Ring Systems General

Various publications from the Chemical Abstracts Service can be nsed to find the name of a known ring system. The most comprehensive sonrce of ring system names is the Ring Systems Handbook (RSH). Entries are in ring analysis order, i.e., according to the following hierarchy of ring data  

A list of common hydrocarbon and heterocyclic parent skeletons and their numbering appears in Table 4.1. 

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The first photochromic spirooxazine compounds synthesized (1970) belonged to the spiroindolinonaphthoxazine ring system. Generally, they were colorless in dilute organic solvents and polymer matrices and became blue upon exposure to UV light. 

Quadridentate ligands can be obtained by condensing suitably substituted aldehydes and ketones with bidentate primary amines, and the resulting Schiff bases closely resemble the porphin ring system. Generally, one of the structural units—N=C—C=N—,—N=C—NHC=N—, N=C—NH—N=C—, —C=N—CH=CH—N=C—, or —C=N— CH2—CH2—N=C—is incorporated into the structure for maximum stability. For example, if or /io-aminobenzaldehyde is condensed with ortho-... 

Thiophenes, synthesis by cyclization reactions and from other ring systems (general review) 85HC(44,1)1. 

Intramolecular reactions between donor and acceptor centres in fused ring systems provide a general route to bridged polycyclic systems. The cts-decalone mesylate given below contains two d -centres adjacent to the carbonyl function and one a -centre. Treatment of this compound with base leads to reversible enolate formation, and the C-3 carbanion substitutes the mesylate on C-7 (J. Gauthier, 1967 A. Belanger, 1968). 

In the case of l,4-ben2oquinone, the product is steam-distilled, chilled, and obtained in high yield and purity. Direct oxidation of the appropriate unoxygenated hydrocarbon has been described for a large number of ring systems, but is generally utilized only for the polynuclear quinones without side chains. A representative sample of quinone uses is given in Table 5. 

In general, five-membered heteroaromatic ring systems with one heteroatom all undergo preferential a rather than /3 electrophilic substitution. This is rationalized in terms of the more effective delocalization of charge in the intermediate (36) leading to a substitution than in the intermediate (37) leading to /3 substitution. 

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