Influence of intramolecular hydrogen bonds in the enzyme-catalyzed regioselective acylation of quinic and shikimic acid derivatives

Selective mono-functionalization of 3-epi, 4-epi-, and 5-epi quinic and shikimic acid derivatives has been accomplished by enzymatic acylation with Candida antarctica lipase A (CAL-A). We propose that the selectivity of this lipase is related to both the inherent receptor selectivity and the degree of intramolecular hydrogen bonding in the ligand. Conformational analysis of the hydroxyl protons has been carried out by ¹H NMR spectroscopy. We have shown that exchange of the hydroxyl protons by acid catalysis provides a useful method for the detection of intramolecular hydrogen bonds. The interpretation of exchange rates and coupling constants determines the direction of the H-bonds as conditioned by the relative acceptor and donor properties of the hydroxyl groups. The selectivity of the acylation agrees fully with the effectiveness of H-bonding networks in polyol compounds and with the higher reactivity of the equatorial hydroxyl groups.     

Novel chiral lipoxygenase substrates: design and synthesis. Part 2

A series of novel lipoxygenase substrates carrying a spacing modifier with a non-ionic hydroxyl terminus have been synthesized in an enantioselective fashion. One of the methylene hydrogens (flanked by the cis,cis-pentadienyl moiety) is replaced by alkyl, aryl and hydroxyl groups. The key steps in the synthesis involved enzymatic transesterification of 1,3-propanediol derivatives and two consecutive cis-selective Wittig olefinations.     

The Identification of New Triterpenoids in Eucalyptus globulus Wood

Eight polyhydroxy triterpenoid acids, hederagenin, (4α)-23-hydroxybetulinic acid, maslinic acid, corosolic acid, arjunolic acid, asiatic acid, caulophyllogenin, and madecassic acid, with 2, 3, and 4 hydroxyl substituents, were identified and quantified in the dichloromethane extract of Eucalyptus globulus wood by comparing their GC-retention time and mass spectra with standards. Two other triterpenoid acids were tentatively identified by analyzing their mass spectra, as (2α)-2-hydroxybetulinic acid and (2α,4α)-2,23-dihydroxybetulinic acid, with 2 and 3 hydroxyl substituents. Two MS detectors were used, a quadrupole ion trap (QIT) and a quadrupole mass filter (QMF). The EI fragmentation pattern of the trimethylsilylated polyhydroxy structures of these triterpenoid acids is characterized by the sequential loss of the trimethylsilylated hydroxyl groups, most of them by the retro-Diels-Alder (rDA) opening of the C ring with a π-bond at C12-C13. The rDA C-ring opening produces ions at m/z 320 (or 318) and m/z 278 (or 277, 276, 366). Sequential losses of the hydroxyl groups produce ions with m/z from [M - 90] to [M - 90*y], where y is the number of hydroxyl substituents present (from 2 to 4). Moreover, specific cleavage in ring E was observed, passing from m/z 203 to m/z 133 and conducting other major fragments such as m/z 189.     

IBX/n-Bu4NBr/CH2Cl2-H2O: a new mild system for selective oxidation of secondary alcohols

A new alternative system for the chemoselective oxidation of secondary hydroxyl group to ketone with IBX/n-Bu₄NBr in CH₂Cl₂-H₂O has been developed. Under the reaction conditions, the secondary hydroxyl group was highly chemoselectively oxidized to the corresponding ketone, in moderate to good yields at rt, in the presence of primary hydroxyl group within the same molecule.     

The adsorption of resorcinol from water using multi-walled carbon nanotubes

Adsorption of resorcinol and other phenolic derivatives on pristine multi-walled carbon nanotubes (MWCNTs) and HNO₃ treated MWCNTs has been investigated in attempt to explore the possibility to use MWCNTs as efficient adsorbents for pollutants. MWCNTs showed higher adsorption ability in a rather wide pH range of 4–8 for resorcinol, while decreased uptake capacity was found for acid-treated MWCNTs. Other phenolic derivatives such as phenol, catechol, hydroquinone and pyrogallol were employed to study the influence of the number and position of hydroxyl groups on the adsorption capacity. The amounts adsorbed by MWCNTs increased with the increasing number of hydroxyl. The substitution of phenol with a hydroxyl in meta-position leads to a much higher absorption ability than substitution in ortho- or para-position, which suggested that MWCNTs possess a great potential in removal of resorcinol from water, as well as the other phenolic derivatives.     

