Aromatic compounds fused ring systems

Another class of compounds is called condensed-ring or fused-ring systems. These structures contain two or more aromatic rings that share a pair of carbon atoms. Examples include naphthalene, anthracene, and phenanthrene, the latter two being isomeric structures. 

This reaction is most often carried out with R = aryl, so the net result is the same as in 14-17, though the reagent is different. It is used less often than 14-17, but the scope is similar. When R = alkyl, the scope is more limited. Only certain aromatic compounds, particularly benzene rings with two or more nitro groups, and fused ring systems, can be alkylated by this procedure. 1,4-Quinones can be alkylated with diacyl peroxides or with lead tetraacetate (methylation occurs with this reagent). 

PET reactions [2] can be considered as versatile methods for generating radical cations from electron-rich olefins and aromatic compounds [3], which then can undergo an intramolecular cationic cyclization. Niwa and coworkers [4] reported on a photochemical reaction of l,l-diphenyl-l, -alkadienes in the presence of phenanthrene (Phen) and 1,4-dicyanobenzene (DCNB) as sensitizer and electron acceptor to construct 5/6/6- and 6/6/6-fused ring systems with high stereoselectivity. 

Shown below are the aromatic ring systems and names of compounds in this book used for Method 1. There are two types of these. The ori/zo-fused and the ortho- and peri-fused rings. For each of the ori/zo-fused ring systems the corresponding saturated ring system and the name obtained by Method 2 is shown. 

Interest in (5,5)-fused ring systems developed in separate, albeit closely related, directions. In the first, chemists continue their quest for new compounds having unique biological, chemical and physical properties. In the second, emphasis is on the classification of aromaticity based upon molecular conformity with Hiickel s rule. An increasing understanding of the fundamental chemistry of heterocyclic ring systems has led to considerable advances in this field, as evidenced by the growing list of applications as biocides and in industrial processes. 

It is easily possible to continue to insert nitrogen atoms into fused ring systems and some important compounds belong to these groups. The purines are part of DNA and RNA and are treated in Chapter 49, but simple purines play an important part in our lives. Coffee and tea owe their stimulant properties to caffeine, a simple trimethyl purine derivative. It has an imidazole ling fused to a pyrimidine ring and is aromatic in spite of the two carbonyl groups. 

From the synthetic point of view, oxidative photocyclization of these two types of enamides has a definite advantage in the construction of a polycyclic ring system with a double bond at the ring juncture in one step from the starting compounds. The photocyclized product can be readily converted to the cis-fused ring system by a facile catalytic reduction or to the fully aromatized lactam on dehydrogenolysis (19,20) as in the case of vV-benzoy-lenamines. 

Many polynuclear aromatic compounds do not contain fused ring systems, e.g., biphenyl and triphenylmethane. Give structures and names of compounds W through 11, formed in the following syntheses of such polynuclear compounds. 

Structures of some aromatic compounds are shown in Figure 21.26. Note that naphthalene has a structure that looks like two benzene rings arranged side by side. Naphthalene is an example of a fused-ring system, in which an organic compound has two or more cyclic structures with a common side. As in benzene, electrons are shared by the carbon atoms that make up the ring systems. 

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