bluuebunny/crossref_metadata_2025 huggingface dataset:
Synthese von optisch aktiven Carotinoiden mit 3,5,6‐Trihydroxy‐5‐6‐dihydro‐β‐Endgruppen
Authors: Buchecker Richard , Marti Urs , Eugster Conrad Hans
Published: December 2004

Syntheses of Optically Active Carotenoids with 3,5,6‐Trihydroxy‐5,6‐dihydro β‐End Groups

For the specification of the relative and absolute configuration in carotenoids with 3,5,6‐trihydroxy‐5‐6‐dihydro β‐end groups, several ionone derivatives and carotenoids bearing this end group were synthesized. Acid‐catalyzed hydrolysis of (3S,5S,6R)– acetoxy‐5,6‐epoxy‐5,6‐dihydro‐β‐ionone (7) and of its (3S,5R,6S)‐isomer (13) gave the diols 8 and 15, respectively, with exclusive inversion at c(5) (Scheme 2). Compared to this, mild acid hydrolysis of caroten‐5‐6‐expoxides in the presence of H2O resulted in the formation of 5,6‐diols with either inversion or retention of the configuration at C(6) (Scheme 3). Spectroscopic data allowed us to distinguish the relative configurations (3R*,5S*,6S*) (see A), (3R*,5R*,6R*) (see B), (3R*,5S*,6R*) (see C), and (3R*,5R*,6S*) (see D), of the 3,5,6‐trihydroxy‐5‐6‐dihydro β‐end groups. Syntheses of the optically active carotene‐hexols 20 and 21 and comparison with published data led to a revision of the structure of mectrazanthin (now formulated as 20), heteroxanthin (now formulated as 28), and further carotenoids with 3,5,6‐trihydroxy end groups.
Structure Revision of the Rauwolfia Alkaloid Raucaffricine
Authors: Schübel Helmut , Treiber Arno , Stökigt Joachim
Published: December 2004

The previous structure of the Rauwolfia alkaloid raucaffricine (vomilenine‐α‐D‐galactoside) has been revised to be vomilenine‐β‐D‐Glucoside by enzymatic and NMR studies.
Comment on the Estimation of the Dimerisation Constant of Caffeine
Authors: Stamm Helmut , Timeus Friedrich
Published: December 2004

Reasons for the low reliability of the

Compare e.g. former results cited in [3][4] with our results in [3].

H‐NMR estimation of the dimerisation constant of caffeine in D2O are discussed. The main reasons are (1) strong influence of small variations in the monomer shifts δ0 which are to be determined by extrapolation, and (2) disturbance by the formation of higher oligomers. The dilemma ist that attempts to minimize errors from (1) increase the influence of (2) and vice versa.
Conformational Equilibria in trans‐1,2‐Diarylethylenes Manifested in their Emission in Solution. Part VI. Heterocyclic Analogues, their Triplets and Exciplexes
Authors: Wismontski‐Knittel T. , Das P. K. , Fischer E.
Published: December 2004

Solutions of heterocyclic analogues of 2‐styrylnaphthalene (2‐St‐N), with benzo[b]thiophene and benzo[b]furan groups replacing the 2‐naphthyl group, exhibit emission anomalies similar to those reported for 2‐St‐N, the most prominent one being a variation of the emission spectra with the excitation wavelength. The exciplexes formed when emission is quenched by N,N‐dimethylaniline (DMA) show a smaller variation of their emission maxima. Ground‐state rotamers may be responsible for the anomalies. The lifetimes of regular fluorescence in toluene are ≤ 2 ns, and our equipment did not allow to determine whether or not the decay is mono‐exponential. Exciplex lifetimes in toluene are 20‐30 ns. The triplets (λ  500‐520 nm) are characterized by short lifetimes (180‐190 ns) and appear to have substantial contributions from twisted configurations.
Bond Angles in Lactones and Lactams
Authors: Nørskov‐Lauritsen Leif , Bürgi Hans‐Beat , Hofmann Peter , Schmidt Helmut R.
Published: December 2004

The carbonyl group in lactones and, to a lesser extent, in lactams tends to show a CCO angle that is larger than the OCO or NCO angle, the difference increasing in magnitude as the ring size decreases. The observed trend provides information on ratios of force constants characterising the flexibility of the s‐cis‐ester group and may be interpreted in terms of incipient chemical reaction to −O(CH2)nCO+. Molecular orbital calculations (MINDO/3, MNDO, EH) for model compounds provide an electronic interpretation of this angle difference in terms of an anomeric interaction between the p‐type lone pair on the carbonyl O‐atom and the antibonding C‐O or C‐N orbital.
Stereoselective Double Functionalization of Iron‐Carbonyl Complexes of 5,6,7,8‐Tetramethylidenebicyclo[2.2.2]oct‐2‐ene. Crystal Structure and Absolute Configuration of (–)‐trans‐μ‐[(2S,5R,7S)‐C,5,6,C‐η:C,7,8,C‐η‐(6,7,8‐trimethylidene‐5‐((Z)‐2‐oxopropylidene)‐2‐bicyclo[2.2.2]octyl acetate)]‐bis(tricarbonyliron)
Authors: Tagliaferri Enrico , Campiche Philippe , Roulet Raymond , Gabioud Raphy , Vogel Pierre , Chapuis Gervais
Published: December 2004

The Friedel‐Crafts monoacylation of trans‐η‐[(1RS,2RS,4SR,5SR,6RS,7SR,8SR)‐C,5,6,C‐η:C,7,8,C‐η‐(5,6,7,8‐tetramethylidene‐2‐bicyclo[2.2.2]octyl acetate)]‐bis(tricarbonyliron) ((±)‐5) is highly stereoselective and yields trans‐η‐[(1RS,2RS,4RS,5SR,6RS,7RS,8SR)‐C,6‐η,oxo‐σ:C,7,8,C‐η‐(6,7,8‐trimethylidene‐5‐((Z)‐2‐oxopropylidene)‐2‐bicyclo[2.2.2]octyl acetate)]‐bis(tricarbonyliron) ((±)‐8) which equilibrates with the trans‐η‐[(1RS,2RS,4RS,5SR,6RS,7RS,8SR)‐C,5,6,C‐η:C,7,8,C‐η‐(6,7,8‐trimethylidene‐5‐((Z)‐2‐oxopropylidene)‐2‐bicyclo[2.2.2]octyl acetate)]‐bis(tricarbonyliron) ((±)‐9) on heating. Optically pure (–)‐9 has been prepared from the corresponding optically pure alcohol (+)‐4. The structure and absolute configuration of (–)‐9 was established by single‐crystal X‐ray diffraction.
Tritylketone und Tritylenone. Beiträge zur sterisch erzwungenen Michael‐Addition und zur diastereoselektiven Aldol‐Addition
Authors: Seebach Dieter , Ertas̨ Mümtaz , Locher Rita , Bernd Schweizer W.
Published: December 2004

