bluuebunny/crossref_metadata_2025_split huggingface dataset:
Fünf‐ und sechsgliedrige cyclische Phosphonsäure‐diamide und‐thioester‐amide sowie Spirobi[oxazaphosphole]
Authors: Neidlein Richard , Buseck Stephan 
Published: December 2004

AbstractFive‐ and Six‐Membered Cyclic Phosphonic Diamides, Thioester‐amides and Spirobi[oxazaphospholes]By reaction of the 1,2‐diaminobenzene derivatives 1a–d with the phosphonoyl dichlorides 2a–g, the 1,3‐dihydro‐1,3,2‐benzodiazaphosphol‐2‐ones 4a–q are obtained. The phosponoyl dichlorides 2a–e, 2g, and 2h yield with naphthalene‐1,8‐diamines (5) the diazaphosphinones 6a–g under similar reaction conditions. The thiazaphosphole oxides 9a–d are accessible by reaction of the phosphonoyl dichlorides 2a–d with 2‐aminobenzenethiol (7). Unexpectedly, the reaction of the dichlorides 2a and 2b with 2‐aminophenol (10) led to the spirobi[oxazaphospholes] 13a and 13b, respectively.
A Novel Heterocyclic Ring System: Synthesis and Spectral Data of 4,8,9b‐Triazacyclopenta[c, d]phenalene
Authors: Hou Dongfeng , Balli Heinz 
Published: December 2004

AbstractThe synthesis and 1H‐NMR and electronic absorption spectra of 4,8,9b‐triazacyclopenta[c, d]phenalene and several of its derivatives are described.
Hydrogenation of the ansa‐Chain of Rifamycins. X‐ray crystal structure of (16S)‐16,17,18,19‐tetrahydrorifamycin S
Authors: Bartolucci Cecilia , Cellai Luciano , Filippo Patrizia Di , Lamba Doriano , Segre Anna Laura , Bianco Armando Doriano , Guiso Marcella , Pasquali Vinicio , Brufani Mario 
Published: December 2004

AbstractThe catalytic hydrogenation of rifamycin S (2) over Pd/C, followed by oxidation with K3[Fe(CN)6], generates a pair of 16,17,18,19‐tetrahydrorifamycins S (3/4), epimeric at C(16). The use of PtO2 as catalyst leads to the hydrogenation also of the C(28)C(29) bond giving, after oxidation by K3[Fe(CN)6], a mixture of the epimers (16R)‐ and (16S)‐16,17,18,19,28,29‐hexahydrorifamycins S (5/6). Furthermore, we synthesized the (16R)‐ and (16S)‐3‐bromo derivatives 7/8 and (16R)‐ and (16S)‐3‐(piperidin‐1‐yl) derivatives 9/10. The determination of the X‐ray crystal structure of the most abundant epimer 4 of the tetrahydrorifamycins allowed the assignment of the absolute configuration at C(16) of all derivative. A Structure‐activity relationship study showed that in general the (16R)‐epimers are more potent inhibitors of bacterial RNA polymerase than the (16S)‐epimers.
Synthesis and Structure of a p‐Terphenoquinone and Paramagnetic Species Derived Therefrom
Authors: Boldt Peter , Bruhnke Detlev , Gerson Fabina , Scholz Markus , Jones Peter G. , Bär Friedrich 
Published: December 2004

Abstract3,5,3″,5″‐Tetra(tert‐butyl)‐p‐terphenoquinone (12b) was synthesized and characterized by its spectroscopic and X‐ray structural data. The ground state of 12b is singlet with a strong contribution of a biradical structure. Paramagnetic derivative of 12b, such as the semiquinone radical anion (12bτ) and its O‐protonated form, the phenoxy radical (18), were studied by ESR and ENDOR spectroscopy.
A Convenient Procedure for he Synthesis o 2,3,4,6‐Tetra‐O‐benzyl‐D‐gluconolactam and D‐Nojirilactam
Authors: Hoos Roland , Naughton Andrew B. , Vasella Andrea 
Published: December 2004

