Cyclization reactions, aromatic

The aminophenols are chemically reactive, undergoing reactions involving both the aromatic amino group and the phenoHc hydroxyl moiety, as weU as substitution on the benzene ring. Oxidation leads to the formation of highly colored polymeric quinoid stmctures. 2-Aminophenol undergoes a variety of cyclization reactions. 

Manufacture of thiophene on the commercial scale involves reactions of the two component method type wherein a 4-carbon chain molecule reacts with a source of sulfur over a catalyst which also effects cyclization and aromatization. A range of suitable feedstocks has included butane, / -butanol, -butyraldehyde, crotonaldehyde, and furan the source of sulfur has included sulfur itself, hydrogen sulfide, and carbon disulfide (29—32). 

During hydrogenation, intermediate aromatic hydroxylamines may undergo various cyclization reactions in molecules containing a suitably disposed carbonyl group, or carbonyl derivative, such as an oxime (13). The cyclized product may or may not maintain the N—OH bond, depending on the solvent, the catalyst, and the electrophilicity of the carbonyl (27,28,29,32,67,68). 

Bond breaking can occur at any position along the hydrocarbon chain. Because the aromatization reactions mentioned earlier produce hydrogen and are favored at high temperatures, some hydrocracking occurs also under these conditions. However, hydrocracking long-chain molecules can produce Ce, C7, and Cg hydrocarbons that are suitable for hydrode-cyclization to aromatics. 

Alkyl radicals generated efficiently from allylsulfones in 80% aqueous formic acid induced a cyclization reaction on aromatic and heteroaromatic compounds to provide polycyclic aromatic and heteroaromatic derivatives (Eq. 7.17).37... 

Electronic factors also influenced the outcomes of these cyclization reactions cyclization of pyrrole 84 to bicyclic amine 85 is catalyzed by the sterically open complex 79a. In this reaction, initial insertion into the Y - H bond occurred in a Markovnikov fashion at the more hindered olefin (Scheme 19) [48]. The authors proposed that the Lewis basic aromatic ring stabilizes the electrophilic catalyst during the hydrometallation step, overriding steric factors. In the case of pyrroles and indenes, the less Lewis basic nitrogen contained in the aromatic systems allowed for the cyclization of 1,1-disubstituted alkenes. 

The Nazarov cyclization of vinyl aryl ketones involves a disruption of the aromaticity, and therefore, the activation barrier is significantly higher than that of the divinyl ketones. Not surprisingly, the Lewis acid-catalyzed protocols [30] resulted only in decomposition to the enone derived from 46,47, and CO. Pleasingly, however, photolysis [31] readily delivered the desired annulation product 48 in 60 % yield. The photo-Nazarov cyclization reaction of aryl vinyl ketones was first reported by Smith and Agosta. Subsequent mechanistic studies by Leitich and Schaffner revealed the reaction mechanism to be a thermal electrocyclization induced by photolytic enone isomerization. The mildness of these reaction conditions and the selective activation of the enone functional group were key to the success of this reaction. 

CHS orchestrates the condensation, cyclization, and aromatization of one p-coumaroyl-CoA and three malonyl-CoA molecules to produce chalcone (Fig. 12.2).22 Transfer of the p-coumaroyl moiety from the CoA-linked starter molecule to Cys 164 within the active site initiates the reaction sequence. Next, the sequential condensation of three acetate units, derived from malonyl-CoA, with the enzyme-bound coumaroyl moiety forms a tetraketide intermediate. Inherent in the condensation reaction is decarboxylation of malonyl-CoA to an acetyl-CoA carbanion that serves as a nucleophile during the successive chain elongation... 

Scheme 9 Photochemical cyclization of diethynylmethanes. Although this reaction is topologically analogous to the cyclization reactions discussed earlier, it does not lead to an aromatic product.

This article will deal with metal-catalyzed cyclization reactions, with reference to oxide and dual-function catalysts. Product cycles may contain five or six carbon atoms. The respective prefixes C5 and will point to the resulting structure (5). The term dehydrocyclization will be applied to reactions that end up with aromatic products the formation of saturated (cycloalkane) rings will henceforth be called cyclization. ... 

A common feature of any cyclization reaction is that a new intramolecular C—C bond is produced that would not have been formed in the absence of the catalyst. Those reactions in which one ring closure step is sufficient to explain the formation of a given cyclic product will be called simple cyclization processes, although their mechanism is, as a rule, complex. We shall distinguish those cases in which any additional skeletal rearrangement step(s) is (are) required to explain the process. Some specific varieties of hydrocarbon ring closure processes are not included. A recent excellent review deals with the formation of a second ring in an alkyl-substituted aromatic compound (12). Dehydrocyclodimerization reactions have also to be omitted—all the more since it is doubtful whether a metallic function itself is able to catalyze this process (13). 

Recentiy published crystal structures of antibody 4C6, an antibody that catalyzes another cationic cyclization reaction (Figure 6), revealed that this antibody has exquisite shape complementarity to its eliciting hapten 5. The active site contains multiple aromatic residues which shield the high-energy intermediate from solvent and stabilize the carbocation intermediates through cation-7r interactions. 

A -tritylaziridine-2-(5)-carboxaldehyde. The application of a novel, sequential, trans-acetalation oxonium ene cyclization has delivered a stereoselective synthesis of the C-aromatic taxane skeleton, and a combinatorial sequence of the regioselective propiolate-ene, catalytic enantioselective epoxidation and carbonyl-ene cyclization reactions has been used to complete the synthesis of the A-ring of a vitamin D hybrid analogue. 

An important exception to this regularity is the cyclization of aromatic alkoxides containing aromatic radical moieties. In these cases, C-0 bond formation is not observed, but C-C bond formation is achieved instead. As Galli and Gentili (1998) pointed out, this is primarily due to the unfavorable thermodynamic driving force for C-0 bond formation compared to C-C bond formation. Thus, the photostimulated reaction depicted in Scheme 7.39 results in the formation of a six-membered carbocycle rather than an octa-membered oxa-heterocycle. The carbocycle is formed in 75% yield (Barolo et al. 2006). This product is a precursor to the thalicmidine biomolecule of the alkaloid group. 

The alkylation of benzene by alkylpotassium compounds has been reported by Bryce-Smith (S9) and is probably due to the increased base strength of organopotassium compounds over organosodium compounds. The potassium hydride eliminated in the cyclization reaction may add to ethylene to form ethylpotassium, which then may react with the aromatic to yield ethane and a benzylic carbanion [Reactions (16) and (17)]. 

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