Tritylketones and Tritylenones. Contributions to the Sterically Enforced Michael Addition and to the Diastereoselective Aldol Addition

Tritylketones are prepared from trityllithium and aldehydes, with subsequent CrO3 oxidation (Scheme 1, 2a‐f). Tritylenones are obtained from the saturated ketones and aldehydes or ketones, preferably by (CH3)3Al‐mediated aldol addition with subsequent dehydration (Scheme 2, products 2e, 4b‐e). The carbonyl of the tritylketone group is sterically protected, but electronically effective (see A‐C); thus, amine‐free enolate solutions can be obtained directly with BuLi; also, exclusive conjugate addition of organolithium derivatives occurs with tritylenones (Schemes 3‐5, products 2d, 5‐7, 15 examples). The lithiumenolates of tritylketones add to aldehydes with practically complete stereoselectivity (Scheme 6, products 10, 9 examples): due to the bulkiness of the trityl group, only the (Z)‐enolates 8 are formed, and the approach of the two trigonal centers in the aldol‐addition step is enforced to occur with relative topicity ul. As a first example of an X‐ray structure determination of a silyl enol ether, the crystal structure of (Z)‐2‐(trimethylsilyloxy)‐1, 1, 1‐triphenyl‐2‐butene (9) is reported. Fortunately, the blocking of the carbonyl group in trityl ketones can be run very specifically (without epimerization at the α‐carbonyl center) by lithium triethylborohydride to furnish, after aqueous workup, primary alcohols and Ph3CH (Eqn. 4, products 13–20, 10 examples); the OH‐group of the aldols 10 must be EE‐, MOM‐ or MEM‐protected (Scheme 8) before this C,C‐bond cleavage can be conveyed. Some of the cleavage products are used for the chemical correlation of the aldol configuration and for the demonstration of the synthetic value of the presented method (see 21–29).
Synthesis of Macrobicyclic Polyamines by Direct Macrobicyclisation via Tripode‐Tripode Coupling
Authors: Dietrich Bernard , Hosseini Mir Wais , Lehn Jean‐Marie , Sessions Richard B.
Published: December 2004

The synthesis of five macrobicyclic polyamines 1–5 is described following a route in which the macrobicycle is formed by the coupling of two tripodal subunits. Such a sequence is appreciably shorter than the stepwise construction via a macrocycle, and may give higher yields, as illustrated by the case of bis‐tren 3, which has been synthesized following both routes.
On Cardioactive Steroids. XVI. Stereoselective β‐Glycosylation of Digitoxose: The Synthesis of Digitoxin
Authors: Wiesner Karel , Tsai Thomas Y. R. , Jin Haolun
Published: December 2004

Two methods for stereoselective β‐glycosylation of digitoxose were developed. The first achieved stereocontrol by a 1,3‐participation of a N‐methylurethane group under acid catalysis. The second utilized mercuric‐ion catalyzed cleavage of thioglycosides and a 1,3‐participation of a p‐methoxybenzoyl group in a neutral medium. The first highly stereoselective and quite efficient synthesis of digitoxin (C7) was achieved by a combination of these methods. The furyl‐substituted precursor IV of digitoxigenin (Scheme 1) was used as aglycone, and the furan group was converted to the unsaturated lactone of digitoxin by our known oxidation procedure (m‐chloroperbenzoic acid/NaBH4) after the assembly of the carbohydrate portion of the molecule and its deblocking was completed.
Reduktion von 1,2‐bis[(Z)‐(2‐nitrophenyl)‐NNO‐azoxy]benzol: Synthese von Cyclotrisazobenzol ( = (5E,6aZ,11E,12aZ,17E,18aZ)‐5,6,11,12,17,18‐Hexaazatribenzo[aei][1,3,5,7,9,11]cyclododeca‐hexaen)
Authors: Hilpert Hans , Hoesch Lienhard , Dreiding André S.
Published: December 2004

Reduction of 1,2‐Bis[(Z)‐(2‐nitrophenyl)‐NNO‐azoxy]benzene1: Synthesis of Cyclotrisazobenzene ( = (5E,6aZ,11E,12aZ,17E,18aZ)‐5,6,11,12,17,18‐Hexaazatribenzo[aei][1,3,5,7,9,11]cyclododeca‐hexaene)

Na2S reduction of 1,2‐bis[(Z)‐(2‐nitrophenyl)‐NNO‐azoxy]benzene (2) yielded 3 deoxygenated products: the (known) red 2,2′‐((E,E)‐1,2‐phenylenbisazo)dianiline (3, 23%), the orange 2‐[2‐((E)‐2‐aminophenylazo)phenyl]‐2H‐benzotriazol (4, 55%) and the colorless 2,2′‐(1,2‐phenylene)di‐2H‐benzotriazol (5, 13%). The constitutions of 3–5 and of 6, the N‐acetyl derivative of 4, were deduced from their 1H‐NMR spectra (chemical shifts, couplings, and symmetry properties), and the configurations of 3, 4, and 6 at their N,N‐double bonds are assumed to be the same as in 2. Oxidation of 3 with 2 mol‐equiv. of Pb(OAc)4 afforded 5 (47%) and a novel, highly symmetrical macrocycle, called cyclotrisazobenzene (7, 24%). The constitution of 7 as a tribenzo‐hexaaza[12]annulene and its (E)‐configuration at the N,N‐bonds was confirmed by X‐ray analysis. The molecular symmetry expressed by the 1H‐, 13C‐ and 15N‐NMR spectra of 7 reveals a rapid torsional motion around the six N,C bonds. This implies that the N,N‐double bonds in the cyclic 12π‐electron system (or 24π‐electron system if the benzene rings are included) of 7 are highly localized.
Synthesis and Diels‐Alder Reactivity of 1‐(Dimethoxy)methyl‐2,3,5,6‐tetramethylidene‐7‐oxabicyclo[2.2.1]heptane. Preliminary communication
Authors: Métral Jean‐Luc , Vogel Pierre
Published: December 2004

A short synthesis of the title compound 13 is reported. The acetal group in 13 enables one to control the regio‐ and stereoselectivity of the two successive Diels‐Alder additions of the tetraene. The first addition is significantly faster than the second one, thus making 13 a versatile reagent for regio‐ and stereoselective ‘tandem’ cycloadditions.
The Radical Anions of [2.2]Paracyclophane‐1,9‐diene and Some of its Derivatives. An Unexpected Conformational Interconversion
Authors: Bruhin Jürg , Gerson Fabian , Möckel Reinhart , Plattner Georges
Published: December 2004