AbstractSwern oxidation of 2,3,4,5‐tetra‐O‐benzyl‐D‐glucose (1) followed by ammonolysis gave the crystalline amide 3 which was oxidized (DMSO/pyridine · SO3) to yield the oxo‐amide 4 and the hydroxy‐lactams 5 and 6. Cyclization of 4 to the very slowly equilibrating 5 and 6 was completed by treatment with AcOH in CHCl3. The configuration of the hydroxy‐lactams was assigned on the basis of NOEs. Reduction (Et3SiH/BF3 · Et2O) of the hydroxy‐lactams either individually or as a mixture led to 2,3,4,6‐tetra‐O‐benzyl‐D‐gluconolactam (7). The procedure, based upon modifications of a patent, does not require chromatography; the overall yield of 7 from 1 is 43%. Hydrogenolysis of 7 gave D‐nojirilactam (8); benzylation led to the known pentabenzyl‐D‐nojirilactam (9) and to the unsaturated lactam 10.
Complex Formation of PdII Ion with the Tripodal Ligand Tris[2‐(dimethylamino)ethyl]amine
Authors: Anderegg Giorgio , Melichar Zdenek 
Published: December 2004

AbstractThe complex formation of PdII with tris[2‐(dimethylamino)ethyl]amine (N(CH2CH2N(CH3)2)3, Me6tren) was investigated at 25° and ionic strength I = 1, using UV/VIS, potentiometric, and NMR measurements. Chloride, bromide, and thiocyanate were used as auxiliary ligands. The stability constant of [Pd(Me6tren)]2+ in various ionic media was obtained: log β([Pd(Me6tren)] = 30.5 (I = 1(NaCl)) and 30.8 (I = 1(NaBr)), as well as the formation constants of the mixed complexes [Pd(HMe6tren)X]2+ from [Pd(HMe6tren)(H2O)]3+:log K = 3.50 = Cl−) and 3.64 (X− = Br−) and [Pd(Me6tren)X]+ from [Pd(Me6tren)(H2O)]2+: log K = 2.6 (X− = Cl−), 2.8(Br−) and 5.57 (SCN−) at I = 1 (NaClO3). The above data, as well as the NMR measurements do not provide any evidence for the penta‐coordination of PdII, proposed in some papers.
Pteridines. Part C. Structure and nonenzymatic synthesis of aurodrosopterin
Authors: Yim Jeongbin , Kim Sujeong , Walcher Gunter , Pfleiderer Wolfgang 
Published: December 2004

AbstractThe nonenzymatic synthesis of aurodrosopterin (5) from 6‐acetyl‐2‐amino‐3, 7, 8, 9‐tetrahydro‐4H‐pyrimido‐[4,5‐b][1,4]diazepin‐4‐one (3) and 7,8‐dihydrolumazine (4) at pH 3 (HCl) was performed. The identity of the synthesized compound with the natural eye pigment isolated from drosophila heads was confirmed by thin‐layer chromatography on cellulose and by comparisons of the 1H‐NMR and UV/VIS spectra. The nonenzymatic synthesis of a neodrosopterin‐like red pigment from 3 and 2,4‐diamino‐7,8‐dihydropteridine was also carried out, but its identity could not be established. This pigment, called aminodrosopterin, has an absorption peak at 489 nm, which is very close to that of neodrosopterin.
Structures and Chemistry of Methanofullerenes: A Versatile Route into N‐[(Methanofullerene)carbonyl]‐Substituted Amino Acids
Authors: Isaacs Lyle , Diederich François 
Published: December 2004

