RuO4-catalyzed oxidative polycyclization of squalene. Determination of the configuration of the penta-tetrahydrofuranyl diol product

The configuration of the penta-tetrahydrofuranyl diol (penta-THF) product obtained by a single-step, RuO4-catalyzed oxidative polycyclization of squalene, has been determined as cis–threo–cis–threo–trans–threo–trans–threo–trans. The cis–cis–trans–trans–trans sequence for the five contiguous THF rings has been established through extensive 2D-NMR spectroscopic studies carried out both on the intact molecule and on some of its derivatives, including the oxidative cleavage products obtained by degradation of the penta-THF with PCC/AcOH. Four different chemical approaches were devised to determine each of the four threo relationships within each carbon pair connecting adjacent THF rings in the molecule. To this aim, studies have been carried out either on some intermediates of the process leading to penta-THF, obtained by stopping the oxidation of squalene prior to completion, or on a degradation product of the penta-THF, obtained from the latter through a bidirectional double oxidative degradation with PCC.