SOME POLYCYCLIC COMPOUNDS

Although Hiickel s Rule strictly applies only to monocyclic compounds, some polycyclic compounds show aromatic behavior. 

The first chapter discusses the concept of aromaticity, after which there is a description of aromatic substitution reactions. Chapters covering the chemistry of the major functionalized derivatives of benzene follow. A chapter on the use of metals in aromatic chemistry discusses not only the chemistry of Grignard reagents and aryllithium compounds but also the more recent uses of transition metals in the synthesis of aromatic compounds. The penultimate chapter discusses the oxidation and reduction of the benzene ring and the text concludes with the chemistry of some polycyclic compounds. 

Some polycyclic compounds are substances which constitute a health hazard. Benzo-(a)-pyrene, for example, can be carcinogenic. The control findings obtained as mean values varied between 0,005 and 0,04 mg/kg, the maximum values being between 0,015 and 0,1 mg/kg soil dry matter. As a rale, all values were below the reference limits or just reached this level. 

For many years, the only synthetic low-molecular-weight inhibitors of the p53-hdm2 interaction described were not very potent. Only chalcone derivatives (6 - Fig. 15.5-5) [45], some polycyclic compounds (7 - Fig. 15.5-5) [56], and sulfonamides (8 - Fig. 15.5-5) [57] were described. A fungal metabolite, chlorofusin (9 - Fig. 15.5-5), was also described as an inhibitor of the p53-hdm2 interaction [58]. Finally, l,4-benzodiazepine-2-ones were proposed from a computational approach (10 - Fig. 15.5-5) [59]. 

Another strategic device applies specifically to polycyclic compounds. In the interests of simplification we want to remove some of the rings and give an intermediate with a famihar ring structure. We can do this by the common atom approach. In TM 329, mark all the carbon atoms which belong to more than one ring - the common atoms . 

Gore et al.426 have used chloroform as a solvent for acetylation catalysed by aluminium chloride and at 45-55 °C find that a 2-methoxy substituent in naphthalene increases the reactivity of the 1 position 1.72 times, of the 6 position 3.8 times, and of the 8 position, 0.9 times the former and latter of these results indicate a considerable steric effect. Likewise, a 2-bromo substituent caused the reactivity of the 6 and 8 positions to be 0.63 and 0.58 times that of the corresponding positions in the unsubstituted compound. At 20-25 °C the relative reactivities of some polycyclics were as follows427 1-naphthyl, 1.0 3-phenanthryl 0.64 9-phenanthryl, 0.02 1-phenanthryl, 0.29 2-naphthyl, 0.28 2-phenanthryl, 0.12 4-phenanthryl, 0.0085. Some of these results seem to be due to steric hindrance, and the large difference in reactivity of naphthalene and biphenyl seems erroneous. 

Indole-2,3-quinodimethanes [44] 44 are bicyclic outer-ring dienes that are widely used to prepare a variety of heterocyclic polycyclic compounds. These dienes, generated by extrusion of CO2 from lactones, are then trapped by dienophiles. Some examples of Diels Alder reactions of the dienes 44 are reported in Scheme 2.19. 

It has been shown, however, that such catalysts may contain protons, either by design or because of the difficulty in removing all traces of moisture, and these protons have been shown to be superacidic with Hammett acidities up to —18. These protons will also play some role in the catalytic activity of these ionic liquids in practical situations. Ionic liquids in which superacidic protons have deliberately been generated by addition of small amounts of water, HCl or H2SO4 have been used to catalytically crack polyethene under relatively mild conditions. The main products are mixed C3-C5 alkanes, which would be a useful feedstock from waste polyethene recycling. In contrast to other cracking procedures no aromatics or alkenes are produced, although small amounts of polycyclic compounds are obtained. 

Polyene Cyclization. Perhaps the most synthetically useful of the carbo-cation alkylation reactions is the cyclization of polyenes having two or more double bonds positioned in such a way that successive bond-forming steps can occur. This process, called polyene cyclization, has proven to be an effective way of making polycyclic compounds containing six-membered and, in some cases, five-membered rings. The reaction proceeds through an electrophilic attack and requires that the double bonds that participate in the cyclization be properly positioned. For example, compound 1 is converted quantitatively to 2 on treatment with formic acid. The reaction is initiated by protonation and ionization of the allylic alcohol and is terminated by nucleophilic capture of the cyclized secondary carbocation. 

Martin, J. M. L., El-Yazal, J., Francois, J.-P, 1996, Structure and Vibrational Spectrum of Some Polycyclic Aromatic Compounds Studied by Density Functional Theory. 1. Naphtalene, Azolene, Phenanthrene, and Anthracene , J. Phys. Chem., 100, 15358. 

Shaw, R. Golden, D. M., and Benson, S. W., Thermochemistry of Some Six-Membered Cyclic and Polycyclic Compounds Related to Coal. J. Phys. Chem, 1977. 81 pp. 1716-1729. 

The oil companies supplying the rubber industry claim that there is no effect on ageing properties when sulphur is present in an oil (up to 6% is possible). They claim that the refining removes the active mercaptans and sulphides and the remainder of the sulphur is complexed into polycyclic compounds and there is no evidence that this type of sulphur has any effect on cure. This claim may be true in some circumstances. However, it is known from practical experience that with certain polymers and compounds the claim is incorrect, especially when the high sulphur oils are used as test media. Oils supplied to the same oil specification from oil fields in different parts of the world, and meeting all the requirements of the specification may, because of widely different sulphur levels, have serious effects on high temperature ageing. 

A concise collection of some modem methods of radical formation via rupture of C-E, C-G and C-A bonds, as well as from alkenes and cyclopropanes, by metals, organometallic hydrides, and photochemical and electrochemical means, are given in the last Chapter of Giese s book [28]. For some examples of radical generation and reactions leading to cyclic and polycyclic compounds see next Chapter (Heading 6.1.3). 

Solid phase DOS of natural product-like molecules in many cases concerns polycyclic compounds, but macrocycles have also been reported [4j. In some cases, it is difficult to draw a line between DOS and simple derivati-... 

In some cases, side reactions were observed. When 4-chlorophenyl isocyanate reacted with iminophosphorane 135 (R= Pr ), along with the corresponding pteridin-4(377)-one (43%), the hydrolyzed compound, 3-isopropylpteridine-2,4-(177,377)-dione (26%), was also isolated. In cases with additional functional groups in the pteridine substrate such as esters, polycyclic compounds such as the imidazo[2,T ]pteridine derivative 138 (37%) were obtained. Similarly, Wallyl and W(l-methylprop-2-enyl) groups in 137 gave imidazo[2,l- ]pteridines 139 by iodoamination. 

It should be emphasized that in some cases the insertions take place several times sequentially. For example, in carbonylation, the insertion of alkene is followed by CO insertion. Sometimes, further insertions of another alkene and CO take place. Particularly useful is the formation of polycyclic compounds by intramolecular sequential insertions of alkenyl and alkynyl bonds. 

We also discuss the conformations of cycloalkanes, especially cyclohexane, in detail because of their importance to the chemistry of many kinds of naturally occurring organic compounds. Some attention also will be paid to polycyclic compounds, substances with more than one ring, and to cyclo-alkenes and cycloalkynes. 

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