bluuebunny/crossref_metadata_2025_split huggingface dataset:
Diels‐Alder Reactions of [2.2] Paracyclophan‐1‐ene and [2.2] Paracyclophane‐1,9‐diene with 3,6‐Disubstituted 1,2,4,5‐Tetrazines
Authors: de Meijere Armin , König Burkhard 
Published: December 2004

Abstract[2.2] Paracyclophan‐1‐ene (1) and [2.2] paracyclophane‐1,9‐diene (6) apparently act as dienophiles with inverse electron demand and smoothly react with dimethyl 1,2,4,5‐tetrazine‐3,6‐dicarboxylate (2a) at room temperature forming dihydropyridazine adducts, which are dehydrogenated to the pyridazino‐anellated [2.2] paracyclophanes 5a and 8a, respectively. The molecular structure of 5a is determined by X‐ray crystal‐structure analysis. Under more rigorous conditions, phenyl‐substituted derivatives 5b and 8b are obtained from 1 and 6, respectively, with 3,6‐diphenyl‐1,2,4,5‐tetrazine. Compounds 1 and 6 are less reactive dienophiles than other strained cyclic olefins as shown by kinetic measurements.
Coenzyme F430 from Methanogenic Bacteria: Mechanistic studies on the reductive cleavage of sulfonium ions catalyzed by F430 pentamethyl ester
Authors: Lin Shu‐Kun , Jaun Bernhard 
Published: December 2004

AbstractMechanistic questions regarding the reductive cleavage of sulfonium ions by the NiI form of coenzyme F430 pentamethyl ester (F430M) were addressed in a series of kinetic studies and isotope labeling experiments. In neat DMF, methane formation from dialkyl(methyl)sulfonium ions consistently showed a delay time of ca. 1 h. In the presence of excess propanethiol, no delay was observed and methane formation followed pseudo‐first‐order kinetics with a logarithmic dependence of the initial rate on the concentration of propanethiol. From the temperature dependence of the reaction rate, an estimate for the activation parameters of ΔH# = 49 kJ mol−1 and (apparent) ΔS# = –114 J K−1 mol−1 was derived. The observation of deuterium incorporation into methane from (CH3)2CHOD, but not from (CH3)2CDOH, indicates that the fourth H‐entity is introduced into CH4 as a proton, and that free CH3 radicals are not involved. In contrast to the reaction with the homogeneous one‐electron reductant sodium naphthalide, the F430M‐catalyzed reduction of mixed dialkyl(methyl)sulfonium ions showed a pronounced selectivity for the cleavage of MeS over that of alkyl‐S (alkyl ≠ Me) bonds. Mechanisms that are consistent with these results, as well as possible explanations for the time delay and the apparent highly negative entropy of activation, are discussed.
Glycosylidene Carbenes. Part 7. Neighbouring‐group participation by the 2‐benzyloxy group in the glycosidation of strongly acidic hydroxy compounds
Authors: Takahashi Yoshikazu , Vasella Andrea 
Published: December 2004

AbstractTo demonstrate the neighbouring‐group participation of the 2‐benzyloxy group in the glycosidation of phenols and of strongly acidic alcohols by the diazirine 1, we examined the glycosidation of 4‐nitrophenol, 4‐methoxyphenol, (CF3)2CHOH, MeOH, and i‐PrOH by the diazirine 11, derived from the 2‐deoxypyranose 6. Oxidation of the oximes 7 yielded (E)‐ and (Z)‐8. In solution, (E)‐8 isomerised to (Z)‐8. Similarly, the (E)‐configurated mesylate 9, prepared from 8, underwent acid‐catalysed isomerisation to (Z)‐9. Treatment of (Z)‐9 with NH3, followed by oxidation of the resulting diaziridine 10 with I2, yielded the desired diazirine 11. Glycosidation by 11 of the above mentioned hydroxy compounds yielded the glycosides 12–21. In agreement with the postulated neighbouring‐group participation, these glycosidation proceeded without, or with a very low diastereoselectivity, favouring the axial anomers.
Glycosylidene Carbenes. Part 8. Crystal Structure of a Cyclic, N‐Acetylallosamine‐Derived Alkoxy‐alkyl‐diazirine
Authors: Linden Anthony , Vasella Andrea , Witzig Christian 
Published: December 2004

