Incorporation of Fluorinated and Other Substrate Analogues

There are many examples of polyketide metabolites where the common acetate starter unit is replaced by other groups, and a general discussion of these is outside the scope of this review. Numerous studies have been carried out on the use of analogues of the normal biosynthetic precursors to produce novel structures. This type of precursor-directed biosynthesis has been the subject of a recent comprehensive review [60]. 

The introduction of fluorine had several purposes. These were to provide a biosynthetic label for detection of incorporation by 19F NMR to produce structural analogues of the natural metabolite and specifically in this case to inhibit 

The success, albeit limited, of incorporation studies of polyketide assembly intermediates has resulted from feeding these in the form of their NAC thio-esters which structurally mimic the thiol end of the phosphopantetheine moiety found in coenzyme A and the acyl carrier protein component of the PKS. This will be discussed further below, but it has also been shown that there are advantages to feeding starter units in the form of their NAC thioesters. 

Following the success of the hexanoate thioester feeding, we carried out similar experiments with 2H-labelled butyrate, pentanoate and heptanoate. Of these, the pentanoate results almost exactly mirrored those for hexanoate, the others showing only poor incorporation exclusively by prior degradation to acetate. The pentyl analogue (80) of norsolorinic acid was subsequently isolated by preparative hplc separation from the natural metabolite and fully characterised. Thus it appears that the NSA PKS can accept both pentanoate and hexanoate starters with comparable facility. 

To probe the substrate specificity further, the incorporation of the NAC thioester of 6-fluorohexanoate was examined. The 470 MHz 19F NMR spectrum of the metabolite (derivatised as its tetramethyl ether) showed a very strong triplet of triplets at 8 -218.2 ppm, similar to that observed in the thioester. The mass spectrum showed a large amount of a new metabolite of MW 444 (Fig. 12b). In the 500 MHz H NMR spectrum (Fig. 12a), the 6 -methylene signal of the 6 -fluoroanalogue (81) appeared as a doublet of triplets, whereas the 

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