Abstract
The C23–C27 1,3-diol was constructed via either Brown's crotylation - osmylation or regionand stereoselective opening of an 2,3-anhydro-β-ribofuranoside derived from D-xylofuranose. Lewis acid catalyzed epoxide opening with a protected lithiofurfural alcohol followed by oxidative spiroketalization established the C21–C24 spiroketal as a masked α′-allyl aldol. The C19–C20 tri-substituted olefin was synthesized via zirconium catalyzed carboalumination - in situ transmetalation with a higher order cuprate, followed by stereoselective conjugate addition to a spiroenone. The C27–C28 tri-substituted olefin was formed either via coupling of a methyl cuprate with an enoltriflate, mediated by sonication, or via direct coupling between the C17-aldehyde and the C28-vinyl magnesium bromide.
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