Oxidative cyclizations, tetrahydrofurans

In Morimoto s total synthesis of (-t-)-eurylene and (+)-14-deacetyleurylene, the pivotal steps are the construction of trans- and cw-tetrahydrofuran rings via a hydroxy-directed syn oxidative cyclization of acyclic bishomoallylic alcohols promoted by Re(VII) and Cr(VI) oxides, respectively. As depicted below, the trans-THF is achieved by treatment of the triol with the oxorhenium(VII) complex, and the cis-THF is constructed by the use of PCC <00AG(E)4082>. 

Intramolecular alkoxycarbonylation of alkynols is parallel to what has been described for alkenols except that functionalization of the triplebond produces a double bond. No lactone formation is observed in the Pd(II)-catalyzed oxidative cyclization-carbonylation of alkynes. Instead [(methoxycarbonyl)methylene]tetrahydrofurans are selectively formed [134, 135]. Moreover, starting from an enynol, furan-2-acetic ester is obtained resulting from a final aromatization step [136]. 

Permanganese is a common oxidative reagent, the application of which to the asymmetric oxidative cyclization of 1,5-dienes has been reported by Brown (Scheme 3.14). The addition of acetic acid is quite important for the reaction to proceed, and highly functionalized tetrahydrofurans are obtained in a range of 58 to 75% ee, in diastereoselective manner [35]. Another oxidative transformation using KMn04 with a chiral ammonium salt has been investigated. Scheme 3.15 illustrates the asymmetric dihydroxylation of electron-deficient olefins to chiral diols in the... 

An oxidative spiroannulation reaction was carried out for simple phenols and as a result good yields of spiro-compounds containing tetrahydrofuran rings were obtained <02TL3597>. In the stereospecific and enantiospecific total synthesis of the sarpagine indole alkaloid dehydro-16-epinormacusine B, an oxidative cyclization of the alcohol shown below was the key and final step <02OL4681>. 

Cyclization at the hydroxylamine also occurs in the hydrogenation of l-nitro-2-thio-cyanatobenzene (30) to give 2-aminobenzothiazole 3-oxide (31) (Scheme 9.14).157 The hydrogenation over Raney Ni in ethanol at room temperature and 0.3 MPa H2 was complete after several hours with absorption of 2 mol of hydrogen to give crude 31 in 75-88% yield. The infrared spectrum of the product 31 indicated the presence of an equilibrium between 31a and 31b. Under similar conditions but with use of platinum oxide in tetrahydrofuran, 31 and its 5-methyl, 5-methoxy, and 5-chloro derivatives... 

Another way in which tetrahydrofuran frameworks can be constructed is by employing oxidative cyclization of 1,5-dienes <070BC1605>, and an example is shown in the following scheme <07S2751>. 

Oxidative cyclizations of dienes and polyenes mediated by transition-metal-oxo species to yield tetrahydrofurans and tetrahydropyrans have been reviewed by Piccially <2007S2585>. A multigram synthesis of diastereomerically pure tetrahydrofuran diols that applies the oxidation of 1,5-dienes has been published <2007S2751>. 

Self-terminating radical oxygenations are not restricted to cyclic alkynes. Electro-and photochemically generated NO3 can be used for the oxidative cyclization of cycloalkyl-clamped alkynes 67 (Scheme 2.12). This reaction leads to formation of anelated tetrahydrofurans (68 with X = or pyrrolidines (68 with X = NTs,... 

Tetrahydrofuranes. Cekovic, BoSnjak, and Mihailovic have reviewed their work on the oxidative cyclization of aliphatic alcohols to tetrahydrofuranes by oxidation with lead tetraacetate. The reagent should be free of acetic acid. Anhydrous calcium carbonate can be added to neutralize the acetic acid in the oxidant and that formed during the reaction. The reaction involves selective oxygenation of a 8-carbon the actual mechanism is uncertain, but is probably a radical reaction. Direct oxidation of the alcohol is generally of minor importance, but P-fragmentation can be a problem, particularly when a stable benzyl or allyl radical is formed. 

A cobalt(II)-catalyzed oxidative cyclization converted a secondary alcohol to the trans-2,5-disubstituted tetrahydrofuran <03JA14702>. Oxidants such as vanadium(V) complex <03EJO2388>, ruthenium tetroxide <03TL5499> and osmium tetroxide <03AG(E)948> were all employed to convert either homoallyl alcohol or polyenes to molecules that contain... 

