Active Catalyst Species Research Articles

Mechanistic studies of uncatalyzed and ruthenium(III)-catalyzed oxidation of the antibiotic drug chloramphenicol by hexacyanoferrate(III) in aqueous alkaline medium: a comparative kinetic study.

The kinetics of oxidation of the antibiotic drug chloramphenicol (CHP) by hexacyanoferrate(III) (HCF) has been investigated spectrophotometrically both in the absence and presence of ruthenium(III) catalyst in aqueous alkaline medium at 25 °C and at constant ionic strength of 1.10 mol dm−3. The stoichiometry is identical in both cases, i.e. [CHP]/[HCF] = 1:2. The oxidation products were identified by TLC and spectral studies such as GC–MS, IR, and 1H NMR. In both catalyzed and uncatalyzed reactions, the order with respect to the concentration of HCF is unity, whereas the order with respect to the concentration of CHP and the concentration of OH− is less than unity over the concentration range studied. The order with respect to the concentration of Ru(III) is unity. The reaction in the presence of Ru(III) is approximately tenfold faster than the uncatalyzed reaction. The active species of oxidant and catalyst are [Fe(CN)6]3− and [Ru(H2O)5(OH)]2+, respectively. On the basis of experimental results suitable mechanisms are proposed. The reaction constants involved in the different steps of the reaction mechanisms were calculated for both cases. The catalytic constant was also calculated for the catalyzed reaction at different temperatures. The activation parameters with respect to the slow step of the mechanism and thermodynamic quantities are also determined.Graphical abstract Electronic supplementary materialThe online version of this article (doi:10.1007/s00706-014-1208-7) contains supplementary material, which is available to authorized users.

Excited-state dynamics of a ruthenium(II) catalyst studied by transient photofragmentation in gas phase and transient absorption in solution

We report studies on the excited state dynamics of new ruthenium(II) complexes [(η6-cymene)RuCl(apypm)]PF6 (apypm2-NR2-4-(pyridine-2-yl)-pyrimidine, RCH3 (1)/H (2)) which, in their active form [1+-HCl] and [2+-HCl], catalyze the transfer hydrogenation of arylalkyl ketones in the absence of a base. The investigations encompass femtosecond pump–probe transient mass spectrometry under isolated conditions and transient absorption spectroscopy in acetonitrile solution, both on the cations [(η6-cymene)RuCl(apypm)]+ (1+, 2+). Gas phase studies on mass selected ions were performed in an ESI ion trap mass spectrometer by transient photofragmentation, unambiguously proving the formation of the activated catalyst species [1+-HCl] or [2+-HCl] after photoexcitation being the only fragmentation channel. The primary excited state dynamics in the gas phase could be fitted to a biexponential decay, yielding time constants of <100fs and 1–3ps. Transient absorption spectroscopy performed in acetonitrile solution using femtosecond UV/Vis and IR probe laser pulses revealed additional deactivation processes on longer time scales (∼7–12ps). However, the formation of the active catalyst species after photoexcitation could not be observed in solution. The results from both studies are compared to former CID investigations and DFT calculations concerning the activation mechanism.

Synergetic effect between Cu0 and Cu+ in the Cu-Cr catalysts for hydrogenolysis of glycerol

Active species of Cu-Cr catalysts, prepared by an epoxide-assisted sol–gel route, were investigated for the hydrogenolysis of glycerol to 1,2-propanediol. Structural characterization of the catalysts was performed by means of N2 physisorption, X-ray diffraction, H2-temperature programmed reduction, high-resolution X-ray photoelectron spectroscopy, NH3-temperature programmed desorption, and N2O titration. On the basis of the characterizations, the copper species on the calcined Cu-Cr catalysts and the reduced Cu-Cr catalysts were assigned. Combined with reaction results, it was found that there was not similar trend in copper metal surface area and glycerol conversion, indicating that a two-site (Cu0 and Cu+) mechanism existed in the hydrogenolysis of glycerol over Cu-Cr catalysts. Besides, the maximum conversion of glycerol was obtained when the surface Cu0/Cu+ ratio increased from 3.1 to 6.6, whereas decreased with increasing sequentially to 15.7, demonstrating that the appropriate surface Cu0/Cu+ ratio was required for optimum hydrogenation activity. Thus, the synergetic effect between the Cu0 and Cu+ was considered to be responsible for the high catalytic activity in the hydrogenolysis of glycerol, and that CuCr2O4 played a critical role in the glycerol hydrogenolysis reaction since it could function as a “hydrogen delivery bridge”.

