Activation of molecular oxygen and its use in stereoselective tetrahydrofuran-syntheses from δ,ε-unsaturated alcohols

Bishomoallylic alcohols (pent-4-en-1-ols) underwent efficient oxidative cyclizations, if treated with 0, and bis{2,2,2-trifluoromethyl-1-[(1R,4S)-1,7,7-trimethyl-2-(oxo-kappa O)bicyclo[2.2.1]hept-3-yliden]ethanolato-kappa O}cobalt(II) in solutions of 2-propanol at 60 degrees C. Ring closures occurred diastereoselectively and afforded 2,3-trans- (96% de), 2.4-cis- (similar to 60% de), and 2,5-trans-substituted (> 99% de) (phenyl)tetrahydrofur-2-ylmethanols as major components. Formation of bicyclic compounds and a 2,3,4,5-substituted oxolane was feasible as exemplified by syntheses of oxabicyclo[4.3.0]nonylmethanols and a derivative of natural product magnosalicin in 61-72% (90-99% de). The effectiveness of tetrahydrofuran synthesis was critically dependent on (i) solvent, (ii) reaction temperature, (iii) initial cobalt concentration, (iv) chain length between hydroxyl and vinyl groups, and (v) substitution at reacting entities. A sequence is proposed for rationalizing observed selectivities.