The radical anions of [2.2]paracyclophane‐1,9‐diene (2) and its 1,10,12,13,15,16‐hexadeuterio derivative 2‐D6, as well as those of 4,5,7,8‐tetramethyl[2.2]paracyclophane‐1,9‐diene (3) and its 12,13,15,16‐tetradeuterio derivative 3‐D4, have been studied by ESR spectroscopy. The coupling constants for 2−· at 178 K are 0.422 mT for four equivalent olefinic protons and 0.046 and 0.020 mT, each for a set of four equivalent aromatic protons. This hyperfine pattern is consistent with either benzene ring bearing two pairs of equivalent protons and it points to a lowering of the anticipated D2h symmetry. The ESR spectra of 2−· are strongly temperature dependent, due to modulation of the two coupling constants of 0.046 and 0.020 mT; these have opposite signs and average to 0.013 mT at 273 K. The experimental findings are interpreted in terms of a transition state of D2h symmetry, 33 kJ/mol above two interconverting equivalent conformations of lower symmetry. Several pieces of evidence suggest that this symmetry is D2, i.e., the benzene rings in 2−· are twisted in opposite directions about the vertical axis. Temperature dependence of the ESR spectra, resulting from modulation of the hyperfine interactions with the aromatic protons, is also observed for 2‐D6−· and 3−·. In the case of 3−·, the olefinic protons are, as expected, only equivalent in pairs, the pertinent coupling constants being 0.560 and 0.325 mT. Upon standing at low temperatures, 2−· and 3−· gradually convert into the radical anions of [2.2]paracyclophane (1) and its 4,5,7,8‐tetramethyl derivative, respectively. At higher temperatures, cleavage of one bridging chain in 2−· also occurs, with the formation of the radical anion of (E)‐4,4′‐dimethylstilbene (7). Both reactions of 2−· must involve the transient radical anion of [2.2]paracyclophane‐1‐ene (4) as proved by the observation of the spectra of 1−· and 7−· with 4 as the starting material.
Acid‐Catalysed Rearrangements of α‐Vinylcyclobutanones
Authors: Jackson David A. , Rey Max , Dreiding André S.
Published: December 2004

The BF3 · Et2O‐ and the CH3SO3H‐catalysed rearrangements of 10 α‐vinylcyclobutanones have been examined. With little acid, the β,β‐dialkyl derivatives 1 were transformed into linear dienones 2 and 3; with more acid, they were converted into cyclopentenones 4 by Nazarov cyclisation of initially formed 2/3. The β‐monoalkyl (including the β,γ‐dialkyl) derivatives 7 rearranged only with a high acid concentration to afford the cyclopentenones 8 by 1,2‐acyl migration. In the case of 7a, the cyclopentenone 8a was accompanied by the unexpected constitutional isomer 9a, which is explained by a reversible interconversion of the cyclobutanone 7a with its isomer 19 via a cyclopropane intermediate like 18. In the case of the β,β‐dialkyl derivative 5, which contains an α‐isobutenyl (instead of an α‐vinyl) group, the acid‐catalysed rearrangement product was the bicyclo[3. 1. 0]hexanone derivative 6.
Rh‐Catalyzed Oxidation of Anthracenes with tert‐Butyl Hydroperoxide. Kinetic Aspects
Authors: Müller Paul , Bobillier Christiane
Published: December 2004

The polar effects of substituents on reactivity in oxidation of 2‐substituted anthracenes with tert‐butyl hydroperoxide (TBHP)/Rh(PPh3)3Cl have been investigated and compared with those obtained with TBHP/VO(acac)2 and chromic acid. The anthracene reactivities obtained from competition experiments are correlated with Hammett's σp‐constants. The P‐values are ‐2.60 for chromic acid and 0.72 for TBHP/VO(acac)2. A poor correlation with p = −0.17 (r = 0.756) was obtained for TBHP/Rh (PPh3)3Cl. It is concluded that the Rh‐catalyzed reaction does not consist in electrophilic oxygen transfer to the anthracene.
Photochemische und chemische Synthesen biologisch aktiver 3‐Oxazoline
Authors: Pfoertner Karl‐Heinz , Montavon François , Bernauer Karl
Published: December 2004

Photochemical and Chemical Syntheses of Biologically Active 3‐Oxazolines

Using the photoreaction of 2H‐azirines with carbonyl compounds, the 2,2‐dimethyl‐3‐oxazolines 5a, 5b, and 6a have been obtained which bear a mono‐ or dichlorophenyl substituent at C(4) of the five‐membered heterocycle and pyridyl at C(5). These compounds exhibit interesting activities against phytopathogenic fungi. Because 6a had been selected for field testing, its photochemical preparation was supplemented by a chemical synthesis. However, the overall performance of 6a in the field was inferior to some of the recently introduced agricultural fungicides of the ergosterol biosynthesis inhibitor type. Although 16 additional 3‐oxazolines with varied substituents have been prepared, the fungicidal activity found with 6a could not be improved.
Photochemical reactions. 143rd communication. Photochemistry of 5,6‐epoxy‐1,3‐dienes in the ionone series. Influence of a hydroxy group in the 7‐position
Authors: Goldener Urs , Scheller Markus E. , Mathies Peter , Frei Bruno , Jeger Oskar
Published: December 2004

The synthesis and photolyses of the epoxydiene (E)‐5 are described. On triplet excitation (λ > 280 nm, acetone), (E)‐5 undergoes initial cleavage of the C(5)O bond leading to the intermediate c. Presumably an H‐shift (c → e) followed by the fragmentation of the 1,4‐diradical e leads (via the enol 37) to the diketones (E)‐6 and (Z)‐12. Alternatively cleavage of the C(6)C(7) bond of c furnishes the diradical intermediate d which reacts by recombination leading to (E)‐13A + B, 16, and 17A + B, or by an H‐shift to the enol intermediate 38. The latter undergoes an aldol‐type reaction to (E/Z)‐14A + B and (E/Z)‐15A + B, as well as a photochemical [2 + 2]‐cycloaddition to 18. On singlet excitation (λ = 254 nm, MeCN), (E)‐5 undergoes photocleavage to the carbene intermediates f and g. The vinyl carbene f reacts with the adjacent double bond furnishing the cyclopropene 22 as the main product. From the carbene intermediate g, compounds 23, 24, and 25 arise by carbene insertion into the neighboring CC or CH bond. Furthermore, the diastereomer of the starting material, the epoxydiene (E)‐20, is formed via the ylide intermediate h.
Conformational Studies on Potential β‐Turn‐Forming Model Peptides
Authors: Bode Karsten , Goodman Murray , Mutter Manfred
Published: December 2004

