NHC-Catalyzed [2 + 4] Annulation of Alkynyl Ester with Chalcone

Abstract

An NHC-catalyzed [2 + 4] cyclization of alkynyl ester with α,β-unsaturated ketone to form a pyran scaffold was developed successfully. The cheap and easily available starting materials, mild reaction conditions, moderate to excellent yields, and high atom economy make this strategy attractive for the syntheses of highly substituted 4H-pyran derivatives.

Progress in Electrochemically Empowered C−O Bond Formation: Unveiling the Pathway of Efficient Green Synthesis

“…[5] Other recently developed methods for the synthesis of furan derivatives are titanium-mediated domino crosscoupling/cyclodehydration and aldol addition/ cyclocondensation, [6] copper catalyzed coupling reaction between ketones and aromatic olefins, 7 gold nanaoparticles promoted cycloisomerizations of conjugated allenones, [8] metal free Iodine catalyzed synthesis to achieve the furan derivative. [9] The six membered saturated cyclic analogues of ether is known as pyran which has been recently synthesized by various traditional pathways by the modification of different synthetic strategies such as NHC-catalyzed [2 + 4] cyclization of alkynyl ester with α,β-unsaturated ketone, [10] Prins-type cyclization of cyclopropane carbaldehydes in the presence of 3-buten-1-ol [11] and Lewis-acid promoted bicyclization between cyclopropane carbaldehydes with quinone methides/esters etc [12] Oxepine is the one carbon homologated analogue can be synthesized via intramolecular CÀ O bond formation using Ullmann-ether reaction or cyclodehydration process from aliphatic or aromatic alcohol under harsh conditions. [13] However, recent advanced protocols for oxipin synthesis are Au-catalyed cyclization of (oalkynyl)phenoxyacrylates in mild conditions, green visible-lightinduced [6 + 4] cycloaddition reactions of ortho-quinone methides with pentafulvenes.…”

Carbene catalyzed C(sp3)–Cl activation of chlorinated solvents for benzyne chlorination

“…N-Heterocyclic carbenes (NHCs) have been developed as versatile organocatalysts 1 that enable unique transformations with high efficiency and selectivity. Besides their most studied interactions with aldehydes or carboxylic acid derivatives, 2 NHCs have also shown remarkable catalytic activity in reactions with imines, 3 silanes, 4 borate esters, 5 etc . 6 To date, investigating new modes of NHC organocatalysis has remained one of the most important tasks in this area.…”