Meso-tetrahydrofurans

Miyafuji and Katsuki95 reported the desymmetrization of meso-tetrahydrofuran derivatives via highly enantioselective C-H oxidation using Mn-salen catalysts. The optically active product lactols (up to 90% ee) are useful chiral building blocks for organic synthesis (Scheme 8-48). 

Katsuki and coworkers have developed a family of salen-metal complexes capable of effecting a C—H oxidation at activated positions. meso-Tetrahydrofurans may be oxidized to the lactol in good yield and excellent enantioselectivity using iodosylbenzene as the stoichiometric oxidant and a Mn-salen complex as catalyst [Eq. (10.45)]. " Meso acylpyrrolidines behave similarly, providing slightly lower enantioselectivities using a similar catalyst [Eq. (10.46)]d ... 

Although it is well known that cyclic ethers are readily oxidized to the corresponding lactols or lactones, their asymmetric desymmetrization was not examined until quite recently. However, desymmetrization of prochiral or meso-cydic ethers is expected to be a useful tool for organic synthesis, since many prochiral or raeso-cyclic ethers are available in bulk. Recently, Miyafuji and Katsuki have reported the desymmetrization of 4-terf-butylcyclotetrahydropyran and meso-tetrahydrofurans with the chiral (salen)manganese(III) complex 13 as catalyst (Scheme 9) [24,25]. The oxidation of the former shows only the modest enantioselectivity, while the reaction of the latter exhibits excellent enantioselectivity. The low enantioselectivity (48% ee) observed in the oxidation of 4-ferf-butyltet-rahydropyran has been attributed to the participation of enantiomeric twist-boat conformers. Although 4-ferf-butyltetrahydropyran exists in an equilibrium mixture of chair and enantiomeric twist-boat conformers and the equilibrium ratio of the latter is very small, the latter is considered to be more reactive than the former for stereo electronic reasons. One of a-C-H bonds in the twist-boat conformer almost eclipses the -orbital while those in the chair conformer are gauche or anti to the -orbital. 

In 1997, the same group disclosed an example of highly enantioselective C—H bond oxidation reactions. As shown in Scheme 1.61, conformationally fixed 3-oxa-bicycle[3.3.0]-octane 156 was oxidized at the a position under the catalysis of the Mn catalyst C25 ligated by a BINOL-derived salen ligand. Subsequently, the same group further expanded the scope to a meso-pyrrolidine derivative (up to 76% ee) and a meso-tetrahydrofuran derivative (up to 90% ee). Later,... 

The A-trimethylsilylimines 68 (R = t-Bu, Ph, 2-MeCgH4 or 2-BrC6H4), which are prepared by the reaction of non-enolizable aldehydes with lithium bis(trimethylsilyl)amide, followed by trimethylsilyl chloride, undergo pinacolic coupling induced by NbCLt 2THF to yield the vicinal diamines 69 as mixtures of dl- and meso-isomers, in which the former predominate. Another method for the preparation of 1,2-diamines is by the combined action of the niobium tetrachloride/tetrahydrofuran complex and tributyltin hydride on cyanides RCN (R = /-Hu. Ph, cyclopentyl or pcnt-4-en-l-yl) (equation 32)82. 

Several compounds have been proposed for the measurement of the void volume, including sodium nitrate solution, water, deuterium oxide, fructose, acetonitrile, tetrahydrofuran (THF), meso-erythritol, gluconolactone, and 2,4-dinitronaphthol. The elution volume of a number of these compounds has been measured in 10-90% aqueous acetonitrile and acidic-aqueous acetonitrile. The results are given in Figure 3.9 where the volumes in A and B were measured in aqueous acetonitrile and in C and D were measured in aqueous acetonitrile containing 50 mM phosphoric acid. Methanol (a) and deuterium oxide (g) showed two peaks when monitored by a refractive index detector (Figure 3.9C). 

Bis(tetrahydrofurane)(meso-tetraphenylporphinato)iron(II) (bTHF FeTPP) is the only known six-coordinate high-spin Fe(II) complex. Its THF ligands are rather loosely bound. Crystals slowly lose THF when exposed to the atmosphere, while the iron is five-coordinate in a solution in benzene (Reed et al. 1980). The magnetic susceptibilty of bTHF FeTPP is temperature dependent (Lecomte et al. 1986) and the axial Fe—O bonds shrink on cooling from 1.35 A at ambient temperature to 1.29 A at nitrogen temperature. 

Difluorododecanes 29 with potassium rerr-butoxide in tetrahydrofuran eliminate hydrogen fluoride stereoselectivcly the meso and syn compounds give, respectively, (E)- and (Z)-6-fluorododec-6-ene (30).26... 

