bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Wittig and Related Reactions in Natural Product Synthesis
Authors: Nicolaou K. C. , Härter Michael W. , Gunzner Janet L. , Nadin Alan 
Published: July 2007

AbstractOriginally reported in Liebigs Annalen in 1953 (then called Justus Liebigs Annalen der Chemie), the Wittig reaction has evolved to include the Horner–Wittig and Horner–Wadsworth–Emmons reactions and several other variations. Today, these reactions constitute some of the most powerful processes for the construction of carbon–carbon bond frameworks, facilitating the chemical synthesis of myriads of organic molecules both in research laboratories and industrial settings. The Wittig and related reactions were proven particularly useful in the field of total synthesis, where they enabled the construction of highly complex structures and had a significant impact in shaping the art to its present state of sophistication. In this article the authors focus, after a brief introduction, on total syntheses from their own laboratories that employ such processes. These examples, together with the numerous others adorning the chemical literature, illustrate amply the importance of the Wittig and related reactions and point to their continuing prominence in organic synthesis.
1‐Methylgeraniol as Novel Regio‐ and Diastereoselective Probe for Hydrogen Bonding in Hydroxy‐Assisted Oxygen‐Transfer Reactions
Authors: Adam Waldemar , Mitchell Catherine M. , Paredes Rodrigo , Smerz Alexander K. , Veloza L. Angela 
Published: July 2007

AbstractThe chiral 1‐methylgeraniol has been designed and applied for epoxidation reactions as a mechanistic probe, which allows to assess simultaneously in one and the same molecule regio‐ and stereochemical information. Of the two trisubstituted double bonds, the terminal 7,8 one reflects kinetic preferences on account of its greater π nucleophilicity, the 3,4 one with the chiral allylic hydroxy functionality expresses diastereoselective directivity through hydrogen bonding. The combination of regio‐ and stereoselectivity provides a novel and powerful mechanistic tool to differentiate electronic and steric effects of oxidants. Thus, various epoxidizing agents have been employed and the mechanistic implications derived from the product ratios are well in line with those reported, which exemplifies the validity of the probe.
Synthesis of rac‐Midpacamide and the spiro‐Cyclization of Its Precursor
Authors: Lindel Thomas , Hoffmann Holger 
Published: July 2007

AbstractThe alkylidene hydantoin 8 was synthesized as an analogue of the marine natural product oroidin (1) by employing a Horner‐Wadsworth‐Emmons reaction. Treatment of 8 with bromine in acetic acid induced a biomimetic spiro‐cyclization forming the ACD ring system of dibromophakellin (2) in one step. rac‐Midpacamide (10) was obtained from the same precursor through a chemoselective, ruthenium‐catalyzed hydrogenation leaving the brominated pyrrole moiety intact.
trans‐ and cis‐Bicyclo[n.1.0]alk‐2‐ynes of Medium Ring Size
Authors: Detert Heiner , Meier Herbert 
Published: July 2007

AbstractStarting from cycloalkenes, the cis‐ or trans‐cyclopropano anellated cycloalkynes 2–7 of medium ring size were synthesized. The triple bonds were introduced in the final reaction step either by the fragmentation of the corresponding 1,2,3‐selenadiazoles or by direct dehydration of a ketone (Scheme 2). The cis‐fused bicycles 2 and 3 and the trans‐fused bicycles 5, 6 and 7 could be isolated in a pure state. The intermediary existence of the most strained system trans‐bicyclo[7.1.0]dec‐2‐yne (4) was proven by trapping reactions.
Unsaturated Hetero Chains, VI. ω‐alkoxy‐ and ω‐Amino‐Substituted Oligonitriles – Synthesis, Structures, and Reactivity
Authors: Aust Nicola C. , Fröhlich Roland , Hecht Jürgen , Würthwein Ernst‐Ulrich 
Published: July 2007

