bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Germanium–Carbon Bond Cleavage Reaction of (Germylmethyl)amine Derivatives. Mechanism and Stereochemistry
Authors: Terunuma Daiyo , Masuo Koujiro , Kizaki Hiroaki , Nohira Hiroyuki 
Published: July 2006

Abstract
A facile cleavage reaction of the germanium–carbon bond of (germylmethyl)amine derivatives to give a germyl anion and an azomethine derivative was developed. The effects of substituents such as phenyl, benzyl, and ethyl groups on the germanium and nitrogen atoms of the (germylmethyl)amine derivatives on the cleavage reaction, were investigated. The mechanism was also investigated based on the stereochemistry of the cleavage reaction using an optically active organogermanium compound.
Preparation and NMR Study of 7,7′-(α,ω-Alkanediyl)bis[theophylline], 1,1′-(α,ω-Alkanediyl)bis[theobromine], and 1,1′-(α,ω-Alkanediyl)bis[3-methyluracil]
Authors: Itahara Toshio , Imamura Kazuhiko 
Published: July 2006

Abstract
The treatment of theophylline, theobromine, and 3-methyluracil with X(CH2)nX (X = Br or I, n = 1—12) in N,N-dimethylformamide containing sodium hydride gave the corresponding 7,7′-(α,ω-alkanediyl)bis[theophylline], 1,1′-(α,ω-alkanediyl)bis[theobromine], and 1,1′-(α,ω-alkanediyl)bis[3-methyluracil]. The interaction of the theophylline, theobromine, and 3-methyluracil rings of these compounds was studied based on their 1H NMR spectra, and stacking of the two purine rings of 7,7′-(α,ω-alkanediyl)bis[theophylline] was observed.
Footprint Catalysis. X. Surface Modification of Molecular Footprint Catalysts and Its Effects on Their Molecular Recognition and Catalysis
Authors: Shimada Toyoshi , Hirose Rie , Morihara Kensaku 
Published: July 2006

Abstract
Trimethylsilylation with hexamethyldisilazane was applied to a surface modification of silica(alumina) gel catalysts, on which surface the “molecular footprint” catalytic cavities had been marked by the authors’ imprinting method. The modification provided for the more stable catalysts than the unmodified catalyst, retaining their molecular-recognition capabilities nearly intact.
Fluorination Reactions at C-5 of 3-O-Benzyl-6-deoxy-1,2-O-isopropylidenehexofuranoses
Authors: Mori Yoko , Morishima Naohiko 
Published: July 2006

Abstract
Fluorination reactions of 2-O-benzyl-6-deoxy-1,2-O-isopropylidene-α-d-gluco-, β-l-ido-, α-d-allo-, and β-l-talo-furanoses (1, 2, 3, and 4) were investigated. The reaction of 1 with diethylaminosulfur trifluoride (DAST) predominantly produced 2-O-benzyl-5,6-dideoxy-5-fluoro-1,2-O-isopropylidene-α-d-glucofuranose with the retention of the configuration at C-5. Both of the fluorides with retained and inverted configurations were obtained in the fluorination of 2 with DAST. In contrast, only the inversion of the configuration occurred when 3 and 4 were reacted with DAST. The reactions of the methanesulfonates of 3 and 4 with tetrabutylammonium fluoride (TBAF) and those of their trifluoromethanesulfonates with tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF) gave the 5-deoxy-5-fluoro derivatives with the inversion of each configuration and the 5,6-unsaturated derivative. However, the reactions of the sulfonates of 1 and 2 overwhelmingly produced the 4,5- and 5,6-eliminated derivatives. For the reaction in which the steric hindrance of the benzyloxyl group at C-3 and the electronic repulsion of ring oxygen to the SN2 displacement with fluoride anion are significant, such as the fluorination of 1 with DAST, the SNi mechansim is reasonable.
Reaction Pathway and Transition State of the Zn-Promoted Barbier-Type Reactions of Benzaldehyde and Benzophenone with Allylic Iodides
Authors: Yamataka Hiroshi , Nishikawa Kazuyoshi , Hanafusa Terukiyo 
Published: July 2006

