Parent fused carbocycles

Three different fused systems have been reported in the literature thieno[3, 2 4,5]thieno[2,3-c]pyridine (f/r-thiophenes), thieno[2, 3 4,5]thieno[2,3-c]pyridine (tram- thiophenes) and thicno[3,2-g]thicno [3,2-d pyridine derivatives. So far, the parent thienothienopyridines have only been prepared via the corresponding pyrylium salts (see below) all others are benzo or extended carbocyclic derivatives, for example, thienothienoquinolines. 

Polycyclic parent hydrides. These are classified as bridged polyalkanes (also known as von Baeyer bridged systems, from the nomenclature system developed to name them), spiro compounds, fused polycyclic systems and assemblies of identical rings. The four systems may be either carbocyclic or heterocyclic. In developing their names, the following principles are used. 

Several saturated parent systems shown in Scheme 1, especially (2), (4) and (7), have been incorporated into spiro compounds or have been fused with a variety of non-aromatic carbocyclic or heterocyclic rings. Unsaturated compounds with an endocyclic carbon-carbon double bond, formally possible in all systems with three adjacent heteroatoms, are only known in very few cases (e.g. cyclic sulfites and sulfates of enediols) or in the form of benzo or other condensed systems (mainly cyclic sulfites and sulfates of aromatic 1,2-dihydroxy compounds). For details concerning the various types of derivatives, see Section 4.33.4.2. 

The conformational properties of eight-membered rings linearly fused with two benzene rings have been investigated in some detail by dynamic NMR. The parent carbocyclic compound is (452) which exists as a 1 1 mixture of the chair (453) and the boat (probably... 

Studies on benzo-fused analogs of 78 have also been reported. Solutions of unstable dibenzo[b,e]phosphabenzene (phosphaanthracene) (80, R = H)286 and dibenzo[6,d]phosphabenzene (phosphaphenanthrene) (81)287 have been prepared and examined by UV spectroscopy. The overall shape of the spectrum due to 80 (R = H) resembles that of anthracene rather than acridine, whereas that of 81 resembles the phenanthridine more closely than the phenanthrene spectrum. The absorptions of both 80 (R = H) and 81 show the typical strong bathochromic shifts relative to their nitrogen and carbocyclic analogs. 10-Phenyldibenzo[6, e]phospha-benzene (80, R = Ph)288 is more stable than its parent and shows similar UV absorption. 

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