INTERNAL OXIDOSQUALENES - DETERMINATION OF ABSOLUTE-CONFIGURATION ANDACTIVITY AS INHIBITORS OF PURIFIED PIG-LIVER SQUALENE EPOXIDASE

The preparation and characterization of oxidosqualenes 3-(6R,7R), 3-(6 S,7S), 4-(10R,11R), and 4-(10S,11S) is reported. Squalenediol 6 was co nverted into the corresponding mixture of(R)-Mosher esters 8 and 9, wh ich were separated by semipreparative HPLC. Esters 8 and 9 were reduce d to the chiral diols 6-(6R,7S) and 6-(6S,7R), respectively, which wer e finally converted into the corresponding epoxides 3-(6R,7R) and 3-(6 S,7S). A similar procedure was used for the preparation of chiral epox y derivatives 4-(10R,11R) and 4-(10S,11S) from esters 10 and 11, respe ctively. The determination of the absolute configuration of these epox ides was carried out by using the method reported by Ohtani et al. (J. Am. Chem. Sec. 1991, 113, 4092), which was adapted to the case of rac emic mixtures from synthetic origin. For this purpose, the (R)-Mosher esters derived from the enantiomers of squalenediols 6 or 7 were used. The validity of this approach was confirmed by the absolute configura tion found for the three squalenediols 6-(6R,7R), 6-(6S,7S), 7-(10R,11 R), and 7-(10S,11S) formed in the Sharpless asymmetric dihydroxylation of squalene (Crispino, G. A.; Sharpless, K. B. Tetrahedron Lett. 1992 , 33, 4273). Results on the inhibitory activity of oxidosqualenes 3-(6 R,7R), 3-(6S,7S), 4-(10R,11R), and 4-(10S,11S) using purified squalene epoxidase (SE) fi om pig liver showed that epoxide 3-(6S,7S) was the best inhibitor within the compounds assayed (IC50 = 6.7 mu M), althoug h oxidosqualene 4-(10R,11R) also exhibited a moderate inhibitory activ ity (IC50 = 25 mu M). The inhibition elicited by the epoxy derivative 3-(6S,7S) was competitive with respect to squalene (K-i = 2.7 mu M). T his activity is comparable to that reported for the most potent compet itive SE inhibitors described so far. Finally, incubation of oxidosqua lene 3-(6S,7S) with purified SE led to the formation of dioxidosqualen e 22-(3S,6S,7S), whereas its regioisomer 23-(3S,18S,19S) was not detec ted. In contrast, incubation of epoxide 3-(6R,7R) under the same condi tions afforded a mixture of dioxides 22-(3S,6R,7R) and 23-(3S,18R,19R) in a 5:12 molar ratio. The fact that oxidosqualenes 3 and 4 have been found in nature, and our previous results showing that racemic dioxid e 23 is a potent inhibitor of oxidosqualene-lanosterol cyclase in rat liver microsomes (Abad, J. L.; et al. J. Org. Chem. 1993, 58, 3991), c onfers a potential physiological relevance to the results reported her ein.