Nitrogen doping of ash-free coal and effect of ash components on properties and oxygen reduction reaction in fuel cell

Nitrogen doping of ash-free coal (HPC) and raw coal in a stream of ammonia was studied in application to a cathode catalyst for a fuel cell. The relation to structure morphology, surface properties and catalytic activity for the oxygen reduction reaction (ORR), the effects of addition of the ash components of the raw coal to HPC and the comparison with carbonization in a stream of helium were determined. Tests were carried out with linear sweep voltammetry measurement in streams of oxygen and argon gases and a scan region of 0.04–1.04V using a three-electrode setup. Inorganic elements dominant in the ash of the raw HPC coal were iron, aluminum, potassium, magnesium, silicon and calcium. The nitrogen doping of HPC at 773–1073K decreased the oxygen content and increased the nitrogen content and the pyridinic nitrogen with the treating temperature compared to the carbonization in a stream of helium based on the XPS analysis. The addition of metals to HPC and subsequent nitrogen doping decreased the nitrogen content of HPC but increased the pyridinic nitrogen in the nitrogen distribution except for magnesium and calcium due to interaction with the pyridinic nitrogen. From the Raman analysis, the nitrogen doping at 773–1073K decreased the ID/IG ratio (deficient carbon degree) compared to the carbonization, but the addition of the ash components, especially 0.5% Fe+0.5% Al, increased it, resulting from the formation of many edges of deficient carbons in the presence of the ash. The TEM observation showed that the nitrogen doping of HPC at 1073K created onion-like fullerene structures but the addition of iron and aluminum destroyed this structure and formed a nanocarbon structure. The presence of iron and aluminum increased the pyridinic nitrogen and the disordered defect carbons formed along the edges of the graphite layers. Furthermore, the nitrogen doping at 1073K increased the onset potential of the ORR from 0.37 (carbonizing) to 0.67V. The addition of both iron and aluminum further increased the onset to 0.79V for the ORR which was the same potential as that of the raw HPC containing the ash components. The presence of ash in the raw coal, especially iron and alumina, promoted the electronic performance for the oxygen reduction. Consequently, the nitrogen-doped combined iron and aluminum with nanocarbons were the active species of the HPC catalysts for the ORR.

One‐pot Synthesis of Ordered Mesoporous NiCeAl Oxide Catalysts and a Study of Their Performance in Methane Dry Reforming

AbstractOrdered mesoporous NiAl and NiCeAl catalysts with different Ce/Al molar ratios were facilely synthesized by using the improved evaporation‐induced self‐assembly method. The characterization results confirmed that the ordered mesoporous structure was well sustained in the Ce‐incorporated NiAl materials (Ce/Al molar ratio&lt;4 %). Compared with NiAl mesoporous materials, Ce‐incorporated mesoporous materials demonstrated higher specific surface areas, larger pore volumes, and more uniform pore sizes. The catalytic test conducted by using methane dry reforming revealed that compared with NiAl catalysts, all the Ce‐promoted catalysts demonstrated improved initial catalytic activity, which was due to the high dispersion and the high reduction degree of active Ni species in NiCeAl catalysts. The stable alumina framework, confinement effect of ordered mesopores, and high oxygen mobility contributed to the improved catalytic stability of NiCeAl catalysts. For comparison, the mesoporous NiCeAl catalyst (denoted as NiCeAl‐IMP) was also prepared by using the conventional impregnation method. The agglomeration of Ni particles was observed during the stability test for the Ni‐impregnated catalyst, which accelerated the rate of carbon deposition. The NiCeAl catalyst (Ce/Al molar ratio=1 %) demonstrated excellent resistance to the formation of graphitic carbon species owing to the redox property, while a large amount of graphitic carbon species was deposited over the NiAl sample, which was responsible for deactivation.