AbstractThe crystal structure of 2‐acetamido‐1,5‐anhydro‐1‐azi‐3‐O‐benzyl‐4,6‐O‐benzylidene‐1,2‐ dideoxy‐D‐allitol (6) is reported, and the structural parameters of the diazirine moiety are compared with those of other free diazirines for which structural information has been determined or calculated previously. Lengthening of the NN bond and shortening of the CN bonds can be observed as the substituents at the C‐atom of the diazirine ring become more electronegative and/or stronger π‐electron donors. This is consistent with descriptions of the bonding nature of diazirines and similar observations made on substituted cyclopropanes.
Synthesis and Biological Evaluation of 2‐(2‐Deoxy‐β‐D‐ribofuranosyl)pyridine‐4‐carboxamide
Authors: Joos Pieter E. , Esmans Eddy L. , Dommisse Roger A. , Bruyn André De , Balzarini Jan M. , De Clercq Eric D. 
Published: December 2004

AbstractA new protected 2‐deoxy‐D‐ribose derivative, 5‐O‐[(tert‐butyl)diphenylsilyl]‐2‐deoxy‐3,4‐O‐ isopropylidene‐aldehydo‐D‐ribose (5), was synthesized starting from 2‐deoxy‐D‐ribose. This compound was coupled with 2‐lithio‐4‐(4,5‐dihydro‐4,4‐dimethyloxazol‐2‐yl)pyridine giving a D/L‐glycero‐mixture 7 of 5‐O‐[(tert‐butyl)diphenylsilyl]‐2‐deoxy‐1‐C‐[4‐(4,5 ‐dihydro‐4,4‐dimethyloxazol‐2‐yl)pyridin‐2‐yl]‐3,4‐O‐isopropylidene‐ D‐erythro‐pentitol. The mixture 7 was 1‐O‐mesylated with methanesulfonyl chloride and subsequently treated with CF3COOH/H2O and ammonia to afford the α/β‐D‐anomers 10 of 2‐(2‐deoxy‐D‐ribofuranosyl)pyridine‐4‐carboxamide. Both anomers were purified and separated by HPLC and identified by NMR and DCI‐MS. Anomer β‐D‐10 was evaluated against a series of tumor‐cell lines and a variety of viral strains. No antitumor or antiviral activity was observed.
A New General Approach to Enantiomerically Pure Cyclic and Open‐Chain (R)‐ and (S)‐α,α‐Disubstituted α‐Amino Acids
Authors: Obrecht Daniel , Spiegler Clive , Schönholzer Peter , Müller Klaus , Heimgartner Heinz , Stierli Friedrich 
Published: December 2004

AbstractA wide range of cyclic and open‐chain α,α‐disubstituted α‐amino acids 1a‐p were prepared. The racemic N‐acylated α,α‐disubstituted amino acids were resolved by coupling to chiral amines 15‐18 derived from (S)‐phenylalanine to form diastereoisomers 19/20 or 21/22 that could be separated by crystallization and/or flash chromatography on silica gel (Scheme 3). Selective cleavage via the 1,3‐oxazol‐5(4H)‐ones 10a‐p gave the corresponding optically pure α,α‐disubstituted amino‐acid derivatives 11 or 12 in high yield (Scheme 3). The absolute configurations of the α,α‐disubstituted amino acids were determined from X‐ray structures of the diastereoisomers 20, 21g′, 22d.
Strukturänderungen bei der Reduktion von 9‐(Diphenylmethyliden)fluoren mit Natrium‐Metall zu Schichten (R2O)Na⊕‐verknüpfter Kohlenwasserstoff‐Dianionen
Authors: Bock Hans , Ruppert Klaus , Herdtweck Eberhardt , Herrmann Wolfgang A. 
Published: December 2004

