Condensed polycyclic skeletons

For the construction of highly condensed polycyclic skeletons, the cascade consisting of two intramolecular Heck-type reactions and a 67r-electrocyclization as performed with the... 

The Danishefsky group has reported the rearrangement of azaspiroindolenine 161 to access the polycyclic skeleton 162, a core ring system that has been further elaborated to phalarine 163 [93, 94]. As outlined in Scheme 40, the requisite indolenine 161 was prepared via ring opening of the spirooxindole 158, followed by intramolecular condensation of the keto-tethered aniline 160. 

The condensation of several rings with one another leads to the appearance of additional interaction depending both on the nature of the individual rings and on the stereochemistry of their linkage. The rigidity of a polycyclic skeleton leads to the interdependence of conformational effects in... 

These data, in conjunction with previous studies (12) showed that stable char contains mainly condensed aromatic structure with intermittent paraffinic groups. This structure is formed by successive dehydration, rearrangement, loss of carboxyl, carbonyl, and paraffinic groups, formation of free radicals, and condensation of the carbon skeleton to polycyclic aromatic structures. 

Polyquinanes. Polycyclic compounds with a skeleton of only 5-membered carbocyclic rings. Among natural products there are some tri- and tetraquinanes derived from the isoprenoid pathway. Different condensation types exist. Triquinanes are often condensed in a linear way, such as capnellene, complicatic acid, the coriolins, the hirsutanes, and phellodonic acid. Different triquinanes are isocomene, modhephene and pentalenene. The only known natural tetraquinanes are the crinipellins, isolated from mushrooms. 

In the first chapters of this book we have mentioned polycyclic compounds. Reactions in which polycyclic compounds are formed have been known for more than a hundred years, but their mechanisms remained puzzling for a long period of time. These reactions were even called reactions without a mechanism . The best known of this type of reactions, is the Diels-Alder addition, named in the honor of Otto Diels and Kurt Alder in which two alkene molecules are condensed so that two new o-bonds are formed. Good examples are reactions in which the norbornene skeletons are obtained. 

On the way to further extension of the scope of Cp Co "-catalyzed C-H functionalization, Glorius and coworkers developed a condensation reaction of 2-arylpyridine derivatives and diazoesters to form unique polycyclic heteroaromatics having 6//-pyrido[2,l-a]isoquinolin-6-one skeletons (Scheme 10.15) [38]. The reaction is achieved by the combination of a bench-stable Co precatalyst [Cp Co(CO)l2] [39], a silver salt (AgSbFg), and an acetate source (KOAc) in trifluoroethanol. The in situ-formed Cp Co catalyst is proposed to play a dual role in this condensation reaction. First, it promotes formal carbene insertion into the ortho C-H bond through pyridine-directed C-H metalation, cobalt-carbene formation, carbene insertion into the aryl-Co bond, and protodemetalation. Second, it acts as a Lewis acid to facilitate nucleophilic attack of the pyridine moiety to the ester group, which eventually leads to the product through aromatization and elimination of methanol. The thus-synthesized polycyclic heteroarenes exhibit bright and color-tunable fluorescence in solution and in the solid state. 

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