Synthesis and desaturation of monofluorinated fatty acids 1

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Abstract
A series of monofluoro C16 and C18 fatty acids have been synthesized and used as mechanistic probes for fatty acid desaturation. Only fluoroolefinic products are obtained when these compounds are processed by an in vivo Saccharomyces cerevisiae ω9 desaturating system as determined by 1H-decoupled 19F NMR and GC–MS analysis. No evidence for fluorohydrin formation has been found when either methyl (R,S)-9- or 10-fluoropalmitate (stearate) 3a,b and 5a,b was incubated with the ω9 desaturase. On desaturation α- and β-fluorine substituent effects (kH/kF) of magnitude 6.2 and 2.4, respectively, have been measured by direct competition experiments between 3a and 3b and between methyl 16-fluoropalmitate 3c and 3b. These results do not support the involvement of discrete hydroxylated and carbocationic intermediates in fatty acid desaturation. Substantial apparent steric effects have been observed for monofluorostearoyl substrates 5c–f bearing a fluorine distal from the site of initial oxidation. In the case of (R,S)-methyl 12-fluorostearate 5f, we show that both enantiomers are desaturated at comparable rates.
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