Abstract
The carbonyl-14C kinetic isotope effects and the substituent effects on the reactivity of the Zn-promoted Barbier-type reaction of allyl iodide (3-iodo-1-propene) with benzaldehyde and benzophenone were determined in THF at 25 °C. The observed normal carbon isotope effects as well as positive Hammett ρ values suggest that the reactions go through a direct nucleophilic addition mechanism. No indication of the occurrence of electron transfer was obtained by enone isomerization and dehalogenation probe experiments. Diastereoselectivity in the reaction of crotyl iodide (1-iodo-2-butene) with benzaldehyde was low and independent of the substituent on aldehyde, consistent with the six-membered cyclic transition state.
An NMR Study on Ion-Pair Structures of Pyridine–Butyllithium Adducts
Authors: Nakamura Norimasa , Yoshino Akihiro , Takahashi Kensuke 
Published: July 2006

Abstract
The 13C and 7Li NMR spectra of the title adducts were observed in THF. In a temperature range from 20 to −80 °C the adducts existed as contact ion pairs in THF. Below −70 °C, the adduct derived from pyridine or 4-phenylpyridine showed three 7Li signals related to three different associated species in solutions. On the other hand, the adduct derived from 2- or 3-phenylpyridine did not give such multiplet signals at −70 °C. The results are related to the interaction between nitrogen and lithium, and the extent of negative-charge migration into the substituent (phenyl ring) in the adduct.
Specific Transport of Copper(II) Ions Across Liquid Membrane by Schiff Base Ligands
Authors: Ameerunisha Sardar , Srinivas Bhamidi , Zacharias Panthappally S 
Published: July 2006

Abstract
Schiff base ligands derived from the condensation of 2-hydroxy-5-methylbenzene-1,3-dicarbaldehyde with substituted anilines are found to be excellent carriers for the specific transport of copper(II) ions through a liquid membrane.
Dissociation Kinetics of 1,5,9-Triazacyclododecane-1,5,9-triacetate and 1,4,8,12-Tetraazacyclopentadecane-1,4,8,12-tetraacetate Complexes of Cerium(III)
Authors: Choi Ki-Young , Kim Ki Sung , Kim Ju Chang 
Published: July 2006

Abstract
The dissociation kinetics of DOTRA=H3dotra (1,5,9-triazacyclododecane-1,5,9-triacetic acid) and PETA = H4peta (1,4,8,12-tetraazacyclopentadecane-1,4,8,12-tetraacetic acid) complexes of Ce(III) ion have been studied in an aqueous solution of 0.1 M (NaClO4) at 25.0 ± 0.1 °C using Cu(II) ions as a scavenger. The dissociation rate constant for the acid-catalyzed pathway decreases in the order, [Ce(peta)]− &gt; [Ce(dotra)] &gt; [Ce(teta)]− &gt; [Ce(nota)] &gt; [Ce(dota)]− according to the present and literature data. The rate constant is significantly affected by internal cavity and chelate ring sizes.
A New α,α′-Diiodination of Ketones Using Iodine-Cerium(IV) Ammonium Nitrate
Authors: Horiuchi C Akira , Takahashi Eiji 
Published: July 2006

Abstract
The direct iodination of some ketones using iodine-cerium(IV) ammonium nitrate in acetic acid or acetonitrile gave the corresponding α,α′-diiodoketones in good yield. In the case of cyclododecanone (4), 3-pentanone (5), and 5-nonanone (6), cis- (meso-) compounds [2,12-diiodocyclododecanone (12), 2,4-diiodo-3-pentanone (13), and 4,6-diiodo-5-nonanone (14)] were obtained preferentially.
The Stability Constants for the 18-Crown-6 Complexes with [2.2]Metacyclophanediazonium Salts
Authors: Tsuge Akihiko , Moriguchi Tetsuji , Mataka Shuntaro , Tashiro Masashi 
Published: July 2006

Abstract
Transannular interaction between two aromatic rings in [2.2]metacyclophanes was estimated by the determination of the stability constants (Ks) for 18-crown-6 complexes with their diazonium salts. It was found out that the Ks is about 10 times as small as that for the corresponding benzenediazonium salt.
One-Step Preparation of α-Chloro-α,β-unsaturated Carbonyl Compounds by the Reaction of Silyl Enol Ethers with TiCl4–LiAlH4–CCl4
Authors: Mitani Michiharu , Kobayashi Yoshinori 
Published: July 2006