The conformational behavior of POE‐bound model peptides Boc‐(L‐Ala)2‐X‐Y‐(L‐Ala)2‐NHPOE (X – Y = L‐Pro‐Gly (I), Gly‐L‐Ile (II); NHPOE = aminopoly(oxyethylene)) as well as of the repetitive hexapeptide of elastin, Boc‐L‐Val‐L‐Ala‐L‐Pro‐Gly‐L‐Val‐Gly‐A‐NHPOE‐M (III) (A = photosensitive 3‐nitro‐4‐(bromomethyl)benzoyl group; NHPOE‐M = aminopoly(oxyethylane) monomethyl ether) has been studied by means of 1H‐NMR and CD spectroscopy. Compounds I and III form a β‐turn with Pro and Gly in positions i + 1 and i + 2, respectively, while an aggregated state for II, has been identified. The results are in good agreement with published prediction codes giving experimental evidence for the dominance of short‐range interactions to establish secondary structure in solution.
The adamantane rearrangement of 1,2‐trimethylenenorbornanes. Part V. Rearrangements of 1,2‐trimethylenenorbornanes initiated by regioselective formation of carbocation centers at C(2) and C(6)
Authors: Klester Alfred Michael , Ganter Camille
Published: December 2004

The behaviour of the regioselectively generated carbocation centers at C(2) and C(6) in 1,2‐trimethylenenorbornanes was investigated in order to study the occurrence or absence of a degenerate rearrangement E⇄M in the adamantane rearrangement of both 1,2‐endo‐ (1) and 1,2‐exo‐trimethylenenorbornane (2) to 2‐endo,6‐endo‐trimethylenenorbornane (3). A degenerate rearrangement E⇄M is inevitably involved inasmuch as a 1,2‐trimethylenenorborn‐2‐yl cation E not only is formed directly as manifested by the conversions of the reactants 4 (C(2), C(3)‐olefin) and 6 (C(2), C(3′)‐olefin), but also indirectly (via F→E) if the leaving group at C(6) to be ionized occupies the endo‐position (6‐endo‐alcohol 8). No degenerate rearrangement E⇄M is operative starting from reactants that lead directly to a 2,6‐trimethylenenorborn‐2‐yl cation G; this is the case with both the ionization of the 6‐exo‐alcohol 10 having the leaving OH‐group in a stereoelectronically favoured configuration to undergo simultaneous C(1), C(2)‐bond migration (→G) as well as the protonation of the olefin 13 which is followed by same reaction pathway.
Synthesen von Dibenzo[b,e][1,4]dioxin‐2,3‐chinonen, einschliesslich der Ecklonochinone A und B sowie der Isoecklonochinone A und B
Authors: Lütolf Walter Leo , Prewo Roland , Bieri Jost H. , Rüedi Peter , Eugster Conrad Hans
Published: December 2004

Syntheses of Dibenzo[b,e][1,4]dioxin‐2,3‐quinones Including the Ecklonoquinones A and B and the Isoecklonoquinones A and B

Oxidation of monomesyloxy‐substituted pyrocatechols with MnO2 in toluene using phase‐transfer conditions leads in high yield to monomesyloxy‐substituted dibenzo[b,e][1,4]dioxin‐2,3‐quinones with loss of one mesyloxy group. In this way, ecklonoquinone A (2), ecklonoquinone B (3), isoecklonoquinone A (43), and isoecklonoquinone B (44) were prepared. Their structures are based on X‐ray analyses of ecklonoquinone‐A leucoacetate (45) and the mesyloxy‐substituted quinone 20. The reddish‐violet dibenzodioxin‐diquinone 49 was prepared from an intermediate of the iso‐series. The parent compound 1 has been synthesized in yields better than 50% from pyrocatechol and methyl 2,5‐dioxo‐2,5‐dihydrobenzoate as oxidant and 2‐methoxypyridin as catalyst. To rationalize the specific effect on the dimerisation step of the mesyloxy group, the intermediacy of 1,4‐quinone monoacetals is proposed. This also applies to a proposed biogenetic scheme.
Bis (2,4,6,8‐cyclononatetraen‐1‐yl)methane
Authors: Sabbioni Gabriele , Neuenschwander Markus
Published: December 2004

Bis (2,4,6,8‐cyclononatetraen‐1‐yl)methanes

Bis (2,4,6,8‐cyclononatetraen‐1‐yl)methanes (2a–c) have been prepared by reaction of all‐cis‐cyclononatetraenide with 1,1‐dichlorodimethyl ether as well as with carbenium ion precursors 9b and 9c. The title compounds 2 are attractive precursors of highly delocalised nonafulvenes of type 3; however, elimination experiments 2→3 failed so far.
Transferts d'hydrogènes dans les cations radicaux ethylène‐acetals dérivés de cyclopentanones et cyclohexanones
Authors: Audier H. E. , Tabet J. C. , Fetizon M.
Published: December 2004

Hydrogen Transfer during Decompositions of Molecular Ions of Acetals Derived from Substituted Cyclopentanones and Cyclohexanones

The isomerisation process of ethylene‐acetal radical ions of low internal energy has been reinvestigated in more detail. The ring contraction of cyclohexane derivatives into methylcyclopentanes is quite general for these ions. The relative rates of [1, n] H transfers to the C free radical resulting from CC cleavage α to the acetal group plays the most important part in the mechanism.
Photochemical reactions. 144th Communication. Photochemistry of 5,6‐epoxy‐1,3‐dienes in the ionone series: Influence of a methoxy group in position 7
Authors: Goldener Urs , Frei Bruno , Jeger Oskar
Published: December 2004

On triplet excitation (λ > 280 nm, acetone), the epoxydiene (E)‐5 undergoes initial cleavage of the C(5)O bond of the oxirane and subsequent cleavage of the C(6)C(7) bond leading to the diradical intermediate e which reacts by recombination furnishing the cyclic compounds (E/Z)‐6, (E/Z)‐7,8, and 9. Alternatively, a H‐shift leads to the aliphatic methyl‐enol ether 10 which undergoes a photochemical [2+2]‐cycloaddition to compounds 12 and 13, the main products on triplet excitation of (E)‐5. On singlet excitation (λ = 254 nm, MeCN), (E)‐5 undergoes cleavage to the carbene intermediates f and g. The vinyl carbene f reacts with the adjacent double bond furnishing the cyclopropene 14 as the main product. From the carbene intermediate g, the methyl‐enol ether 15 arises by carbene insertion into the neighboring CH bond. Furthermore, the diastereomer of the starting material, the epoxydiene (E)‐16, and compounds 17A+B are formed via the ylide intermediate h. Finally, the cyclobutene 18 is the product of an electrocyclic reaction of the diene side chain.
Synthèse de l'ériodermine
Authors: Pulgarin César , Gunzinger Jan , Tabacchi Raffaele
Published: December 2004