AbstractThe reaction of C60 with oxadiazole 13 afforded the dimethoxymethanofullerene 7 in 32% yield as a 6‐6‐ring‐bridged isomer with a closed transannular bond. A literature survey showed that all 6‐6‐ring‐bridged methanofullerenes are σ‐homoaromatic with a closed transannular bond (6‐6‐closed) and all 6‐5‐ring‐bridged are π‐homoaromatic with an open transannular bond (6‐5‐open). The preference for 6‐6‐closed and 6‐5‐open structures is not due to substituent effects but is best explained with the conservation in these isomers of the favorable bonding seen in C60 with higher double‐bond character at 6‐6 bonds and higher single‐bond character at 6‐5 bonds. Reaction of C60 with diazo diester 15 gave the fullerene diester 14 which was hydrolyzed with BBr3 in benzene to the methanofullerenecarboxylic acid 10, a versatile synthon for the preparation of amphiphilic fullerene derivatives. Treatment of 10 with alcohols and amino acid esters under DCC coupling conditions afforded the esters 5 and 17 and the amino‐acid derivatives 11 and 12, respectively.
Anatolioside E: A New Acyclic Monoterpene Glycoside from Viburnum orientale
Authors: Çaliş Ihsan , Yürüker Ayşen , Rüegger Heinz , Wright Anthony D. , Sticher Otto 
Published: December 2004

AbstractA new open‐chain monoterpene glycoside, anatolioside E (1), was isolated from the leaves of Viburnum orientale in addition to three known acyclic monoterpene glycosides, betulalbusides A (2) and B (3), and 2(E)‐2,6‐dimethyl‐2,7‐octadien‐1,6‐diol‐6‐O‐β‐D‐glucopyranoside(4). The structure of anatolioside E (1) was elucidated on the basis of chemical and spectral data as 6‐O‐[β‐D‐glucopyransoyl‐(1‴‴ → 6‴″)‐2‐(E), 6(R), 2,6‐dimethyl‐6‐hydroxy‐2,7‐octadienoyl‐(1‴″ → 2″″)‐β‐D‐glucopyranosyl‐(1″″ → 6‴)‐2‐(E), 6(R), 2,6‐dimethyl‐6‐hydroxy‐2,7‐octadienoyl‐(1‴ → 4″)‐α‐L‐rhamnopyranosyl‐(1″″ → 2′)‐β‐D‐glucopyranosyl]linalool.
Glycosylidene Carbenes. Part 13. Synthesis and thermolysis of representative 1‐azi‐glycoses
Authors: Vasella Andrea , Witzig Christian , Waldraff Christian , Uhlmann Peter , Briner Karin , Bernet Bruno , Panza Luigi , Husi René 
Published: December 2004

AbstractIn the context of the hypothesis postlating a heterolytic cleavage of a CN bond during thermolysis of alkoxydiazirines (Scheme 1), we report the preparation of the diazirines 4, 5, 7, and 8, the kinetic parameters for the thermolysis in MeOH of the diazirines 1 and 4–9, and the products of their thermolysis in an aprotic environment. The diazirines 4, 57, and 8 (Scheme 2–5) were prepared from the known hemiacetals 10, 19, 34 (prepared from 31 in an improved way), and 42 according to an established method. The oximes 11, 20, 35, and 43 were obtained from the corresponding hemiacetals as (E/Z)‐mixtures; 43 was formed together with the cyclic hydroxylamine 44. Oxidation of 11, 35, and 43 (N‐chlorosuccinimide/1,8‐diazabicyclo[5.4.0]undec‐7‐ene (NCS/DBU) or NaIO4) gave good yields of the (Z)‐hydroximolactones 12, 36, and 45, while the oxime 20 led to a mixture of the (E)‐ and (Z)‐hydroximolactones 21 and 22, which adopt different conformations. Their configuration was assigned, inter alia, by a comparison with the enol ethers 28 and 29, which were obtained, together with 30, from the reaction of the diazirine 5 with benzaldehyde and PBu3. Treatment of the hydroximolactone O‐sulfonates 13, 23, 37, and 46 with NH3/MeOH afforded the diaziridines 15, 25, 38, and 47 in good yields, while the (E)‐sulfonate 24 decomposed readily. Oxidation of the diaziridines gave 4, 5, 7, and 8, respectively. Thermolysis of the diazirines 1 and 4–9 in MeOH yielded the anomeric methyl glycosides 50/51, 16/17, 26/27, 52/53, 39/40, 48/49, and 54/55, respectively. A comparison of the kinetic data of the thermolysis at four different temperatures shows the importance of conformational and electronic factors and is compatible with the hypothesis of a heterolytic cleavage of a CN bond. An early transition state is evidenced by the absence of torsional strain by an annulated 1,3‐dioxane ring. Thermolysis of 1 in MeCN at 23° led mostly to the diasteroisomeric (Z,Z)‐, (E,E)‐, and (E,Z)‐lactone azines 56, 57, and 58 (Scheme 6), which convert to 56 under mild conditions, and to 59 (3%). The benzyloxyglucal 59 was obtained in higher yields (18%), together with 44% of 56–58, by thermolysis of solid 1. Similarly, thermolysis at higher temperatures of 4 in toluene, THF, or dioxane and of 9 in CH2Cl2 or THF yielded the (Z,Z)‐lactone azines 60 and 61, respectively, the latter being accompanied by the dihydro‐oxazole 62.
Single‐Crystal Structure of the Solvent‐Separated Radical Ion Pair [9,10‐diphenylanthracene.⊖][Na⊕(THF)6] and its implication for cation solvation
Authors: Bock Hans , John Andreas , Näther Christian , Havlas Zdenek , Mihokova Eva 
Published: December 2004

