Quinolines, photochemical synthesis

Several investigators discovered the photochemical rearrangement of quinoline Af-oxides to the corresponding benzoxazepines and then to indoles. For example, Kobayashi and colleagues synthesized for the first time 2- and 3-trifluoromethylindoles via this route (Scheme 15, equation 1) [104]. In a series of papers, Kaneko and coworkers described the photochemical synthesis of indoles from quinoline Af-oxides via benzoxazepines, which were not isolated (equation 2) [105-109]. A similar photolysis of 5-(alkoxycarbonylamino)isoquinoline 2-oxides afforded l-alkoxycarbonyl-4-hydroxyindoles (equation 3) [110]. Irvine s group reported similar photochemical syntheses of indoles from quinoline Af-oxides albeit in low yields [111]. 

This way of rationalization is substantiated by the photolytic behavior of, -unsaturated aldehydes (478) (Scheme 162). Only in one of the five cases studied was 479 observed this result is in accordance with the difficult intramolecular addition of alkoxyl radicals in the Cy6/Cy7 case. Since thiyl cyclizations appear efficient even in the Cy6/Cy7 case one might expect to observe easy photocyclizations of unsaturated thiones also in this case the easy photochemical synthesis of quinolines from A -(o-styryl)thioamides appears to be an excellent illustration of this view. Here, the sulfur analog of 479 would be an intermediate. To complete the analogy, one may expect the converse P-scission reaction to be efficient with radicals that readily open such as oxyranylalkyl ones (see Section VIII.2) in this way an interesting macrolide synthesis was described by Carlson. Photolysis of epoxycyclanone (480)... 

SCHEME 45 l2-catalyzed photochemical synthesis of benzimidazo[l,2-a]quinoline. 

Photocyclization of 1-benzylideneisoquinolines to the Aibexao[d,e,g -quinolines is a well-studied example of the above stilbene-phenanthrene cyclization and therefore has been extended to the development of new photochemical methods for the synthesis of aporphine alkaloids (11-14). In this section, photocyclization of 2-acyl-1 -benzylideneisoquinolines to dehy-droaporphines and their subsequent conversion to aporphines and oxo-aporphines are summarized. 

Takenori M, Akiya O (2011) Synthesis of 2,4-diiodoquinolines via the photochemical cyclization of o-alkynylaryl isocyanides with iodine. J Oig Chem 76 1163-1166 Wang XS, Li Q, Wu JR, Zhang MM (2009) Green method for the synthesis of benzo[/] pyrimido[4,5-6]quinoline derivatives eatalyzed by iodine in aqueous media. Synth Commun 39 3069-3080... 

A direct access to the synthesis of dihydroquinolines (eq 22) was developed. The vinyl quinone mono- or di-SES-imide, accessible by oxidation of the corresponding aminophenol or phenylenediamine, undergoes a thermal 67r-electrocyclization in the presence of a polar nonprotic additive. Dehydrodesulfinylation of the SES group will lead to the quinoline. The vinyl quinone monoimide substrate can also provide the protected indole by photochemical cyclization. 

The photochemical generation of quinolines and benzimidazoles from anilines (a reaction known since more than 100 years, compare Section 6.1.1.1) has been further extended to the use of mesoporous, iV-doped Ti02 under both UV and visible light. " An arenesulfonic acid functionalized mesoporous silica decorated with titania has been claimed for the one-pot photocatalytic synthesis of quinolines. ... 

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