Metal‐Free Radical 5‐exo‐dig Cyclizations of Phenol‐Linked 1,6‐Enynes for the Synthesis of Carbonylated Benzofurans

Abstract

A new metal-free radical 5-exo-dig cyclization of phenol-linked 1,6-enynes with O2, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), and tBuONO is described. With this general method, carbonylated benzofurans can be accessed through incorporation of two oxygen atoms into the product from O2 and TEMPO through dioxygen activation and oxidative cleavage of the N-O bond, respectively.

Cyclizations of Benzo-Fused Substrates Involving Two Multiple Bonds, Including Heteroatom-Substituted Unsaturated Bonds

“…In 2015, Li reported the radical cyclization between aromatic vinyl ethers and alkynes with air, TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), and tert -butyl nitrite (eq ). In this reaction, 51a was converted to dicarbonylated benzofuran 52 . The mechanism of this reaction has been shown as Scheme .…”

“…The mechanism of this reaction has been considered as follows (Scheme 16). Initially, the visible light irridation of Mes-Acr- O-and 18 O-labeling experiments. In 2017, Kumar reported the visible-light photoredox catalyst induced metal-free radical cyclization between aromatic vinyl ethers (or enamines, thio vinyl ethers) and alkynes with sodium triflinate (eq 21).…”

Iodine-Promoted Radical Cyclization in Water: A Selective Reaction of 1,6-Enynes with Sulfonyl Hydrazides

“…Endowed with unsaturated C–C bonds, enynes are desirable to privilege building blocks, and have been extensively devoted to direct and selective tandem cyclizations across CC and CC bond in a one-step operation . For the transition-metal-catalyzed cycloadditions of enynes, the major mechanism involves the formation of metal cyclopropyl carbenes intermediates by 5- exo cyclization, which has been widely accepted for the cycloisomerization of 1,6-enynes. − Among them, a range of well-known reactions concern the alkyne activation of the substrates followed by the addition of the alkene and then convert into cyclic compounds. In this respect, our group has reported the radical cyclization reactions of 1,6-enynes for the synthesis of CF 3 -containing heterocycles and phosphorated fluorene derivatives by transition metal catalysis .…”

Visible-Light-Induced Cyclization/Aromatization of 2-Vinyloxy Arylalkynes: Synthesis of Thio-Substituted Dibenzofuran Derivatives

“…Recently, the radical cascade reaction of 1,6-enynes has emerged as an attractive approach for the synthesis of heterocycle derivatives . Li’s group reported Tempo or nitro radical induced cyclization of 2-vinyloxy arylalkyne which provided two different types of benzofurans (Scheme a and b) . Jiang and Sun’s groups achieved the construction of the sulfonylated benzofuran skeleton by using sodium sulfinate and DMSO as the source of sulfonyl in a similar radical pathway, respectively (Scheme c and d).…”