Benzofused systems

Among the most common unsaturated units, there are mono (poly) cyclic aromatic hydrocarbons, heterocycles, benzofused systems, and olefinic and acetylenic groups, typically paired with various fullerene derivatives. The extent of conjugation/interaction between these units determine the polymer solution/solid-state electronic structure, which in turn control polymer properties, such as, optical absorption/emission, redox... 

Benzofused Systems Carbocyclic Systems A De Novo Class of Photo-Enediynes... 

A diversity of biological effects are possessed by benzofused six-membered heterocycles. These range from antimicrobial activity to cardiovascular, CNS, and inflammation-influencing agents. It can be inferred that the ring system itself is primarily a molecular scaffold upon which to assemble the characteristic pharmacophore for the various receptors involved. It is interesting also to note that the range of bioactivities involved differ substantially from those seen with the benzofused five-membered heterocycles described in Chapter 10. 

The mass spectra of benzofused pseudoazulenes 65 and 80 -83 show the mass ion as the base peak, and intense peaks are observed that correspond to the doubly charged molecular ion. The authors197 have attributed this fact to the aromaticity of these systems. This conclusion, however, cannot be drawn for all pseudoazulenes since the relative intensity of the mass ion for 26b is only 5.5%,55 for 4-methyl-4H-cyclopenta[b]quinoline 64%, 14 for 5-methyl-5/f-cyclopenta[c]quinoline7.0%,123andfor2-phenacyl-9-benzoyl-2H-indeno[2,1-fo] pyridine 18%.105... 

While not formal carbanions, arynes have been found to react with electrophilic centers. The first report of an aryne reacting with an epoxide has been published. While the yield of this reaction is not good, it suggests that this could be a viable route for the synthesis of benzofused heterocyclic ring systems <07SL1308>. 

Malonate-substituted cyclopropanes are often considered as synthetic building blocks due to the specific reactivity of their carbocyclic ring systems. For instance, total synthesis of ( )-bruguierol A was achieved in several steps that involved cyclopropanation of 2-(2-ally 1-4-methoxyphenyl)-2-methyl-1,3-dioxolane with dimethyl diazomalonate and [Rh2(esp)2] as catalyst (eq 44). The characteristic 2,3-benzofused 8-oxabicyclo-[3.2.1]octane core of bruguierol was constructed via a Sc(OTf)3-catalyzed intramolecular [3+2] cycloaddition of the cor-... 

Particularly, benzofused Cp derivatives such as indene and fluorene had serious impact on the development of catalyst precursors for stereospecific olefin polymerization [17]. However, fusing aromatic ring systems to the cyclo-pentadienyl ring can sometimes cause problems with regard to the bonding situation between metal and ligand because the tendency of the benzofused ring to preserve its aromatic six-electron system leads to a haptotropic shift as depicted in Scheme 6.2. 

TbfnCl2(2,6-Me2C6H30) (XVI) is well soluble in dichloromethane and THF, is of low solubility in toluene, and is completely insoluble in aliphatic solvents. It could be isolated in 21% yield as red crystals with four molecules per unit cell in the monoclinic crystal system P2i/c. As listed in Table 6.3, the Ti-Ct distance measures 2.0701 A, and the Ti-Cl bond lengths amount to an average of 2.270 A (Fig. 6.12). Like in compound XV also in XVI it is to assume that face-to-face n-stacking interactions take place between the benzofused ring C12-C17 and the phenoxide ring C30-C35 as the distance between its respective centroids measures 3.62 A. 

The same principles of resonance, steric considerations, and directing power of substituents apply to larger polycyclic systems, derived from naphthalene by additional benzofusion, such as anthracene and phenanthrene (Section 15-5). For example, the site of preferred electrophilic attack on phenanthrene is C9 (or CIO) because the dominant resonance contributor to the resulting cation retains two intact, delocalized benzene rings, whereas all the other forms require disruption of the aromaticity of either one or two of those rings. 

An efficient total synthesis of bruguierol A was accomplished from commercially available (S)-epichlorydrin (Scheme 18.57) [47]. The key step is a catalytic tandem intramolecular hydroalkoxylation/hydroarylation reaction to construct a 2,3-benzofused 8-oxobicyclo[3.2.1]octane system. Bruguierol A belongs to a family of compounds termed bruguierol A to C, isolated from the stem of the Bruguiera gym-morrhiza tree. 

While oxopyrazines, oxopyridazines, and their benzofused derivatives are susceptible to protonation at aza-nitrogen, the corresponding pyrimidones are not, since in the 0x0 or thiono tautomers their nitrogen atoms give rise to an amidine structure whose conjugated system would be dismpted by further protonation. Other heterocyclic systems have yet to be explored. 

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