Polycyclic polyenes, structure

TABLE 16. Experimentally determined structure parameters for small polycyclic polyenes (distances in A, angles in degrees)... 

The organization of Part Two is according to structural type. The first section, Chapter Seven, is concerned with the synthesis of macrocyclic compounds. Syntheses of a number of heterocyclic target structures appear in Chapter Eight. Sesquiterpenoids and polycyclic higher isoprenoids are dealt with in Chapters Nine and Ten, respectively. The remainder of Part Two describes syntheses of prostanoids (Chapter Eleven) and biologically active acyclic polyenes including leukotrienes and other eicosanoids (Chapter Twelve). 

Linear and branched molecules, as well as some of the monocyclic ones, are identified only by their IUPAC names if their structure is immediately obvious. In the absence of accepted trivial or easy-to-read systematic names, larger polycyclic dienes and polyenes with rather unwieldy IUPAC names have been given numbers (4th column of the Table), which refer to the formula scheme following Table 1. 

As Hiickel formulated it, the An + 2 rule applies only to monocyclic systems. However, as a practical matter it can be used to predict the properties of polycyclic conjugated polyenes, provided the important VB structures involve only the perimeter double bonds, as in the following examples ... 

Vibration spectroscopy is also able to measure the concentration of ion radicals (by estimation of the band intensities). Moreover, the IR intensities of some bands in the fingerprint region for organic ion radicals may be much larger than the intensities of the bands for the neutral parent molecules. The examples are polycyclic aromatic hydrocarbons or linear polyenes and their ion radicals. The vibration patterns of the intensity-carrying modes are closely related to the electronic structure of the ion radicals (Torii et al. 1999 and references therein). 

Of polycyclic non-benzenoid hydrocarbons by far the most numerous and the most investigated series consists of the azulenes. The parent compound is azulene itself, bicyclo[5,3,0]decapentaene (VI). Like benzene it can be drawn as a polyene in two equivalent Kekule forms. It has a peripheral conjugated system involving 10 TT-electrons. Its structure is discussed on the following pages. 

The majority of the polycyclic alkanes in nature derive from the terpenoid family (C5 isoprene units) with the principal structural variations of six and five fused rings, formed by cyclization of the polyene open terpenoids (Ciq, C15, C20, C25 - up to C4.0). 

In view of the obvious differences in the structure of the wave-functions for the linear and polycyclic conjugated polymers (linear polyacenes), let us analyze two representative members of both classes. In Table 3.5 values of the above described parameters are shown for polyene C50H52 and polyacene C5oH2g that consists of 12 benzene rings. The values of that describe the contributions to the correlation energy obtained from different Z-layers in the cue-CCSD method are also shown in the table. 

Metabolic cyclization routes of polyenes leading to terpenes [1] inspired chemists to synthesize steroids and polycyclic structures [2]. Metal catalysis, chiral auxiliaries, substrate control, and other stoichiometric methods reach some level of success [3], but only recently Rendler and MacMillan published independently two approaches using organo-SOMO catalysis to synthesize these features (Scheme 10.1) [4], as a continuation of their work on the asymmetric cyclization of aldehydes [5]. 

Tandem cyclization reactions to form polycyclic structures have also been reported (Schemes 6.87 and 6.88). Some of these reactions can strongly resemble the polyene cyclizations that lead to steroids. Further ene-yne reactions can be found in Section 6.2.5. 

Substituted benzene compounds belong to a class of conjugated compounds called arenes. Examples include benzene, naphthalene, anthracene, and phenanthrene. The common structural feature of arenes is a monocyclic or polycyclic system of k electrons that results in a special stability called aromaticity. As a result, aromatic compounds are much less reactive in electrophilic addition reactions than we would expect based on the reactivity of polyenes. 

At nearly the same time MacMillan and coworkers developed a new protocol for SOMO-catalyzed intramolecular arylation of enolizable aldehydes (Scheme 4.7) [33]. In these studies the required oxidation step was accomplished by tris-phenanthroline complexes of iron]111) bearing non-nucleophilic counterions, such as Fe(phen)3-(PFd3. Higher degrees of enanhoselectivities were obtained compared to oxidations with CAN (see 34, Scheme 4.7). Using this method a simple three-step-access to (-)-tashiromine was elaborated by the authors. For theorehcal calculahons of this transformation see Reference [34], By extension of this concept to suitable requisite tethered polyenes the authors were able to estab-hsh a powerfijl cascade reaction leading to defined configured polycyclic structures (36, Scheme 4.7). The oxidation step in this process was achieved by slow addition of Cu(OTf)2/TFA sodium salt [35]. 

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