The effect of (natural) polyols on the initial colour of heavy metal- and zinc-free poly(vinyl chloride)

The propensity of (natural) polyols dispersed in heavy metal- and zinc-free PVC sheets to improve the initial colour, e.g. the colour during the first minutes of exposure to high processing temperatures (short-term stability), has been investigated. It is shown using W(Lab) values that the initial colour improves upon addition of polyols containing primary hydroxyl groups. The polyols act as HCl scavengers, presumably via an acid-catalysed S_N2 substitution of the primary hydroxyl groups by chloride ions. In contrast, polyols with only secondary or tertiary hydroxyl groups accelerate the thermal degradation of PVC. Notwithstanding, the efficacy of the (natural) polyols containing primary hydroxyl groups will be reduced if the polyol is susceptible to competitive acid-catalysed intramolecular cyclodehydration reactions under the processing conditions. This is substantiated by a comparison of the behaviour of mannitol and 3,4-di-O-methyl-d-mannitol. The methylated derivative, which is less prone to undergo intramolecular cyclodehydration, improves the initial colour of heavy metal- and zinc-free PVC sheets more significantly than mannitol itself.     

Direct Ferrier rearrangement on unactivated glycals catalyzed by indium(III) chloride

Anhydrous InCl₃ has been shown to efficiently catalyze the Ferrier rearrangement by a direct allylic substitution of the hydroxyl group at C-3 position of glycals to afford the corresponding 2,3-unsaturated glycosides in high yields at ambient temperature. This methodology obviates the need for protecting and/or activating the C-3 hydroxyl group of glycals. The reaction works in equal ease with both 4,6-di-O-benzyl-d-glucal and 4,6-di-O-benzyl-d-galactal. The mildness of InCl₃ makes this approach compatible for glycosyl acceptors with acid labile groups. The generality of the reaction has been demonstrated with a diversity of alcohols, phenols, and sugar nucleophiles.     

(S)-Mandelic acid enolate as a chiral benzoyl anion equivalent for the enantioselective synthesis of non-symmetrically substituted benzoins

A strategy for the enantioselective synthesis of non-symmetrically substituted benzoins from (S)-mandelic acid and aromatic aldehydes has been developed. This strategy is based on a diastereoselective aldol reaction of the lithium enolate of the 1,3-dioxolan-4-one derived from (S)-mandelic acid and pivalaldehyde with aromatic aldehydes, which gives the corresponding aldols in good yields. Subsequent hydroxyl group protection as MEM ethers, basic hydrolysis of the dioxolanone ring, oxidative decarboxylation of the α-hydroxy acid moiety, and hydroxyl group deprotection provides chiral non-symmetrically substituted benzoins with high enantiomeric excesses.     

Catalyseurs bifonctionnels—IV: Synthese et action catalytique de steroïdes comportant une fonction alcool et un noyau imidazole

The binding of two side chains bearing an hydroxyl and imidazole terminal group on 6β,17β-diamino steroïds, in the normal and d-homo series, is described. The catalytic effect of these products on the hydrolysis of p-nitro-phenyl acetate is studied. A slight acceleration is observed with compounds in which hydroxyl and imidazole groups are attached to the steroid skeleton.     

Synthesis and properties of thermoplastic propyl-etherified amylose

Amylose was etherified with 1-bromopropane in DMSO. The degree of substitution (DS) was varied by altering the feed ratio of 1-bromopropane. The structures of the products were characterized by IR and 1H-NMR spectroscopy. When the molar feed ratio of 1-bromopropane to hydroxyl groups of amylose was beyond 7.5, the hydroxyl groups were completely substituted with propyl ether groups. The etherified amylose with DS 1.9 showed a glass transition temperature (T_g), and that with DS 2.3 or 3.0 showed both T_g and melting temperature (T_m) (DS 3.0 means complete substitution). The etherification imparted melt processability and solubility in nonpolar organic solvent to amylose.     