Abstractω‐Alkoxy‐substituted 1‐oxa‐3,5‐diazahexatrienes (5), 1‐oxa‐3,5,7‐triazaoctatetraenes (7), and 1‐oxa‐3,5,7,9‐tetraazadecapentaenes (9) were synthesized by condensation of 1,3‐dialkoxy‐2‐azapropenylium salts (3) or 1,5‐dialkoxy‐2,4‐diazapentadienylium salts (8) withN‐acyl‐ andN‐alkoxycarbonylamidines (4). It is the nature of the leaving group which mainly determines the chain length of the products obtained. – Compounds7show ring‐chain tautomerism involving the cyclic form6in dependence of the substitution pattern. All ω‐alkoxy substituted oligontriles5, 7and9may easily converted to the corresponding ω‐amino compounds11, 14, and15by reaction with secondary amines10. Reaction of5a, bwith the primary amine12leads to the triazinone13. The lithiated ketimine16converts5band7dto the corresponding 1‐oxa‐3,5,7‐triazaoctatetraenes17and 1‐oxa‐3,5,7,9‐tetraazadecapentaenes18. Helical geometries dominate the structures of the new oligonitriles in the crystalline state as derived from X‐ray analyses of compounds5a, 7c, 9a, and18. Push‐pull substitution effects, which should be strong in the amino derivatives11, 14, and15, have no significant influence on the structure of the oligonitriles.
A Butyrolactone → 1,3‐diol Strategy for the Obtention of Tolypothrix Polyethers – Total Synthesis of the Tolypothrix Pentaether from Enantiomerically Enriched S‐Glycidol
Authors: Priepke Henning , Brückner Reinhard 
Published: July 2007

AbstractPermethyl ether 1 from Tolypothrix conglutinata was synthesized following the retrosynthetic analysis of Scheme 1. According to it, we combined aldehyde 3 [prepared from S‐glycidol (7); cf. Schemes 2, 3] and lactone 4 (also prepared from S‐glycidol (7); cf. the preceding communication[2] and Scheme 4) to the enantiopure and diastereopure trimethoxy lactone 2 (Scheme 5). The CO group of this lactone was removed through the Criegee rearrangement of a derived lactol peroxonosylate (Scheme 6); thereby, the CCO bond of lactone 2 was replaced with complete retention of configuration by one COH bond of the resulting syn‐configured diol 18. Protecting group manipulations (Schemes 6, 7) and oxidation of the resulting alcohol 21 to aldehyde 31 (Scheme 10) initiated the three final operations: C2‐homologation with Ph3PCH‐CHO (→ enal 32), tosyl hydrazone formation (→ 34), and reduction/concomitant CC bond migration. They led to the target ether 1 as a 89:11 mixture with the CHCH‐richer ether 36.
An Unorthodox Pathway to the 1,2‐Dithiin System
Authors: Huisgen Rolf , Kalwinsh Ivars , Morán Joaquín R. , Nöth Heinrich , Rapp Jochen 
Published: July 2007

AbstractTetracyanoethylene combines with two molecules of thiobenzophenone in refluxing benzene to give the tetrasubstituted 1,2‐dithiin 8 (21–29%) besides the corresponding thiophene derivative 9 (40–52%). The X‐ray analysis of the ruby‐red 8 reveals a half‐chair conformation with a torsion angle of 58.9° at the disulfide bond. The thermal desulfurization 8 → 9 (benzonitrile, 100°C) proceeds with t1/2 = 26.7 h, whereas the sulfur loss induced by triethyl phosphite is a billion times faster. The mechanisms of the formation of 8 and its sulfur extrusion are discussed in the light or recent literature.
1,3‐Dipolar Cycloadditions, 99. Nitrones and Thiones: Cycloadditions and Surprising Metathesis Reactions
Authors: Giera Henry , Huisgen Rolf 
Published: July 2007

AbstractNitrone 4 combines with alicyclic thioketones at room temp. affording 1,4,2‐oxathiazolidines. Steric hindrance in both reactants affects the cycloaddition/cycloreversion equilibria, but the mobility is still high; the low π‐LUMO energies of CS are responsible for the superdipolarophilic character. Nitrone 4 and thiobenzophenone enter into a metathesis reaction which was traced back to a sulfhydrogenolysis pathway; H2S acts as a catalyst. An analogous metathesis between nitrone 4 and benzaldehyde involves a hydrolysis.
Antibiotics from Gliding Bacteria, LXXX. Chivosazoles A–F: Novel Antifungal and Cytotoxic Macrolides from Sorangium cellulosum (myxobacteria)
Authors: Jansen Rolf , Irschik Herbert , Reichenbach Hans , Höfle Gerhard 
Published: July 2007

