A silicon-tethered tandem radical cyclisation–trapping strategy to the fully substituted cyclopentene ring in viridenomycin

Treatment of α-bromosilyl ether 27 derived from the substituted cyclopentenol 26 and (bromomethyl)chlorodimethylsilane, with 1,1′-azobis(cyclohexanecarbonitrile) in n-heptane at 100 °C, in the presence of allyltri-n-butylstannane leads to the silicon heterocycle 28 by way of a stereocontrolled tandem radical cyclisation (to 11)–trapping process. Oxidative cleavage of the carbon–silicon bond in 28, using H2O2–KF in THF–MeOH, next led to the corresponding cyclopentane diol 29a, which was then elaborated to the β-hydroxy ester 30b using five steps. Oxidation of 30b using Moffatt conditions, followed by methylation of the resulting enol ester 32a gave the fully functionalised cyclopentene ring in the antitumoural antibiotic substance viridenomycin 1 isolated from Streptomyces viridochromogenes and Streptomyces gannmycicus.