A cascade double 1,4-addition/intramolecular annulation strategy for expeditious assembly of unsymmetrical dibenzofurans

Abstract

Existing synthetic routes for accessing dibenzofuran core have intrinsic regioselectivity, limiting the substitution patterns available in heteropolycyclic arene products. Here we report a double 1,4-conjugate addition/intramolecular annulation cascade reaction between propargylamines and two equivalents of imidazolium methylides that allows efficient access of structurally versatile dibenzofurans. This transition metal-free protocol proceeds smoothly under bench-top air atmosphere and offers easy manipulation of substituents on the dibenzofuran core, and also provides good-to-excellent product yields with good functional group tolerance, particularly the –Br and –Cl groups which are often incompatible with existing metal-catalyzed C–C and/or C–O bond ring-forming processes. It is worth noting that ladder-type π-systems with all-arene quarternary carbon structure can be straightforwardly generated upon simple late-stage functionalization.

Cascade Annulation Strategy for Expeditious Assembly of Hydroxybenzo[c]chromen-6-ones and Their Photophysical Property Studies

“…9 Annulation of the ortho-alkynyl quinone methide (o-AQM) 10 intermediate offers a versatile strategy for the rapid assembly of heterocyclic compounds, particularly for the synthesis of chromene-related systems (Scheme 1b). 11 This approach allows for the efficient and streamlined construction of diverse structures within the chromene family. In 2018, Du and co-workers disclosed a versatile Lewis acid-catalyzed double annulation of o-AQM with electron-rich phenols for assembling synthetically and biologically interesting tetracyclicbridged dioxabicyclo [3.3.1]nonane skeletons (Scheme 1bb(i)).…”

Copper-Catalyzed Cascade Annulation of o-Hydroxyphenyl Propargylamines with Pyrazolin-5-ones to Access Pyrano[2,3-c]pyrazoles

“…Despite impressive innovations, these reactions were mainly limited to the chemistry of o -HPPAs with active methylene compounds for the synthesis of chromene and benzofuran derivatives (Scheme b). Kwong et al have recently developed several new cyclization of o -HPPAs with β-naphthol, pyridinium ylide, dimethyl 3-oxoglutarate water, dipyridinium methylide, and imidazolium methylides . However, the obtainable products were still limited to chromene and benzofuran derivatives (Scheme b).…”

“…Kwong et al have recently developed several new cyclization of o-HPPAs with β-naphthol, 11 pyridinium ylide, 12 dimethyl 3-oxoglutarate 13 water, 14 dipyridinium methylide, 15 and imidazolium methylides. 16 However, the obtainable products were still limited to chromene and benzofuran derivatives (Scheme 1b). Thus, expansion in the scope of products readily accessible via the cascade reaction of o-HPPAs still urgently requires exploration, which will result in new discoveries in the field of o-QMs.…”

Radical-Induced Cascade Annulation/Hydrocarbonylation for Construction of 2-Aryl-4H-chromen-4-ones

“…In addition, transition-metal-catalyzed annulations often require the use of highly toxic carbon monoxide or expensive palladium catalyst and ligand at high temperatures. Inspired by our pioneering results, in which multireactive propargylamines displayed unique feature of cyclization, and in continuation of our interest in developing new synthetic protocols to build valuable heterocyclic frameworks [ 58 , 59 , 60 , 61 , 62 , 63 , 64 ], we herein disclose a cascade annulation of propargylamines for the synthesis of 2-aryl-4 H -chromen-4-ones under green and operationally simple metal- and solvent-free conditions, which is unprecedented in previous works ( Scheme 1 g). This cascade process presumably involves a sequence of radical-induced C-N cleavage, followed by C-O coupling, intramolecular 6- endo-dig annulation, thermal hemolytic cleavage, and oxidative hydrocarbonylation.…”