Structural Changes on Reduction of 9‐(Diphenylmethylidene)fluorene by Sodium Metal to Sheets of (R2O)Na⊕‐ Connected Hydrocarbon DianionsThe single‐crystal structure of 9‐(diphenylmethylidene)fluorene (monoclinic, space group P21/n, Z = 4) proves a CC bond length of 135 pm and a twisting of the two Ph rings out of the methylidene‐fluorene plane by 70° and 83°. Its reduction in Et2O solution by a Na mirror under Ar yields dark green crystals with a metallic luster. Their structure, determined in a N2 flow cooled to 223 K (monoclinic, space group C2/c, Z = 4), reveals sheets of hydrocarbon dianions, connected by sodium(diethylether) links. The Na centers are sixfold coordinated as half‐sandwiches to the fluorene benzo rings and threefold allylic to the Ph rings. In the dianion of 9‐(diphenyl‐methylidene)fluorene, the central CC bond is stretched by 13 pm to 148 pm length, and the two molecular cyanine‐like halves are twisted by 51° relative to each other. Both the contact‐ion multiple structures as well as the structural changes on twofold reduction are discussed by way of comparison with literature examples and based on semiempirical calculations.
Syntheses of Novel Isopenam and Isocephem Antibiotics. Preparation of a retinamido derivative of a highly strained β‐lactam as potent anticancer agent
Authors: Hakimelahi Gholam H. , Shiao Min‐Jen , Hwu Jih Ru , Davari Hady 
Published: December 2004

AbstractSyntheses of the cis‐configurated isopenam 9 (Scheme 1), isocephem 14 (Scheme 2), and isocephem 19 (Scheme 3) are described. The key step in the preparation of 14 and 19 involved a Pummerer‐type rearrangement of the corresponding sulfoxides 12 and 18. These β‐lactams were found to possess biological activity against several pathogenic microorganisms in vitro. The electronic activation of the lactam moiety of 19 remarkably enhanced its biological activity. A retinoic moiety was attached to 19 via an amino linker. The resultant retinamido‐β‐lactam 21 showed significant cytostatic activity in tracheal organ cultures obtained from vitamin‐A‐deficient hamsters.
Synthesis, Isolation, and NMR‐Spectroscopic Characterization of Fourteen (Z)‐Isomers of Lycopene and of Some Acetylenic Didehydro‐ and Tetradehydrolycopenes
Authors: Hengartner Urs , Bernhard Kurt , Meyer Karl , Englert Gerhard , Glinz Ernst 
Published: December 2004

AbstractEight (Z)‐isomers of lycopene were prepared by stereocontrolled syntheses and fully characterized by 1H‐NMR, 13C‐NMR, mass, and UV/VIS spectroscopy: (5Z)‐, (7Z)‐, (15Z)‐, (5Z,5′Z)‐, (7Z,7′Z)‐, (7Z,9Z)‐, (9Z,9′Z)‐, and (7Z,9Z,7′Z,9′Z)‐lycopene. Six additional (Z)‐isomers, namely (9Z)‐, (13Z)‐, (5Z,9′Z)‐, (9Z,13′Z)‐, (5Z,9Z,5′Z)‐, and (5Z,13Z,5′Z)‐lycopene, were isolated in small quantities from isomer mixtures by semiprep. HPLC and were identified by 1H‐NMR spectroscopy.
Synthesis and Structure of [4.5.5.5]Fenestranes ( = tetracyclo[5.4.1.04, 12.09, 12]dodecanes)
Authors: Hirschi Daniel , Luef Wolfgang , Gerber Peter , Keese Reinhart 
Published: December 2004

AbstractThe preparation and the X‐ray structure analysis of cis, trans, cis, cis‐10‐hydroxy‐[4.5.5.5]fenestrane‐1‐methanol (8b) is reported. The measured bond angles and bond lengths of the central C(C)4 fragment are better reproduced by calculations with the AM 1 than by the MNDO method.
Sydnones as Masked Hydrazines for Heterocycle Formation: Reactions of 3‐(2‐substituted phenyl)sydnones with HCl
Authors: Gelvin Christopher R. , Turnbull Kenneth 
Published: December 2004

