Stereoselective synthesis of the C(25)-C(36) segment of arenicolides A and B: determination of the configuration of the trisubstituted epoxide.

Two diastereomeric epoxides 4a and 4b corresponding to the C(25)-C(36) fragment of arenicolides A and B were synthesized in a stereoselective manner involving the Pd(0)-catalyzed stereospecific methoxy substitution reaction of epoxy unsaturated ester 6 with B(OMe)(3) as the key step. Comparison of the (1)H NMR spectra of the synthetic compounds with that of arenicolide A revealed that the configuration of the epoxide in arenicolides A and B is 30R and 31R.