Cyclitols solutions

A tandem enzymatic aldol-intramolecular Homer-Wadsworth-Emmons reaction has been used in the synthesis of a cyclitol.310 The key steps are illustrated in Scheme 8.33. The phosphonate aldehyde was condensed with dihydroxyacetone phosphate (DHAP) in water with FDP aldolase to give the aldol adduct, which cyclizes with an intramolecular Horner-Wadsworth-Emmons reaction to give the cyclo-pentene product. The one-pot reaction takes place in aqueous solution at slightly acidic (pH 6.1-6.8) conditions. The aqueous Wittig-type reaction has also been investigated in DNA-templated synthesis.311... 

Remarkable selectivity has been observed in the oxidation of molecules containing several secondary hydroxyl groups, such as al-dopentopyranosides, 6-deoxyaldohexopyranosides, cyclitols, and various anhydro derivatives. As indicated previously, attack usually occurs at relatively hindered hydroxyl groups. When aqueous solutions of benzyl /3-D-arabinopyranoside, benzyl jS-D-ribopyranoside,... 

Of all the polyhydroxy compounds studied in aqueous solution,63 os-inositol exhibits the greatest mobility. The ept- and aUo-inositol and o s-quercitol also Bhow considerable movement, but to a lower degree other cyclitols are Iobs mobile. Reducing sugars and alditols generally show very little or no movement in the presence of Mg29 and alkali metal ions all move in the presence of Ca , Sr , and Ba1 , but the rates are only moderate. Table III gives the relative mobilities of several polyhydroxy compounds in aqueous solutions of various metal ions. 

By extrapolation of the changes in the chemical shifts in the NMR spectrum of epi-inositol, the stability constant of the calcium complex has been calculated to be ca. 3M"1—i.e., about 70% of the cyclitol is present as a cationic complex in 1M calcium chloride solution. However, the extrapolation cannot be accurately carried out because there are secondary changes in chemical shifts caused by weaker complexing at two oxygen atoms. 

From the change of pH caused by successive additions of cyclitols to a borate solution, the equilibrium constants of complex formation have been calculated66 they show that the complexes are formed from cyclitol and borate in a 1 1 ratio. The stability of the tridentate borates depends on the steric disposition of the free hydroxyl groups in the complex the more of these in axial positions, the less stable the complex. This is illustrated by the values of the equilibrium constant in the following two series which are arranged by decreasing number of free axial hydroxyl groups in the complex scyWo-quercitol, 5.0 epf-quercitol, 310 m-quercitol, 7900 myoinositol, 25 epi-inositol, 700 and m-inositol, 1.1 X 106. When the constitution of a cyclitol allows the formation of both the tridentate and the classical, cis-1,2 type of complex, the former predominates. 

The usual sugar-reagents do not serve to detect cyclitol spots (except those of inososes). Ammoniacal silver nitrate has been widely used, but the best results90 are obtained with separate solutions of silver nitrate in acetone and sodium hydroxide in ethanol.91 The Scherer test has been adapted for use on paper chromatograms,91 and other reagents are useful in certain instances.89-90... 

The reduction of inososes is a method much used for the preparation of new cyclitols. Catalytic hydrogenation in neutral, aqueous solution yields predominantly the isomer with the newly formed hydroxyl group in the axial position,210 whereas the product from a reduction by sodium amalgam usually contains considerable quantities of both the possible epimers.211 Recently, sodium borohydride has been used for reducing inososes62,68 211 in some cases, but not in others, it provides more of the equatorial isomer than does sodium amalgam. 

Only one diketone (2,5-diketo-ma/o-inositol) of the cyclitol series has been isolated in the crystalline state. However, there is ample evidence that a number of other cyclitol diketones have been obtained. Thus, solutions of dearfro-inositol and Zewo-inositol which have been oxidized by Acetobacter... 

Induced Shifts (p.p.m.) of Proton Signals of Some Cyclitols on Addition of LaCIj Solution... 

The original purpose was to cyclize this compound to a cyclitol derivative by treatment with potassium in xylene. However, a crystalline diene derivative (115) was formed which, after hydrolysis of the methylene groups, gave crystalline l,6-dideoxy-D-(/ireo-2,5-hexodiulose (116). It showed no indications of the presence of a double bond and reduced Fehling solution in the cold. The location of the two keto groups at C-2 and C-5 in this compound and in the diulose (118), obtained later, was proved by ozonization of their parent diene compound (115) to yield D-threaric acid. The compound (116) showed no mutarotation and was converted into a crystalline bis[(p-nitrophenyl)hydrazone] and di-(p-nitrobenzoate). 

The axial or equatorial positions of protons on a cyclitol ring (or of acetoxy methyl protons) may also be deduced from chemical-shift data. Geometric (or spectroscopic) proton equivalence produces characteristic spectral simplification, often serving to distinguish more symmetrical from less symmetrical configurations. Variable-temperature spectra are useful for studying equivalence due to time-averaging they reveal many other kinds of equilibria if these exist in the solution of the sample. " ... 

The following examples, selected from cyclitols containing four or five hydroxyl groups, Ulustrate many of the approaches already mentioned. In the nine Figures provided for these examples, chemical shift (delta) values in p.p.m. for solutions in chloroform-d are based on an internal reference of tetramethylsilane, and delta values for the solutions in deuterium oxide are approximate figures, based on an external reference of tetramethylsilane or an internal reference of deuterium hydrogen oxide. 

Measurements in solution include those of infrared and ultraviolet spectra, infrared hydrogen-bonding, " and dipole moments. Sometimes, suitable solvents for such measurements on cyclitols cannot be found. 

Hedgley and Fletcher found that isomerizations take place when solutions of cyclitol acetates or 1,5-anhydroalditol acetates in liquid hydrogen fluoride are left to stand. These isomerizations evidently occur via acetoxonium ions, because, as indicated in Section 1,2 (p. 132), such ions are quite readily formed from cis-diol diacetates in liquid hydrogen fluoride. 

Density measurements of mannitol in potassium or sodium acetate solutions have revealed the nature of solute-solvent interactions in these mixtures and the kinetics of oxidation of several hexitols (and cyclitols) by molecular oxygen, catalysed by copper compounds, has been studied. ... 

There appears to be only one example of halohydrins obtained by direct action of halogen acids on the cyclitols d-quercitol was heated at 100° with a solution of HCl (saturated at 10°), and a very small amount of substance, m.p. 198-200°, was obtained which had an analysis corresponding to a monochlorohydrin 100) and also one, m.p. 155°, that appeared to be a trichlorohydrin, C6H7Cl3(OH)2. 

The cyclitols can form complexes with metals similar to those of the glycitols. The formation of an insoluble reaction product with basic lead acetate is a means of removing m /o-inositol almost quantitatively from solution 123). 

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