Hydroquinone, chiral

The effects of pH on electrokinetic velocities in micellar electrokinetic chromatography was studied by using sodium dodecyl sulfate solutions [179]. Micellar electrokinetic capillary chromatography with a sodium dodecyl sulfate pseudostationary phase has been used to determine the partition constants for nitrophenols, thiazolylazo dyes, and metal chelate compounds [180]. A similar technique was used to separate hydroquinone and some of its ether derivatives. This analysis is suitable for the determination of hydroquinone in skin-toning creams [181]. The ingredients of antipyretic analgesic preparations have also been determined by this technique [182], The addition of sodium dodecyl sulfate improves the peak shapes and resolution in chiral separations by micellar electrokinetic chromatography [183]. 

At atmospheric pressure the Diels-Alder adducts of 1,4-benzoquinones are often not stable under the conditions of reaction and undergo an isomerization leading to the corresponding hydroquinones (Scheme 4). Due to the acceleration at high pressure the temperature of reaction can be lowered so that the secondary isomerization does not proceed and the primary Diels-Alder adduct can be isolated in good yields. The diastereoselectivity at high pressure induced by a chiral auxiliary, however, is with a diastereomeric excess of d.e. = 36%, only moderate. 

A Californian sponge of the Halichondriidae family contained a sulfated sesterterpene hydroquinone and five sulfated sesterterpenes. The structures of the halisulfates 1-5 (359-363) were determined by interpretation of spectral data and a structure was proposed for halisulfate 6 (364). The halisulfates are antimicrobial and antiinflammatory [318]. The absolute configuration of halisulfate 3 (361), which was also isolated from Ircinia sp. from the Philippines, has been determined by application of the chiral amide method and by chemical degradation techniques [319]. Halisulfate 7 (365) is a sesterterpene sulfate from a Coscinoderma sp. from Yap, Micronesia [320]. 

The tetrafunctional alcohol pentaerythritol is a popular core in dendrimer chemistry it has been modified into a tetrakis chromium phenylcarbene 85, which underwent a quadruple benzannulation upon reaction with 3-hexyne. The reaction proceeded with only moderate diastereoselectivity in terms of the planes of chirality formed demetalation by mild oxidative work-up gave the tetrakis-hydroquinone derivative 86 (Scheme 33) [76]. 

TABLE 5.1 Chiral Homopotymers with Side Chains Based on Asymmetric Esters ofTerephthalic Acid and Hydroquinone... 

The chiral side chain polymers derived from asymmetric esters of terephthalic acid and hydroquinone can form (in a broad temperature range, including ambient temperature) an unusual mesophase (the isotropic smectic phase, IsoSm ) characterized by high transparency and optical isotropy within the visible wavelength range, combined with a hidden layered smectic ordering and some elements of helical superstructure at shorter dimensions of 10 to 250 nm. The short-pitch TGB A model seems to be the most adequate for the mesophase structure. 

Clathrates provide cavities of a specific size and shape and therefore they can be used very effectively for separating gases with different sizes of molecules. For example, urea clathrates have been used to separate linear from branched hydrocarbons. The hydroquinone clathrate can be used to store and deliver radioactive krypton. In addition, if the host is chiral, there can be chiral discrimination so that one enantiomer of a guest is enclosed in the clathrate structure in preference to the other enantiomer. The trapped guest molecules can be liberated, for example, by solution in an apolar solvent, at the convenience of the user,... 

Calixquinones can be easily reduced to the corresponding calixhydroquinones (Zn/HCl or Na2S204) . The calix[8]hydroquinone 57b, however, was prepared from the octabenzyl ether 57a obtained by one-pot condensation in a mixture with the analogous calix[6]- and -[7]arene. Oxidation of 57b to the respective octaquinone was not reported, but the ewrfo-ether 57c was obtained by exhaustive 0-propylation prior to the cleavage of the benzyl ether groups . Inherently chiral derivatives of a calix[4]arene monoquinone have been obtained by 1,4-addition of various nucleophiles to the quinoid system. 

Thus, the aldol shown, which is susceptible to Sharpless-type epoxidation, has been obtained from phytal and the protected hydroquinone (ref. 120). Formation of the epoxide presumably with a chiral peracid (or perhaps with a conventional peracid relying on the asymmetry of the substrate) and then cleavage reductively in t-butyl methyl ketone containing lithium aluminium hydride led to a diol. The benzylic hydroxyl group of this was hydrogenolysed to afford the hydroquinone dimethyl ether in 85% yield. Ceric ammonium nitrate (CAN) oxidation afforded the intermediate benzoquinone hydrogenation of which was reported to result in 2R,4 R,8 R-a-tocopherol by, presumably, avoidance of a racemisation step. 