Study on the basic centers and active oxygen species of solid-base catalysts for oxidation of iso-mercaptans

It is a challenge to remove mercaptans as well as to keep octane value in clean gasoline production. In this work, gas–liquid–solid heterogeneous base-catalyzed oxidation of tert-butyl thiol by molecular oxygen was investigated. The reactivity and stability of modified MgO catalysts were studied. The catalysts were further characterized by XRD, FT-IR, CO2-TPD, H2-TPD, O2-TPD and EPR. Compared with commercial cobalt phthalocyanine catalyst (CoPc catalyst), the modified MgO catalyst displayed an enhanced stability to the oxidation of tert-butyl thiol, and the catalytic lifetime is 10h longer than that of CoPc catalyst at 3.0h−1 of LHSV. It was found that the active sites of the catalysts are defects and basic centers. In addition, the basic sites responsible for the reactivity are mainly the medium and strong basic centers. It is interesting that the part of O3C2−−Mg3C2+ provides medium basic centers, however, the other part of it supplies as strong basic centers. It was demonstrated that superoxide anions O2− served as the active oxygen species.

Spectroscopic Investigation and Reactivities of Ruthenium(III) Catalyzed Oxidation of Anticholinergic Drug Atropine Sulfate Monohydrate by Hexacyanoferrate(III) in Aqueous Alkaline Media: A Mechanistic Approach

The kinetics of oxidation of an atropine sulfate monohydrate (ASM) by hexacyanoferrate (HCF) (III) has been investigated spectrophotometrically in presence of ruthenium(III) catalyst in aqueous alkaline medium at 25°C. The stoichiometry is 1: 2. The reaction is first order in HCF(III) and has less than unit order in ASM concentrations. The effect of ionic strength and dielectric constants were studied on the rate of reaction. The active species of oxidant and catalyst are Fe(CN)6 3– and [Ru(H2O)5(OH)]2+, respectively. A suitable mechanism is proposed. The catalytic constant (KC) was also calculated. The activation parameters and thermodynamic quantities are also determined and discussed.

Mechanism of Mo-catalyzed C–S cleavage of thiophene

The Mo(PMe3)6-catalyzed C–S cleavage of thiophene was studied using density functional theory (DFT). A comparison of several possible pathways shows that, the five- and four-coordinated species are determined as two most active catalyst species, and this reaction is the most likely to undergo consecutively the dissociation of two PMe3 groups from Mo(PMe3)6 to give the four-coordinated Mo(PMe3)4, the binding of thiophene to Mo(PMe3)4 in η1-S-mode to give Mo(PMe3)4(η1-S-thiophene), the transformation of Mo(PMe3)4(η1-S-thiophene) into the Mo(PMe3)4(η2-C2,S-thiophene), and the cleavage of the C–S bond of thiophene to give the ring-opened species Mo(PMe3)4(κ2-C2,S-thiophene). The butadiene–thiolate complex (η5-C4H5S)Mo(PMe3)2(η2-CH2PMe2) may generate in the ring-opened species through the departure of one PMe3 group, hydrogen transfer and structural transformation. The thiophene adduct Mo(PMe3)3(η5-thiophene) may originate in Mo(PMe3)4(η2-C2,C3-thiophene) by removing one PMe3 group and changing the η2-coordination mode into η5-coordination mode. The highest formation energy barrier of the thiophene adduct and butadiene–thiolate complex is 20 and 10.4 kcal/mol, respectively, which is easy to cross and supports the facts that the two species can be experimentally observed.

Ruthenium‐Catalyzed Transvinylation – New Insights

AbstractThe use of ruthenium complexes in transvinylation catalysis has been well established since the 1980s. However, the reaction mechanism and the active catalyst species, which is presumed to contain ruthenium carbonyl carboxylate entities, have so far remained elusive. In this work the synthesis and characterization of three novel ruthenium complexes comprising ruthenium carbonyl carboxylate structural motifs including two single crystal structures as well as the crystal structures of two known ruthenium complexes are reported. These new complexes and four known ruthenium complexes with appropriate structural motifs were applied in transvinylation catalysis. Mechanistic studies including identification and characterization of the active species, isotope labeling experiments and examination of the regio‐ and stereoselectivity of the transvinylation reaction are presented, resulting in the proposal of a probable reaction mechanism, which is supported by DFT calculations on the B3LYP/6‐31G* level of theory.magnified image

Ligand‐Modified Rhodium Catalysts on Porous Silica in the Continuous Gas‐Phase Hydroformylation of Short‐Chain Alkenes–Catalytic Reaction in Liquid‐Supported Aldol Products