Synthesis of Eriodermin

The total synthesis of eriodermin (=2,7‐dichloro‐4‐formyl 3‐hydroxy‐8‐methoxy‐1,6‐dimethyl‐11H‐dibenzo[b,e] [1,4]dioxepin‐11‐one) is described.
Photochemical reactions. 145th Communication. Carbonyl vs. epoxyketone photochemistry: Photolysis of 1,2;3,4‐diepoxy‐2,6,6‐trimethyl‐1‐cyclohexyl methyl ketone
Authors: Phaff Rox , Bischofberger Norbert , Mathies Peter , Petter Walter , Frei Bruno , Jeger Oskar
Published: December 2004

The synthesis and photolysis of the title compound 3 is described. Irradiation (λ > 280 nm, MeCN) of the di‐epoxyketone 3 leads predominantly to γ–H abstraction. Cyclization furnishes the cyclobutanols 22–24, while cleavage gives compound 25, presumably via the allene‐oxide intermediate 36. Further, products 27 and 28 are formed by Norrish fragmentation and by initial cleavage of the C(α)O bond of the oxirane, respectively. The structures of the products 22–25, 27, and 28 were assigned on the basis of the spectral data of the photolysis products of the 13C‐labelled diepoxyketone[6,6‐dimethyl‐13C2]‐3 and by X‐ray analysis of the compounds 24 and 35, the latter being the p‐nitrobenzoate of 22.
Unusual Acid‐Catalyzed Rearrangement of Two α,α′‐Diimino‐oximes
Authors: Fuhrer Walter , Rasetti Vittorio , Rihs Grety
Published: December 2004

2‐[2‐(Alkylimino)‐2‐phenylethylidene]pyrrolidines (vinamidines, 3–6) were obtained either via activation of the corresponding vinologous amide 1 with Meerwein salt and subsequent treatment of the intermediate 2 with an amine, or more directly by acid‐catalyzed condensation of the Schiff bases derived from acetophenone with 2‐ethoxy‐1‐pyrroline. Nitrosation of these vinamidines led to α,α′‐diimino‐oximes. In two cases (10, 11), these oximes underwent acid‐catalyzed rearrangement with formation of a 5,6,7,8‐tetrahydroimidazo[1,2‐a]pyridine ring system (12, 13). X‐Ray analysis of one of these products (13) and also of one of the vinamidine salts (6) are presented.
Oberflächenmodifikation von Kohlenstoff‐Elektroden durch Zweikomponenten‐Polymere, aufgebaut aus Epoxidharzen und einem Derivat von Vitamin B12
Authors: Ruhe Annette , Walder Lorenz , Scheffold Rolf
Published: December 2004

Surface Modification of Carbon Electrodes by Polymers Consisting of Epoxy Resins and a Derivative of Vitamin B12

A derivative of vitamin B12 containing a 3,5‐diaminobenzoyl group in the c side‐chain has been synthesized. It was used as the amine component in the formation of an optically active cross‐linked epoxy‐polymer. Surface modification of carbon electrodes (basal plane graphite and carbon felt) has been achieved by impregnating them with solutions of the monomers followed by thermal curing at 120°C. Electrochemical responses of the Co(II)/Co(I) redox system from sub‐mono to multilayer coverages have been observed. The polymer exhibits catalytic activities typical for monomeric vitamin B12 as shown by spectroscopic and electrochemical techniques. Polymermodified electrodes with large specific surfaces have been used in organic electrosynthesis, i.e. the exclusive formation of valeronitrile by reductive cross‐coupling of ethyl iodide and acrylonitrile.
Metal Complexes with Macrocyclic Ligands. Part XXI. A nonlinear relationship between the stability of Cu2+ complexes and their complexation rate with 1,4,8,11‐tetraazacyclotetradecane
Authors: Wu Yi‐he , Kaden Thomas A.
Published: December 2004

The kinetics of the reaction (1) between 1,4,8,11‐tetraazacyclotetradecane (Cy) and a series of Cu(II) complexes CuL (L) = glycolate, malonate, succinate, picolinate, glycinate, iminodiacetate, nitrilotriacetate, N‐(2‐hydroxyethyl)ethylenediaminetriacetate, ethyienediamine, 1,3‐diaminopropane, diethylenetriamine and N,N‐bis(3‐aminopropylamine) were studied spectrophotometrically at 25° and I = 0.5 (KNO3). From the analysis of the log kobs/log [L] profiles obtained at different pH values the resolved bimolecular rate constants k (Table 3) were obtained by a nonlinear curve‐fitting procedure. For nearly all systems studied, the rate constant k, describing the reaction between the 1:1 complex CuL and the monoprotonated form of the macrocycle CyH, was obtained. The nonlinear relationship between log k and log KCuL and its nature is discussed. It is shown that the inverse relationship between reactivity and stability described by others is only a special case of the more general Eqn. 3 described here.
The Synthesis of 4,7‐Bis(dialkylamino)tricyclo[5.2.1.04,10]deca‐1(10),2,5,8‐tetraenes and their Reduction with Alkali Metal
Authors: Butenschön Holger , de Meijere Armin
Published: December 2004

The synthesis and characterization of the novel 4,7‐bis(dialkylamino)tricyclo[5.2.1.04,10]deca‐1(10), 2,5,8‐tetraenes 12 from 1,4,7‐trihalotriquinacenes 8 and secondary amines is reported. The structural and electronic characteristics of these as well as the acepentalene dianion (32−) and some related systems as determined by semiempirical (MNDO) calculations are discussed. Thereby, 32− should be a triply etheno‐bridged trimethylenemethane dianion exhibiting Y‐delocalization favored over the formation of a peripheral 10π‐electronic system. Attempts directed towards the generation of 32− by reacting tetraenes 12 with Na led to the formation of tris(dialkylamino)triquinacenes 9, presumably by a kind of reduction/disproportionation mechanism.
Etudes sur les composés organométalliques. Partie XXI. Contribution à l'étude de la décomposition du tétracyclohexyltitane
Authors: Meyer Edouard Marc , Jacot‐Guillarmod André
Published: December 2004

Studies on Organometallic Compounds. Contribution to the Study of Thermal Decomposition of Tetracyclohexyltitanium

Thermal decomposition of tetracyclohexyltitanium proceeds in a different manner for the solid compound and for etheral solution. We propose an explanation of the decomposition mechanisms based on radical reactivities and compare our results with previous literature data.
The Crystal Structure of Enniatin B
Authors: Kratky Christoph , Dobler Max
Published: December 2004