AbstractThe myxobacterial metabolites chivosazole A (1) and its variants (2–6) were discovered in Sorangium cellulosum, strain So ce12, which simultaneously provides the broad spectrum antibiotic sorangicin as well as the sorangiolides and disorazoles. The antifungal and cytotoxic chivosazoles (1–6) are novel glycosides of 6‐deoxyglucopyranose derivatives and an aglycon that includes an oxazole in its 31‐membered macrolide ring. The aglycon itself, chivosazole F (7), was formed by strain So ce885 and showed similar activity antibiotic and cytotoxic.The biogenetic origin of the structural elements in chivosazole F (7) was studied by feeding experiments with [1‐13C]‐, [1,2‐13C]acetate, [methyl‐13C]methionine and [1‐13C]serine. Accordingly, the aglycon 7 is a polyketide assembled by condensation of nine acetate units, one serine unit and a further seven acetate units. While C‐1 of serine is part of the macrolide ring, the amido to hydroxyl part of the serine together with C‐1 of the adjacent N‐acetyl unit form the 1,3‐oxazole ring. C‐ and O‐methyl groups are derived from methionine.
An Attempted Synthesis of 2,6‐Bis(phenylazo)barbaralane
Authors: Quast Helmut , Becker Christian , Peters Eva‐Maria , Peters Karl , von Schnering Hans Georg 
Published: July 2007

AbstractTreatment of the unsaturated ketone 6 with phenylhydrazine fails to afford the unsaturated phenylhydrazone 3 but gives the tricyclic phenylhydrazoketone exo‐8, which is oxidised with iron(III) chloride to yield the phenylazoketone exo‐9. The configuration of exo‐9 is elucidated with the help of an X‐ray diffraction analysis. The unsaturated phenylazo compound 14 is obtained from Meerwein's diketone 10 in two ways. Iodination of the phenylhydrazone 11 in the presence of pyridine followed by elimination of pyridinium iodide from the bispyridinium salt 13 with aqueous sodium hydroxide in dimethyl sulphoxide furnishes 14 in low yield. A somewhat better yield is more conveniently achieved when the dibromodiketones exo,exo‐ and exo,endo‐12 are allowed to react with phenylhydrazine in the presence of pyridine. Bromination of 14 with N‐bromosuccinimide in cyclohexane as solvent affords the labile dibromide 15. Both reagents employed, viz. the zinc/copper couple in refluxing tetrahydrofuran and butyllithium at low temperature, fail to convert 15 into the hoped‐for bis(phenylazo)barbaralane 1 but instead give rise to the formation of the isomer 17, either by cyclisation of the intermediate anion 16 or by rearrangement of 1.
Cycloallenes, 11. 1‐Phenyl‐1,2‐cyclononadiene: Preparation and Dimerisation
Authors: Christl Manfred , Moigno Damien , Peters Eva‐Maria , Peters Karl , Von Schnering Hans Georg 
Published: July 2007

Abstract1‐Phenyl‐1,2‐cyclononadiene (10) was prepared by addition of dibromocarbene to 1‐phenylcyclooctene and treatment of the resulting 9,9‐dibromobicyclo[6.1.0]nonane 7 with methyllithium. In addition to 10, the 9‐bromo‐9‐methylbicyclononanes cis‐ and trans‐8 as well as the bicyclopropylidene 9 were formed. The dimerisation of 10 proceeded slowly already at room temperature and gave rise to the 1,2‐bismethylenecyclobutane derivatives cis‐ and trans‐11. From the complex mixture that resulted on thermolysis at 200°C of cis‐ and trans‐11 only their isomer 12 could be isolated. Heating of a benzene solution of tetracyanoethylene and cis‐ and trans‐11 afforded the Diels‐Alder adduct 13 of cis‐11, which rearranged to the benzodicyclononene derivative 14 on thermolysis.
New Biphenylenophanes and Biphenylophanes – 1,8‐Dimethylbiphenylene by Continuous Vacuum Pyrolysis
Authors: Laufenberg Susanne , Feuerbacher Nina , Pischel Ivo , Börsch Oliver , Nieger Martin , Vögtle Fritz 
Published: July 2007