AbstractLigand‐modified Rh complexes were physically adsorbed on the surface of porous silica. The resulting materials were subjected to the continuous gas‐phase hydroformylation of C2 and C4 alkenes. The ligands used for catalyst modification were bidentate phosphorus ligands known from the literature, namely, sulfoxantphos (1) and a benzopinacol‐based bulky diphosphite 2. The tested catalyst materials were active and, in particular, selective as in comparable homogeneous liquid‐phase experiments. Long‐term stability experiments over 1000 h on stream showed minor deactivation. A significant increase in the catalyst mass after the reaction was detected by weighing and thermogravimetric analysis. By using headspace‐GC–MS, the mass increase could be attributed to high‐boiling compounds, which are formed in situ during the catalytic reaction itself and accumulate inside the pores of the support. Evidence is given that the initially physisorbed catalyst complexes dissolve in the high‐boiling aldol side‐products, which are suitable solvents for the active catalyst species and provide a liquid‐phase environment held by capillary forces on the support.

Kinetics and mechanistic study of manganese(II)-catalyzed cerium(IV) oxidation of thiamine hydrochloride in aqueous perchloric acid medium by stopped flow technique

The kinetics of the manganese(II)-catalyzed oxidation of thiamine hydrochloride by cerium(IV) in aqueous perchloric acid medium at a constant ionic strength of 1.10 mol dm−3 was studied spectrophotometrically at 15, 25, 35, and 45 °C by the stopped flow technique. The reaction between thiamine hydrochloride and cerium(IV) in the acid medium exhibits 1:3 stoichiometry. The main products were identified by spot test, IR, 1H NMR, and GC–MS studies. The reaction is first order in cerium(IV) and manganese(II) and has less than unit order in thiamine hydrochloride. As the acid concentration increases the rate of reaction decreases. The added product cerium(III) retards the rate of reaction. The active catalyst and oxidant species were identified as [Mn(H2O)4]2+ and [Ce(OH)]3+, respectively. A probable mechanism involving free radicals and the formation of a complex between substrate and catalyst is proposed. The reaction constants, activation parameters, and thermodynamic quantities are calculated and discussed.

Oxidation of d-Glucose by Silver(III) Periodate Complex in the Presence of Ru(III)/Os(VIII) as a Homogeneous Catalyst: A Comparative Mechanistic Study (Stopped Flow Technique)

The kinetics of the oxidation of ruthenium(III) (Ru(III)) and osmium(VIII) (Os(VIII)) catalyzed oxidation of d-glucose (d-Glu) by silver(III) periodate complex (DPA) in aqueous alkaline medium at 298 K and constant ionic strength 0.003 mol·dm−3 was studied spectrophotometrically. The reaction between d-Glu and DPA in alkaline medium exhibits 1:2 stoichiometry in both catalyzed reactions (d-Glu:DPA). The main products were identified as D-arabinonic acid and formic acid by spot tests, GC–MS spectra and chromatographic techniques. The reaction orders with respect to various species concentrations were determined. Also, the active species of catalyst and oxidant have been identified. Probable mechanisms were proposed. The activation parameters with respect to the slow step of the mechanism were computed and discussed and thermodynamic quantities were also calculated. It has been observed that the catalytic efficiency for the present reaction is in the order Os(VIII) > Ru(III).

Separation and characterization of the active species in Ti-doped NaAlH4

The catalytic mechanism of doped complex hydrides for hydrogen storage remains unconfirmed. Here, we report a simple method to separate the active species of Ti-based catalysts in NaAlH(4) by filtration using tetrahydrofuran (THF) as solvent. The results show that the average particle size of the obtained Al-Ti active species is 30-50 nm.