The X‐ray crystal structure of enniatin B (trigonal (hexagonal axes), a = 14.626 Å, c = 16.309 Å, space group R3) is described and the conformation compared to results of other investigations.
Diastereoselektivität der Geruchswahrnehmung von Alkoholen der Iononreihe
Authors: Schulte‐Elte Karl H. , Giersch Wolfgang , Winter Beat , Pamingle Hervé , Ohloff Günther
Published: December 2004

Diastereoselective Odor Perception of Alcohols in the Ionone Series

The characteristic odor of the diastereoisomers 1 and 2 of 1‐(2,2,6‐trimethylcyclohexyl)‐3‐hexanol is configuration dependent, the trans‐alcohol 1 being identified as the sensorily active component. Structure modification of model 1/2, for example substitution on C(2), C(13), and C(14) (ionon numbering) by CH3 groups, introduction of double bonds in the 3‐ or 4‐position, and isosteric substitution of C(7) by an O‐atom, leads to analogues revealing an unequivocal relation between stereochemistry and odor. The specific odor of alcohol 1 is generally released when all substituents are in an equatorial position; the resulting analogy with the molecular size and shape of odoriferous steroids suggests that the release of the particular scent can be correlated with a steroid‐resembling receptor event.
Nucleophilic 1,2‐Shifts of Carboxamide Groups in the Benzil‐Benzilic Acid Type Rearrangement of 4‐Aryl‐2,3‐dioxobutyramides and of Quinisatine
Authors: Gowal Heike , Spiess Anita , Ballenegger Marc , Due Laurent , Moll Hans , Schlunke Hans‐Peter , Dahn Hans
Published: December 2004

4‐Aryl‐2,3‐dioxobutyramide hydrates 1, undergo the benzil‐benzilic acid rearrangement to form (substituted) benzyltartronate monoamides 2. For compound 1a (Ar = Ph), it is demonstrated by isotopic labelling that the reaction occurs exclusively by migration of the CONH2 group. Kinetic measurements with 1a‐c and with the cyclic amide quinisatine 6 show that the rearrangement of the carboxamide group, proceeding via an alkali‐catalysed step, can reach a plateau in the kobs./[OH−] diagram (cf. the Fig.), due to complete formation of a mono‐anion, and a further increase of rate attributable to the rearrangement of a bis‐anion. Comparisons suggest that rearrangements involving an amide group are slower than those involving an ester group, and that, for this effect (as for others), the pre‐equilibrium deprotonation of the hydrate is more important than a specific migration tendency.
Nucleophilic addition to C,Cdouble bonds. Part IX. Kinetics and mechanism of the base‐catalyzed intramolecular cyclization of olefinic alcohols
Authors: Tombo Gerardo M. Ramos , Ganter Camille
Published: December 2004

The base‐catalyzed intramolecular cyclization of polycyclic olefinic alcohols of type a (10‐endo‐hydroxy‐anti9,10‐tricyclo [4.2.1.12,5]dec‐7‐en‐9‐ones (type h), anti9,10‐tricyclo[4.2.1.12,5] dec‐3‐en‐9‐endo‐ols (type j), and anti10,11‐tricyclo[4.3.1.12,5]undec‐3‐en‐10‐endo‐ols (type 1)) to the ethers d and f, resp., has been studied. A mechanism for the nucleophilic addition of the corresponding alkoxide anion b to the isolated C,Cdouble bond is discussed. It is proposed that b is formed (fast acid/base equilibrium) in the first step. For the subsequent reaction sequence, there are two well distinguishable pathways: (a) Compounds with an additional carbonyl group (h) cyclize via a homoenolate‐like intermediate c, which is protonated stereoselectively on the exo‐side by the hydroxylic solvent. (b) Compounds without a carbonyl group (j and l) cyclize 102‐104 times slower, and the reaction proceeds via a carbanion‐like transition state e. The proton transfer from the hydroxylic solvent is clearly coupled with the C,Obond formation. Steric compression in the olefinic alcohols a influences the cyclization rate: (a) Alcohols with a smaller ring (h, X = CH2CH2) cyclize 70–200 times faster than the ones with a larger ring (1, X = CH2CH2CH2). (b) Replacement of the H‐atom at the carbinol C‐atom by a CH3 group enhances the rate of ether formation by a factor of 50–100. Due to through‐bond interactions between the C,C‐double bonds, olefinic alcohols with an additional endocyclic C,C‐double bond (h and j, X = CHCH) cyclize 20–300 times faster than the corresponding monoolefinic ones (h and j, X = CH2CH2).
Formation of the 5‐Azoniafulvene Ion and its Benzo‐annellated Analogue from N‐Mannich Bases of Pyrrole and Indole
Authors: Burger Ulrich , Bringhen Alain O. , Wirthner Philippe J. , Schärer Jean‐Claude
Published: December 2004

1‐Dialkylaminomethylpyrroles are shown to behave in many respects like aminals. Acylation by an acid chloride, for instance, occurs normally at the amine‐type N‐atom rather than at the pyrrole ring. Spontaneous cleavage of the resulting quaternary acylammonium salts affords the 5‐azoniafulvene ion (3). This higly reactive iminium ion, and its benzo‐annellated analogue (4) can be trapped by electron rich aromatic compounds such as N‐methylpyrrole or N,N‐dimethylaniline. More elaborate N‐Mannich bases are accessible by addition of indoles to enamines.
Ion‐Transport Properties of Ionophores in Asymmetric Membranes
Authors: Morf Werner E. , Ruprecht Heidi , Simon Wilhelm
Published: December 2004

The selectivity and the permeability of neutral‐carrier‐based membranes for substrate ions not only depend on the complexation behavior of the ionophores used, but also on their environment. In asymmetric membranes, ionophores may generally exhibit asymmetric transport properties. Such asymmetric bulk membranes were prepared from two PVC half‐membranes incorporating synthetic cation‐carriers in different plasticizers. The results of potentiometric and electrodialytic studies are discussed on the basis of a theoretical analysis of the carrier‐mediated electrical properties of asymmetric bulk and bilayer membranes.
Stereoselective Double Friedel‐Crafts Acylation of trans‐μ‐(2,3,6,7‐Tetramethylidenebicyclo[3.2.1]octane)bis(tricarbonyliron). Crystal Structure of trans‐μ‐{(1RS,2RS,3SR,5RS,6SR,7SR)‐C,2,3,C‐η:C,6,7,C‐η‐[(Z,Z)‐1,1′‐(3,7‐Dimethylidenebicyclo[3.2.1]octane‐2,6‐diylidene)di(2‐propanone)]}bis(tricarbonyliron)
Authors: Alan Pinkerton A. , Schwarzenbach Dieter , Gabioud Raphy , Vogel Pierre
Published: December 2004

