Flavanone, desaturation

A desaturation reaction forming a double bond between C-2 and C-3 of the C-ring is involved in the formation of both flavones and flavonols, and the respective substrates involved, (25)-flavanones and (2R,3R)-DHFs, differ only in the presence or absence of the 3-hydroxyl (Figure 3.2). 

The direct desaturation of carbon adjacent to heteroatoms, notably oxygen and nitrogen, has also been observed. These reactions include the desaturation of a tetrahydrofuran ring in the biosynthesis of aflatoxin and sterigmatocystin by a specific P450 enzyme, the P450-catalyzed desaturation of flavanones to flavones, and the conversion of ethylcarbamate to vinyl carbamate . In some instances, the desaturation may involve the carbon and the heteroatom instead of two carbon atoms. The CYP2B1-catalyzed oxidation of testosterone to androstenedione, which involves oxidation of the 17-hydroxy to a 17-keto function, is possibly such a reaction, because only... 

Chalcones (e.g., 9) are key intermediates in the formation of several major groups of flavonoids (see Fig. 11.2). In some plants, they are converted into aurones (such as 11). In other instances, chalcones undergo reduction of the exocy-clic double bond to produce dihydrochalcones (such as 12). Chalcones and flavanones (e.g., 10) exist in equilibrium in in vitro systems. However, as only flavanones with a (25)-configuration are known to occur naturally, this interconversion appears to be enzymatically controlled and does not involve racemization. Desaturation of flavanones can yield flavones (such as 4 and 5), whereas introduction of oxygen at the 3-position gives dihydroflavonols (flavanonols) (such as 13) which, in turn, are the precursors of flavonols (e.g., 6-8), anthocyanins (e.g., 1-3), and condensed tannin precursors (Ebel and H lbrock, 1982 Hahlbrock, 1981 Haslam 1974) (Figs. 11.7 and 11.8). 

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