Radical Intramolecular Arylation of Pyridinium Salts: A Straightforward Entry to 7‐Hydroxypyrido[2,1‐a]isoquinolinylium Salts

Abstract

The synthesis of 7‐hydroxypyrido[2,1‐a]isoquinolinylium salts can be achieved in moderate yields by intramolecular radical arylation of pyridinium salts obtained from substituted pyridines and o‐bromophenacyl bromides.

Radical Addition to Iminium Ions and Cationic Heterocycles

“…The reaction of substituted α,2-dibromoacetophenones with pyridines affords the corresponding pyridinium salts which undergo a similar radical cyclization to furnish tricyclic pyridinium salts. The method was extended to a broad range of azinium salts (e.g., isoquinolinium, quinolinium, phenantridinium, phthalazin-2-ium) which afford the related polycyclic heteroaromatic compounds in moderate yields (Scheme 15) [37]. The presence of electron-withdrawing substituents in the azinium moiety results in a decreased yield whereas electron-releasing groups on both aromatic rings facilitate the arylation process.…”

Cascade C–H Annulation Reaction of Benzaldehydes, Anilines, and Alkynes toward Dibenzo[a,f]quinolizinium Salts: Discovery of Photostable Mitochondrial Trackers at the Nanomolar Level

“…The existing synthetic methods for the dibenzo­[ a , f ]­quinolizinium core are mainly based on photochemical cyclization, acid-mediated cyclization, ring-closing metathesis, and radical intramolecular arylation. , Most recently, for example, a visible-light-mediated gold-catalyzed reaction between quinoline-substituted aryl diazonium salts and silylaryl alkynes has been reported . However, these methods are all laborious and suffer from drawbacks such as uneasy attainability of starting materials, narrow substrate scope, as well as multiple steps with poor overall yields.…”

Recent Advances in the Synthesis of Azonia Aromatic Heterocycles

“…Finally, we reported a synthesis of 7-hydroxybenzo­[ a ]­quinolizinium derivatives by intramolecular radical arylation of the appropriate N -[2-(2-bromophenyl)-2-oxoethyl]­pyridinium salts. Reactions were carried out using 2 equiv of both tris­(trimethylsilyl)­silane (TTMSS) and AIBN in m -xylene under reflux (Scheme ). The procedure was extended to other polycyclic systems such as dibenzo­[ a , f ]­quinolizinium, dibenzo­[ a , h ]­quinolizinium, isoquino­[1,2- a ]­phthalazin-7-ium, and isoquino­[2,1- f ]­phenanthridinium salts.…”