AbstractIn general, reaction of 3‐(2‐substituted phenyl)sydnones with HCl gives products derived from cleavage of the sydnone ring to the corresponding hydrazine and subsequent cyclization to the side chain. In one case, 3‐(2‐aminophenyl)sydnone (43), the product obtained, l‐amino‐lH‐benzimidazole (47), apparently results from nucleophilic interception by the side chain prior to complete cleavage of the sydnone ring.
Asymmetric Synthesis of α‐Amino Acids and α‐N‐Hydroxyamino Acids from N‐Acylbornane‐10,2‐sultams: 1‐chloro‐1‐nitrosocyclohexane as a practical [NH] equivalent
Authors: Oppolzer Wolfgang , Tamura Osamu , Deerberg JöRg 
Published: December 2004

AbstractSuccessive treatment of N‐acylsultams 3 with sodium hexamethyldisilazide, 1‐chloro‐1‐nitrosocyclohexane (1), and aq. HCl gave diastereoisomerically pure, crystalline N‐hydroxyamino‐acid derivatives 5. These were converted into various amino acids 7, N‐hydroxyamino acids 8, and an N‐Boc‐amino acid 9. (S, S)‐Isoleucine (17) and (S, S)‐2‐acetamido‐3‐phenylbutyric acid (23) were obtained from N‐crotonoylsultam 15 via 1,4‐addition of an organomagnesium or organocopper reagent followed by enolate ‘amination’ with 1.
Organometallic Derivatives of Cycloproparenes
Authors: Müller Paul , Bernardinelli Gérald , Jacquier Yvan 
Published: December 2004

AbstractThe tricarbonylchromium complex of cyclopropa[b]naphthalene (3a) is deprotonated at C(1) with BuLi. Quenching of the resulting anion with Mel results in a 1:1 mixture of stereoisomeric methyl derivatives 6a and 6b. Peterson olefination of the bis‐silylated complex 3b affords the complexed alkylidenecycloproparene 5 in low yield. The tricarbonylchromium complex of cyclopropa[b]anthracene (10) is accessible via bis‐hydro‐desilylation of 9. Its X‐ray structure is also reported. All attempts to synthesize a tricarbonylchromium complex of benzocyclopropene (11a) failed.
Vicinal 2H/1H Isotope Shifts for 6Li‐NMR and the Aggregation Behavior of Phenyllithium
Authors: Eppers Oswald , Günther Harald 
Published: December 2004

AbstractVicinal deuterium‐induced 6Li‐NMR isotope shifts have been found for (D5)phenyllithium. These shifts are to low field, and their magnitude varies from 7 to 19 ppb with different solvents and donor molecules. On the basis of the isotopic fingerprints observed in the 6Li‐NMR spectra of C6H56Li/C6D56Li mixtures, the aggregation behaviour of phenyllithium under different solvent conditions and in the presence of amines as donor ligands (TMEDA, PMDTA) was studied. Tetramers and dimers are observed in Et2O, dimers as well as momomers in Et2O/TMEDA and in THF, and momomers in THF/PMDTA. The hitherto unidentified species in Et2O/TMEDA was shown to be a monomer.
7‐Deazaguanosine: Synthesis of an oligorbonucleotide building block and disaggregation of the U‐G‐G‐G‐G‐U G4 structure by the modified base
Authors: Seela Frank , Mersmann Karin 
Published: December 2004

