Recent Applications of Quinolinium Salts in the Synthesis of Annulated Heterocycles

Quinoline derivatives are frequently found in natural products and biologically active compounds; however, construction of quinoline fused polyheterocycles is a challenging goal in synthetic organic chemistry. In this regard, quinolinium salts meet the demand to a great level, as they can be synthesized readily and employed effectively for rapid construction of the condensed heterocyclic core. The present review focuses on recent (2015–2021) applications of different quinolinium salts, which react with suitable partners to access diverse annulated products. Most of the reactions discussed here involve easily available starting materials, are operationally simple, offer high atom-efficiency, and are environmentally benign. Mechanistic aspects of representative transformations have also been highlighted to better understand the reaction pathways.

1 Introduction

2 Annulation Involving N-Alkyl Quinolinium Salts

2.1 Reaction with Alkenes

2.2 Reaction with Alkynes/Arynes

2.3 Reaction with Phenolic Compounds

2.4 Reaction with Cyclic/Acyclic Diketones

2.5 Reaction with Amines/Cyclic Amines

2.6 Reaction with Enamines

2.7 Reaction with Isocyanoacetates

2.8 Reaction with Cyclopropanes

2.9 Ring Expansion Reactions

3 Annulation Involving Quinolinium Zwitterionic Tosylates

3.1 Reaction with Alkynes/Arynes

3.2 Reaction with Allenes/Ketenes

3.3 Reaction with Aldehyde-Amino Acid (Azomethine Ylide)

3.4 Reaction with Sulfonium Salts

3.5 Reaction with Diazoacetate

4 Annulation Involving Quinolinium Zwitterionic Thiolates

4.1 Reaction with Sulfonium Salts

4.2 Reaction with Sulfenes

4.3 Reaction with Arynes

5 Annulation Involving Quinoline N-Oxides

5.1 Reaction with Diynes and Ynones

5.2 Lactonization Involving Acrylate

6 Annulation Involving N-Iminoquinolinium Salts

6.1 Reaction with Allenoates

6.2 Reaction with Hydroxymethylallyl Carbonate

7 Miscellaneous Cyclizations

8 Conclusions