RuO4-catalyzed oxidative polycyclization of the Cs-symmetric isoprenoid polyene digeranyl. An unexpected stereochemical outcome

The RuO₄-catalyzed oxidative polycyclization of digeranyl, a C_s-symmetric tetraene possessing a repetitive 1,5-diene structural motif, has been studied. The required substrate has been synthesized by Ti(III)-mediated tail-to-tail homocoupling of geranyl bromide. The process afforded two hitherto unknown isomeric tris-tetrahydrofuran products possessing unexpected all-threo cis-trans-cis and cis-trans-trans relative configuration. The new stereochemical outcome is explained based on previously formulated chelation or steric control models on the basis of structural differences between digeranyl and previously studied isoprenoid polyenes farnesyl acetate, geranylgeranyl acetate and squalene.