AbstractBuilding blocks derived from 7‐deazaguanosine (c7G, 1) were prepared for solid‐phase oligoribonucleotide synthesis. Compound 1 was converted into the isobutyurl derivative 2b and the (dimethylamino)methylidene compound 3 (Scheme 1). After tritylation (→ 4a, b), silylation was studied with regard to regioselectivity. It was found that the triisopropylsilyl group in combination with the (dimethylamino)methylidene residue gave the highest 2′ ‐selectivity (→ 5e). The 2′ ‐O ‐silyl derivative 5e was reacted with PCl3 affording the 3′ ‐phosphonate 7 which was used in solid‐phase oligoribonucleotide synthesis. Oligonucleotides derived from U‐G‐G‐G‐G‐U with an increasing number of c7G residues instead of G were synthesized. Aggregation was studied by polyacrylamidegel electrophoresis and CD Spectroscopy. Disaggregation of the G4‐structure of U‐G‐G‐G‐G‐U was observed when c7G replaced G, demonstrating that guanine N(7) participates in the aggregation process.
The Conformation of Medium‐Sized Organosilicon Heterocycles: First evidence for the existence of a boat‐boat conformation in a 12H‐dibenzo[d,g][1,3,2]dioxasilocin
Authors: Fuhrer Hermann , Meier Hansrudolf , Rist Günther , Pastor Stephen D. , Shum Sai P. , Clarke Frank H. 
Published: December 2004

AbstractX‐Ray Crystal‐structure analysis of 2,4,8,10‐tetrakis(1,1‐dimethylethyl)‐6,6‐dimethyl‐12H‐dibenzo[d,g][1,3,2]‐dioxasilocin showed that its eight‐membered organosilicon heterocycle adopts a boat‐boat (BB) conformation in the solid state (Figs. 2 and 3).
Synthese von selektiv N‐ funktionalisierten Polyamin‐Derivaten
Authors: Fiedler Wolfgang J. , Hesse Manfred 
Published: December 2004

Synthesis of Selectively N‐Functionalized Polyamine DerivativesA threefold differently protected derivative 8 of the triamine spermidine (2) has been prepared in three steps starting from propane‐1,3‐diamine (1). The protected spermidine derivative 8 was converted to its spermine analogue 12, a useful polyamine intermediate. In a convergent way, the fourfold differently protected derivative 18 of the tetraamine spermine (3) has been obtained by coupling the two different and separately prepared propane‐1,3‐diamine units 15 and 17. Spermidine derivative 19 and spermine derivative 20, both selectively protected at both primary amino groups, have been Prepared from the free polyamines 2 and 3, respectively, in a direct approach.
Template‐Assembled Synthetic Proteins (TASP). Cyclic Templates with Incorporated Turn‐Inducing Mimics
Authors: Earnest Ivan , Kalvoda Jaroslav , Francotte Eric , Rihs Grety , Fritz Hans , Blommers Marcel J. J. , Raschdorf Fritz , Sigel Christoph , Mutter Manfred 
Published: December 2004

AbstractThe 8‐amino‐5,6,7,8‐tetrahydronaphth‐2‐oic acid (1), 8‐(aminomethyl)‐5,6,7,8‐tetrahydronaphth‐2‐oic acid (2), and 8‐(aminomethyl)naphth‐2‐oic acid (3) were synthesized in their protected forms as turn‐inducing dipeptide mimics. Two of them (2 and 3) were incorporated into a novel type of cyclic, peptide‐based structures (see 21 and 34–36) designed as templates for the synthesis of TASP molecules.
Modification of Cyclosporin A (CS): Generation of an enolate at the sarcosine residue and reactions with electrophiles
Authors: Seebach Dieter , Beck Albert K. , Bossler Hans G. , Gerber Christian , Ko Soo Y , Murtiashaw C. William , Naef Reto , Shoda Shin‐Ichiro , Thaler Adrian , Krieger Manfred , Wenger Roland 
Published: December 2004