AbstractThe synthesis of 1,8‐disubstituted biphenylenes by pyrolysis of azo compounds has been optimized. By means of a continuously working pyrolysis apparatus, the cis isomer 2 can be reliably produced on a gram scale. The chiral syn‐[2.2]biphenylenophane 12 has been synthesized by an intermolecular cyclization reaction, subsequent oxidation of the sulfur atoms and pyrolytic extrusion of sulfur dioxide. In an analogous manner, the [2.2]biphenylophane 13 was obtained. The separation of the racemates of the hydrocarbons 12 and 13 could be achieved using a stationary chiral HPLC phase. Compound 12 preferentially adopts a syn conformation in the solid state. The CD spectra of 12 and 13 are compared.
Photochemistry of 11,12‐bis(Aminomethyl)‐9,10‐dihydro‐9,10‐ethenoanthracene in Solution and in the Solid State
Authors: Scheffer John R. , Ihmels Heiko 
Published: July 2007

AbstractThe photochemistry of 11,12‐bis(aminomethyl)‐9,10‐dihydro‐9,10‐ethenoanthracene (4) was investigated in solution and in the solid state. Direct irradiation of 4 gave the singlet state photoproduct 5, whereas the triplet derived photoproduct could only be obtained by irradiating the bis(hydrochloride) salt of 4 in the presence of xanthen‐9‐one to give semibullvalene 6‐(HCl)2. Solid state photolysis of 4 resulted in the formation of the pyrrole derivative 7, which is likely due to steric interactions in the solid state.
A Convenient Synthesis of γ,δ‐Unsaturated N‐Formylenamines by Wittig Alkenylation of γ,δ‐Unsaturated Diformamides
Authors: van Vliet Michiel C. A. , Meuzelaar Gerrit J. , Bras Jérǒme , Maat Leendert , Sheldon Roger A. 
Published: July 2007

Abstractγ,δ‐Unsaturated N‐formylenamines (5–7) were prepared in two steps from readily available γ,δ‐unsaturated alcohols. The alcohols were converted into the γ,δ‐unsaturated diformamides (N,N‐diformylamines, 1–4) by tosylation, followed by reaction with sodium diformamide. The γ,δ‐unsaturated diformamides were applied in a Wittig reaction under mild conditions to give the title compounds with a conjugated enamide group in moderate to high yields.
Special Calixarenes by Directed Syntheses
Authors: Böhmer Volker 
Published: July 2007

AbstractThe synthesis of special calixarenes and calixarene‐like macrocyclic compounds via stepwise procedures and fragment condensation is reviewed. Among the compounds available are exo‐ and endo‐calixarenes consisting of different phenolic units, having bridges other than methylene or showing inherent chirality (Cn‐symmetry). Macrobi‐ and ‐tricyclic molecules such as bridged calixarenes, double calixarenes, bicyclocalixarenes and annelated calixarenes are also described. Single crystal X‐ray structures are reported for several compounds. Some properties like conformational barriers, pKa values or the ability to complex metal ions are discussed, showing the potential of these compounds which may (only) be produced by directed synthesis.
Bimolecular Formation of Radicals by Hydrogen Transfer, 13. Uncatalyzed Transfer Hydrogenation of Azulene and the Initiation of the Azulene‐Naphthalene Rearrangement
Authors: Keller Friedrich , Beckhaus Hans‐Dieter , Rüchardt Christoph 
Published: July 2007

