Stereoselective syntheses of functionalized cyclic ethers via (Schiff-base)vanadium(V)catalyzed oxidations

(Schiff-base)vanadium(V) complexes catalyze the oxidation of Br- (formation of Br-2) and the stereoselective synthesis of functionalized tetrahydrofurans from substituted bishomoallylic alcohols. In both instances, tert-butyl hydroperoxide (TBHP) serves as primary oxidant. The oxidation of Br- was applied as the key step for stereo- and 6-endo-selectively constructing the 2,2,3,5,6,6-substituted tetrahydropyran nucleus of the marine natural product aplysiapyranoid A starting from an adequately substituted bishomoallylic alcohol. In the absence of Br-, 1-alkyl-, 1-vinyl-, and 1-phenyl-5,5 -dimethyl-substituted bishomoallylic alcohols are selectively oxygenated to furnish 2,5-cis-configured tetrahydrofurans as major products. 2- Or 3-substituted omega,omega-dimethyl-substituted bishomoallylic alcohols afford trans-disubstituted tetrahydrofurans under these conditions. Oxidation of substituted 4-penten-1-ols, i.e., substrates with a terminal pi-bond, proceeds with a preference for formation of trans-disubstituted tetrahydrofurans. According to data from (i) V-51 NMR spectroscopy, (ii) mass spectrometry, (iii) a structure-selectivity survey, (iv) competition kinetics, and (v) a stereochemical analysis, the oxygen atom transfer onto a bishomoallylic alcohol occurs in a peroxide- and alkenol-loaded (Schiff-base)vanadium(V) complex.