Polycyclic anti-aromaticity

Dewafr-Huckel aromaticity rule. In a polycyclic alternant hydrocarbon a cycle with (4n + 2) n-centres is aromatic whereas a cycle with An jt-centres is anti-aromatic. 

In this chapter we concentrate on reduction processes of carbon-rich systems. The formation of anions and radical anions of -conjugated monocyclic systems, cyclophanes, bowls and fullerenes is described. Carbon-rich compounds can be reduced directly by contact with alkali metals Li, Na, K, Rb and Cs, which have a low reduction potential. Proton, carbon and lithium NMR and EPR spectroscopies are the main methods used to gain a better understanding of the mono- and polycyclic systems in solution. Special attention will be given to modes of electron delocalization, aromaticity, anti-aromaticity, as well as aggregation, bond formation and bond cleavage processes of diamagnetic electron transfer products. Electrochemical reductions will be briefly discussed. 

The notion of aromaticity and anti-aromaticity is extended to polycyclic aromatic... 

The 4/2+2 rule solved the mystery of the profound difference between benzene, [10]annulene, [14]-annulene, and [ISjannulene on one side and the 4/2 monocyclic systems, like elusive cyclobutadiene and puckered cyclooctatetraene, on the other side. Attempts were made to extend the 4/2+2 rule to polycyclic systems, for which it was not initially designed. Of numerous attempts in this direction, we will mention only that of Platt,who proposed that the 4/2+2 rule be applied to molecular periphery. It turns out that Platt s generalization of the Hiickel 4/2+2 rule is correct when one restricts attention to benzenoid hydrocarbons. For example, the perimeter rule correctly classifies pyrene (which has 16 Jt-elec-trons), perylene (which has 20 //-electrons), and coronene (which has 24 //-electrons) as aromatic as they have 14 or 18 //-electrons on the perimeter. But the perimeter rule does not give a correct answer for the non-benzenoid systems illustrated in Figure 10. The structure on the left, which has 14 //-electrons on the periphery, instead of being aromatic, as will be seen later, is in fact fully anti-aromatic . On the other hand, the structure on the right (corannulene), which has 15 //-electrons on the periphery, is not... 

Figure 85. Molecular graphs of additional anti-aromatic structures and polycyclic conjugated hydrocarbons, for which it is difficult to determine by inspection if their RE is positive or negative.

Methods for the synthesis of the biologically active dihydrodiol and diol epoxide metabolites of both carcinogenic and noncarcinogenic polycyclic aromatic hydrocarbons are reviewed. Four general synthetic routes to the trans-dihydrodiol precursors of the bay region anti and syn diol epoxide derivatives have been developed. Syntheses of the oxidized metabolites of the following hydrocarbons via these methods are described benzo(a)pyrene, benz(a)anthracene, benzo-(e)pyrene, dibenz(a,h)anthracene, triphenylene, phen-anthrene, anthracene, chrysene, benzo(c)phenanthrene, dibenzo(a,i)pyrene, dibenzo(a,h)pyrene, 7-methyl-benz(a)anthracene, 7,12-dimethylbenz(a)anthracene, 3-methylcholanthrene, 5-methylchrysene, fluoranthene, benzo(b)fluoranthene, benzo(j)fluoranthene, benzo(k)-fluoranthene, and dibenzo(a,e)fluoranthene. 

Metabolites of carcinogenic polycyclic aromatic hydrocarbons (PAH) such as ( ) trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahy-drobenzo[a]pyrene (BPDE) and 7,8,9,10-tetrahydroxytetrahydroben-zo[ a] pyrene (BPT) participate in tt binding interactions with nucleotide bases (1-19) which lead to the reversible formation of... 

Even the 1,2-dihydrodiol derivatives of polycyclic aromatic hydrocarbons are converted to the corresponding epoxydiols with MCPBA. The reaction is stereoselective only in some cases. The trans-dihydrodiols (6) give the antiepoxide (7), whereas the cfs-dihydrodiols (8) give a mixture of anti- (9) and syn-epoxy compounds (10). The anti- and syn-diol epoxides of benz[a]anthracene and benzo[a]pyrene have been prepared by this method.10... 

The aromatic polycyclic diones hypericin and pseudohypericin, which are present in plants of the family Hypericum (St. John s wort), have been accredited with anti-HIV activity for more than a decade. They possess the capacity to inactivate HIV virions directly as well as to interfere with their assembly or processing. Hypericin has received continued interest as a potential therapeutic agent and was more recently found to interact with preintegration complexes (PICs) and thus affect the proviral DNA integration process (De Clercq, 2000). 

The directly bonded C—coupling constants have been used to give a measure of aromaticity in mono- and polycyclic hydrocarbon ring systems.Cyclo-octatetraene reacts with methylene dichloride and methyl-lithium to afford a mixture of syn and anti-9-chlorobicyclo[6,l,0]nona-2,4,6-triene (40 and 41) which, on treatment with a 30% lithium dispersion in tetrahydrofuran, gives cyclonona-tetraenide, which is isolated as the tetraethylammonium salt. The chemical shift and the coupling constant (7i3c-ih = 137 c./sec.) observed for the lithio-salt (42) are indicative of the aromatic character of the ring, in accord with the predictions of Hiickel s Aromaticity rule. 

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