The stereoselective preparation of rfl .v-2-hydroxymethyl-5-subslituted tetrahydrofurans from 4-pentenols is possible by oxidative cyclization catalyzed by cobalt(ll) salts. Both cobalt(II) trifluoroacetate and cobalt acetylacetonate have been employed for the cyclization, but better yields are obtained with Co(modp)2 (l)45. 

Oxidative cyclization of selected 5,6-dihydroxyalkenes to ci.v-2,5-disubstituted tetrahydrofurans has been realized with chromium(VI) reagents. In fact, when Collins reagent or pyridinium chlorochromate are employed at 20 °C for 3 min, the cyclization affords the cis-2,5-products in moderate yield. The resulting (tetrahydrofuranyl)diols are diastereomerically pure by H- and i5C-NMR spectroscopy. Moreover, a comparison of the reaction mixture with authentic 2,5-trans samples indicates a stereoselectivity of 99.5%47. 

For effecting oxidative cyclizations of the types noted, superior results have been obtained with use of the combination lead tetraacetate-iodine, a method introduced by the Ciba-Basel group Ch. Meystre, Heusler, Kalvoda, P. Wieland, Anner, and Wettstein. Known as the hypoiodite reaction because the combination generates HOI, the method appears to be simpler and more efficient than lead tetraacetate alone, and than any of three other methods not employing lead tetraacetate. An improved procedure is as follows. Pregnenolone acetate (5) is first reduced with lithium tri-f-butoxyaluminum hydride in tetrahydrofurane to 3 8-acetoxy-20 -hydroxy-A -pregnene (6) the 20a-isomer is formed in minor amount. At the end... 

Chiral enolates in which the auxiliary is in the ester portion provide still another route to optically active lactams. Early results indicated that little asymmetric induction was obtained with menthyl enolates. Use of the enolate obtained from 24 did lead to high levels of asymmetric induction. Treatment of 24 with lithium diisopropylamide in tetrahydrofuran, followed by addition of imine 25, gives cf -/(-lactam 26 in 79% yield and 91%ee98. Optically active /3-lactams can be prepared by addition of chiral iron enolates (see Section D.l. 1.1.3.2.) to imines99-101. Addition of aluminum enolate 27 to imine 28, followed by oxidative cyclization with iodine and an amine, affords /(-lactam 29 in 54% yield and >95% ee. 

Permanganate- and perruthenate-promoted oxidative cyclization of hexa-1,5-dienes are usually key steps in the total synthesis of naturally occurring molecules containing tetrahydrofuran rings <04JOC3368>, <04SL1437>, <04TL303>. 

Oxidative cyclization of S,e-unsaturated tertiary alcohols. Tetrahydrofuran units are formed in moderate yields by this oxidation. 

In 1965, Klein and Rojahn reported that 1,5-dienes are converted into tetrahydrofurans when treated with potassium permanganate <65X2353 >. This oxidative cyclization leads specifically to cis-... 

Syntheses of 0-methylthalisopavine (59b) have been recorded. In one of these the construction of the carbon-nitrogen skeleton was effected by oxidative cyclization of the lactam (58) to the lactam (59a) by vanadium oxyfluoride in acetonitrile, in 40% yield. Reduction of (59a) by diborane in tetrahydrofuran gave the tertiary amine (59b). An alternative synthesis involved the cyclization, with sulphuric acid, of the acetal (60), prepared from deoxyveratroin. " ... 

The present oxygenation reaction catalyzed by cobalt(II) complex with the combined use of molecular oxygen (Oxidant) and 2-propanol (Reductant) was also applied to several oxidation reactions as secondary alcohols into ketones, the direct preparation of ketones from vinylsilanes, and stereoselective oxidative cyclization of 5-hydroxyalkenes into tetrahydrofuran derivatives.il... 

In addition to the syntheses of carbocycles in Scheme 3-38, oxygen-, silicon-, and aluminum-containing heterocycles have also been prepared by [2+2+1] cycloadditions. An interesting tetrahydrofuran synthesis involves stoichiometric metallacycle formation by oxidative cyclization of Ni(0) with two equivalents of norbomadiene, followed by oxygen atom insertion with N2O (Scheme 3-39). 

---