Tocopherol carries six methyl groups three on the hydroquinone ring and three on the side chain. The three chiral centers are all of R configuration (2-R, 4 -R, 8 -R). The most significant chiral center at C2 is as stable to oxidative cleavage as the remote centers at C4 and C8, but it disappears, of course, upon oxidation to the quinone (see Scheme 7.2.1). The UV spectrum shows a maximum at 284 nm (e = 30,000) in petrol ether and at 292 nm in ethanol. Quantitative determination in food occurs by extraction with ether, HPLC, and reduction of Fe(III) ions to Fe(II). The iron(II) concentration is then determined colorimet-rically with 2,2 -dipyridyl = 520 nm) or 4,7-diphenyl-1,10-phenanthrolin. Solutions of free tocopherol fluoresce in the UV( ), whereas neither tocopherol acetate nor the quinone shows any fluorescence (Lang, 1974 Isler and Bmbacker, 1982). This phenomenon is not understood. 

P8 M is not the only polymer forming the isotropic smectic phase. To date, we have observed formation of that phase for a half-dozen chiral polymethacrylates and polysiloxanes. Table 5.1 summarizes the chemical structure and phase behavior of synthesized side-chain homopolymers, which carry chirally substituted side chains derived from asymmetric esters of terephthalic acid and hydroquinone. Such a structure with alternating orientation of carboxylic link groups seems to favour the formation of the IsoSm phase, whereas isomeric derivatives of p-hydroxybenzoic acid, where all carboxylic links have the same orientation, form only conventional Sm A and Sm C phases. Molar mass of all the synthesized homo- and copoly(meth)acrylates is within the range of 1 to 2-10 g mol the poiysilox-anes have the average degree of polymerization, p 35. 

Channel Inclusion Compounds, p. 223 Chiral Guest Recognition, p. 236 Clathrate Hydrates, p. 274 Crown Ethers, p. 326 Crystalline Microporous Silicas, p. 380 Cyclodextrins, p. 398 Hofmann-Type Clathrates, p. 645 Hydrogen Bonding, p. 658 Hydroquinone, p. 679... 

Kim [213] studied the effect of the coimter ion on the HKR of epiehlorhydrine, 1,2-epoxybutane, 1,2-epoxyhexane and epoxystyrene. The salen polymer catalyst were synthesized by eopolymerization of salen bearing chloromethyl groups with sodium phenoxide derivatives of hydroquinone, 1,3,5-trihydroxybenzene or l,l,l-tris(p-hydroxyphenyl)ethane in presence of A-methylpyrrolidine and NaH in THF (Sehemes 143 and 144). Co(lI) type polymeric chiral salen ligands were formed by reaction of the corresponding salen ligand with hydrous Co(ll) acetate. To obtained Co(IIl) polymeric chiral... 

As expected from the depicted mechanism, early attempts to control the stereoselectivity of the MBH reaction was focused on the application of chiral amines (Fig. 4.48). Thus, using high pressure conditions (5 kbar) to accelerate the reaction and a C -symmetric DABCO derivative 245 (15 mol%), product 241a (R =Me, R sq-NO CgH ), was obtained in 45% yield and 47% ee (1 mol% hydroquinone, THF, 30°C) [318]. When used with pyrrolizidine derivative 246 (10 mol%, acetonitrile, 0°C) improved results (17-93% yield, 39-72% ee) were obtained in reactions between methyl or ethyl vinyl ketone (237a R =Me and 237b R =Et) and aromatic aldehydes. The presence of NaBF as co-catalyst was required to achieve these results, due to the coordination of aldehyde and hydroxy group of the catalyst to the alkali metal, which fixed the orientation for the attack of the nucleophile to the electrophile in the transition state [319]. 

Isopropanol s. under Cobalt (II) trifluoroacetoacetonate Chiral crown ether s. under NaOCl Hydroquinone s. under Pd(OAc)2 Benzoquinone s. under Pd(OAc)2... 

Asym. diene synthesis. Butadiene bubbled into a reactor containing startg. chiral butenolide and a little hydroquinone at — 78°, and the stoppered vessel heated at 210° for 24h (lR,6S,7R)-7-methyl-8-oxabicyclo[4.3.0]non-3-en-9-one. Y 63% (dia-stereoisomerically pure). F.e. and regiospecific conversion of isoprene in the presence of AICI3 s. R.M. Ortuno et al.. Tetrahedron 44,1711-9 (1988) ibid. 45,1833-8 (1989) asym. 1,3-dipolar cycloaddition to 5-menthyloxy-2(5/f)-furanones s. B. de Lange, B.L. Feringa, Tetrahedron Letters 29, 5317-20 (1988). 

Figure 4.9 Molecules that form clathrate structures via a hexagonal motif, hydroquinone (4.2) and Dianin s compound (4.3) (note the chiral centre - marked with an asterisk).

With hydrogen peroxide or, better, t-butylhydroperoxide as the oxidant, the combination of palladium acetate and hydroquinone forms an efficient catalyst system, which does not require other cocatalysts. Using t-butylhydroperoxide as the oxidant, it is possible to react a series of different carboxylates with alkenes in methylene chloride solution. For instance, (5)-0-acetylmandelic acid was reacted with cyclohexene to give the addition product with a modest chiral induction (de ca. 20%). Also, intramolecular cyclization could be achieved for example, 2-cyclopenten-l-acetic acid gave mainly the allylic lactone in good yield, accompanied by a small amount of the double bond isomer (Scheme 9). 

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