AbstractThe one‐electron transfer to large π‐delocalized hydrocarbons provides an interesting possibility to crystallize solvent‐separated ion‐pair salts containing optimally solvated cations. Accordingly, the reduction of 9,10‐diphenylanthracene in aprotic THF solution at a sodium metal mirror allows to grow dark‐blue prismatic crystals of its radical anion and sixfold THF‐solvated sodium cation. The structure of the radical anion is very similar to that recently published for the neutral molecule. According to AM1 hypersurface calculations based on the structural data, the phenyl twist angles obviously must be determined by lattice packing, and the negative charge is delocalized predominantly within the anthracene π system. The counter cation [Na⊕(THF)6], reported ordered for the first time, shows nearly octahedral coordination within a rather densily packed solvent shell. Due to the strong repulsions between the solvent molecules, its isodesmically calculated solvation enthalpy is smaller than that of the analogous dimethoxyethane complex [Na⊕(DME)3].
Computational Approach to the Receptor‐Bound Conformation of Cyclosporin A
Authors: Köck Matthias , Müller Gerhard , Kessler Horst 
Published: December 2004

AbstractThe conversion of the conformation of cyclosporin A (CsA) observed in CHCl3 to the receptor‐bound state is investigated by two molecular‐mechanics methods, template forcing and dynamic forcing. The conformations of CsA in CHCl3 and complexed with LiCl in THF as determined by NMR are used as starting structures. The transition starting from the CsA/CHCl3‐derived conformation is hindered by steric interactions of two side chains (MeBmt1 and Val5). While starting with the CsA/LiCl‐derived conformation, the conversion is facile. It is illustrated that these calculations, which are of artificial character, using only the starting and final structures of the observed conformational transition during the receptor‐binding event, allow an insight into the interactions between the substrates and receptor in terms of an induced fit.
NMR of Terminal Oxygen. Part 13. 17O‐NMR spectra of C‐nitroso compounds, thionitrites and NO+ Ion: Resonance effects in ON—X compounds and correlation with CD spectra
Authors: Dahn Hans , Péchy Peter , Flögel Rainer 
Published: December 2004

AbstractThe 17O‐NMR signals of four true C‐nitroso compounds 1–4 appear at particularly low field (1550–1265 ppm), whereas the dimers (azodioxy type) resonate at ca. 400 ppm and the ‘isonitroso compounds’ (  quinone‐oximes; 5 and 6) at ca. 250 ppm. S‐Nitroso compounds (  thionitrites; 8 and 9) show shift values of ca. 1300 ppm, not far from C—NO; the NO+ ion is much stronger shielded (474 ppm). The results, together with those for higher‐shielded nitroso compounds X—NO (X  RO, R2N, Cl, O−) are discussed in terms of (a) resonance stabilization through n‐donation from X(π‐bond order, approximated by the known barriers of rotation around the X—N bond) and of (b) electronic excitation energies ΔE. The latter are approximated by long‐wave (symmetry‐forbidden) UV/VIS absorptions and confirmed, where available, by the maxima of the curves of circular dichroism (CD); the CD curve of thionitrite 9 has been measured. It is found that the δ(17O) values of X—NO depend both on bond order and on ΔE, which could not be separated. The higher shielding of NO+ compared with X—NO is explained on the basis of anisotropy effects, which differ between sp and sp2 systems.
Stereoselektive Synthesen von (Z)‐(10‐Methoxy‐4H‐benzo[4,5]cyclohepta[1,2‐b]thiophen‐4‐yliden)essigsäure
Authors: Waldvogel Erwin 
Published: December 2004