AbstractFrom the uncatalyzed transfer‐hydrogenation of azulene 1 with the hydrogen donors 9,10‐dihydroanthracene (4), 7H‐benz[de]anthracene (5) and 9,9‐diphenyl‐10,10‐dideuteroanthracene (6) at 340–375°C in diphenyl ether, two isomeric octahydroazulenes and, in addition, naphthalene and tetralin are obtained. From product studies, kinetics, isotopic labelling experiments and semiempirical MO calculations, a nonchain stepwise radical mechanism is proposed, which is initiated by H‐atom transfer from the donor to azulene. The proposed mechanism for the formation of naphthalene and tetralin via azulene‐naphthalene rearrangement under these conditions is a combination of the Scott mechanism and the Alder “walk” mechanism, which are initiated by the hydrogen‐transfer step.
Electrosynthesis of α‐Aminonitriles: Anodic Cyanation of N‐Substituted Tetrahydroquinolines and N‐Phenylpiperidines
Authors: Le Gall Erwan , Hurvois Jean‐Pierre , Renaud Tristan , Moinet Claude , Tallec André , Uriac Philippe , Sinbandhit Sourisak , Toupet Loic 
Published: July 2007

AbstractThe electrochemical cyanation of N‐alkyl‐substituted cyclic six‐membered amines including N‐alkyl‐1,2,3,4‐tetrahydroquinoline and N‐phenylpiperidine derivatives has been investigated. The reaction gives the corresponding α‐aminonitriles and was carried out at a graphite felt anode percolated with a methanolic solution of the amine, containing six equivalents of sodium cyanide per mol of substrate, and lithium acetate as the supporting electrolyte. In the case of N‐substituted tetrahydroquinolines 1a–f, cyanation occurs regioselectively at the ring and the position of substitution depends on the nature of the N‐alkyl substituent. The stereoselectivity of the cyanide addition was investigated using various 3‐methyltetrahydroquinolines 8a–c as substrates. In all cases, the major stereoisomers 9a–c possess a cis configuration in which the 2‐cyano group has an axial disposition and the 3‐methyl group is equatorial. In a similar fashion, various N‐phenylpiperidines substituted at the ring by either one or two methyl groups were oxidized. In all the selected amines the introduction of the cyanide group occurs stereospecifically in the position α to the nitrogen atom. The cis or trans relative configuration of the cyanated compounds is discussed separately for each substrate. It is noteworthy that in all cases the cyano group was in an axial position.
Product Analysis in the Iodine(III)‐Promoted Oxidation of Carbohydrate‐Derived Cyclic Enol Ethers: A Mechanistic Study
Authors: Harders Jan , Garming Alfons , Jung Alexander , Kaiser Volker , Monenschein Holger , Ries Monika , Rose Lars , Schöning Kai‐Uwe , Weber Thomas , Kirschning Andreas 
Published: July 2007

AbstractA study on the mechanism of the well‐documented hypervalent iodine‐mediated allylic oxidation of glycals leading to 2,3‐dihydro‐4H‐pyran‐4‐ones is presented. Notable features are the isolation of ring‐contracted by‐products 6 and 7, which are produced upon oxidation of per‐O‐benzylated glycal 4, as well as the characterization of carbohydrate‐derived tetrahydrofurfurals 12a and 13a, which are formed by the conformation‐dependent oxidation of glycals 9a and 10b. In addition, the iodine(III)‐mediated oxidation process has been studied by in situ NMR spectroscopy of lyxo‐configured glycals 14a,b. Intermediate alkylphenyliodonium species 19b,d and 2‐enoyranosides 16a and 20a have been characterized by their NMR signals. These data support a plausible mechanism that is initiated by electrophilic attack of the iodine(III) reagent on the electron‐rich enol ether double bond of the glycal. This is followed by the breaking of a bond β,γ‐positioned in relation to the carbohydrate‐bound iodine and subsequent reductive elimination of iodobenzene. Thus, depending on the glycals employed, a number of diverse oxidation products may be formed.
A Practical Synthesis of 3‐Methoxyflavones
Authors: Deng Bo‐Liang , Lepoivre Jozef Arsène , Lemière Guy , Dommisse Roger , Claeys Magda , Boers Frank , De Groot Alex 
Published: July 2007

