Birch-Donovan hypothesis

Early work on the formation of cyanidin (19) - , quercetin (14)18-20 (—)-epicatechin (21) and phloretin (23) demonstrated quite clearly the biosynthetic pattern postulated by the Birch-Donovan hypothesis, i.e. ring A of the flavonoids is derived by a head to tail condensation of three acetate (or malonate) units, while ring C and the three carbon chain originate from a phenylpropanoid precursor. All subsequent experimental observations show that chalcones, with their respective isomeric ilavanone, occupy a central position as intermediates in flavonoid biosynthesis. Thus Grisebach and his collaborators have demonstrated the incorporation of appropriately labelled C labelled chalcones flavanones)... 

Figure 6.8. The Birch-Donovan hypothesis the biosynthesis of pinosylvin...

In 1907, Collie proposed that polymers of ketene (CH2=C=O) might be precursors of such compounds as orsellinic acid, a common constituent of lichens. The hypothesis was modernized in 1953 by Birch and Donovan, who proposed that several molecules of acetyl-CoA are condensed (Eq. 21-19) but without the two reduction steps required in biosynthesis of fatty acids (Fig. 17-12).329 As we now know they were correct in assuming that the condensation occurs via malonyl-CoA and an acyl carrier group of an enzyme. The resulting p-polyketone can react in various ways to give the large group of compounds known as polyketides. 

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