Benzopyrilium cation

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. 

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]. 

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

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