Internal Oxidosqualenes: Determination of Absolute Configuration and Activity as Inhibitors of Purified Pig Liver Squalene Epoxidase

Article abstract of DOI:10.1021/jo00117a014

The preparation and characterization of oxidosqualenes 3-(6R,7R), 3-(6S,7S), 4-(10R,11R), and 4-(10S,11S) is reported.Squalenediol 6 was converted into the corresponding mixture of (R)-Mosher ester 8 and 9, which were separated by semipreparative HPLC.Esters 8 and 9 were reduced to the chiral diols 6-(6R,7S) and 6-(6S,7R), respectively, which were finally converted into the corresponding epoxides 3-(6R,7R) and 3-(6S,7S).A similar procedure was used for the preparation of chiral epoxy derivatives 4-(10R,11R) and 4-(10S,11S) from esters 10 and 11, respectively.The determination of the absolute configuration of these epoxides was carried out by using the method reported by Ohtani et al. (J.Am.Chem.Soc. 1991, 113, 4092), which was adapted to the case of racemic mixtures from synthetic origin.For this purpose, the (R)-Mosher esters derived from the enantiomers of squalendiols 6 or 7 were used.The validity of this approach was confirmed by the absolute configuration found for the threo squalenediols 6-(6R,7R), 6-(6S,7S), 7-(10R,11R), 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-(6R,7R), 3-(6S,7S), 4-(10R,11R), and 4-(10S,11S) using purified squalene epoxidase (SE) from pig liver showed that epoxide 3-(6S,7S) was the best inhibitor within the compounds assayed (IC 50 = 6.7 μM), although oxidosqualene 4-(10R,11R) also exhibited a moderate inhibitory activity (IC 50 = 25μM).The inhibition elicited by the epoxy derivative 3-(6S,7S) was competitive with respect to squalene (K_i = 2.7 μM).This activity is comparable to that reported for the most potent competitive SE inhibitors described so far.Finally, incubation of oxidosqualene 3-(6S,7S) with purified SE led to the formation of dioxidosqualene 22-(3S,6S,7S), whereas its regioisomer 23-(3S,18S,19S) was not detected.In contrast, incubation of epoxide 3-(6R,7R) under the same conditions affored 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 dioxide 23 is a potent inhibitor of oxidosqualene-lanosterol cyclase in rat liver microsomes (Abad, J.L.; et al.J.Org.Chem. 1993, 58, 3991), confers a potential physiological relevance to the results reported herein.