Keywords: Oxidation / Tetrahydrofuran / Catalysis / Asymmetric synthesis / Vanadium Vanadium(V) complexes 4 have been prepared from tridentate Schiff-base ligands 3 and VO(OEt) 3 . All vanadium(V) compounds were characterized (IR, UV/Vis, and 51 V NMR spectroscopy, and in selected examples by X-ray diffraction analysis) and were applied as oxidation catalysts for the stereoselective synthesis of functionalized tetrahydrofurans 2 starting from substituted bis(homoallylic) alcohols 1 (mono-or trisubstituted C−C double bonds). Oxidation of secondary or tertiary 1-alkyl-, 1-vinyl-, or 1-phenyl-substituted 5,5-dimethyl-4-penten-1-ols under optimized conditions [TBHP as primary oxidant and 1,2-(amino)indanol-derived vanadium(V) reagent 4g as catalyst] provided 2,5-cis-configured tetrahydrofurans in synthetically useful yields and diastereoselectivities (22−96% de). On the other hand, trans-disubstituted oxolanes (62%−96 de) were obtained from oxidations of 2-or 3-alkyl-and 2-or 3-phenyl-substituted 5,5-dimethyl-4-penten-1-ols bis(homoallylic) alcohols. Treatment of 4-penten-1-ols (i.e. substrates with monosubstituted olefinic π-bonds) with TBHP and catalytic amounts of vanadium(V) complex 4g furnished trans-disubstituted tetrahydrofurans as major products (20−96% de), no matter whether an alkyl or a phenyl substituent was located in position 1, 2, or 3 of the alkenol chain. The mechanism of this reaction has been investigated in detail. Based on re-
(Schiff base)vanadium(V) complexes 5 with tridentate imine auxiliaries served as catalysts for the oxidation of Br − with tert-butyl hydroperoxide (TBHP) in nonaqueous solvents. This reaction has been applied for the conversion of substituted 4-penten-1-ols into 5-exo-bromo-cyclized products, including a diastereomerically pure heterocyclic precursor used in a synthesis of the all-trans-configured 2,3,4,5-substituted tetrahydrofuran 2-epi-magnosalicin. Treatment of ω-substituted bis(homoallylic) alcohols with the reagent combination of pyHBr, TBHP, and a vanadium(V) catalyst 5 afforded 6-endo-cyclized products, i.e. brominated tetrahydropyrans, as major compounds. The results from 51 V NMR, ESI-MS, and supporting reactivity-selectivity studies indic-
Peroxidase (PO) activity of vanadate(V)-dependent bromoperoxidase (BPO) I (Ascophyllum nodosum) [VBrPO(AnI)] was retained with a half-life time of ~60 days, if stored in H2O2-incubated, morpholin-4-ethane sulfonic acid (MES)-buffered aqueous alcoholic solutions. These conditions were applied for converting bromide and, e.g., methyl pyrrole-2-carboxylate into bromopyrroles with an almost quantitative peroxide yield. δ,ε-unsaturated alcohols furnished β-bromohydrins and products of bromocyclization, i.e., tetrahydrofurans and tetrahydropyrans (70–84 % mass balance), if treated with H2O2, KBr, and VBrPO(AnI) in phosphate-buffered, CH3CN-diluted media.
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