Condensed polycyclic systems

Dehydrogenation (the conversion of alicyclic or hydroaromatic compounds into their aromatic counterparts by removal of hydrogen - and also, in some cases, of other atoms or groups) has found wide application in the determination of structure of natural products containing complex hydroaromatic systems. Dehydrogenation is employed also for the synthesis of polycyclic hydrocarbons and their derivatives from readily accessible synthetic hydroaromatic compounds. The general process is illustrated by the conversion of tetralin into naphthalene. 

Intramolecular cyclisation of a 4-arylbutanoic acid system is also an important step in a convenient synthesis of the polycyclic system, chrysene, which is formulated and described in Expt 6.12. Here, methyl cinnamate is first subjected to reductive dimerisation to give methyl meso-ft,y-diphenyladipate, which is accompanied by some of the ( + )-form. The meso isomer (16) is the most easily isolable and cyclisation occurs smoothly in sulphuric acid to yield the diketone 2,1 l-dioxo-l,2,9,10,ll,18-hexahydrochrysene, which is obtained as the trans form (17) as shown in the following formulation. Clemmensen reduction of this ketone followed by dehydrogenation (in this case using selenium) completes the synthesis of chrysene. 

The synthesis of a tetraphenyl derivative (rubrene, Expt 6.13) of the linearly fused tetracyclic aromatic hydrocarbon naphthacene involves an interesting intermolecular cyclisation process between two molecules of 1-chloro-1,3,3-triphenylpropa-1,2-diene. This substituted allene is formed in situ from 1,1,3-triphenylprop-2-yn-l-ol (Expt 5.41) when the latter is allowed to react with thionyl chloride and the resulting chlorosulphite ester heated with a little quinoline cyclisation occurs spontaneously under these reaction conditions to give rubrene which has an intense red colour. 

The synthesis of 9-phenylphenanthrene (Expt 6.14) illustrates the formation of the phenanthrene system by the cyclisation of a 1,2-diphenylethylene (stilbene). The process involves an allowed photochemical cyclisation which gives initially a dihydrophenanthrene. This is readily dehydrogenated in situ by molecular oxygen in the presence of iodine. 

The required stilbene is usually readily prepared by dehydration of the appropriate alcohol obtained by a Grignard reaction, e.g. 

---

In different types of condensed polycyclic systems, mostly nitrogen-containing systems, the elimination of attached fluorine by thiols has proved a valuable synthetic tool for the introduction of sulfur moieties. In 4-(aminosulfonyI)-7-fluoro-2,l,3-benzoxadiazole(l) the reactivity of the fluorine towards sulfur nucleophiles is sufficiently high that it can be used analytically for the detection of thiols14 (c.g., in peptides). The reaction proceeds almost quantitatively. 

Benzene (1) is the simplest aromatic hydrocarbon upon which our knowledge of aromatic chemistry is based. This hydrocarbon, the alkylbenzenes (2), the arylmethanes [e.g. diphenylmethane (3)], the biphenyls [e.g. biphenyl (4)] and the condensed polycyclic systems [e.g. naphthalene (5) and anthracene (6)] all exhibit chemical reactivity and spectroscopic features which are markedly different from their aliphatic and alicyclic hydrocarbon counterparts. Indeed the term aromatic character was introduced to specify the chemistry of this group of hydrocarbons and their substituted functional derivatives, and it was soon used to summarise the properties of certain groups of heterocyclic compounds having five- and six-membered ring systems and the associated condensed polycyclic analogues (Chapter 8). 

In the investigation of the total synthesis of steroids, not only have new chemical reactions been discovered and ones known earlier modified, but extremely delicate methods for the selective performance of chemical transformations have been developed. Moreover, with the results obtained in total steroid synthesis, the stereochemistry of condensed polycyclic systems has been given a new impetus and further development. 

Intramolecular Cycloaddition of Nonconjugated Enediynes of a Higher Order as a Route to Functionalized Condensed Polycyclic Systems... 

