Benzo-fused five-membered heterocycles

Due to the migration of C=C bond, the conjugated esters formed can accept the attack of a second nucleophile. Thus, by applying different difunctionalized benzenes, benzo-fused five-membered heterocyclic compounds 526 can be prepared... 

As appropriately substituted o-disubstituted benzene derivatives are feadily available, this procedure has found widespread application in the synthesis of benzo-fused five-membered heterocycles. Examples abound in the various chapters in these volumes and the following few examples illustrate the general trend. 

Domino twofold Heck/67t—electrocyclization reactions of 1,2-dihaloal-kenes, -cycloalkenes, and heterocycles to give, in particular, benzo-fused five-membered heterocycles 12SL2735. 

Due to the high electrophilic nature of arynes, they are excellent dipolarophiles and can add to various 1,3-dipoles, leading to the construction of a range of benzo-fused five-membered heterocycles (Scheme 2). The commonly used 1,3-dipoles are nitrones, nitrile oxides, nitrile imines, azomethine imines, azides, diazo compoxmds, etc. Most of these reactions work under mild conditions in the absence of any transition-metal catalyst. 

Figure 1.40 Several other five-membered and benzo-fused five-membered nitrogen-containing aromatic heterocycles.

An example of cycloaddition reactions that involve transformation of five-membered heterocycles with one heteroatom into benzo[c]-fused heterocycles through a sequence of Diels-Alder reactions is presented in Scheme 1. 

For the benzo[c]-fused heterocycles presented in Table 20, the same interesting features are possessed. All the heterocycles have a much more uniform ring bond order than their benzo[6] isomers. Benzene bond order uniformity here is considerably more disrupted by the presence of the five-membered heterocycle (Table 20). According to the ring s bond orders, the most uniform heterocycle is benzo[c]pyrrole with a total bond order deviation of 1.718. 

The principal component analysis model has also been employed for the comparison of aromaticity for benzo-fused five- and six-membered heterocycles <90JPR(332)870>. The loadings for the bicyclic compounds show a close correlation to those of the corresponding monocyclic heterocycles. The data for the characteristics and the scores for thiophene and for benzo[6]thiophene (along with benzene and naphthalene) are listed in Table 20. 

Synthesis of five-membered heterocycles via a TOP sequence from activated alcohols affords 4,5-benzo-fused-2-substituted imidazoles, oxazoles and thiazoles in synthetically useful yields over the presumed oxidation-condensation-oxidation sequence (eq lOS). ... 

Transition-Metal Free Synthesis of BenzO Fused Five- and Six-Membered Heterocycles Employing Arynes... 

TRANSITION-METAL-FE E SYNTHESIS OF BENZO-FUSED FIVE- AND SIX-MEMBERED HETEROCYCLES EMPLOYING ARYNES... 

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