bluuebunny/crossref_metadata_2025_split huggingface dataset:
Derivatives of natural tetrapyrroles for photodynamic therapy, 3. Oxidative degradation studies on porphyrins with chromic acid
Authors: Gauler Rainer , Hesse Ulrike , Risch Nikolaus 
Published: July 2007

AbstractUntil now the composition of the complex porphyrin mixtures of hematoporphyrin derivative (HPD) and Photofrin II could not be clarified in detail. We investigated the applicability of oxidative degradation to the structural assignment of HPD constituents. Therefore, some fundamental work concerning the characterization of the relevant degradation products had to be done. In this article we present the spectral and analytical data of monomeric and dimeric maleimides potentially arising from the corresponding constituents of HPD.
Cycloaddition of Functionalized Nitrile Oxides and Fulminic Acid to [60]Fullerene
Authors: Irngartinger Hermann , Weber Anton , Escher Thomas 
Published: July 2007

AbstractCycloaddition of nitrile oxides to [60]fullerene led to [60]fullereno[1,2‐d]isoxazole derivatives 1b–d. [60]Fullereno[1,2‐d]isoxazole (1a) is the corresponding cycloadduct with fulminic acid. X‐ray structure analyses of compound 1b and 1e were determined.
Phenazin‐5(10H)‐yls, 4. 10‐Ethylphenazin‐5(10H)‐yl · H2O: Crystal Structure, Magnetic Susceptibility and Intermolecular π‐π Interactions (Pimerization)
Authors: Gleiter Marco R. , Krieger Claus , Neugebauer Franz A. , Kremer Reinhard K. 
Published: July 2007

AbstractThe unit cell of 10‐ethylphenazin‐5(10H)yl · H2O consists of eight phenazin‐5(10H)‐yls that are arranged in four independent radical pairsA–D. All pairs show close interplanar distances (3.28–3.36 Å). Furthermore,A, B, andDare characterized by short intermolecular contacts between atoms with significant spin populations. This is not valid for the pairCwith the closest interplanar distance of 3.28 Å. Magnetic susceptibility measurements as a function of temperature indicate complete spin pairing (“pimerization”) between 20 and 100 K. Therefore, the radical pairCprovides evidence that unspecific close interplanar contacts together with a reasonable overlap of the π systems are sufficient to lead to pimerization.
Ethynylation of Aryl Halides by a Modified Suzuki Reaction: Application to the Syntheses of Combretastatin A‐4, A‐5 and Lunularic Acid
Authors: Fürstner Alois , Nikolakis Katharina 
Published: July 2007

AbstractOn treatment with trimethyl borate sodium acetylide undergoes a palladium‐catalyzed cross coupling with functionalized aryl halides or triflates in reasonable to good yields. The ethynylarenes thus obtained serve as building blocks for the formation of the highly effective tubulin polymerization inhibitors combretastatin A‐4 (1) and A‐5 (2) as well as for the synthesis of the plant‐growth regulator lunularic acid (36).
Synthesis of the Isotopically Labelled C‐Terminal Fragment of Zervamicin: An Approach to the Synthesis of Aib‐Containing Peptides
Authors: Ogrel Alexei , Bloemhoff Wim , Lugtenburg Johan , Raap Jan 
Published: July 2007

AbstractThe isotopically labelled C‐terminal fragment of zervamicin, H‐Hyp10‐[4,4‐2H2‐Gln]‐Aib‐Hyp‐Aib‐Pro‐Phl16, has been synthesized in solution by a Fmoc/tert‐butyl strategy in 28% overall yield. The Fmoc group was removed by reaction for 2 min with 0.1 M NaOH in dioxane/methanol/water, (30:9:1, v:v:v). The couplings of the sterically hindered Aib residues were achieved by means of either BOP/DMAP activation or Fmoc‐Aib‐Cl. Acid deprotection of the peptide was performed by reaction with 50% TFA in CH2Cl2 for 30 min without significant cleavage of the acid‐labile Aib‐Pro and Aib‐Hyp peptide bonds.
Transannular Cyclizations of Medium‐Sized Cycloalkynes and Cycloalkynones Induced by Electro‐ and Photochemically Generated NO3 Radicals
Authors: Wille Uta , Plath Christian 
Published: July 2007

