Heteroaromatic rearrangements photochemical

JV-Oxide rearrangements in heteroaromatic compounds are frequently induced by either photochemical or acid anhydride initiated processes, and usually involve formation of a C—O bond at the carbon a to the original N-oxide. 60 One of the few -rearrangements is observed by treating 1,3-dimethylpyrido[2,3-r/Jpyrimidine-2,4(l //,3//)-dione 8-oxide (1 see Section 7.2.2.1.1.5.1.) with acetic acid/acetic anhydride at 90 CC, which gives 6-acetoxy-l,3-dimethylpyrido[2,3-t/]pyrimidine-2,4(l//,3//)-dione (2). When the reaction is run at reflux temperature in trifluoroacctic anhydride/trifluoroacetic acid, a 46% yield each of the a- and -product, 3 and 4, respectively, is obtained.310... 

Heteroaromatic A-imines are photochemically active compounds (see Table III) On irradiation they generally yield products of (a) N-N bond cleavage, (b) ring enlargement, (c) rearrangement. However, the available data do not permit definition of the conditions necessary for selective reactions. Apparently unimportant changes in the substituents and the solvent may lead to different results. Therefore, the photochemical reactions will be discussed separately for the individual A-imines. 

A method for the synthesis of highly substituted aromatic and heteroaromatic hydroxy compounds is the photochemical Wolff rearrangement of an unsaturated a-diazo ketone in the presence of an alkyne. The product, an alkenylcyclobutenone, undergoes ringopening and recyclization to a phenol (equation 60). Three examples of the reaction are the formation of the naphthols 587 and 588 and that of the hydroxybenzofuran 589. Complexes 590 (R = alkyl or aryl r-alkyl or Me3Si), produced from THF, alkynes and... 

Several reviews have been published within the year which are of general relevance to the photoreactions of aromatic compounds. The subjects of these reviews include photochemistry in ionic liquids and in isotropic and anisotropic media, organic synthesis utilizing photoinduced electron-transfer reactions," heteroatom-directed photoarylation processes, photochromism, and photochemical molecular devices. Reviews more directly pertinent to the sections in the present chapter include those of the photoisomerization of five-membered heteroaromatic azoles, the photocycloaddition of benzene derivatives to alkenes, Diels-Alder additions of anthracenes, advances in the synthesis of polycyclic aromatic compounds, diarylethene-based photochromic switches, the photo-Fries rearrangement, and the application of Diels-Alder trapping of photogenerated o-xylenols to the synthesis of novel compounds. " A number of chapters in the two recently published handbooks of photochemistry and photobiology and in the revised edition of the text on photochromism are also pertinent to the current subject matter. 

Rearrangements of aromatic and heteroaromatic nitrenes can be initiated with either heat or light. The thermal reaction is typically induced by flash vacuum thermolysis, with isolation of the products at low temperatures. Photochemical experiments are conducted either under matrix isolation conditions or in solution at ambient temperature. These rearrangements are usually initiated by ring expansion of the nitrene to a seven-membered ring ketenimine or carbodiimide (i.e., an azacycloheptatetraene). 

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