From benzo five-membered heterocycles

From benzo five-membered heterocycles 6.2.5.1 Indazoles (cf. Section 6.1.2.1)... 

As with five-membered heterocycles, phosphorus pentasulfide may replace a ring oxygen atom by sulfur, or it may effect replacement in an acyclic precursor and then bring about cyclization. Thus isochroman-l-one (99) is converted into thioisochroman-l-thione (100) under the same conditions a 3-arylisocoumarin( 101) produces a 3-ary 1-1-thioisocoumarin (102) only, with no replacement of the ring oxygen atom.122 A typical example of thiation, followed by cyclization, is the formation of 2-alkyl- or 2-aryl-3,l-benzo-thiazine-4-thiones (103) from /V-acylanthranilic acid esters (104) by treatment with phosphorus pentasulfide in boiling xylene.122... 

Useful reviews on redox transformations of thiophen derivatives (179 references), on the stereochemistry of carbonyl derivatives of five-membered heterocycles (257 references), on synthetic approaches to dihydrothiophens (135 references), and on biosteric thiophens" have been published. Aspects of thiophen chemistry have been treated in reviews on the synthesis of heterocycles by thermal [2 + 2] cycloaddition reactions of acetylenes and on aspects and perspectives of organic heterocyclic chemistry. A review comparing the chemistry of thieno[2,3-h]- and thieno [3,2-i ]-thiophen with that of benzo[ft] thiophen and quinoline has been published. In Advances in Heterocyclic Chemistry, the development of benzo[6] thiophen from 1968 to 1980 and of selenophen from 1970 to 1980 was presented. Other aspects of thiophen chemistry are treated in chapters on Dewar Heterocycles," on Cyclizations under Vilsmeier Conditions, on Polyfluoroheteroaromatic Compounds, and on Reactions of Benzyne with Heterocyclic Compounds. " Several dissertations treating various aspects of thiophen chemistry have appeared. " ... 

The importance of five-membered heterocycles with one heteroatom, of the benzo and dibenzo condensed systems and of the partially or completely reduced compounds as natural products, pharmaceuticals, and starting materials or auxiliaries for syntheses is much greater than for three- or four-membered heterocycles, apart from oxirane. 

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). ... 

The strength of IMHB and the position of tautomeric equilibrium (Figure 10.9) in ketoenhydrazines derived from benzo[fe] annelated five-membered heterocycles 15 can be modulated by varying heteroatom and the substituents R -R [9, 81-83]. Compounds of the indole series 15 (X = NMe) exist in the enolimine... 

The validity of the Hammett relationship log K/K0 = pa, has been extensively investigated for five-membered heteroaromatic compounds and their benzo analogues. The ratio p(heterocycle) / p(benzene) is closest to unity for thiophene. Judged from work on the polarographic reduction of nitro compounds, the ability to transmit electronic effects is HC = CH = S < O < NH. 

The formation of pyrrole and its benzo analogs from other heterocyclic systems can occur in any of several general ways. In this section we will consider reactions which change the ring size and composition. In Section 3.06.6, reactions where the five-membered nitrogen-containing ring remains intact will be considered. 

For benzo[. ]thiophene, there is a it-MO delocalization between the two aromatic rings. For benzo[r]thiophene, the orbitals of the five-membered rings are localized on the heteroatom, C-1 and C-3, and there is no it-MO delocalization on the heterocyclic five-membered rings. These results are in agreement with the theoretical aromaticity of these molecules as are theoretical results from the reactivity indexes. 

A series of four- and five-membered benzoheterocycle uracils derived from tying back the 4 and 5, as well as the 2 and 3, aromatic positions were disclosed. The benzoheterocycles obtained from linking aromatic positions 4 and 5 were developed by Bayer in 2003 [101]. The differentiating feature between these benzo-heterocyclic uracils and earlier ones discussed in Section 3.3.1 is the replacement of the N-methyl group with an amino group in the uracil heterocycle, as exemplified by 93 and 94 (Fig. 3.26). 

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