AbstractTransannular radical cyclizations of medium‐sized cycloalkynes and cycloalkynones induced by addition of NO3 radicals to the C–C triple bond were studied. In the reaction of NO3 with medium‐sized cycloalkynes oxidation of the triple bond to a carbonyl group occurred and formation of cis‐fused bicyclic alkanones by transannular cyclization was observed. In the reaction of NO3 radicals with medium‐sized cycloalkynones α,β‐epoxy ketones were formed in a transannular reaction sequence which can be formally considered as a retro Eschenmoser‐Ohloff fragmentation reaction. The scope and limitations of these reactions are presented and reaction mechanisms are proposed.
The Effect of Pressure on the Cycloadditions of Cyanoacetylene to Furan Derivatives – [2.2](2,5)Furanoparacyclophane, [8](2,5)Furanophane, and Furan
Authors: Breitkopf Volker , Bubenitschek Peter , Hopf Henning , Jones Peter G. , Klärner Frank‐Gerrit , Schomburg Dietmar , Witulski Bernhard , Zimny Bernd 
Published: July 2007

AbstractAt 1 bar and 160°C the reaction of cyanoacetylene (1) with [2.2](2,5)furanoparacyclophane (3) produced the unexpected “ring‐enlarged” ketones 6–11. In the reaction of 1 with [8](2,5)furanophane (4) comparable products 21 and 22 were observed, in addition to the products 19 and 20 expected from a consecutive Diels‐Alder addition, Alder‐Rickert cleavage process and the Diels‐Alder addition of 1,4‐dicyano‐1,3‐cyclobutadiene (2a) to 4, respectively. In the reaction of 1 with the parent furan 5 only the (2:1) and (1:2) Diels‐Alder adducts 23, 25, 26, and 27 were found. High‐pressure experiments and the reactivity of 2‐cyano‐7‐oxabicyclo‐[2.2.1]hepta‐2,5‐diene (24), which was prepared independently by flow‐thermolysis of the (1:2) Diels‐Alder adducts 26 and 27, provide evidence that the (2:1) adducts 20, 23, and 13 are probably formed by a sequence of Diels‐Alder and [2 + 2] cycloadditions rather than by the reverse sequence starting with [2 + 2] cyclodimerization of 1 followed by Diels‐Alder reaction with cyclobutadiene 2a as postulated by the analogy to the trimerization of 1 and the cycloaddition of 1 to paracyclophane. The high‐pressure experiments led us to propose a new mechanism of formation of the “ringenlarged” ketones 6–11.
α‐Amino Acids and N‐Protected α‐Amino Aldehydes by Stereoselective Additions of a Chiral Vinyllithium Reagent to Sulfonylimines
Authors: Braun Manfred , Opdenbusch Kersten 
Published: July 2007

AbstractThe chiral vinyllithium reagent (S)‐1b, readily generated from dibromoalkene (S)‐1a by bromine/lithium exchange, adds stereoselectively to mesitylsulfonylimines 2a–f so that the diastereomers 3 are obtained in ⩾98% d.e. after column chromatography. The bromoalkenes 3a–d are submitted to ozonolysis in methanol to give α‐mesitylsulfonylamino esters (S)‐8a–d which can be hydrolyzed to deliver N‐protected α‐amino acids in >95% e.e. On the other hand, α‐mesitylsulfonylamino aldehydes 12a–d are available when bromoalkenes 3a, d–f are first debrominated (→ 11a–d) and subsequently ozonized. In order to avoid reacemization, the aldehydes 12a–d are not purified but submitted to a Mukaiyamatype aldol addition whereby hydroxyesters 15a–d are formed as single diastereomers in a chelate‐controlled reaction. The relative configuration of the esters 15a–d is proven by conversion into the oxazolidinones 16a–d whose optical purity is determined to exceed 92% e.e. by 1H‐NMR measurements in the presence of chiral shift reagents. The sulfonylimine 21a and a series of para‐substituted derivatives 21b–h are also allowed to react with the vinyllithium reagent 1b to give mixtures of diastereomers 22/23. The logarithms of the diastereomeric ratios 22:23 correlate with Hammett's σ‐values.
Convenient Syntheses of 2,3,4,6‐Tetra‐O‐Alkylated D‐Glucose and D‐Galactose
Authors: Käsbeck Ludwig , Kessler Horst 
Published: July 2007