The early Escherunoser-Stork results indicated, that stereoselective cyclizations may be achieved, if monocyclic olefins with 1,5-polyene side chains are used as substrates in acid treatment. This assumption has now been justified by many syntheses of polycyclic systems. A typical example synthesis is given with the last reaction. The cyclization of a trideca-3,7-dien-11-ynyl cyclopentenol leads in 70% yield to a 17-acetyl A-norsteroid with correct stereochemistry at all ring junctions. Ozonolysis of ring A and aldol condensation gave dl-progesterone (M.B. Gravestock, 1978 see p. 279f.). 

Fused polycyclic systems are very numerous and diverse. They are named using the structures and names of their smaller components and the concept of ortho-fusion, which is purely a formal operation encountered in nomenclature. This concept is essential to the naming of larger systems, and is the formation of one bond by the condensation of two bonds belonging to two different cyclic systems, one of them being a mancude ring. 

In an extensive study into the application of the decarboxylative approach to azomethine ylides, Giigg reported the construction of numerous, complex polycyclic systems via an intramolecular protocol. Thiazolidine-4-carboxylic acid (263) was shown to react with 264 in refluxing toluene to furnish a 2 1 mixture of 265 and 266 in 63% yield (81). The reaction is assumed to occur via condensation of the aldehyde and amino acid to generate the imine 267, followed by cyclization to 268. Subsequent thermal decarboxylation of the ester generates either a syn dipole leading to 265 from an exo transition state, or an anti dipole and endo transition state generating adduct 266 (Scheme 3.90). 

Alkyl- or aryl-dibenzothiophenes are conveniently prepared from the 2-arylthio-cyclohexanones, which are readily cyclized and dehydrogenated to yield the respective 1-, 2-, 3- or 4-substituted dibenzothiophenes (382 equation 9 Section 3.15.2.3.2). More complex polycyclic systems are available, using suitable aryenethiols, such as naph-thalenethiols, and 2-bromo-l-tetralone to synthesize the appropriate 2-arylthio ketones. Diaryl sulfides can be converted to dibenzothiophene derivatives in satisfactory yields by photolysis in the presence of iodine (equation 10) (75S532). Several alkyldibenzothiophenes with substituents in the 2- and/or 3-positions were prepared in satisfactory yield by the condensation of dichloromethyl methyl ether with substituted allylbenzo[6]thiophenes (equation 11) (74JCS(P1)1744). 

Virtually all types of metal ions have been complexed with macrocyclic ligands.2-7 Complexes of transition metal ions have been studied extensively with tetraaza macrocycles (Chapter 21.2). Porphyrin and porphyrin-related complexes are of course notoriously present in biological systems and have been receiving considerable investigative attention (Chapter 22).8 Macrocyclic ligands derived from the Schiffbase and template-assisted condensation reactions of Curtis and Busch also figure prominantly with transition metal ions.6,7 The chemistry of these ions has been more recently expanded into the realm of polyaza, polynucleating and polycyclic systems.9 Transition metal complexes with thioether and phosphorus donor macrocycles are also known.2... 

Benzo(a)pyrene (Figure 1.14) is the most studied of the polycyclic aromatic hydrocarbons (PAHs), which are characterized by condensed ring systems ( chicken wire structures). These compounds are formed by the incomplete combustion of other hydrocarbons, a process that consumes hydrogen in preference to carbon. The carbon residue is left in the thermodynamically favored condensed aromatic ring system of the PAH compounds. 

There are only a few synthetic approaches toward these polycyclic systems. The coupling product from diazotized anthranilic acid and 2-hydroxy-1,4-naphthoquinone is transformed into 372 by heating with acetic anhydride (83IJC(B)95). The same quinone reacts with citral and, depending on the reaction conditions, several products are formed. In a triethylamine-catalyzed reaction, the angular quinones 373 and 374 are formed, whereas acid-catalyzed condensation gives the linear compound 375 (82SC195 ... 

Bonchev, D. and Balaban, A.T. (1981). Topological Centric Coding and Nomenclature of Polycyclic Hydrocarbons. I. Condensed Benzenoid Systems (Polyhexes, F isenes).J.Chem.Inf.Com-put.Sci.,21,223-229. 

---