Stereoselective Syntheses of (Z)‐(10‐Methoxy‐4H‐benzo[4,5]cyclohepta[1,2‐b]thiophen‐4‐ylidene)acetic AcidTwo stereoselective syntheses for the antiinflammatory compound 1 ((Z)‐isomer) are described. In the first approach (Strategy A, Scheme 1) the stereoselective synthesis of 1 was realized via the bicyclic compound 11 under thermodynamic conditions, followed by a thiophene annelation with retention of the double‐bond geometry (Schemes 2–4). Optimized conditions were necessary to avoid (E/Z)‐isomerization during annelation. In the second approach (Strategy B, Scheme 1), diastereoisomer 17b was obtained selectively from a mixture of the diastereoisomers 17b and 18b by combining thermodynamic epimerization and solubility differences (Scheme 5). Diastereoisomer 17b was converted into the tricyclic compound 23 using a novel thiophene annelation method which we described recently (Scheme 6). In a final step, a stereospecific ‘syn’‐elimination transformed the sulfoxide 24 into the target compound 1 (Scheme 7). To avoid (E/Z)‐isomerization, it was necessary to trap the sulfenic acid liberated during the reaction. The key reactions of both approaches are highly stereoselective (> 97:3).
Construction de complexes tétranucléaires de ruthénium à ossature planaire par condensation de deux unités diruthénium à l'aide de ligands pontants: Synthèse et structure moléculaire de [Ru4(CO)8{μ2‐P(Cy)2}4] et de [Ru4(CO)8{μ4‐P(Cy)}2{μ2‐P(Cy)2}2] (Cy = cyclohexyl)
Authors: Béguin Alain , Rheinwald Gerd , Stoeckli‐Evans Helen , Süss‐Fink Georg 
Published: December 2004

Assembly of Tetranuclear Ruthenium Complexes with Planar Metal Core by Condensation of Two Diruthenium Units Using Bridging Ligands: Synthesis and Molecular Structure of [Ru4(CO)8{μ2‐P(Cy)2}4] and [Ru4(CO)8{μ4‐P(Cy)}2{μ2}2](Cy = Cyclohexyl)The dinuclear complexes [Ru2(CO)6{μ‐P(Cy)2}2] (1) or [Ru2(CO)4{μ‐(HCO2)}2{P(Cy)2H}2] (2) react in THF solution at 160° to give the tetranuclear complexes [Ru4(CO)8{μ2‐P(Cy)2}4] (3) and [Ru4(CO)8{μ4‐P(Cy)}2{μ2‐P(Cy)2}2] (4), as well as the trinuclear complex [Ru3(CO)7(μ2‐H){μ2‐P(Cy)2}3] (5). If the reaction is performed at 200°, the bicapped tetranuclear species 4 is obtained in a higher yield, whereas 3 and 5 are formed in trace amounts only. The phenyl derivatives [Ru2(CO)6{μ‐P(Ph)2}2] (6) or [Ru2(CO)4{μ‐(EtCO2)}2{P(Ph)2H}2] (7) react in a similar manner to give only the complex [Ru4(CO)8{μ4‐P(Ph)}2{μ2‐P(Ph)2}2] (8), analogous to 4. The molecular structure of 3 consists of a planar framework of four Ru‐atoms, each Ru—Ru bond being bridged by a μ2‐dicyclohexylphosphino ligand. The complex 4 represents a planar rectangular Ru core, both faces being capped by μ4‐cyclohexylphosphinidene ligands and two opposite edges being bridged by μ2‐dicyclohexylphosphino ligands.
An Easy and Fast Conversion of N2‐[(tert‐Butoxy)carbonyl]‐L‐amino Acids to Corresponding Amino‐aldehydes
Authors: Zlatoidsky Pavol 
Published: December 2004