The effect of poly(methacrylic acid) on electrosurface properties of titanium dioxide in aqueous suspensions

The electrosurface properties and aggregation stability of aqueous titanium-dioxide suspensions are studied in wide ranges of medium pH values and poly(methacrylic acid) concentrations. Using the 2pK model, the ionization constants of surface hydroxyl groups of titanium dioxide are found and it is established that the adsorption of poly(methacrylic acid) macromolecules substantially decreases the fraction of neutral (uncharged) hydroxyl groups and affects the ratio between the positively and negatively charged surface hydroxyl groups.     

Determination of organic hydroxyl and amino compounds with o-sulphobenzoic anhydride

The determination of alcoholic hydroxyl groups by acylation with o-sulphobenzoic anhydride in dioxane medium is proposed. Quantitative esterification was generally obtained by refluxing for 2–3 h. No interferences were found from water, aldehydes, ketoncs, ethers, esters and phenols ; the reagent appeared to be of the phthalic anhydride type. Results for 26 hydroxyl and some amino compounds by the proposed reagent are given.     

A Simple Synthesis Route for Selectively Methylated β-Cyclodextrin Using a Copper Complex Sandwich Protecting Strategy

This communication reports a novel synthesis route for the preparation of monofunctionalized β-cyclodextrin in a single stage. The approach involves only the in-situ protection of secondary hydroxyl groups as an excellent alternative to the classical procedure involving a series of five steps of protection and deprotection of hydroxyl groups (both primary and secondary ones) belonging to β-cyclodextrin.     

Mild generation of o-quinodimethanes via fluoride induced 1,4-elimination of α-(o-trimethylsilylmethyl)benzylesters: stereoselective synthesis of 19-nor steroids and RU486 precursors

19-Nor steroids and RU486 tricyclic synthetic intermediates were stereoselectively prepared by an intramolecular Diels-Alder reaction involving an o-quinodimethane possessing a pro-17 unique chiral stereocenter, substituted by a protected hydroxyl group. The o-quinodimethane was generated in mild conditions by fluoride induced 1,4-elimination of α-(o-trimethylsilylmethyl)benzylesters and the present methodology allows a flexible access to α,α′-disubstituted o-quinodimethanes, as shown by the 11β-substituted steroid approach. The IMDA diastereoselections reported herein were highly dependent on the nature of the hydroxyl protective group and the diastereoselectivities superior to those observed with the thermolysis of the corresponding benzocyclobutenes.     

NMR and ab initio studies of Mg,Ca, Zn,Cu alizarin complexes

Complexation of Mg²⁺, Ca²⁺, Zn²⁺ and Cu²⁺ with alizarin, characterized by NMR and theoretical studies (DFT, B3LYP/6-31G(d,p) PCM and NMR GIAO B3LYP/IGLOIII), indicated that the metal ion is attached to two hydroxyl groups in a bidentate mode. The alternative complexation in the bidentate mode, with only one hydroxyl group and a carbonyl oxygen, is less possible as it has a higher energy.     

Sulfonate protecting groups. Synthesis of O- and C-methylated inositols: d- and l-ononitol, d- and l-laminitol, mytilitol and scyllo-inositol methyl ether

Syntheses of d- and l-ononitol, d- and l-laminitol, mytilitol and scyllo-inositol methyl ether starting from myo-inositol are described. One or two of the myo-inositol 1,3,5-orthoformate hydroxyl groups were protected as tosylates. These mono or ditosylates served as key intermediates for the preparation of O- and C-methyl inositols. Racemic 2,4-di-O-tosyl-myo-inositol 1,3,5-orthoformate was resolved as its diastereomeric camphanates. Use of sulfonate groups for the protection of inositol hydroxyl groups resulted in substantial improvement in the overall yield of O- and C-methyl inositols.     