AbstractConvenient syntheses of the 2,3,4,6‐tetra‐O‐benzylated and ‐allylated D‐glucopyranoses 1 and 2 and the corresponding D‐galactopyranoses 3 and 4 are described. The D‐glucose derivatives 1 and 2 were obtained from inexpensive sucrose by peralkylation and subsequent acid hydrolysis. In this reaction sequence an alkylated D‐fructofuranosyl cation is generated which was trapped by different nucleophiles to afford 4‐benzyloxymethylenefurfural (8) and the alkylated D‐fructosides 7, 8 and 10. On the other hand 2,3,4,6‐tetra‐O‐benzyl‐D‐galactose 3 was synthesized by oxidative cleavage of ethyl 2,3,4,6‐tetra‐O‐benzyl‐1‐thio‐α,β‐D‐galactopyranoside (11) with aqueous N‐bromosuccinimide. The alternative route for the preparation of 2,3,4,6‐tetra‐O‐allyl‐D‐glucopyranoside (2) via p‐methoxybenzyl β‐D‐glucoside is less attractive. However, this route was used for the synthesis of 2,3,4,6‐tetra‐O‐allyl‐d‐galactose (4).
Synthesis of rac‐ar‐Curcumen‐15‐al
Authors: Weyerstahl Peter , Schlicht Volker 
Published: July 2007

AbstractStarting from p‐tolunitrile (4) via alkylation with the bromoacetal 12 (→ 8), and with MeI (→ 9), hydrolysis to 10, Wittig reaction (→ 7) and DIBAH reduction, the rac‐ar‐curcumen‐15‐al (2) was obtained. The cumin‐like, citrus odor of rac‐2 is similar to that of the isolated (−)‐2.
1,3‐Dipolar Cycloadditions, 96. – Cycloadditions of Alicyclic Thione S‐Oxides with Alicyclic Thiones
Authors: Huisgen Rolf , Mloston Grzegorz , Polborn Kurt , Sustmann Reiner , Sicking Willi 
Published: July 2007

AbstractThione S‐oxides (sulfines) harbor the allyl anion MO of 1,3‐dipoles, but fail to undergo 1,3‐cycloadditions to the usual dipolarophiles with a CC multiple bond. Thioketones stand the test as superdipolarophiles. The thione S‐oxides 3 and 9 combined with the thioketones 5 and 7 at 80–100°C, furnishing spiro‐1,2,4‐oxadithiolanes in high yields despite steric hindrance. The structures of the cycloadducts were clarified by their spectra and an X‐ray analysis of 11. Becke3LYP/6‐31G* calculations of transition structures and products for the cycloadditions of thioformaldehyde S‐oxide to thioformaldehyde support the observed regiochemistry. The activation energy amounts to 3.9 kcal mol−1, in contrast to EA = 17.4 kcal mol−1 for the corresponding addition to ethylene. The transition structures are in accordance with concerted cycloadditions.
Palladium‐Catalyzed Synthesis of Propellanes with Hexaarylethane Structure
Authors: Dyker Gerald , Körning Jutta , Bubenitschek Peter , Jones Peter G. 
Published: July 2007

AbstractAreno‐anellated [3.3.n]propellanes withn= 2, 3, and 4 are easily accesible by palladium‐catalyzed domino coupling processes of the pentalene system8with various aryl iodides. The X‐ray structure investigations reveled a noteworthy elongation of the central CC single bond of these propellanes to ca. 162 pm.
2(3H)‐Oxazolones from α‐Hydroxy Amides and Keteneylidenetriphenylphosphorane via a Phoshorus Ylide Cascade
Authors: Löffler Jonas , Schobert Rainer 
Published: July 2007

AbstractUpon heating in xylene α‐hydroxyamides 3 react with the cumulated phosphorus ylide 1 to give the substituted 2(3H)‐oxazolones 8 in 40–80% yield. The reaction proceeds via an addition/cyclization/intermolecular‐Wittig olefination sequence, which implies three different types of phosphorus ylides of increasing “ylide activity”. Evidence for the proposed mechanism is provided by trapping the intermediate ylide species with suitable carbonyl compounds. This technique may also be used to exclusively prepare the 2,4‐oxazolidinediones 13 in good yields.
Reactions of Sodium Cyanodithioformate with Monosubstituted Hydrazines
Authors: Drexler Kerstin , Dehne Heinz , Reinke Helmut , Michalik Manfred 
Published: July 2007

AbstractSodium cyanodithioformate (1) reacts with methylhydrazine (2) only in the presence of H2S to give the new, zwitterionic thiooxamic acid 1‐(2‐methylhydrazide) 2‐(2‐methylhydrazone) (3) in good yield. The reaction of 1 with phenylhydrazine (5) gives a mixture of the zwitterionic thiooxamic acid 1‐(2‐phenylhydrazide) 2‐(2‐phenylhydrazone) (6b) and the non‐zwitterionic glyoxylamide bis(phenylhydrazone) (7). The mechanism of the formation of 3 and 6b is discussed. X‐ray structural data of 3 and 6b are given.
A Novel Pentaannulation Reaction of Iminium Ions
Authors: Ofial Armin R. , Mayr Herbert 
Published: July 2007

