Highly condensed ring systems

The anthraquiaone vat dyes can be classified into several groups on the basis of their chemical stmctures (/) benzanthrone dyes, (2) indanthrones, (3) anthrimides, (4) anthrimidocarbazoles, (5) acylaminoanthraquinones, (6) anthraquinoneazoles, (7) anthraquiaone acridones, (8) anthrapyrimidines, and 9) highly condensed ring systems. Most currendy (1993) available dyes have been known for many decades, and very few new dyes have been commercialized since the 1970s. Recendy, research and development efforts have focused on improved manufacturing of traditional vat dyes. 

It should be possible to develop ring design heuristics that can generate clear and aesthetic layouts for even highly condensed ring systems such as cubane. Although there is no substitute for an RTD to lay out and orient traditional systems (steroids), the RTD approach alone is inadequate because the presence of substituents can affect the optimal layout. 

One of the most important condensed ring systems is indole. Whether the indole nitrogen is substituted or not, the favored site of attack is C-3 of the heterocyclic ring. Bonding of the electrophile at that position permits stabilization of the intermediate by the nitrogen without disruption of the benzene aromaticity. Indole can exist in two tautomeric forms, the more stable enam-ine and the 3-H-indole or imine forms. C-2 to C-3 pi-bond of indole is more capable of cycloaddition reactions then the other pi bonds of the molecule. Inter molecular cyclo additions are not favorable, whereas intramolecular variants are often high-yielding. 

There is normal diene reactivity in highly condensed aromatic systems such as perylene and benzanthrene here the conjugated double bonds belong to different rings, the positions at which the philodiene is added being indicated by arrows in the following formulae ... 

Pyrridazinoindoles can be considered as azaanalogs of different caibolines, and especially p- and y-, the condensed ring system of which represent the basis of many compounds of high physiological activity. Therefore the unified aromatic system of isomeric pyrridazinoindoles containing three nitrogen atoms in different positions and their derivatives have attracted great attention of researchers. 

Since the six carbons shown above have 10 additional bonds, the variety of substituents they carry or the structures they can be a part of is quite varied, making the Diels-Alder reaction a powerful synthetic tool in organic chemistry. A moment s reflection will convince us that a molecule like structure [XVI] is monofunctional from the point of view of the Diels-Alder condensation. If the Diels-Alder reaction is to be used for the preparation of polymers, the reactants must be bis-dienes and bis-dienophiles. If the diene, the dienophile, or both are part of a ring system to begin with, a polycyclic product results. One of the first high molecular weight polymers prepared by this synthetic route was the product resulting from the reaction of 2-vinyl butadiene [XIX] and benzoquinone [XX] ... 

The chemistry of 1,2,5-thiadiazole systems (RC)2N2S has been extensively investigated. " In addition to the condensation reactions of sulfur halides with 1,2-diaminobenzenes, this ring system is obtained in high yields by the reaction of S4N4 with acetylenes (Section 5.2.6). For example, the reaction of S4N4 with diphenylacetylene produces 3,4-diphenyl-1,2,5-thiadiazole in 87% yield. 

---