Benzene ring fusion

Another study16 investigates the effect of benzene ring fusion on the reactivity of 1,2-Oxathietane. Ab initio calculations were performed using the 3-21G and the 6-31G basis sets, at Hartree-Fock and MP2 calculational levels. It was found that the allowed (8s + 2s) cycloreversion is unfavorable energetically. A subsequent experimental and theoretical study17 favors biradical intermediates in the valence tautomerism of benzoxathiete to monothio-o-benzoquinone. 

The effect of one substituent on the reactivity of another is generally similar to that observed in the corresponding polysubstituted benzenes. However, the partial bond fixation in an azole can lead to differential effects in the mutual interactions of substituents, similar to those found in naphthalene where the benzene ring fusion induces bond fixation. 

Analogous compounds derived by fusion of a benzene ring to a pyrrole furan or thio phene nucleus are called indole benzofuran and benzothiophene... 

Benzo-fusion to a heterocyclic ring involved in tautomerism has the effect of steering the equilibrium in directions which tend to retain the full aromaticity of the benzene ring. 

In valence bond terms the pyrazine ring may be represented as a resonance hybrid of a number of canonical structures (e.g. 1-4), with charge separated structures such as (3) contributing significantly, as evidenced by the polar character of the C=N bond in a number of reactions. The fusion of one or two benzene rings in quinoxaline (5) and phenazine (6) clearly increases the number of resonance structures which are available to these systems. 

The fusion of a benzene ring to pyrazine results in a considerable increase in the resistance to reduction and it is usually difficult to reduce quinoxalines beyond the tetrahydroquinoxa-line state (91). Two possible dihydroquinoxalines, viz. the 1,2- (92) and the 1,4- (93), are known, and 1,4-dihydroquinoxaline appears to be appreciably more stable than 1,4-dihydropyrazine (63JOC2488). Electrochemical reduction appears to follow a course anzdogous to the reduction of pyrazine, giving the 1,4-dihydro derivative which isomerizes to the 1,2- or 3,4-dihydroquinoxaline before subsequent reduction to 1,2,3,4-tetra-hydroquinoxaline (91). Quinoxaline itself is reduced directly to (91) with LiAlH4 and direct synthesis of (91) is also possible. Tetrahydroquinoxalines in which the benzenoid ring is reduced are well known but these are usually prepared from cyclohexane derivatives (Scheme 30). 

Quantitative data are available on the effect on electrophilic substitution reactions of the fusion of a benzene ring to the b face of a furan or thiophene ring. The overall effect is to decrease reactivity this decrease is much more pronounced in the case of fusion to a furan than to a thiophene ring. As a consequence the overall reactivities of benzo[Z)]furan and benzo[Z)]thiophene are approximately equal 71AHC(13)235). 

If the NHj group is eliminated first, as in the fusion of eolehieine with potassium hydroxide, and the product is oxidised with permanganate, terephthalie and trimellitic (benzene 1 2 4-tricarboxylic) acids are formed, which should come from a third benzene ring. 

Fusion of an additional heterocyclic ring onto a benzodiazepine is well known to considerably increase potency. This increase in potency is apparently maintained when the benzene ring is replaced by thiophene. Thiophene aminoketone 161 is converted to the benzodiazepine analogue 164 via chloroacetamide 162 and then glycine derivative 163 by the same sequence as that used in the benzene series. Treatment of the product 164 with phosphorus pentasulfide gives the thio-amide 165 reaction of that intermediate with hydrazine leads to the amino amidine 166. Conden-... 

In another type of aromatic hydrocarbon, two or more benzene rings are fused together. Naphthalene is the simplest compound of this type. Fusion of three benzene rings gives two different isomers, anthracene and phenanthrene. 

Most electrophilic substitutions in benzimidazole (31 R = H) occur primarily in the 5-position. In multiple bromination the order followed, 5 > 7 > 6,4 > 2, parallels molecular orbital calculations. In benzimidazole itself the 4(7)- and 5(6)-positions are tautomerically equivalent. Fusion of a benzene ring deactivates C-2 to electrophilic attack to such an extent that it is around 5000 times less reactive than the 2-position of imidazole. Strong electron donors at C-5 direct halogenation to the 4-position, whereas electron-withdrawing groups favor C-4 or C-6 substitution (84MI21). 

Fluoran (1) is the commonly used name for the spiro [isobenzofuran- 1,9 -xanthen]-3-one. Benzo[a]fluoran (2) has the benzene ring fused to the 1-and 2-positions of the xanthene moiety. Fusion at the 3- and 4-positions gives benzo[c]fluoran (3). Numbering of the atoms is employed as shown in 1-3. 

Six-membered ring ADC compounds can be generated by oxidation of the corresponding cyclic hydrazides. Pyridazine-3,6-dione (12) and phthalazine-1,4-dione (13, R = H), often called diazaquinones,4 are stable in solution only at low temperature, but can be generated, and intercepted at higher temperatures.39-43 Fusion of an extra benzene ring increases stability44 and the tetracyclic compound 14 is relatively stable.45 Substituted phthala-zine-l,4-diones have been widely studied because of their involvement in... 

The fusion of the benzene ring at the 3-4 bond of the furoxan system dramatically decreases the energy barrier of thermal interconversion, with the consequence that, in benzofuroxan derivatives 10a and 10c, the equilibrium is active at room temperature [6, 7, 10] (Scheme 6.2). 

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