Semimicro determination of hydroxy and amino compounds using pyromellitic dianhydride

The PMDA method for determining hydroxyl and primary and secondary amino groups was applied on the semimicro scale. A sample containing 0.4 to 0.6 mEq of amine or hydroxyl was reacted with 25 ml of 0.04 M PMDA in a routine analysis. The unconsumed PMDA was titrated with 0.08 N base. For colorless clear samples, the phenolphthalein endpoint was used; for colored or turbid samples, a potentiometric titration is recommended. Further scaling down of the method may be feasible by increasing the $\text{PMDA}\text{OH}$ ratio.     

Mechanisms of water photooxidation at n-TiO2 rutile single crystal oriented electrodes under UV illumination in competition with photocorrosion

Photoetching is known to compete with water photooxidation at n-TiO₂ rutile electrodes in contact with aqueous H₂SO₄ solutions under UV illumination and anodic bias. A mechanism based on the generation of bridging hydroxyl species from the adsorption of water molecules at photoinduced bridging oxygen vacancies is proposed in order to explain the competition between both photoreactions. This mechanism, designated as Redox Photooxidation (RP) Mechanism, correlates the atomic arrangement of the TiO₂ surface with its photocatalytic activity, considering that the first step for water photooxidation is the photogeneration of bridging oxygen/hydroxyl radicals associated with intrinsic bandgap surface states, via inelastic transfer of free valence band holes to bridging oxygen/hydroxyl groups, depending on the electrolyte pH. The critical distance between adjacent bridging oxygen/hydroxyl radicals allows their covalent bonding with generation of surface-bound peroxide species, which are further photooxidized leading to oxygen evolution. The RP mechanism allows to explain literature experimental results concerning surface modifications of n-TiO₂ rutile during photoetching in competition with water photooxidation, as well as their dependence on crystal orientation. The photogeneration of chemisorbed peroxo species, intermediates of the oxygen evolution reaction, detected by MIRIR spectroscopy, as well as experimental results obtained from PL and DEMS experiments are also interpreted in the light of the RP mechanism. A comparative analysis with the nucleophilic attack (NA) Mechanism, an alternative model proposed recently to explain photoelectrochemical water oxidation at n-TiO₂ rutile, is presented.     

Reactivity of Aliphatic and Phenolic Hydroxyl Groups in Kraft Lignin towards 4,4′ MDI

Several efforts have been dedicated to the development of lignin-based polyurethanes (PU) in recent years. The low and heterogeneous reactivity of lignin hydroxyl groups towards diisocyanates, arising from their highly complex chemical structure, limits the application of this biopolymer in PU synthesis. Besides the well-known differences in the reactivity of aliphatic and aromatic hydroxyl groups, experimental work in which the reactivity of both types of hydroxyl, especially the aromatic ones present in syringyl (S-unit), guaiacyl (G-unit), and p-hydroxyphenyl (H-unit) building units are considered and compared, is still lacking in the literature. In this work, the hydroxyl reactivity of two kraft lignin grades towards 4,4′-diphenylmethane diisocyanate (MDI) was investigated. ³¹P NMR allowed the monitoring of the reactivity of each hydroxyl group in the lignin structure. FTIR spectra revealed the evolution of peaks related to hydroxyl consumption and urethane formation. These results might support new PU developments, including the use of unmodified lignin and the synthesis of MDI-functionalized biopolymers or prepolymers.     

Submicro methods for the analysis of organic compounds : The determination of hydroxyl groups

Various procedures were examined for the determination of organic hydroxyl groups. Conventional acetylation methods could not be used on the submicro scale, but the spectrophotometric method based on esterification with 3,5-dinitrobenzoyl chloride followed by extraction of the ester and colour development with alkaline acetone proved reasonably satisfactory for aliphatic alcohols. Acidic hydroxyl groups e.g. in phenolic compounds, were titrated accurately with 0.01 M tetrabutylammonium hydroxide in benzene-methanol solution in a pyridine medium ; visual end-points or potentiometricend-points with glass-silver or glass-in-stream platinum electrode pairs were used depending on the strength of the acidic group. Some differentiating titrations were possible. A general submicro bromination method for phenols was not feasible.