Abstract
Reaction of silyl enol ethers in a system composed of TiCl4, LiAlH4, and CCl4, which generates dichlorocarbene, produced α-chloro-α,β-unsaturated carbonyl compounds in an one-step process.
Electrochemical Reduction of Dichlorosilanes in the Presence of 2,3-Dimethylbutadiene
Authors: Kunai Atsutaka , Ueda Takafumi , Toyoda Eiji , Ishikawa Mitsuo 
Published: July 2006

Abstract
Electrolytic reduction of dichloromethylphenyl-, dichlorodiphenyl-, dichlorodi-o-tolyl-, and dichlorodi-p-tolylsilanes in the presence of 2,3-dimethylbutadiene afforded 1-methyl-1-phenyl-, 1,1-diphenyl-, 1,1-di-o-tolyl-, and 1,1-di-p-tolyl-3,4-dimethyl-1-silacyclopent-3-ene, respectively, while the similar reaction of chloromethyldiphenylsilane with 2,3-dimethylbutadiene afforded 1-(methyldiphenylsilyl)- and 1,4-bis(methyldiphenylsilyl)-2,3-dimethyl-2-butene. A reaction mechanism leading to these products has been discussed.
Deprotection of Acetals and Silyl Ethers Using Some π-Acceptors
Authors: Tanemura Kiyoshi , Suzuki Tsuneo , Horaguchi Takaaki 
Published: July 2006

Abstract
Hydrolysis of dodecanal dimethyl acetal and dodecyl silyl ethers in MeCN–H2O was examined using a catalytic amount of π-acceptors such as 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), tetracyanoethylene (TCNE), 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQF4), and chloranil (CA). Cleavage of dodecyl triethylsilyl ether with TCNQ and CA was caused by room light. By application of DDQ-catalyzed deprotection of acetals to hydrolysis of tetrahydropyranyl ethers, the corresponding alcohols were obtained in quantitative yields.
Specific Affinity Labeling of μ Opioid Receptors by S-Activated Enkephalin Analog Containing p-Nitrophenylalanine
Authors: Yasunaga Teruo , Shimohigashi Yasuyuki , Kodama Hiroaki , Miyazaki Masaya , Nagaishi Masaya , Ohno Motonori , Kondo Michio 
Published: July 2006

Abstract
p-Nitrophenylalanine (Phe(p-NO2)) was incorporated in place of Phe4 of [d-Ala2, Leu(CH2S-Npys)5]enkephalin. In the rat brain receptor binding assay, Npys-containing Phe(p-NO2)4- and Phe4-enkephalins exhibited almost unchanged affinities for μ receptors (IC50 = 16—21 nM). However, when the membrane was incubated with each analog, Phe(p-NO2)4-enkephalin (EC50 = 6.90 nM) could label the μ receptors about three times more effectively than Phe4-enkephalin.
Molecular and Crystal Structures of Complexes of Stable Free Radical BDPA with Benzene and Acetone
Authors: Azuma Nagao , Ozawa Takehiro , Yamauchi Jun 
Published: July 2006

Abstract
The crystal structures of two complexes of the 9-(9H-fluoren-9-ylidenephenylmethyl)-9H-fluoren-9-yl stable radical with benzene (BDPA–Bz) and acetone (BDPA–Ac) have been determined by X-ray diffraction at room temperature. They are an isostructure of a monoclinic system with a space group of P21/n. For BDPA–Bz, a = 9.502(2), b = 14.632(2), c = 19.452(2) Å, β = 93.64(1)°, V = 2699(1) Å3, Z = 4, Dx = 1.220 g cm−3. For BDPA-Ac, a = 9.657 (1), b = 14.675(2), c = 18.812(2) Å, β = 95.388(9)°, V = 2654.4(6) Å3, Z = 4, Dx = 1.190 g cm−3. The final R values are 0.043 and 0.062 for 5.47 and 3.93 ratio of the observed reflections versus the parameters for BDPA–Bz and BDPA–Ac, respectively. The BDPA molecules in both crystals have the same propeller shape with an approximated C2 symmetry. The molecular packing of these crystals does not show a one-dimensional stacking, in spite of the magnetic one-dimensionality elucidated by the magnetic susceptibilites. The exchange pathways are discussed based on the McConnell’s spin-density Hamiltonian.
Plasma Reduction of CO2 with H2O for the Formation of Organic Compounds
Authors: Ihara Tatsuhiko , Kiboku Mitsuo , Iriyama Yu 
Published: July 2006