AbstractThe preformed iminium salts (1a–1d)‐SbCl react with (1,1‐dimethylallyl)trimethylsilane 2 in dichloromethane at ambient temperature to give the pyrrolidinium salts (3a–3d)‐SbCl in high yield. In addition, small amounts of the ene products 4c‐SbCl and 4d‐SbCl have also been detected.
Synthesis of Bicyclic Homochiral Dienes by Allylic Rearrangement of Cyclohexenols – Suitable Building Blocks for The Synthesis of Nagilactones
Authors: Westermann Bernhard , Dubberke Silke 
Published: July 2007

AbstractA very short and straightforward synthesis towards highly functionalized dienes is described. The homochiral starting material, unsaturated β‐oxo ester 2, can be prepared by enzyme‐catalyzed saponification. The utility of the dienes has been demonstrated in a cycloaddition with N‐phenyltriazolidinone as the dienophile. The Diels‐Alder reaction is highly diastereoselective, yielding only one diastereomer.
Bicyclic and Tricyclic Compounds with β‐Amino Alcohol Groups as Chiral Ligands in the Enantioselective Reaction of Diethylzinc with Aldehydes
Authors: Aurich Hans Günter , Biesemeier Frank , Geiger Michael , Harms Klaus 
Published: July 2007

AbstractReduction of the enantiomerically pure allyloxy esters 9 or ent‐9 with DIBAH afforded the corresponding aldehydes which were treated in situ with chiral or achiral hydroxylamines 8 to give nitrones 10. These underwent a spontaneous intramolecular 1,3‐dipolar cycloaddition, affording the bicyclic compounds 11 and 12, respectively. In an analogous manner, a mixture of the tricyclic compounds 14 and 15 was prepared. Treatment of compound 16 with cyclohexene oxide afforded a mixture of diastereomers 17 and 18. Diastereomers 14 and 15 as well as 17 and 18 could be separated by chromatography. X‐ray analyses of compounds 11Ff, 17, and 11Af · HCl were performed. The bicyclic and tricyclic compounds were used as chiral ligands in the reaction of diethylzinc with aldehydes 19, in particular with benzaldehyde (19a). Using bicyclic compounds with a tertiary β‐hydroxyalkyl substitutent at the N atom as ligands, ee's in the range of 78 to 95% were found. Whereas for the best ligands 11Ae and Af the enantioselectivity in the reaction of 4‐tolualdehyde was only slightly decreased, with the aliphatic aldehydes 19c and d distinctly lower enantioselectivities were determined.
Synthese und Konformation von Hexahydro‐isochino[1,2‐b][3]benzazepinen
Authors: Clemens Martina , Meise Werner , Himmel Klaus , Jansen Martin 
Published: July 2007

Synthesis and Conformation of Hexahydro‐isoquino[1.2.‐b][3]benzazepinesThe synthesis of the six hexahydro‐isoquino[1,2‐b][3]benzazepines 13a–f is described; their conformation is investigated by NMR spectroscopic methods, X‐ray analysis and dehydrogenation with mercury(II) acetate.
An Intramolecular 1,3‐Dipolar Cycloaddition Involving One Alkyne and Two Nitrone Groups
Authors: Aurich Hans Günter , Biesemeier Frank , Harms Klaus 
Published: July 2007

AbstractThe chiral nonracemic compound 4 was prepared from ethyl (S)‐(–)‐lactate and 1,4‐dibromo‐2‐butyne. Reduction with DIBAH at −72°C and subsequent treatment of the resulting dialdehyde with N‐methylhydroxylamine yielded the dinitrone 5, which underwent two consecutive intramolecular cycloadditions to give the chiral nonracemic compound 7. The structure of 7 was confirmed by an X‐ray analysis. Reductive ring opening afforded compound 8.
Phenazin‐5(10H)‐yls, 5. – Intermolecular π‐π Interactions (Pimerization) oftert‐Butyl‐Substituted Phenazin‐5(10H)‐yl Radicals in The Solid State: Syntheses, Crystal Structures and Magnetic Susceptibility Measurements
Authors: Gleiter Marco R. , Kanellakopulos Basil , Krieger Claus , Neugebauer Franz A. 
Published: July 2007

