Chromen-2-one

Spiro-2//-chromenes of type 11 are photochromic On irradiation, the 2//-pyran ring is opened reversibly involving a ring-chain valence isomerization. This leads to the formation of coloured zwitterionic merocyanines 12 which recyclize spontaneously  

Coumarin 13 (27/ -l-benzopyran-2-one), the 1-benzopyrylium ion 14 and the flavylium ion 15 (2-phenyl-l-benzopyrylium ion) derived from flavane (2-phenyl 2i/-chromene) are important derivatives of 277-chromene. They are discussed in chapters 6.7 and 6.8. 

2//-Chromen-2-one possesses the trivial name coumarin. X-ray analysis shows coumarin to be nearly planar (see Fig. 6.6)  

Its spectroscopic properties correspond to those of the parent compound 2//-pyran-2--one. Coumarin has the character of an enol lactone rather than that of a heteroarene, as is shown by its JR and NMR spectra  

Characteristic reactions of coumarin are, accordingly, additions to the C-3/C-4 double bond and nucleophilic opening of the lactone function [24]. 

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