The Friedel‐Crafts mono and double acylations of trans‐μ‐[(1RS,2RS,3SR,5RS,6SR,7SR)‐C,2,3,C‐η:C,6,7,C‐η‐(2,3,6,7‐tetramethylidenebicyclo[3.2.1]octane)]bis(tricarbonyliron) (4) are highly stereoselective and yield trans‐μ‐{(1RS,2RS,3SR,5RS,6SR,7RS)‐C,2,3,C‐η :C,6,7,C‐η‐[(Z)‐1‐(3,6,7‐trimethylidenebicyclo[3.2.1]‐oct‐2‐ylidene)‐2‐propanone]}bis(tricarbonyliron) (5) and trans‐μ‐{(1RS,2RS,3SR,5RS,6SR,7SR)‐C,2,3,C‐η :C,6,7,C‐η‐[(Z,Z)‐1,1′‐(3,7‐dimethylidenebicyclo [3.2.1] octane‐2,6‐diylidene)di(2‐propanone)]}bis(tricarbonyliron) (6) whose structure has been established by single‐crystal X‐ray diffraction.
2,5‐Chinonmethide des 1,6‐Methano[10]annulens mit S,N‐ und N,N‐Acylketenacetal Struktur
Authors: Neidlein Richard , Hartz Georg
Published: December 2004

2,5‐Quinonemethides of 1,6‐Methano[10]annulene with the Structures of S,N‐ and N,N‐Acylketene Acetals

Investigation on the chemical reactivities of 2‐(tert ‐butoxy)‐1,6‐methano[10]annulene and the syntheses of the compounds 2, 4, 6–8, 11, 13–22 are described; the results of 1H‐ and 13C‐NMR‐spectroscopic measurements are reported.
(Cholestanyloxycarbonyl)benzyl esters as peptide substituents: Conformational properties of fully protected oligo‐L‐lysines with C‐terminal (cholestanyloxycarbonyl)benzyl and benzyl ester moieties
Authors: Toniolo Claudio , Bonora Gian Maria , Moretto Vittorio , Schneider Conrad H. , Rolli Hanspeter , Jung Marina , Izdebski Jan
Published: December 2004

This study involves L‐lysine oligo peptides, protected at the N‐terminus by the Nps and at the ε‐amino functions by Boc groups. Two series were prepared from dimer to octamer, one containing the p‐[(cholestan‐3β‐yloxy)carbonyl]benzyl, the other one the benzyl ester group at the C‐terminus. Conformational analyses were performed by IR absorption. The occurrence of the intermolecular β‐structure in the solid state and in CH2Cl2 solution was demonstrated for the highest oligomers. The relative stabilities of the self‐associated species were determined by adding a variety of polar solvents to the CH2Cl2 solutions. The cholestanyl‐containing peptides have a lower propensity to self‐aggregate than the benzyl‐ester analogues. Self‐aggregation and decreasing solubility run in parallel. It was also directly shown that soluble urea derivatives may disrupt intermolecular H‐bonds in CH2Cl2, a point of practical interest, particularly in solid‐phase peptide synthesis.
Optical Resolution of (RS)‐Pantolactone through Amide Formation
Authors: Fizet Christian
Published: December 2004

The optical resolution of (RS)‐pantolactone (1) is carried out through formation of diastereoisomeric amides. These are separated by a single hot washing with CH2C12 or CHC13. The used asymmetric amine, (1R)‐3‐endo‐aminoborneol (4), is readily accessible and can be recovered almost quantitatively after resolution.
Der Sterische Verlauf der nucleophilen Cyclopropanöffnung an den vier im Spiro(methylcyclopropan)‐Ring Diastereoisomeren Lanugonen J (Spirocoleonen)
Authors: Buss Peter , Prewo Roland , Bieri Jost. H. , Rüedi Peter
Published: December 2004

The Stereochemistry of the Nucleophilie Opening of the Cyclopropane Ring in the four Diastereoisomeric Lanugons J (Spirocoleons)

Methanolysis of the spiro(methylcyclopropane) moiety in the four diastereoisomeric lanugons J 3 (13S, 15R), 4 (13R,15R), 5 (13S,15S), and 6 (13R,15S) is shown to proceed stereospecifically with inversion of the configuration at the attacked C‐atom (C(15)). The resulting epimeric 2‐methoxypropyl‐substituted hydroxy‐1,4‐benzoquinones (royleanones) 7a/8a (monomethoxy derivatives) and 7b/8b (dimethoxy derivatives) could be separated by HPLC using the recently described ‘Non‐Aqueous Cation Exchange System’. Respective pairs of 1H‐NMR spectra exhibit minute but significant differences in the ABMX3 part of the 2‐methoxypropyl group. The final structure assignment is based on a single‐crystal X‐ray analysis of the dimethoxyroyleanone 7b, whose absolute configuration was established by chiroptical correlation with known abietanoids. Thus, methanolysis of 3 and 4 ((R)‐configuration at the reaction centre) yielded the royleanones 7a and 7b with (S)‐configuration, whereas analogous treatment of 5 and 6 ((S)‐configuration at the reaction centre) led to the corresponding derivatives 8a and 8b both having the (R)‐configuration in the 2‐methoxypropyl side chain.
Über Dienketene aus o‐Chinolacetaten
Authors: Quinkert Gerhard , Kleiner Erna , Freitag Bernd‐Jürgen , Glenneberg Jürgen , Billhardt Uta‐Maria , Cech Franz , Schmieder Klaus R. , Schudok Clemens , Steinmetzer Hans‐Christian , Bats Jan W. , Zimmermann Gottfried , Dürner Gerd , Rehm Dieter , Paulus Erich F.
Published: December 2004