AbstractA new method for the preparation of N2‐[(tert‐butoxy)carbonyl]‐L‐amino‐aldehydes from N2‐[(tert‐butoxy)carbonyl]‐L‐amino acids based on reduction of mixed anhydrides with LiAl(t‐BuO)3H is described.
1:1 Metal Complexes of Bivalent Cobalt, Nickel, Copper, Zink, and Cadmium with the Tripodal Ligand tris[2‐(dimethylamino)ethyl]amine: Their stabilities and the X‐ray crystal structure of its copper(II) complex sulfate
Authors: Anderegg Giorgio , Gramlich Volker 
Published: December 2004

AbstractThe complex formation by Co2+, Ni2+, Cu2+, Zn2+, and Cd2+ with tris[2‐(dimethylamino)ethyl]amine (N(CH2CH2NMe2)3, Me6tren) was investigated at 25° and at an ionic strength of 1, using VIS spectroscopy and potentiometric measurements. The stability constants of these complexes are compared with those of tris(2‐aminoethyl)amine (N(CH2CH2NH2)3, tren), obtained under the same conditions. The values of the constants for Me6tren are much lower than those for tren, due to the bulky Me substituents. The values of the constants can be correlated with the ability of the individual metal ions to adopt coordination number 5. This appears to be easier for Cu2+ and Co2+ than for Cd2+ and Zn2+ and is very difficult for Ni2+. The 1:1 complexes [ML(H2O)]2+ are monoprotonic acids whose pKs values are similar or lower than those of the corresponding aquametal ions. The X‐ray crystal structure of the copper(II) complex [Cu(SO4)(Me6tren)] · 8H2O reveals pentacoordination at the central ion. The UV/VIS spectra of the aqueous solutions of the Co2+, Ni2+, and Cu2+ 1:1 complexes confirm that the same coordination number is present also in these complexes.
Studies Directed toward the Synthesis of Phomenoic Acid. Part 1. Enantioselective synthesis of the C(1)‐to‐C(6) segment
Authors: Hassan Hammed H. A. M. , Tamm Christoph 
Published: December 2004

AbstractBased on a retrosynthetic analysis, a concept for the synthesis of all stereoisomers of the C(1)‐to‐C(6) segment of phomenoic acid has been developed. Both enantiomers of the chiral synthon 9 were prepared starting from rac‐epoxy‐diester 10. They were converted to both enantiomers of the epoxyalcohol, 16 and 21, respectively, using Sharpless epoxydation. They served as building blocks for the synthesis of the tetrol derivatives 20 and 22, respectively. All four stereoisomers were obtained in optically pure form. They contain the correct assembly of protected functional groups, allowing selective deprotection in view of further transformations.
Asymmetric Catalysis by Vitamin B12: The isomerization of achiral cyclopropanes to optically active olefins
Authors: Troxler Thomas , Scheffold Rolf 
Published: December 2004

AbstractAchiral spiroactivated cyclopropanes are isomerized to optically active (R)‐(cycloalk‐2‐enyl)‐Meldrum's acids ( = (R)‐5‐(cycloalk‐2‐enyl)‐2,2‐dimethyl‐1,3‐dioxane‐4,6‐diones) in high yield and ee's up to 86% by catalytic amounts of cob(I)alamin in polar protic solvents.
Synthesis of Enantiomerically Pure D‐ and L‐(Heteroaryl)alanines by asymmetric hydrogenation of (Z)‐α‐amino‐αβ‐didehydro esters
Authors: Masquelin Thierry , Broger Emil , Müller Klaus , Schmid Rudolf , Obrecht Daniel 
Published: December 2004