AbstractSome 3‐methoxyflavones were known to possess antipicornavirus properties. In order to study the structure‐activity relationship, attempts to prepare 3‐methoxyflavone by literature methods were unsuccessful. We have modified the Baker‐Venkataraman rearrangement, using a solid base and a phase‐transfer catalyst. After searching optimal reaction conditions and a study of the intermediate formed in a model reaction, good yields were obtained for the 3‐methoxyflavones 7, 21–32. Further some of the corresponding 7‐O‐n‐butyl derivatives 4, 33–37 could be isolated as side products.
Vinylogous β‐Carotenes: Generation, Storage, and Delocalization of Charge in Carotenoids
Authors: Broszeit Guido , Diepenbrock Felix , Gräf Oliver , Hecht Dirk , Heinze Jürgen , Martin Hans‐Dieter , Mayer Bernhard , Schaper Klaus , Smie Andreas , Strehblow Hans‐Hennig 
Published: July 2007

AbstractIn this paper, we report on the synthesis of β‐carotenes of variable chain length with between 5 and 23 double bonds (1–7). These oligoenes were prepared by McMurry and Wittig reactions. The tetradecapreno‐β‐carotene 7 would seem to be the longest β‐carotene yet reported. Furthermore, we investigated the electronic properties using cyclic voltammetry and photoelectron spectroscopy (UPS) to generate open‐shell and closed‐shell ions of carotenoids in solution and in the solid state, respectively. With increasing chain length (⩾ 11 double bonds), even the generation of tetracations and tetraanions could be observed by cyclic voltammetry. Extending the number of conjugated bonds causes the potentials to converge to limiting values. All electron‐transfer processes occur in one‐electron steps, which are close to each other in pairs. The potential difference between the first oxidation potential and the first reduction potential is a linear function of the reciprocal chain length. Despite the different techniques used (CV and UPS) and the different condensed phases, there is an excellent correspondence between the energies of the radical cation states generated by the two methods. This shows that the radical cation formation is principally determined by the chain length. The structures of the ion states were investigated using semiempirical methods at the NDDO level. Charge delocalization and bond relaxation are not identical and do not utilize the same number or the same kind of atoms. It can be shown that from the length of 20 double bonds onwards, the effective conjugation length for doubly‐charged cations converges slowly to a limiting value.
Mukaiyama Reactions of a Silyl Enol Ether with Heteroatom‐Substituted β‐Formyl Esters
Authors: Angert Hubert , Czerwonka Regina , Reißig Hans‐Ulrich 
Published: July 2007

AbstractHeteroatom‐substituted aldehydes 3, 4, and 5 were subjected to Mukaiyama reactions with silyl enol ether 10. The reaction of the new trifluoromethyl‐substituted aldehyde 3 with 10 afforded γ‐lactone 11 with excellent trans selectivity. In contrast, the known benzyloxy‐substituted aldehyde 4 and 10 provided γ‐lactone 13 without any selectively in the presence of BF3 as the promoting Lewis acid; use of TiCl4 induced exclusive cis selectivity. Carbamoyl‐substituted aldehyde 5 and silyl enol ether 10 were combined under different conditions, although good diastereoselectivity could not be achieved. The results obtained are discussed in terms of the electronic effects of the substituents and their ability to form chelates.
Electrochemical Oxidation of 4‐Ethenyl[22]paracyclophane: Cyclic Voltammetry, Electrosynthesis of Polymers, Analysis by Liquid Chromatography, and Mass Spectrometry
Authors: Gollas Bernhard , Hesse Ingrid , Lotz Ralf , Pasch Harald , Speiser Bernd , Zagos Ioannis 
Published: July 2007

Abstract4‐Ethenyl[22]paracyclophane is oxidized electrochemically at a platinum electrode in acetonitrile/0.1 M tetra‐n‐butylammonium hexafluorophosphate. Its behavior in electro‐oxidation is compared to that of styrene. Cyclic voltammetry proves diffusion controlled, direct one‐electron oxidation of the hydrocarbon. The product of preparative scale electrolysis is analyzed by elemental analysis and various spectroscopic techniques. Bond formation occurs at the vinyl group, resulting in analogues of polystyrene. Polymers are separated on a reversed phase chromatography column, and online coupling to electrospray ionization mass spectrometry as well as a combination of size exclusion chromatography and MALDI‐MS prove the formation of species up to the undecamer. The determination of the molecular product masses allows the assignment of the polymer end groups. A mechanistic hypothesis for the radical cation induced electropolymerization of the monomer is developed from the electro‐chemical and structural results.