moderate yields. The reaction mechanism included generation of azomethine ylides, β-C-H functionalization of pyrrolidine, and sequential [3 + 2] cycloaddition. The obtained spirooxindole derivatives were investigated by in vitro evaluation against mouse colon cancer cells CT26 and human liver cancer cells HepG2 by MTT assay.

Direct (het)arylation of tetrahydroisoquinolines via a metal and oxidant free C(sp3)-H functionalization enabled three component reaction

An unprecedented method for the direct arylation and heteroarylation of tetrahydroisoquinolines under metal and oxidant free conditions is reported. The arylation reactions occurred via a C(sp3)-H functionalization enabled three component condensation of tetrahydroisoquinolines, 9-fluorenone imine, and arenes without involving a pre-functionalization/pre-derivatization step. A wide range of arenes and heteroarenes participated in the reaction to provide structurally diverse arylated tetrahydroisoquinolines with good to excellent yields.

Regio- and Enantioselective (Het)arylation of β-Alkenyl Pyrroline to α-Aryl-β-alkenyl Pyrrolidines

Regio- and enantioselective direct arylation of β-alkenyl pyrroline is reported. A wide range of electron-rich arenes and heteroarenes reacted under mild conditions with different β-alkenyl pyrrolines to provide structurally diverse α-aryl-β-alkenyl pyrrolidines with very good yields and excellent regioselectivity. Enantioselective reaction in the presence of Lewis acids and chiral phosphoric acids provided the desired arylated product with 73% enantiomeric excess.

The molecular diversity scope of 4-hydroxycoumarin in the synthesis of heterocyclic compounds via multicomponent reactions

4-Hydroxycoumarins are some of the most versatile heterocyclic scaffolds and are frequently applied in the synthesis of various organic compounds. 4-Hydroxycoumarin-based compounds are important among heterocyclic structures due to their biological and pharmaceutical activities. In this study, we provide an overview on the recent applications of 4-hydroxycoumarin in multicomponent reactions for the synthesis of various heterocyclic compounds during the time period of 2015-2018.