Abstract
The reduction of CO2 with H2O was carried out by means of a microwave plasma reaction in order to obtain organic compounds. The main products obtained in a cold trap were determined to be oxalic acid and H2O2 by an analysis of reversed-phase chromatography using UV and conductivity detectors.
13C NMR and ESR Studies on a Polydiacetylene Having Acetylenic Carbons in the Side Chains
Authors: Hayamizu Kikuko , Okada Shuji , Tsuzuki Seiji , Matsuda Hiro , Masaki Atsushi , Nakanishi Hachiro 
Published: July 2006

Abstract
The 13C NMR spin-lattice relaxation times and ESR spectra were measured for a polydiacetylene obtained by the solid-state polymerization of a tetrayne with n-C14H29 alkyl substituents (14-4A-14). Poly(14-4A-14) has two different side chains. One side chain contains two conjugated acetylenic bonds directly bound to the main backbone conjugated system; the other is an alkyl chain. Although the unpaired electron density of poly(14-4A-14) measured by ESR is less than that in trans-polyacetylene, paramagnetic effects may contribute to the long 13C relaxation times of unsaturated carbons, which provides insight concerning the π-electron structure on the conjugated system.
Thermotropic Liquid Crystals Based on Oligosaccharides. n-Alkyl 1-O-β-d-Cellobiosides
Authors: Ma Yung-Dae , Takada Akihiko , Sugiura Makoto , Fukuda Takeshi , Miyamoto Takeaki , Watanabe Junji 
Published: July 2006

Abstract
n-Alkyl 1-O-β-d-cellobiosides with an alkyl length ranging from 7 to 18 in carbon number m were synthesized, and their thermal properties were studied in comparison with those of the already-known monosaccharide homologues, n-alkyl 1-O-β-d-glucopyranosides. The cellobiosides were found to exhibit a thermotropic mesophase at higher temperatures than the glucopyranosides. Depending on m and thermal history, the cellobiosides showed various textures such as homeotropic, transient bâtonnets, polygonal, and fan-shaped textures. Like in the case of the monosaccharide homologues, the difficulty of aligning the samples prohibited a full classification of the mesophase, but the X-ray diffractograms of the unaligned cellobiosides indicated that their mesophase belongs to smectic A(or less possibly, smectic C). The m dependence of the lamella thickness was much weaker than (about half) that of the monosaccharide homologues. Possible molecular arrangements of the cellobiosides in the mesophase were presented.
Apparent Molar Volumes and Apparent Molar Adiabatic Compressions of Water in Some Alcohols
Authors: Sakurai Masao , Nakamura Kunio , Takenaka Nobuo 
Published: July 2006

Abstract
The ultrasonic velocities in dilute alcoholic solutions of water have been measured in the temperature range from 5 to 45 °C. The solvent alcohols used were methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol, 2-methyl-2-propanol, 1-pentanol, and 2-methyl-2-butanol. In addition, the densities of 2-butanol and 2-methyl-1-propanol solutions of water were also measured. The apparent molar volumes and apparent molar adiabatic compressions of water in these alcohols have been evaluated. The limiting partial molar quantities are less than the molar quantities of pure water; this phenomenon is generally more pronounced for lower-chain or branched-chain alcohols. The results are discussed in connection with the hydrogen-bonding interactions, that is, the self-association of the solvent alcohols and the alcohol–water interactions.
Equilibria between Contact and Solvent-Separated Ion Pairs of 1,1-Diphenylethylene Dimer Dianion and Tetraphenylethylene Dianion
Authors: Matsui Hideki , Yoshino Akihiro , Takahashi Kensuke 
Published: July 2006