AbstractIntroduction oftert‐butyl groups into the phenazine frame‐work was accomplished by treatment of 5‐acetyl‐5,10‐dihydrophenazine (2a) withtert‐butyl chloride in the presence of AlCl3. Starting from the 2,8‐ or 3,7‐di‐tert‐butyl‐substituted derivatives2cand2b, a series of phenazin‐5(10H)‐yl radicals (1c–i) was synthesized and characterized by ESR and EN‐DOR spectroscopy. With the exception of1c, all phenazin‐5(10H)‐yls were obtained in crystalline form, and for1d–fthe long‐wavelength absorption band at λ ≈ 870 nm indicates intermolecular π‐π interactions in the solid state. For1d,1eand1hthe crystal structure could be determined. The unit cell of1dconsists of eight phenazin‐5(10H)‐yls. Surprisingly, four of them are arranged in radical pairs, whereas the other four lie independently in the lattice. In agreement with this structure, the magnetic susceptibility results correspond to a content of 50% monoradical and an almost complete spinpairing in the radical pairs up toT= 220 K. In1e, the four phenazin‐5(10H)‐yls in the unit cell are arranged in two independent radical pairs,AandB, which are characterized by close interplanar distances and short intermolecular contacts between atoms with significant spin populations. Accordingly, the susceptibility data indicate strong spin‐pairing at low temperature. Due to extensive steric shielding of the phenazin‐5(10H)‐yl framework, the crystal structure of1hgives no evidence of any π‐π interactions between adjacent radicals. As expected, the magnetic susceptibility of1hcorresponds to that of an ordinary monoradical.
Reactions of 2‐(Organylseleno)propenenitriles with Dimethyl Ethynedicarboxylate
Authors: Döpp Dietrich , Sturm Thomas 
Published: July 2007

Abstract(2‐tert‐Butylseleno)propenenitrile (12a) has been prepared and treated with dimethyl ethynedicarboxylate (3) and methyl propiolate (18) to give the 2,3‐dihydroselenophenes 16 and 21, respectively. In contrast, 2‐(ethylseleno)‐, 2‐(methylseleno)‐ and 2‐(phenylseleno)propenenitrile (12b–d) tend to form polysubstituted butadienes 17b–d with 3 by net organoylseleno group transfer from 12b–d to C‐2 of 3 following attachment of C‐3 of 17 to C‐3 of 3. Five‐membered selenonium ylides are proposed as intermediates.
Carbohydrate‐Based Crown Ethers
Authors: Miethchen Ralf , Fehring Volker 
Published: July 2007

AbstractChiral 12‐crown‐4 ethers (13–16) and 18‐crown‐6 ethers (21–24) containing two and three ethylene‐bridged D‐glucopyranose moieties, respectively, have been synthesized. 2‐O‐(Acetoxyethyl)‐3,4,6‐tri‐O‐benzoyl‐α‐D‐glucopyranosyl fluoride (5), generated by treatment of 2,3,4‐tri‐O‐benzoyl‐1,2‐O‐ethylene‐β‐D‐glucose (4) with HF/nitromethane/acetic anhydride was the key building block. It was used as a glycosyl donor and/or – after modification – as an acceptor component in head‐to‐tail‐type glycosylations. Thus, the openchain precursors 9/11, 10/12, 17/18 and 19/20 of the crown ethers were prepared from reaction of 5 with 7, 11 and 12, respectively, in BF3‐catalyzed glycosylations. The stereoselectivity of the glycosylations could be influenced by the use of different solvents. α‐Selective syntheses of the crown ethers 14 (from the thioglycoside 11) and 23 (from the thioglycoside 20) were achieved in dichloromethane using NIS/trifluoromethanesulfonic acid for activation. In acetonitrile, the thioglycoside 11 yielded predominantly the 12‐crown‐4 ether 13, while thioglycoside 18 formed a mixture of the α‐and β‐anomers 21 and 22 under the same activation conditions.
Synthesis and Application of Isothiocyanato Derivatives of Nucleosides
Authors: Zehl Andrea , Cech Dieter 
Published: July 2007

