Biosynthesis Kuhn-Roth oxidation

The first report of the incorporation of radioactively labelled mevalonic acid into dendrobine (82) was by Yamazaki et al. (224). Sodium [2— CJmevalonate [( )-463 ] was administered to stems of D. nobile by the cotton-wick method. After 12 days, the plants were extracted and radioactive dendrobine (82) was isolated by column chromatography. The total incorporation was 0.012%. Subsequent Kuhn-Roth oxidation led to acetic acid that showed the expected activity assuming the biosynthesis pathway a leading to the picrotoxanes via a cadalene as precursor (Scheme 55, pathway c). 

From these results it was deduced that any radioactivity in the acetic acid obtained by Kuhn-Roth oxidation of [2-acetate-labeled erythroskyrin must be derived from C(26). The acetic acid contained 4.45% of the total activity, and this was shown by the Schmidt degradation procedure to be located in the methyl group. The acetic acid obtained by Kuhn-Roth oxidation of [1- " C]- and [2- C]malonate-derived erythroskyrin was inactive. These results proved that the biosynthesis of erythroskyrin involved the condensation of valine and a polyketide moiety. More specifically, the results were taken as evidence that erythroskyrin is biosynthesized from valine [N(l) and C(4) to C(8)], one acetate [C(25) and C(26)], and nine malonate [C(2), C(3), C(9) to C(24)] units. This conclusion must, however, be considered as tentative, particularly as no direct evidence was obtained on the incorporation of from the various substrates into C(2) and... 

The biosynthesis of ochratoxin A was, therefore, reduced to a study of the origin of the isocoumarin acid, 7 this investigation was carried out by Steyn et al. (1970). Sodium [1- " C]acetate was added to resting cultures of A. ochraceus. On acid hydrolysis of the labeled ochratoxin A, the isocoumarin acid 7 was found to contain all the activity. Kuhn-Roth oxidation of compound 7 gave acetic acid which contained 20% of the total activity. Schmidt... 

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