Transition‐Metal‐Free Synthesis of Nitrogen Containing Heterocycles With Fully Substituted N‐fused Pyrrole Rings

Abstract

The efficient synthesis of C-2 alkylated indolizine, pyrrolo[1,2-a]isoquinoline, and pyrrolo[1,2-a]quinoline derivatives utilizing as ingle reactivity platform is described. Employing a[ 3 + +2] dipolar cycloaddition reaction between heterocyclic N-ylides and allenoates, the incorporation of C-2 alkyl substituents was achieved resulting in various N-fused heterocycles with af ully substituted pyrrole ring. This metal-free route employing allenoates as ad ipolarophile solves the longstanding limitations of functional diversity at the C-2 position and the efficient creation of the pyrrolo[1,2-a]quinoline motif.Scheme1.Approaches toward C-2 alkylated pyrrolo-fused heterocycles.Scheme4.Synthesis of pyrrolo[1,2-a]quinolines. Isolated yields after column chromatography are shown.

Recent Applications of Quinolinium Salts in the Synthesis of Annulated Heterocycles

“…Meanwhile, the reaction of allenoates 22 with quinolinium salts 21 under metal-free conditions was studied by Bakshi and Singh. 42 The incorporation of C-2 alkyl substituents was achieved by employing allenoates as dipolarophile via [3+2] cycloaddition reaction to result in pyrrolo[1,2- a ]quinoline scaffold 23 (Scheme 6 ). Both quinoline and picoline reacted smoothly with allenoates to form the corresponding fused heterocycles 23a – f in good to excellent yields (up to 99%).…”