AbstractStrong bases (lithium diisopropylamide (LDA) or BuLi) convert cyclosporin A (CS) to hexalithio derivative containing a Li alkoxide, four Li azaenolate, and one Li enolate units. The Li6 compound is solubilized in tetrahydrofuran (THF) by addition of excess LDA or LiCl. Reactions with electrophiles (alkyl halides, aldehydes, ClCO2R, CO2, (RS)2, D2O) at low temperatures give products containing new side chains in amino‐acid residue 3 of the cyclic undecapeptide (see 1–13, Schemes 1, and 2, and Figs. 1 and 2) in moderate to high yields and, with Re‐ or Si‐selectivities, depending upon the conditions of lithiation of up to 7:1, Pure CS derivatives (Scheme 2, Table 1 in the Exper. Part) can be isolated by column chromatography. N‐Alkylations or cleavage of the peptide backbone by carbonyl addition occur only at higher temperatures and/or with prolonged reaction times (see 14 and 15 in Scheme 4). Very little or no epimerization of stereogenic centers occurs under the conditions employed. Possible reasons for the feasibility of these surprizing conversions of CS are discussed (Schemes 4 and 5 and Fig. 3). For comparision, [MeAla3]CS (2b) and [D‐MeAla3]CS (2a) were also prepared by conventional peptide synthesis in solution (Schemes 6 and 7). Their 1H‐ and 13C‐NMR spectra are compared with those of CS (Table 2 in the Exper. Part).
Über Diels‐Alder‐Reaktionen des C60‐Fullerens Vorläufige Mitteilung
Authors: Krätler Bernhard , Puchberger Michael 
Published: December 2004

On Diels‐Alder Reactions of the C60‐FullereneWe report on the regioselective [4+2] cycloaddition of Buckminsterfullerene C60(1) at room temperature with 2,3‐dimethylbuta‐1,3‐diene and with the monoterpene 7‐methyl‐3‐methylideneocta‐1,6‐diene (= myrcene) and on the spectroscopic characterization of the corresponding crystalling monoadducts 2 and 3. According to these experiments, 1 acts as a reactive dienophile, which can be functionalized regioselectively under mild and controlled conditons.
Derivatisierung von PQQ und PQQ‐Triester für den Nachweis von PQQ
Authors: Martin Pierre , Winkler Tammo 
Published: December 2004

Formation of derivatives of PQQ and PQQ‐Triester for the Detection of PQQPQQ is an organic redox/cofactor found in a class of enzymes called quinoproteins. It has been reported that materials like milk, egg yolk, vinegar, beer, serum, urine, and biological tissues contain little amounts of PQQ. To facilitate studies on PQQ, a simple detection of PQQ could be extremely useful. For this purpose, we describe in this paper some adducts of PQQ or PQQ‐triester with 1,2‐diamines, 2,4‐dihydroxyaniline, pyrrole, indole as well as an oxidation product of PQQ. UV and fluorimetric detections of these colored products show that this method is relatively sensitive. The reaction of PQQ‐triester with (MeO)3P to the phosphole adduct 13 is of great importance: the use of [32P]‐labeled trialkyl phosphites will make this reaction an extremely sensitive method to detect PQQ.
Glycosylidene Carbenes Part 12. A new synthesis and some reactions of spirooxiranes
Authors: Vasella Andrea , Dhar Preeti , Witzig Christian 
Published: December 2004

AbstractThe diazirine 1, upon thermolysis or photolysis in either acetone or cyclohexanone, at different concentrations, yield the spiro epoxides 2 and 3, and 4 and 5, respectively (Scheme 1). Yield of 2 and 3 depended both on the temperature and the concentration, and correlated inversely with the yield of the major by‐product, the enol‐derived glycoside 6. Other by‐product were the benzyloxglycal7 and the lactone azines 8. ZnCl2‐Promoted methanolysis of 2 under mild condition yielded mixture of the uloside 9 and 10 (1.2:1); similarly, 4 yielded 11 and 12 (1.8:1; Scheme 2). More strongly acidic conditions converted 11 into 12, evidencing that ZnCl2‐promoted methanolysis proceeds under kinetic control, which is rationalized. The diazirine 13, upon thermolysis of Photolysis in either acetone of cyclohexanone, yielded the α‐D‐configurated spiro epoxides 14 and 16, and the α‐D‐configurated dihydrooxazoles 15 and 17, respectively (Scheme 3), which are either formed by ring‐opening of ß‐D‐epoxides, by competitive interception of the initially formed, hypothetical addition products of the intermediate carbene to the ketones. The glycosylidene carbenes, derived from 1 or 13 are not very reactive towards ketones, yields are good only when sterically unhindered ketones are used in large excess.
7‐Deazaadenosine: Oligoribonucleotide building block synthesis and autocatalytic hydrolysis of base‐modified hammerhead ribozymes
Authors: Seela Frank , Mersmann Karin , Grasby Jane A. , Gait Michael J. 
Published: December 2004

