Sequential application of stereoselective syn-oxidation methodologies to natural product synthesis: A potentially biomimetic approach to the C12–C21 bistetrahydrofuran region of monensin

(Schiff‐base)vanadium(V) Complex‐Catalyzed Oxidations of Substituted Bis(homoallylic) Alcohols − Stereoselective Synthesis of Functionalized Tetrahydrofurans

“…Oxidation of 3-phenyl-and 3-isopropylsubstituted bis(homoallylic) alcohols 14 and 15 with TBHP in the presence of catalyst 4g afforded diastereomerically pure ( 1 H NMR spectroscopy) 2,3-trans-configured tetrahydrofurans trans-19 (76%) and trans-20 (54%). 16% of trans-configured tetrahydropyran trans-25 was obtained from isopropyl-substituted alkenol 15, but no tetrahydropyran 24 from the oxidation of 5-methyl-3-phenylhex-4-enol (14).…”

“…This reagent was superior to alternative methods which required, for example, [VO(acac) 2 ], [2,26] molybdenum() complexes, [66] and more recently [Cp 2 TiCl 2 ], [67] as catalysts. No direct comparison of the peroxyvanadium() complex based tetrahydrofuran formation with highly diastereoselective oxidations using stoichiometric amounts of acylrhenates() [14] or oxidations which require the reagent combination of cobalt() diketonate complexes, molecular oxygen and TBHP [68] can be made at this point because the sets of substrates in these studies did not match. Finally, it is worth mentioning that mCPBA is a reliable but poorly selective oxidant for converting bis(homoallylic) alcohols into functionalized tetrahydrofurans (Supporting Information).…”

“…The first route follows a transition metalϪperoxo (H 2 O 2 as primary oxidant) or Ϫperoxy pathway (alkyl hydroperoxides as active oxygen source, Figure 1, left). [13,14] The second mechanism proceeds via high-valent transition metalϪoxo compounds (Figure 1, right). [14] Both mechanisms open perspectives for diastereoselective tetrahydrofuran syntheses, since their facial selectivities for π-bond oxygenation may, depending on the substrate, be complementary.…”

“…[13,14] The second mechanism proceeds via high-valent transition metalϪoxo compounds (Figure 1, right). [14] Both mechanisms open perspectives for diastereoselective tetrahydrofuran syntheses, since their facial selectivities for π-bond oxygenation may, depending on the substrate, be complementary. [8,15] From a historical point of view, the reagent combination of [VO(acac) 2 ] [acac ϭ acetylacetonato(Ϫ1)] and TBHP was the first and in the following years most frequently applied oxidant for converting bis(homoallylic) alcohols, by a transition metal catalyzed oxidation, into functionalized tetrahydrofurans.…”

Pyridinium chlorochromate (PCC) oxidation of bishomoallylic tertiary alcohols. A structure–reactivity study

“…This process has recently been used in the synthesis of polyether natural products [7,8]. Three different mechanisms have been proposed as illustrated in Figure 2: (2 + 2) or (3 + 2) cycloaddition processes and Cr(VI)-mediated epoxidation [3,6,9].…”