Aromatic compounds, addition cyclization reactions

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). 

Aromatic aldehydes react very easily with tetramic acid under acidic conditions to give 3-benzylidene compounds (41). The yields are moderate, because often there are subsequent reactions. As a,/3-unsaturated carbonyl compounds, (41) react in a Michael addition with excess tetramic acid to form (67), but it can also react with other acyclic and cyclic 1,3-dicarbonyl compounds. In these reactions the aryl substituents may vary over a wide range. Thus, (67) and (68) can be cyclized with ammonium acetate to afford pharmacologically interesting compounds (70) and (71) (90TH1). The latter are dihydropyridines. Curiously, (69) does not cyclize under these conditions. (See Fig. 32.)... 

The major classes of photochemical reaction for aromatic compounds are nucleophilic substitution and a range of processes that lead to non-aromatic products—valence isomerization, addition or cycloaddition reactions, and cyclization involving 6-electron systems. These five general categories of reaction will be described in the following sections, together with a few examples of more specific processes. 

A Lewis-acid-promoted cyclization of ethene tricarboxylate derivatived aromatic compounds 1162 provides a route to isochroman-3-ones 1163 via a Friedel-Crafts intramolecular Michael addition protocol. The substrate must possess two ///f to-positioncd electron donating groups in order for the reaction to proceed (Equation 452) <20040BC3134>. 

The analogous primary amine had been earlier studied by Wubbels and gave only the photo-Smiles reaction, but this process was not observed for the present system and the reaction is suggested to proceed by a photo-electron transfer mechanism to yield the radical ion pair. Photoaddition of N-H bonds to aromatic compounds has been known for some years, and the intramolecular process which leads to cyclization products has been reported for the reaction of 9-(aj-anilinolalkyl)phenanthrenes (220). The formation of the spiro-compounds (221) by addition of the N-H bond across the phenan-threne 9,10-positions is suggested to arise from the intramolecular exciplex in... 

An alkene unit can also be used to generate a cation and cyclize to form a polycyclic aromatic compound. When 229 reacted with acetaldehyde, 230 was produced and this was cyclized by treatment with sulfuric acid to give hydronapthalene (231). This modification does not incorporate the additional ring of the cyclic ketones used in the Bogert-Cook reaction it is sometimes called the Bogert synthesis. 

Peramal et al. have used reactions of aromatic amines with a,fl-unsaturated carbonyl compounds via Michael addition, cyclization and aromatizationby conventional heating or MWI to afford the corresponding qtrinolines (71) in high yields as shown in Scheme 6.27 [66]. 

In addition to the works previously reported [50] Bi(0Tf)3 x H20 proved to be as active as scandium triflate in the EC acylation of aromatic compounds with carboxylic acids in the presence of perfluoroalkanoic anhydrides using solvent-free conditions (Equation 31) [60]. Under these conditions no reactions were reported with aromatics less activated than benzene. As previously reported [21], Bi(0Tf)3 xH20 was shown to be recoverable in good yields with no loss of activity. In addition, Bi(NTf2)3 [61] is proved to be an excellent catalyst for the intramolecular EC cyclization of 4-arylbutyric acids [62]. 

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