Abstract
Temperature-dependent 1H NMR chemical shifts of the two title carbanions have been measured in three solvents and with three counter ions. The equilibria between two types of ion pairs of the carbanions have been discussed in terms of solvent, counter ion, and temperature.
Ionization Potentials of Iodide, Bromide, and Thiocyanate Anions in Solvents: Solvent–Solvent Interactions in the Solvation Sphere
Authors: Watanabe Iwao , Tanida Hajime , Maya Keiichi , Ikeda Shigero , Yokoyama Yu 
Published: July 2006

Abstract
The solution-phase ionization potentials of I−, Br−, and SCN− ions in 16 solvents including water have been measured by a photoelectron emission spectroscopic technique. The values obtained as the threshold energy Et for halides are found to be correlated well with the Mayer–Gutmann acceptor number and better with the Riddle–Fowkes modified acceptor number of the solvent indicating that the electronic states of the halides are predominantly stabilized through the Lewis acid–base interaction. The reorganization energies ΔGr of solvent after the photoionization of the halides are obtained from the Et values. The experimental ΔGr values are reproduced by using a simple model which incorporates the dipole–dipole repulsion and the bond formation energies in the first solvation layer. The solvation mechanism of SCN− is somewhat different from those of the halides because its Et depends less on the acceptor number and its ΔGr values in alcohols are about half of those for the halides.
Kinetics and Mechanisms of Photoinduced Electron-Transfer Reaction of Zinc Myoglobin. Intracomplex vs. Intermolecular Quenching Controlled by Conformational Change Associated with Charge and Steric Bulk of Quenchers
Authors: Tsukahara Keiichi , Asami Satoko , Okada Mihoko , Sakurai Takeshi 
Published: July 2006

Abstract
Photoinduced electron transfer (ET) between zinc myoglobin (ZnPPMb) and a variety of quenchers, such as hexacyanoferrate(III) ([Fe(CN)6]3−) and hexaammineruthenium(III) ([Ru(NH3)6]3+) ions, cationic viologens, copper(II) protein (stellacyanin), and metmyoglobins, has been studied in aqueous degassed solutions. The excited triplet state of ZnPPMb (*ZnPPMb) was quenched by [Fe(CN)6]3− in a self-associated complex. Both quenching rate constant and formation constant of the self-associated complex decrease with increasing ionic strengths. The thermal backward ET reaction for this system was not observed; it is most likely that the backward ET step is much faster than the quenching reaction. All of the cationic quenchers examined in this work did not form a self-associated complex with *ZnPPMb, and the intermolecular quenching occurred. The thermal backward ET reaction was observed for these cationic quenchers. Not only photoinduced ET but also thermal backward ET reactions were insensitive to the driving force of the reactions, suggesting that the reactions are controlled by conformational changes in ZnPPMb. The quenching rate constants increase with increasing ionic strength for the cationic quenchers. The effects of poly-l-lysine hydrochloride, sodium poly-l-glutamate, and sodium cyclo-hexaphosphate were also examined. The active site of the *ZnPPMb toward both anionic and cationic quenchers is assumed to be the positively charged site near the heme pocket.
Studies on Bitter Peptides from Casein Hydrolyzate. XIV. Bitter Taste of Synthetic Analogs of Octapeptide, Arg–Gly–Pro–Phe–Pro–Ile–Ile–Val, Corresponding to the C-Terminal Portion of β-Casein
Authors: Nakatani Masaru , Nakata Takashi , Kouge Katsushige , Okai Hideo 
Published: July 2006

Abstract
In order to elucidate the relationship between the chemical structure and bitter taste of the C-terminal portion of β-casein, some analogues, in which the phenylalanine residue was substituted by d-phenylalanine, lysine, glycine, glutamic acid or l-pyrenylalanine, were synthesized. Sensory analyses and CD spectra showed that the location of a hydrophobic amino acid with the l-configuration between the two proline residues should be important for this series of peptides to produce a strong bitterness.
Regioselective Allylation of Ketenes Promoted by SmI2
Authors: Miyoshi Norikazu , Takeuchi Seiji , Ohgo Yoshiaki 
Published: July 2006

