Recent advances in the synthesis of dibenzofurans

This review reports recent accesses to the dibenzofuran nucleus described in the literature since 2008. This article starts with synthesizing dibenzofurans by creating the C-O bond of the furan ring. In the following section, we evoke the formation of dibenzofurans by cyclizing diarylether derivatives. The last part of this update concerns the construction of dibenzofurans from benzofuran or phenol derivatives. Representative examples showing the scope of these processes illustrate new approaches and biological activities of dibenzofurans. Reaction mechanisms explaining certain dibenzofuran formation are described, as suggested by their authors.

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

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.

Double functionalization of 2-amino-2′-hydroxy-1,1′-biaryls: synthesis of 4-nitro-dibenzofurans and benzofuro-indoles

A TM-free double functionalization of 2-amino-2′-hydroxy-1,1′-biaryls (see Scheme above) has been presented for the synthesis of 4-nitro-dibenzofurans.

Cu-catalyzed reaction of 1,2-dihalobenzenes with 1,3-cyclohexanediones for the synthesis of 3,4-dihydrodibenzo[b,d]furan-1(2H)-ones

The Cu(I)-catalyzed reaction of 1-bromo-2-iodobenzenes and other 1,2-dihalobenzenes with 1,3-cyclohexanediones in DMF at 130 °C using Cs(2)CO(3) as a base and pivalic acid as an additive selectively delivers 3,4-dihydrodibenzo[b,d]furan-1(2H)-ones with yields ranging from 47 to 83%. The highly regioselective domino process is based on an intermolecular Ullmann-type C-arylation followed by an intramolecular Ullmann-type O-arylation. Substituted products are accessible by employing substituted 1-bromo-2-iodobenzenes and substituted 1,3-cyclohexanediones as substrates. Reaction with an acyclic 1,3-diketone yields the corresponding benzo[b]furan.

Synthesis of dibenzofurans via palladium-catalyzed phenol-directed C-H activation/C-O cyclization

A practical, Pd(0)/Pd(II)-catalyzed reaction was developed for phenol-directed C-H activation/C-O cyclization using air as an oxidant. The turnover-limiting step of the process was found to be C-O reductive elimination instead of C-H activation. This reaction can tolerate a variety of functional groups and is complementary to the previous methods for the synthesis of substituted dibenzofurans.

One-pot N-arylation of indoles directly from N-arylsulfonylindoles via consecutive deprotection and S(N)Ar Reactions with activated Aryl halides

An efficient one-pot step by step t-BuOK-mediated procedure for the synthesis of N-arylindoles has been developed in moderate to good yields. The protocol involves the consecutive deprotection of N-arylsulfonylindoles as latent indoles and subsequent S(N)Ar reactions with activated aryl halides. This tandem reaction affords an efficient and convenient preparation of N-arylindoles that benefit from prior indoles protection by arylsulfonyl group, and can shorten a reaction sequence and improve synthetic efficiency.