On Dien‐Ketenes from o‐Quinol‐Acetates

A detailed picture of the photochemistry of o‐quinol‐acetates is presented. (RS)‐6‐Acetoxy‐6‐methyl‐, (RS)‐6‐acetoxy‐2,6‐dimethyl‐, (RS)‐6‐acetoxy‐5,6‐dimethyl‐, (RS)‐6‐acetoxy‐2,4,6‐trimethyl‐, (RS)‐6‐acetoxy‐2,3,4,6‐tetramethyl‐, and (RS)‐6‐acetoxy‐2,3,4,5,6‐pentamethyl‐2,4‐cyclohexadien‐1‐ones serve as representative educts. There are two separate main photochemical routes conveniently designated as 1(π*, n) or 3(π*, π) tracks. The latter may also be attained by sensitization and leads to phenols. The former, by α‐cleavage furnishes dienketens as indispensable phototransients. Photolysis of dien‐ketens follows one or more of three reaction channels, each of which yields a particular type of photoproduct: heat‐induced monocyclization affords 2,4‐cyclohexadien‐1‐ones, heat‐induced bicyclization stereoselectively furnishes bicyclo[3.1.0]hex‐3‐en‐2‐ones, and multi‐step addition of protic nucleophiles stereoselectively gives 1,4‐, 1,6‐ and/or 1,2‐adducts. By X‐ray analysis or NOE studies, the structure of isolated photoproducts is established. Conventional spectroscopy at low or flash spectroscopy at normal temperature yield information on the formation and decay of kinetically unstable intermediates. Photoproduct composition depends on the pattern of substitution of the educts, on the solvents, and on the nucleophiles that might be present. Substituents primarily exert an influence upon the population of the various conformers of the dien‐keten. Solvents affect the rate of the divers reaction paths competing for the phototransient. Nucleophiles play more than a trivial role when adducts are formed. With the detailed view of a dien‐keten's role on hand, the photoproduct from a given o‐quinol‐acetate or more general from a linear conjugated cyclohexadienone is now predictable.
Partial Purification and Characterization of Raucaffricine β‐D‐Glucosidase from Plant Cell‐Suspension Cultures of Rauwolfia serpentina BENTH
Authors: Schübel Helmut , Stöckigt Joachim , Feicht Richard , Simon Helmut
Published: December 2004

A novel highly substrate‐specific Rauwolfia enzyme, raucaffricine β‐D‐glucosidase, was isolated from cell‐suspension cultures of R. serpentina. The enzyme has been purified ca. 1350‐fold, its major characteristics such as Mr = 66600 ± 5%, pH optimum 5.1, temperature optimum 38°, and inhibition of its activity by glucose and fructose were investigated. Its limited distribution in different cell cultures and differentiated plants indicates that the enzyme is present in significant amounts exclusively in cultured Rauwolfia cells.
Synthesis of anti‐9‐Bromo‐ and anti‐9‐Chlorobicyclo[4.2.1]nonatriene via Bicyclo[4.2.1]nonatriene→Bicyclo[4.2.1]nonatriene Rearrangement
Authors: Goldstein Melvin J. , Barren James P.
Published: December 2004

The first syntheses of 9‐bromo‐ and 9‐chlorobicyclo[4.2.1]nona‐2,4,6‐trienes were each achieved in three stereoselective steps from the 9,9‐bis(selenophenyl) derivative 9 in 79% overall yield for the bromide and 64% for the chloride. A deuterium‐labeling experiment reveals the first rearrangement of a bicyclo[4.2.1]nonatriene which leaves the ring system intact.
Remarkable Enhancement of the Rate of Cathodic Reduction of Hydrocarbonate Anions at Palladium in the Presence of Caesium Cations
Authors: Spichiger‐Ulmann Martine , Augustynski Jan
Published: December 2004

Steady‐state polarization curves, representative of the electrochemical reduction of HCO ions at a smooth palladium cathode, obtained in CsHCO3 and NaHCO3 solutions, are compared. The net current densities, associated with the electroreduction of HCO anions, measured in 1M CsHCO3 solution are shown to be up to 9 times larger than those observed in 1M NaHCO3 solution. This result is interpreted in terms of a direct involvement of the Cs+ cations in the reaction at the cathode through the formation of ion pairs with the HCO anions.
C45‐ und C50‐Carotinoide. 4. Mitteilung. Synthese von optisch aktiven cyclischen C20‐Bausteinen und von (2R,2′R)‐2,2′‐Bis(4‐hydroxy‐3‐methyl‐2‐butenyl)‐β,β‐carotin ( = C.p. 450)
Authors: Wolleb Heinz , Pfander Hanspeter
Published: December 2004

C45‐ and C50‐Carotenoids: Synthesis of Optically Active Cyclic C20‐Building Blocks and of (2R,2′R)‐2,2′‐Bis(4‐hydroxy‐3‐methyl‐2‐butenyl)‐β,β‐carotene ( = C.p. 450)

The synthesis of the optically active C20‐building blocks (R)‐26 and (R)‐39 and of the optically active cyclic C50‐carotenoid C.p. 450 (3) starting from (−)‐β‐pinene is reported.
Sequence dependence of secondary‐structure formation part IV Helix‐forming potential of amphiphilic oligopeptides containing aib residues
Authors: Mutter Manfred , Altmann Karl‐Heinz , Flörsheimer Andreas , Herbert JüRgen
Published: December 2004

The conformational properties of Aib‐containing oligopeptides having the propensity to adopt amphiphilic helical conformations were investigated by CD spectroscopy in solution. The peptides CF3COOH.H‐Pro‐Glu‐[Ala‐Aib‐Glu‐Aib‐]4Gly‐OH (I), HCl.H‐Pro‐Ala‐Aib‐[Gul‐Ala‐Ala‐Aib‐]2Glu‐Ala‐Aib‐Gly‐PEGM (II), and CF3COOH.H‐Ala‐Aib‐[Glu‐Glu‐Ala‐Aib‐]3PEGM (III) were synthesized according to the general principles of the liquid‐phase method for peptide synthesis. Peptides I‐III exhibit helical conformations in CF3CH2OH, MeOH, and in H2O at acidic pH; however, at pH 7, only II forms a stable helix, whereas I and III are predominantly in an unordered conformation. Some general features for the construction of amphiphilic helices are discussed in the light of the experimental data.
Stereoselective Reduction of δ‐Hydroxy‐β‐ketoesters
Authors: Kathawala Faizulla G. , Prager Bernhard , Prasad Kapa , Repič Oljan , Shapiro Michael J. , Stabler Russell S. , Widler Leo
Published: December 2004

The reduction of δ‐hydroxy‐β‐ketoesters 1 was investigated with three different reducing agents. In several instances, high selectivity in favor of syn‐1,3‐diols was observed.
Konfigurationsbestimmung von geometrischen Isomeren mittels selektiver 13C{1H}‐NOE‐Differenzspektroskopie
Authors: Neidlein Richard , Kramer Walter , Ullrich Volker
Published: December 2004

Application of Selective 13C{1H}‐NOE Difference Spectroscopy in Determination of the Configuration of (Z)/(E)‐Isomers

Heteronuclear 13C{1H}‐NOE studies have been performed to elucidate the configuration at the imino double bond in N,N′‐Dicyanoquinonediimines. Selective low‐power irradiation of the neighbouring ring protons in compounds 1–5 increases the signal intensities of CN as well as of the imino‐C‐atoms in the 13C‐NMR spectra. The increase of signal intensity can easily be detected by the difference spectroscopy. The results of this investigation are in good agreement with a recently published study on the stereochemistry of N,N′‐Dicyanoquinonediimines, based on the interpretation of 13C‐chemical shifts.