Abstract3′‐Deoxy‐3′‐isothiocyanatothymidine (4), 3′,5′‐protected 2′‐deoxy‐2′‐isothiocyanatouridine (8), and 2′,3′‐dideoxy‐2′‐isothiocyanatouridine (14), respectively, have been prepared from the corresponding 2′‐ or 3′‐amino derivatives. SnCl2, Bu3SnH, and dithiothreitol were employed to reduce the 2′‐ or 3′‐azido derivatives to give the aminonucleosides which served as starting materials for the thiocarbonylation. Thiocarbonylation, by either a mixture of carbon disulfide and mercuric oxide or N,N′‐thiocarbonyldiimidazole, leads directly to the target molecules in high yields. Furthermore, the direct transformation of azidonucleosides into the corresponding isothiocyanto derivatives via treatment of phosphanimine nucleosides with CS2 is described. Th activity of the isothiocyanate group is used for covalent coupling of different linking molecules or dyes to the carbohydrate moiety of the nucleosides.
Regioselective Olefination of Cross‐Conjugated 3‐Thioxosteroids by [3+2] Cycloaddition
Authors: Fiedler Beate , Weiß Dieter , Beckert Rainer 
Published: July 2007

AbstractReaction of 3‐thioxoandrosta‐1,4‐dien‐17‐one (1) with the diazo compounds 2a–i affords 4′‐substituted spiro[17‐oxoandrosta‐1,4‐diene‐3,2′‐[1,3,4]thiadiazolines], which undergo a two‐step ring contraction with extrusion of nitrogen and sulfur, leading to the substituted 3‐methylen‐ and 3‐propylidenandrosta‐1,4‐dien‐17‐ones 3a–i in good yields.
Synthetic Microbial Chemistry, XXX. Synthesis of Acetophthalidin, a Fungal Metabolite which Inhibits the Progression of the Mammalian Cell Cycle
Authors: Nomoto Shin , Mori Kenji 
Published: July 2007

AbstractAcetophthalidin [(±)‐3‐acetyl‐5,7‐dihydroxyphthalide (1)] was synthesized from 6,8‐diacetoxy‐3‐methylisocoumarin (8) by its epoxidation followed by hydrolysis.
Thermal and Electron Transfer Induced Reactions of Enediynes and Enyne‐Allenes, 5. First Isolation of Two Rotamers About the PC Bond in a Phosphane Oxide
Authors: Schmittel Michael , Kiau Susanne 
Published: July 2007

AbstractThe thermal cyclization of the phosphane oxide substituted enyne‐allene 1d results in the formation of 4d, an unknown double‐bond shift isomer of the 11H‐benzo[b]fluorene system. Because of the completely hindered rotation about the PC bond, two rotamers could be isolated and identified on the basis of their spectral data. No thermal interconversion between the two rotamers was observed.
Flash‐Vacuum Pyrolysis of Stabilised Phosphorus Ylides, 9. Preparation and pyrolysis of β,γ‐dioxo ylides, β,β′,γ,γ′‐Tetraoxo Ylides and Hexaoxo Bis(ylides)
Authors: Aitken R. Alan , Karodia Nazira 
Published: July 2007

AbstractSix examples of the β,β′,γ,γ′‐tetraoxo phosphorus ylides 6 have been prepared. While they are found, upon flash‐vacuum pyrolysis (FVP) at 500°C to decompose to give no useful products, conventional pyrolysis by distillation at 200°C gives the trioxo alkynes 12 and 13 where R1 and R2 are alkoxy groups. Three examples of the previously unknown hexaoxo bis(ylides) 7 have been obtained but, with eight contiguous C=X functions, these are too fragile to give any useful product upon FVP. The pyrolysis of the simpler β,γ‐dioxo ylides 9 used to prepare 6 and 7 has also been examined for the first time and at 600°C it gives the alkynes 11 in low yield for R1 = OMe or OEt but not Ph. The fully assigned 13C‐NMR spectra of all the ylides 6, 7, and 9 are presented.
Enzymatic Synthesis of Peptide Conjugates: Tools for the Study of Biological Signal Transduction
Authors: Kappes Thomas , Waldmann Herbert 
Published: July 2007

AbstractGlyco‐, phospho‐, nucleo‐ and lipoproteins play key roles in biological signalling cascades, by which chemical signals are passed across the cell membrane and further to the cell nucleus. For the study of the biological phenomena associated with these protein conjugates, structurally well‐defined peptides containing the characteristic linkage region of the peptide backbone with the carbohydrate, the lipid or the phosphoric acid ester can be applied advantageously. The multifunctionality and the pronounced acid‐ and base‐lability of lipo‐, phospho‐, nucleo‐ and glycopeptides, however, renders their synthesis a formidable challenge to conventional organic synthesis. This review highlights the potential of enzymatic transformations, in particular for the removal of protecting groups, as central techniques for the construction of complex and sensitive biologically relevant peptide conjugates under particularly mild conditions and with high selectivity.