Polycyclic aromatics, regioselective

Aromatic rings can be reduced without difficulty. Major problems connected with these reductions concern maintenence of other functions, control of regioselectivity in polycyclic aromatics, and control of stereochemistry. 

Pressure influences the regioselectivity and the endo-exo diastereoselectivity of the cycloadditions. All the cycloadducts were converted into polycyclic aromatic hydrocarbons by treatment over a Pd/charcoal catalyst. This approach provides a new and efficient route to a broad variety of polycyclic aromatic hydrocarbons [36]. 

The use of the stannylquinones 81 results in the regioselective formation of 1,4-naphthoquinones or 9,10-anthraquinones 82 [40]. Highly-oxygenated angularly-fused polycyclic aromatic compounds are prepared by the ring enlargement [41]. (Scheme 29)... 

A new cascade reaction of aromatic aldehydes with terminal conjugated alkynes produces a range of polycyclic aromatic hydrocarbons.195 The effect of temperature on regioselectivity is discussed. 

Scheme IS dq>icts a high yield, general method for specific ortho allgriation of polycyclic aromatic hydrocarbons. In this example, biphenyl is subjected to reductive methyladon followed by oxidative rearrangement with trityl tetrafiuoioborate to give 2-methylbipbenyl. In unsymmetrical substrates the regioselectivity is poor, phenanthiene gives a 3 2 mixture of 4-methyl- and 1-mediyI-phenanlhrene.

A considerable degree of control of the regioselective hydrogenation of polycyclic aromatic hydrocarbons has been achieved through the choice of catalyst and optimized conditions. The review by Nishimu-ra and Takagi and Rylander s monographs furnish many examples and references. - ... 

The regioselectivity of the partial hydrogenation of polycyclic aromatics depends on both the substrate structure and the catalyst used. The hydrogenation of phenanthrene, (28) over Raney nickel or palladium takes place by way of the initial formation of the 9,10 dihydro species, 29 (Eqn. 17.27). 

The reaction of polycyclic aromatic hydrocarbons with [hydroxy(tosyloxy)iodo]benzene in the presence of trimethylsilyl isothiocyanate leads to the regioselective thiocyanation of an arene nucleus, as illustrated by the reaction of anthracene shown in Scheme 3.213 [274],... 

In this section, we shall use resonance forms to predict the regioselectivity and reactivity of polycyclic aromatic molecules (Section 15-5), using naphthalene as an example. Some biological impfications of the reactivity of these substances will be explored in Section 16-7. 

Resonance structures aid in predicting the regioselectivity of larger polycyclic aromatic hydrocarbons... 

In Summary Naphthalene is activated with respect to electrophilic aromatic substitution favored attack takes place at Cl. Electrophilic attack on a substituted naphthalene takes place on an activated ring and away from a deactivated ring, with regioselectivity in accordance with the general rules developed for electrophilic aromatic substitution of benzene derivatives. Similar considerations apply to the higher polycyclic aromatic hydrocarbons. 

Among polycyclic hydrocarbons, there is a considerable interest in poorly studied compounds having a benzopentalene skeleton. The presence of a fused aromatic ring is expected to restrict possible rearrangements, therefore ensuring relatively high regioselectivity. 

Intermolecular and intramolecular photocycloaddition and photoaddition to aromatic rings in the electron-donor and electron-acceptor systems were discussed in this chapter. The highly stereoselective and regioselective photocycloaddition is a synthetically useful method for the construction of polycyclic carbon-skeleton compounds, including natural products. New aspects for the stereoselective intermolecular and intramolecular photocycloaddition reactions via exciplexes in less... 

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