Dedicated to Duilio Arigoni on the occasion of his 75th birthday A series of 18-fluoro thiastearates were prepared and incubated with a yeast D9-desaturating system. The relative efficiency of desaturase-mediated sulfoxidation was monitored via 19 F-NMR analysis of the sulfoxide products, and a strong preference for oxo transfer to the S-atom occupying the 9-position was confirmed. The oxidation profile obtained in this manner matched that of analogous experiments with non-fluorinated substrates. These results form the basis of a versatile 19 F-NMR-based method for mapping the position of the putative diiron oxidant relative to substrate, and has potential application to the study of membrane-bound desaturases in vitro.1. Introduction. ± Fatty acid desaturases are an important superfamily of enzymes found in a wide variety of aerobic organisms [1]. The first desaturation reaction to be studied in detail was the conversion of stearoyl CoA to oleyl CoA as it occurs in baker×s yeast (Scheme 1, a) [2]. Interest in this particular enzyme system has risen recently with the discovery that enhanced stearoyl CoA D9 desaturase (SCD) activity in mammals occurs in a number of disorders, including obesity, diabetes, and other metabolic diseases [3]. Thus, the development of mechanism-based SCD inhibitors would have potential therapeutic significance. A major stumbling block to progress in this area is that stearoyl CoA D9 desaturase is membrane-bound and difficult to isolate in purified form. However, some important mechanistic information has been obtained on the structurally related yeast enzyme [4], which can be conveniently studied in vivo by an appropriate experimental design. A KIE study showed that the H-atom at C(9) of substrate is removed first in an isotopically sensitive step, followed by rapid cleavage of the CÀH bond at C(10) [5]. This result correlates well with the observation that desaturase-mediated oxo transfer occurs most efficiently when a S-atom occupies the 9-position of a thiastearoyl substrate (Scheme 1, b) [6]; the enantioselectivity of sulfoxidation matches the stereochemistry (syn, pro-R) of the corresponding parent reaction [7] [8]. Taken together, these results strongly suggest that the putative diiron oxidant involved in yeast D9 desaturase mediated oxidation is asymmetrically located between C(9) and C(10) of substrate.The success of the thia-analogue approach in providing critically important information on desaturase-active-site topology prompted us to consider modifications to the probe that would obviate the need to isolate milligram amounts of product for

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