Controlled tin catalyzed hydrolysis of 3-acryloxypropyltrimethoxysilane with mono- and multi-functional mercaptans

Mechanistic investigations were initiated to further our understanding of the reaction pathways for hydrolysis and condensation of organofunctional alkoxysilanes in organic media. Previous work has shown that addition of mercaptans to solutions that contain dibutyltin dilaurate (DBTDL), which functions as a catalyst, and 3-acryloxypropyltrimethoxysilane can greatly reduce the rate of silane hydrolysis and condensation. Gas chromatography/mass spectrometry (GC/MS) hydrolysis studies were carried out to understand how mercaptan structure and concentration affected the relative hydrolysis rate of the acrylate silane in solution. Detailed nuclear magnetic resonance spectroscopy (NMR) and electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry (MS) were carried out on simple mixtures of DBTDL and select mercaptans, and with varying mercaptan concentrations, to elucidate which significant tin complexes can form. These studies showed that mercaptans readily undergo ligand exchange reactions with the labile carboxylate groups of DBTDL and that an excess of mercaptans reduced the amount of proposed active catalyst species. It was found that an excess of mercaptan functional group was necessary to prevent significant and rapid hydrolysis of the organofunctional silane in organic media. A minimum of 1:4 DBTDL carboxylate group:mercaptan functional group was required for most mercaptans examined. A tetramercaptan, pentaerythritol tetrakis(3-mercaptopropionate), was shown to be effective in deactivating the DBTDL catalyst, even at relatively low concentrations of tetramercaptan. The findings in this work provide further support for the proposed mechanism for catalyzed hydrolysis and condensation. The tin metal was determined to be predominately 4-fold coordinated in the various tin mercaptide compounds studied. Evidence for transient tin hydroxide reactive intermediates was detected using mass spectrometry, as upon the addition of water, oligomeric condensation products were observed. However, the lifetime of these reactive intermediates precluded their direct observation by either mass spectrometry or NMR. Oligomeric and cyclic tin mercaptide species were shown to form when multifunctional mercaptans were combined with DBTDL in solution.

Biodiesel production using calcium manganese oxide as catalyst and different raw materials

The use of heterogeneous catalysts for biodiesel production aims to simplify the production process as well as to reduce purification costs and related environmental impacts. Calcium manganese oxide was recently identified by the authors as an interesting heterogeneous catalyst for biodiesel production from animal fat; however, the difference between this and other catalysts, the catalyst activation/deactivation mechanisms, its behaviour in the synthesis using different raw materials as well as the impacts of its use on product quality remained unclear. Therefore, the present work: (i) compared biodiesel production using calcium manganese oxide and other catalysts (CaO and NaOH); (ii) studied the reasons leading to activation/deactivation of the heterogeneous catalyst; (iii) analysed biodiesel heterogeneous synthesis using calcium manganese oxide and different raw materials (lard, waste frying oil and a mixture); and (iv) evaluated raw material and catalyst impact on the product quality. Considering the use of different catalysts, the results showed that, after 8h of reaction, product purity was similar using the different catalysts, being 92.5wt.% using both NaOH and calcium manganese oxide and 93.8wt.% using CaO. The active species of the heterogeneous catalysts were CaO, in the case of calcinated calcium carbonate, and Ca0.9Mn0.1O, in the case of calcinated calcium manganese oxide. Because the deactivating species were different for both catalysts, the calcium manganese oxide required lower activation temperature, which should be an advantage. When using different raw materials, the reaction was slower when lard was used for biodiesel synthesis. Leaching results showed that catalytic behaviour was heterogeneous but further purification of biodiesel might be needed. Using heterogeneous catalysis, after 8h of reaction, independently of the raw materials used, similar results were obtained regarding the reaction progression; the quality was generally maintained but the water content of the product was improved compared to the homogeneous process.

High Yield Preparation of Carbon Nanotube Arrays of Good Quality by CVD

AbstractMulti‐walled carbon nanotube arrays (CNTAs) of good quality are prepared in a high yield of ∼7 wt.% by CVD at 850 °C, using rectified cyclohexane as the carbon source. All the CNTAs forming on the inner wall of a quartz tube reactor are of good quality, but those forming in the zone 40–60 cm away from the entrance of the furnace show the best degrees of graphitization and alignment. The length of these CNTAs is ∼200 µm, their average growth rate during the CVD reaction (8 min) is 25 µm min−1, and the content of MWCNTs and iron are 93.7 wt.% and 3.9 wt.%, respectively. The effect of CVD reaction temperature on the yield and quality of the CNTAs is also investigated and explained by knowledge of the chemical kinetics of reaction; four main chemical reactions are suggested for the growth of CNTAs during the process of CVD reaction, and the ratios of the active carbon species (A) to the active Fe catalyst species (C) (A:C) and the other carbon species (B) (A:B). The reasons for the high yield of CNTAs of good quality are discussed briefly.