Abstract
Ketenes react with various allylic halides mediated by 2 equiv samarium(II) diiodide (SmI2) to the ketenes to afford allylated ketones in good yields. In the reaction with γ-substituted allylic halides, the regioselectivity is influenced by the olefinic geometry of allylic halides. By using γ-substituted (E)-allylic halides, the allylation proceeds on the less hindered site (α-position) of allylic groups predominantly and the tendency was enhanced by the addition of HMPA.
Effects of the Cross-Linking Structure on the Water Sorption Ability of Poly(vinyl carboxylate)
Authors: Matsuguchi Masanobu , Sadaoka Yoshihiko , Shinmoto Masafumi , Sakai Yoshiro 
Published: July 2006

Abstract
Water sorption behavior of four kinds of polymerized vinyl carboxylate, i. e., vinyl benzoate, vinyl methacrylate, vinyl crotonate, and vinyl cinnamate was investigated. The dielectric measurement and quartz crystal microbalance technique were adopted for this purpose. The polymers were hydrophobic and the water sorption ability was enhanced by cross-linking. The dielectric constant of sorbed water was small compared with that of liquid water and the value slightly increased with increasing the amount of sorbed water. The orientation polarization of sorbed water also slightly increased with increasing the amount of sorbed water. It was proved that the cross-linking structure affected the water sorption behavior of the polymer.
pH-Responsive Guest Binding of Polypeptide Containing a Cyclodextrin at the Terminal
Authors: Nagata Yoshitomo , Aso Toshihiro , Kinoshita Takatoshi , Tsujita Yoshiharu , Yoshimizu Hiroaki , Minoura Norihiko 
Published: July 2006

Abstract
Poly(γ-methyl l-glutamate-co-l-glutamic acid) containing a β-cyclodextrin at the terminal (MG/GA-CyD) was prepared and the relationship investigated between the conformation of the polypeptide and inclusion capability of the β-cyclodextrin by circular dichroism and fluorescence spectroscopy in aqueous solution containing 8-anilino-1-naphthalenesulfonate (ANS) as a guest molecule. The terminal β-cyclodextrin of MG/GA-CyD could not include ANS at higher pH, where the polypeptide almost formed a random coil. This was attributed to the terminal β-cyclodextrin being concealed in the random coil of the polypeptide. At low pH, however, random coil-to-α-helix transition of the polypeptide exposed the terminal β-cyclodextrin to the aqueous phase resulting in the ANS-inclusion capability. These results imply that control of the polypeptide conformation leads to conformational switched guest binding of the terminal cyclodextrin.
Highly Twisted t-Butoxycarbonyl Groups in Hexa-t-butyl Benzenehexacarboxylate. The Structural Origin of Steric Hindrance-Induced Dual Fluorescence
Authors: Yasuda Masahide , Kuwamura Goro , Nakazono Takumi , Shima Kensuke , Inoue Yoshihisa , Yamasaki Noritsugu , Tai Akira 
Published: July 2006

Abstract
Comparative X-ray crystallographic analyses of tetra-t-butyl 1,2,4,5-benzenetetracarboxylate (1) and highly congested hexa-t-butyl benzenehexacarboxylate (2) have shown that t-butoxycarbonyl groups in 2 are much more twisted with respect to the benzene ring and bond-elongated in the peripheral positions than those in 1 or hexamethyl benzenehexacarboxylate. Such deformations and the staggered carbonyl orientation, all arising from the steric hindrance, may be responsible for the unusual dual-fluorescence behavior of the sterically congested hexaester 2.
Studies of the Selective O-Alkylation and Dealkylation of Flavonoids. XVI. Demethylation of 2′-Methoxyacetophenones with Anhydrous Aluminum Chloride or Bromide in Acetonitrile
Authors: Kawamura Yasuhiko , Takatsuki Hisanori , Torii Fumihito , Horie Tokunaru 
Published: July 2006

Abstract
Demethylation of 2′-methoxyacetophenones with anhydrous aluminum chloride in acetonitrile was studied to survey its scope and limitations. The mechanism which the reaction proceeds via sterically constrained intermediates was proposed from the substituent effects. Additionally, dealkylation of 2′-benzyloxy-, 2′-ethoxy-, and 2′-isopropoxyacetophenones with two demethylating reagents, hydrochloric acid in acetic acid and anhydrous aluminum bromide in acetonitrile, was studied. It was found that the reactivity was greatly affected by the steric factor between the alkoxyl group and reagent. This behavior may have wide application in selection of protecting groups in organic synthesis.