186 



FLAVONOIDS AND RELATED COMPOUNDS 



In the quinoid form they are rapidly oxidized by air and destroyed. Therefore they are 

 most safely prepared in slightly acidic solution. Since they are both glycosides and salts, 

 the anthocyanins are the most water-soluble of all the flavonoids. On standing or warm- 

 ing in neutral aqueous solution a colorless isomer ("pseudobase") is formed: 



The hydroxylation patterns of the anthocyanins are very much like those of the fla- 

 vones. Methylation is generally restricted to the 3' and 5' hydroxyl groups. Hirsutidin 

 is a rare exception, having a 7-methoxyl group. Glycosylation is also more restricted 

 in the anthocyanins than in the other flavonoids. If only one sugar is present, it is in- 

 variably in the 3-position (except for glycosides of apigenidin, which lacks a 3-hydroxyl 

 group). If two hydroxyls are glycosylated (which never happens in the flavones) they are 

 at the 3 and 5 positions. These 3, 5-diglycosides are the most common and best known 

 anthocyanins. The sugars which combine seem to be the same ones used in the flavone 

 glycosides. Sometimes acids are present esterifying hydroxyl groups of the ring itself 

 or of the sugar. 



The three basic types of anthocyanins, like the catechins, depend on the hydroxyla- 

 tion of ring B. All of them have 5, 7-hydroxyl (or methoxyl) on ring A. 



B 



\V 



r—^^ 



^ 



OH 



pelargonidin cyanidin delphinidin 



Additional variation is introduced by methylation, giving the structures: 



OCH. 



ochr. 



OCH. 



OCHi 



peonidin 



petunidin 



malvidin 



Hirsutidin, as mentioned above, has the B ring as in malvidin with an additional 7-meth- 

 oxyl group. Apigenidin has no 3-hydroxyl group: 