NMO NMP Nu PPA PCC PDC phen Phth PPE PPTS Red-Al SEM Sia2BH TAS TBAF TBDMS TBDMS-C1 TBHP TCE TCNE TES Tf TFA TFAA THF THP TIPBS-C1 TIPS-C1 TMEDA TMS TMS-C1 TMS-CN Tol TosMIC TPP Tr Ts TTFA TTN N-methylmorpholine N-oxide jV-methyl-2-pyrrolidone nucleophile polyphosphoric acid pyridinium chlorochromate pyridinium dichromate 1,10-phenanthroline phthaloyl polyphosphate ester pyridinium p-toluenesulfonate sodium bis(methoxyethoxy)aluminum dihydride (3-trimethylsilylethoxy methyl disiamylborane tris(diethylamino)sulfonium tetra-n-butylammonium fluoride f-butyldimethylsilyl f-butyldimethylsilyl chloride f-butyl hydroperoxide 2,2,2-trichloroethanol tetracyanoethylene triethylsilyl triflyl (trifluoromethanesulfonyl) trifluoroacetic acid trifluoroacetic anhydride tetrahydrofuran tetrahydropyranyl 2,4,6-triisopropylbenzenesulfonyl chloride 1,3-dichloro-1,1,3,3-tetraisopropyldisiloxane tetramethylethylenediamine [ 1,2-bis(dimethylamino)ethane] trimethylsilyl trimethylsilyl chloride trimethylsilyl cyanide tolyl tosylmethyl isocyanide meso-tetraphenylporphyrin trityl (triphenylmethyl) tosyl (p-toluenesulfonyl) thallium trifluoroacetate thallium(III) nitrate... 

A remarkable synthesis of meso- and racemic chimonanthine has recently been published (30). The sodium salt of the urethane (XVI) was dimerized by oxidation with iodine in tetrahydrofuran. Two diastereo-isomeric dimers (XVII R = CH2CH2.CC Et) were obtained XVIIa, mp 243°-245°, and XVIIb, mp 214°-216°. Reduction of the former with LiAlH4 in tetrahydrofuran yielded an isomer of calycanthine (mp 240°-243°) which was regarded as retaining one of the indole nuclei. Similar reduction of the second isomer (XVIIb) yielded optically inactive chimonanthine (mp 202°-203°) whose IR- and mass-spectra were identical with those of the natural base. 

A Pd(ll) catalyst system with an oxazoline ligand 44 has been described that allows the desymmetrization of meso-Z-alkyl-2-propargylcyclohexane-l,3-diols in an asymmetric cyclization-carbonylation reaction. The products which contain a chiral quaternary carbon were obtained in excellent yields with high ee s (Scheme 56) <2006T9988>. 7-Hydroxy terminal <2005JOC3099> and internal <2006TL2793> alkenes can be converted to tetrahydrofurans by Pd(0)-catalyzed carboetherification reactions combined with a coupling of aryl or vinyl halides. 

Models which allow interpretation of X-ray diffraction data in terms of orbital populations, radial dependence of the orbitals and LCAO coefficients are discussed. They are applied to experimental data on iron(II) phthalocyanine, iron(II) meso-tetraphenylporphyrin and its bis-pyridyl and bis-tetrahydrofurane derivatives. The diffraction studies indicate that the first two complexes are intermediate spin complexes with differing ground states while the last two are respectively low- and high-spin iron(II) compounds. A difference between the two intermediate spin complexes is thought to be related to the effect of the crystalline environment. This interpretation implies that the leading contributor to the ground... 

Until very recently this type of conversion had not been observed for phosphine functionalised ferrocenes, although under certain conditions (solvent, acid, photochemical intiation, for example) similar conversions have been reported for related systems. [182] The bis-planar chiral ferrocenyldiphosphine bis(l-(diphenylphosphino)-T)-indenyl)iron(II) was found to undergo an isomerisation from the meso isomer to the rac isomer in tetrahydrofuran solvent at room temperature. [183] Among attempts to understand the mechanism of this... 

Recently QU1NAP was used as the chiral ligand to alkylate the allyl acetate 18. The enantioselectivity is enhanced by the addition of the crown ether 15-crown-5, but the enantiomeric excess is still moderate (up to 47 % ee)S9. The diphenyl phosphinoaryloxazoline ligand 20 exhibits best enantioselectivities for these substrates and demonstrates its broad versatility for the enantiose-lective alkylation of allyl acetates. Almost complete chirality transfer is achieved77 (for a detailed discussion see the section entitled meso-Tt-Allylpalladium Complexes). Dimethyl-formamide and tetrahydrofuran serve equally well as solvents, but with the latter solubility problems may arise. Higher temperature enhances reactivity at the expense ot selectivity (at 80 °C 65% ee). 

---