Iron(0) Particles: Catalytic Hydrogenations and Spectroscopic Studies

Ironing it out: The simple pre-catalyst system FeCl3/EtMgCl was applied to the hydrogenation of various alkenes and alkynes at room temperature. Domino iron-catalyzed allylation/hydrogenation reactions were performed under 1–4 bar H2. Spectroscopic studies determined the heterogeneous nature of the active catalyst species, its oxidation state (Fe0), and the local structure and size of the formed particles.

Metal–Ligand Cooperation in Catalytic Intramolecular Hydroamination: A Computational Study of Iridium–Pyrazolato Cooperative Activation of Aminoalkenes

The present study comprehensively explores diverse mechanistic pathways for intramolecular hydroamination of prototype 2,2-dimethyl-4-penten-1-amine by Cp*Ir chloropyrazole (1; Cp*=pentamethylcyclopentadienyl) in the presence of KOtBu base with the aid of density functional theory (DFT) calculations. The most accessible mechanistic pathway for catalytic turnover commences from Cp*Ir pyrazolato (Pz) substrate adduct 2⋅S, representing the catalytically competent compound and proceeds via initial electrophilic activation of the olefin C=C bond by the metal centre. It entails 1) facile and reversible anti nucleophilic amine attack on the iridium-olefin linkage; 2) Ir-C bond protonolysis via stepwise transfer of the ammonium N-H proton at the zwitterionic [Cp*IrPz-alkyl] intermediate onto the metal that is linked to turnover-limiting, reductive, cycloamine elimination commencing from a high-energy, metastable [Cp*IrPz-hydrido-alkyl] species; and 3) subsequent facile cycloamine liberation to regenerate the active catalyst species. The amine-iridium bound 2 a⋅S likely corresponds to the catalyst resting state and the catalytic reaction is expected to proceed with a significant primary kinetic isotope. This study unveils the vital role of a supportive hydrogen-bonded network involving suitably aligned β-basic pyrazolato and cycloamido moieties together with an external amine molecule in facilitating metal protonation and reductive elimination. Cooperative hydrogen bonding thus appears pivotal for effective catalysis. The mechanistic scenario is consonant with catalyst performance data and furthermore accounts for the variation in performance for [Cp*IrPz] compounds featuring a β- or γ-basic pyrazolato unit. As far as the route that involves amine N-H bond activation is concerned, a thus far undocumented pathway for concerted amidoalkene → cycloamine conversion through olefin protonation by the pyrazole N-H concurrent with N-C ring closure is disclosed as a favourable scenario. Although not practicable in the present system, this pathway describes a novel mechanistic variant in late transition metal-ligand bifunctional hydroamination catalysis that can perhaps be viable for tailored catalyst designs. The insights revealed herein concerning the operative mechanism and the structure-reactivity relationships will likely govern the rational design of late transition metal-ligand bifunctional catalysts and facilitate further conceptual advances in the area.

Solid state 13C NMR characterisation study on fourth generation Ziegler–Natta catalysts

In this study, solid state 13C NMR spectroscopy was utilised to characterize and identify the metal–ester coordination in active fourth generation (phthalate) Ziegler–Natta catalysts. It is known that different donors affect the active species in ZN catalysts. However, there is still limited data available of detailed molecular information how the donors and the active species are interplaying. One of the main goals of this work was to get better insight into the interactions of donor and active species. Based on the anisotropy tensor values (δ11, δ22, δ33) from low magic-angle spinning (MAS) 13C NMR spectra in combination with chemical shift anisotropy (CSA) calculations (δaniso and η), both the coordinative metal (Mg/Ti) and the symmetry of this interaction between metal and the internal donor in the active catalyst (MgCl2/TiCl4/electron donor) system could be identified.

Hydrophobic N,N-Diarylammonium Pyrosulfates as Dehydrative Condensation Catalysts under Aqueous Conditions

Oil-soluble N,N-diarylammonium pyrosulfates as nonsurfactant-type catalysts for the dehydrative ester condensation under aqueous conditions are described. Preheat treatment of dibasic sulfuric acid with bulky N,N-diarylamines generates water-tolerant salts of pyrosulfuric acid as active catalyst species. The present catalysts in water can also widely be applied to unusual selective esterifications and dehydrative glycosylation.