AbstractHomogeneous asymmetric hydrogenation of a wide range of methyl and tert‐butyl (Z)‐2‐(acylamino)‐3‐(heteroaryl)acrylates (see 1a–f and 2a–d, f, g, resp.) catalyzed by diphosphinerhodium catalysts was studied for the synthesis of enantiomerically pure 3‐furyl‐, 3‐thienyl‐, and 3‐pyrrolylalanines (see 3a–f, and 4a–d, g; Scheme 1). The precursors, the (Z)‐α‐amino‐α,β‐didehydro esters 1a–f and 2a–d, f, g were prepared in high yields using the phosphorylglycine‐ester method (Scheme 1). Isomerically pure (Z)‐α‐amino‐α,β‐didehydro esters were required to obtain the highest enantiomeric excesses (ee's) in the asymmetric hydrogenation, and the tert‐butyl‐ester strategy was beneficial in terms of both getting pure (Z)‐α‐amino‐α,β‐didehydro esters and obtaining high ee's in the hydrogenation. Finally, in contrast to the methyl‐ester series, deprotection of the tert‐butyl esters 4a–d, g was easily performed using CF3CO2H without any racemization.
A Reinvestigation of the α‐Alkylation of 4‐Monosubstituted 2‐phenyloxazol‐5(4H)‐ones (‘azlactones’): A general entry into highly functionalized α,α‐disubstituted α‐amino acids
Authors: Obrecht Daniel , Bohdal Udo , Ruffieux Ruth , Müller Klaus 
Published: December 2004

AbstractNovel, more reliable and general reaction conditions for the α‐alkylation of 4‐monosubstituted 2‐phenyloxazol‐5(4H)‐ones ( = 4‐monosubstituted 2‐phenyl‐azlactones) rac‐2 to 4,4‐disubstituted 2‐phenyloxazol‐5(4H)‐ones rac‐1 were found (Scheme 2). Thus, a whole range of highly functionalized rac‐1 were prepared in medium‐to‐good overall yields (40‐90%, see Table). Azlactones rac‐1 are ideal precursors for the synthesis of optically pure α,α ‐disubstituted (R)‐ and (S)‐α‐amino acids.
Synthetic Routes to the Cyclo[n]carbons
Authors: Diederich François , Rubin Yves , Chapman Orville L. , Goroff Nancy S. 
Published: December 2004

AbstractCyclo[n]carbons (cyclo‐Cn) are n‐membered monocyclic rings of sp‐hybridized C‐atoms with unique electronic structures resulting from two perpendicular systems of conjugated π‐orbitals, one in‐plane and one out‐of‐plane. Several synthetic approaches to generate cyclo‐C18 from stable precursors were investigated. In a six‐step sequence from anthracene, tris(anthraceno)hexadehydro[18]annulene 5 was prepared and shown by laser‐desorption time‐of‐flight mass spectrometry to generate C18 in three successive retro‐Diels‐Alder reactions, together with anthracene as the by‐product. The attempted preparation of C18 by flash‐vacuum pyrolysis of 5 using solvent‐assisted sublimation only afforded anthracene next to polymers. The reaction of 1,6‐bis(triisopropylsilyl)hexa‐1,3,5‐triyne with [Co2(CO)8] followed by exchange of two CO groups for a bridging bis(diphenylphosphino)methane (dppm) ligand gave Co complex 22 which, after removal of the silyl groups, was oxidatively cyclized to afford the very stable trimeric and tetrameric macrocycles 6 and 7, complexes with cyclo‐C18 and cyclo‐C24, respectively. An X‐ray crystal structure established the identity of 6 and showed that the butadiyne units within the C18 core are considerably bent. Attempts to free cyclo‐C18 from the coordinating metal atoms in 6 did not succeed, presumably due to the steric shielding of the Co‐atoms by the dppm ligands. Low‐temperature matrix isolation studies using IR and UV/VIS spectroscopy showed that irradiation of carbon oxide 2 (C24O6) leads to ketene intermediates and, by subsequent loss of six CO molecules, presumably to cyclo‐C18.