We have developed two syntheses of vicenistatin and its analogues. Our first-generation strategy included the rapid and sequential assembly of the macrocyclic lactam by using an intermolecular Horner-Wadsworth-Emmons reaction between the C3-C13 fragment and the C1-C2, C14-C19 fragment, followed by an intramolecular Stille coupling reaction. The second-generation strategy utilized a ring-closing metathesis of a hexaene intermediate to generate the desired 20-membered macrolactam. This second-generation strategy made it possible to prepare synthetic analogues of vicenistatin, including the C20- and/or C23-demethyl analogues. Evaluation of the cytotoxic effect of these analogues indicated the importance of the fixed conformation of aglycon for determining the biological activity of the vicenistatins.
Factors determining the course of reaction (i.e., usual coupling vs. abnormal cine-substitution) in the Stille coupling of trisubstituted iodoalkene and trans-vinylstannane were investigated. The palladium-catalyzed reaction with triphenylarsine and lithium chloride in N-methylpyrrolidone gave the normal Stille coupling product in good yield and good selectivity, while the use of N,N-diisopropylethylamine and cuprous iodide in toluene afforded the cine-substituted product exclusively. It is proposed that transmetalation between vinylpalladium and vinylstannane species determines the course of reaction.
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