Benzopyrilium

An Au(iii) catalyst has been used for the cyclization of <9-alkynylbenzaldehydes to benzopyrilium salts, which are readily oxidized to the corresponding azaphilones (Equation (97)).346 Similar transformations have been realized with catalytic AgSbF6 347... 

Note that each of the six common anthocyanidins has the basic structure of the flavylium cation (2-phenyl benzopyrilium 1.31). 

The pyrilium cation 9.1, 2-pyrone 9.2, 4-pyrone 9.3, and their benzo-fused analogues the benzopyrilium cation 9.4, coumarin 9.5, chromone 9.6, are the parent structures of a series of six-membered ring heterocycles containing one oxygen atom. The impetus for research in this area comes from the enormous number of plant-derived natural products based on the benzopyrilium, coumarin, and chromone structures. 

The red, violet, and blue pigments of flower petals are called anthocyanins, and are glycosides of various benzopyrilium cations. Delphinidin chloride 9.7, for example, is a blue pigment. Khellin 9.8 is a natural product which has found clinical application in the treatment of bronchial asthma and has been the starting point for the design of many totally synthetic chromones with improved biological properties. 

The benzopyrilium salt 28 yields a mixture of the esters 29 and 30, and 31 (Z = and S ) are convertible into the corresponding 32. Finally, It has been shown that cyclopropenium salts 33 also react and may form mixtures of the isomeric products 34 and 35. 

Treatment of benzopyrilium perchlorates (159) with excess hydrazine hydrate in refluxing EtOH has provided 5/f-benzo-2,3-diazepines (160) in 63-75% yield (Scheme 23) <93KGS1475>. Reaction of (159 R = Me, R = C02Et, CN) with one equivalent of hydrazine hydrate aflbrds 2-amino-isoquinolinium perchlorates (161) in 90% yield, which, in the case of the 4-cyano compound, proceeds to the benzodiazepine (160 R = Me, R = CN) in 73% yield. 

In the above examples, enantioselectivity is proposed to arise from formation of a enantiopure metal complex-associated yhde. Suga and coworkers have developed an alternate strategy utilising Lewis acids that bind to an electron-deficient biden-tate dipolarophile. In this approach the2-benzopyrilium-4-olate (9.146) formed by... 

The anthocyanins, the blue, red or violet pigments found in many flowers, are glucosides in which there is a six-membered oxygen-containing ring. Their aglucones (non-sugar portion) or anthocyanidins are hydroxy-lated derivatives of a benzopyrilium nucleus ... 

Barluenga et al. reported a similar type of benzannulations by a metal-free protocol. Treatment of o-alkynylbenzaldehyde 11 with a mixture of IPy2BF4 and HBF4, followed by addition of alkynes 2, afforded the iodonaphthalene product 31 [22]. They proposed that 31 would be obtained from intermediate 33 via the retro [4 - - 2] cycloaddition through the formation of iodine-substimted benzopyrilium cation 32 (Scheme 15.12). Yue et al. also reported L-mediated benzannulations [23]. 

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