AbstractA 7‐deazaadenosine ( = tubercidin; c7A;1) building block for solid‐phase oligoribonucleotide synthesis was prepared. The amino group of1was protected with the (dimethylamino)methylidene residue (→3), and the monomethoxytrityl group was introduced at OHC(5′) (→4). Protection of OHC(2′) was carried out by silylation, showing that use of the (i‐Pr)3Si group resulted in high 2′‐O‐selectivity (→5b, 80%). Reaction of5bwith PCl3afforded the phosphonate 7 which was used in solid‐phase oligoribonucleotide synthesis. The autocatalytic hydrolysis of hammerhead ribozymes using pG‐G‐G‐A‐G‐U‐C‐A‐G‐U‐C‐C‐C‐U‐U‐C‐G‐G‐G‐G‐A‐C‐U‐C‐U‐G‐A‐A‐G‐A‐G‐G‐C‐G‐C as substrate strand (S) and modified G‐C‐G‐C‐C‐G‐A‐A‐A‐C‐U‐C‐C‐C as enzyme strand (E) was studied. When c7A replaced A13or A14, a small decrease of catalytic activity was observed, while modification in position A15enhanced the autocatalytic hydrolysis. The results demonstrate, that the atom N(7) of adenosine in any of these positions is not crucial for ribozyme action.
The Effect of Electron‐Donating and Electron‐Withdrawing Substituents on 1H‐and 13C‐NMR chemical shifts of novel 7′‐aryl‐substituted 7′‐apo‐β‐carotenes
Authors: Hand Elli S. , Belmore Kenneth A. , Kispert Lowell D. 
Published: December 2004

AbstractThe synthesis of 7′‐aryl‐7′‐apo‐β‐carotenes, where aryl (Ar) is Ph, 4‐NO2C6H4, 4‐MeOC6H4, 4‐(MeO2C)C6H4, C6F5, and 2,4,6‐Me3C6H2, is described. NMR Chemical shifts of all H‐ and C‐atoms are presented, together with specific examples of the spectra. In contrast to 1H chemical shifts which, except for HC(8′) and HC(7′), did not differ greatly from those of β,β‐carotene, considerable variations in 13C chemical shifts were observed. Signals of the C(α) atoms of the polyene chain [C(β)C(α)] +n Ar were shielded, those of the C(β) atoms were deshielded, with some exceptions when n = 1; the effects decreased with increasing n.
Synthese und Pyrolyse einer Triafulven‐Vorstufe
Authors: Mühlebach Michel , Neuenschwander Markus , Engel Peter 
Published: December 2004

Synthesis and Pyrolysis of a Triafulvene PrecursorIn view of retro‐Diels‐Alder reactions (RDA reactions), the triafulvene precursor 3 has been prepared in a simple three‐step synthesis by dibromocarbene addition at dibenzo‐barrelene (11→12; 44%), halogen‐Li exchange followed by methylation (12→14, 100%) and HBr elimination (14→3, 62%) (Scheme 3). Reactivity of the so far unknown bridged 1,1‐dibromocyclopropane 12 has been explored, including reductions, allylic rearrangements, and ‘carbene dimerizations’ (Scheme 4). First experiments with respect to the thermal behavior of 3 show that RDA reaction, although occurring in most cases, is not the predominant pathway. When 3 is heated in a sealed tube without solvent, two dimers 26 and 27 are isolated in a total yield of 55% (Scheme 6). On the other hand, gas‐phase pyrolysis of 3 at 400° mainly produces rearranged 28 (56%; Scheme 7). It is assumed that bridged trimethylenemethane 29 is an essential intermediate in thermal rearrangements of 3 (Scheme 8).