Aromatic compounds ring opening

Compound B bas lost its cyclic arrangement of six n electrons and tha-efore its aromaticity. Tbus, ring opening is endothermic. 

Cyclic Hydrocarbons. These are structures in which the carbon atoms form a ring instead of an open chain. They are also called carbocyclic or homocyclic compounds. They are divided into two classes alicyclic (or cycloaliphatic) and aromatic compounds. 

The diazotization of amino derivatives of six-membered heteroaromatic ring systems, particularly that of aminopyridines and aminopyridine oxides, was studied in detail by Kalatzis and coworkers. Diazotization of 3-aminopyridine and its derivatives is similar to that of aromatic amines because of the formation of rather stable diazonium ions. 2- and 4-aminopyridines were considered to resist diazotization or to form mainly the corresponding hydroxy compounds. However, Kalatzis (1967 a) showed that true diazotization of these compounds proceeds in a similar way to that of the aromatic amines in 0,5-4.0 m hydrochloric, sulfuric, or perchloric acid, by mixing the solutions with aqueous sodium nitrite at 0 °C. However, the rapidly formed diazonium ion is hydrolyzed very easily within a few minutes (hydroxy-de-diazonia-tion). The diazonium ion must be used immediately after formation, e. g., for a diazo coupling reaction, or must be stabilized as the diazoate by prompt neutralization (after 45 s) to pH 10-11 with sodium hydroxide-borax buffer. All isomeric aminopyridine-1-oxides can be diazotized in the usual way (Kalatzis and Mastrokalos, 1977). The diazotization of 5-aminopyrimidines results in a complex ring opening and conversion into other heterocyclic systems (see Nemeryuk et al., 1985). 

The forerunner in the Co-catalyzed [2+2+2] cycloaddition domino processes was that identified by Vollhardt and colleagues [273], with their excellent synthesis of steroids. Reaction of 6/4-1 with [CpCo(CO)2] gave compound 6/4-3 with an aromatic ring B via the intermediate 6/4-2. In this process, trimerization of the three alkyne moieties first takes place, and this is followed by an electrocyclic ring opening of the formed cyclobutene to give o-quinodimethane. This then undergoes a Diels-Alder reaction to provide the steroid 6/4-3 (Scheme 6/4.1). 

Some experiments were conducted to determine the influence of the solution pH. The catalytic oxidation of p-coumaric acid has been performed by increasing the initial pH from 3.5 to 7.5 and 11. Although the cleavage of the exocyclic double bond may be pH sensitive we obtained the same major intermediates which are the hydroxybenzaldehyde and the p-hydroxybenzoic acid molecules. The rate of oxidation of these aromatic compounds to ring opening compounds was higher when the initial pH was basic and yielded higher concentrations of maleic and fumaric acids. 

Moreno described the cycloaddition of 2,5-dimethylfuran (42) catalyzed by silica-supported Lewis acids under solvent-free conditions in closed Teflon vessels using a commercial microwave oven (Scheme 9.11) [28, 52]. Under these conditions coordination of the silica-supported catalyst with the oxygen bridge favors ring opening, thus leading to the aromatic compounds in one step. The use of Si (71) gave the best results for aromatic compounds. 

The 1,2,3-thiadiazole 1 possesses three contiguous heteroatoms in a five-membered ring and exists as a remarkably stable neutral aromatic compound. It is isomeric with the ring-opened a-diazothioketone 2 (Equation 1) although there is evidence that it reacts through this intermediate, all structural methods, including X-ray diffraction, point to 1 as the structure for a 1,2,3-thiadiazole. 

Unlike the ring opening of decalin, the RO of naphthenic compounds containing at least one benzene ring (i.e. tetralin, naphthalene, etc.) are much slower due to the presence of aromatic rings. Corma et al.A1 has reported differences in activity... 

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