bluuebunny/crossref_metadata_2025 huggingface dataset:
Early and rapid de novo synthesis of Alzheimer βA4‐Amyloid precursor protein (APP) in activated microglia
Authors: Banati R. B. , Gehrmann J. , Czech C. , Mönning U. , Jones L. L. , König G. , Beyreuther K. , Kreutzberg G. W.
Published: December 2004

Upon acute activation, microglia, the immuneffector cells of the brain parenchyma, express the amyloid precursor protein (APP) that is otherwise prominent in pathological structures related to Alzheimer's disease. In this disease complex amyloidbearing neuritic plaques contain βA4‐amyloid protein, the APP, and numerous inflammatory proteins. The accompanying activation of microglia has mostly been viewed as a secondary reaction to amyloid deposits.

Activation of microglia was performed in a graded fashion. Transection of peripheral nerves such as the facial or sciatic nerve causes a microglial reaction within hours in the nucleus of origin or in projection areas of the CNS. A predominantly glial up‐regulation of APP mRNA and protein could be detected as early as 6 h post lesion not only at the site of affected neuronal cell bodies but also in corresponding projection areas. Its time course suggests rapid transneuronal signalling to glial cells in the projection area. Light and electron microscopy demonstrate that microglia, which are cells of mononuclear phagocyte lineage and comprise up to 20% of all glial cells, are the dominant source for non‐neuronal APP expression. Ultrastructurally, brain perivascular cells within the basal lamina constitutively express APP and thus are a possible source of vascular amyloid.

Additionally, microglia express leukocyte‐derived (L)‐APP mRNA and protein that have recently been described in mononuclear cells of the immune system. Increased L‐APP expression may serve as a potential marker for glial/microglial activation. Such immune‐mediated amyloidogenesis initiated by microglia might have implications for the treatment of neurodegenerative diseases. © 1993 Wiley‐Liss, Inc.
A depolarization‐stimulated, bafilomycin‐inhibitable H+ pump in hippocampal astrocytes
Authors: Pappas Christopher A. , Ransom Bruce R.
Published: December 2004

Relatively little is known about the mechanisms of pHi regulation in mammalian glial cells. We analyzed pHi regulation in rat hippocampal astrocytes in vitro using the pH‐sensitive dye BCECF. All experiments were carried out in CO2/HCO3‐free solutions. Recovery from NH+4‐induced acid loads was strongly dependent on the presence of extracellular Na+ and was inhibited by amiloride and its more specific analog EIPA, indicating the presence of Na+‐H+ exchange in these cells. Removing bath Na+ or adding amiloride caused resting pHi to shift in the acid direction. Even in the absence of bath Na+ or presence of Na+/H+ inhibitors, however, these astrocytes continued to show significant recovery from acid loads. The mechanism of this amiloride‐insensitive and Na+‐independent pHi recovery process was sought and appeared to be a proton pump. In the absence of Na+, recovery from an acid load was completely blocked by the highly specific blocker of vacuolar‐type (v‐type) H+ ATPase, bafilomycin A1 (BA1). In normal Na+containing solutions, exposure to BA1 caused a small acid shift in baseline pHi and slowed recovery rate from NH+4‐induced acid loads by about 32%. The rate of Na+‐independent pHi recovery was increased by depolarization with 50 mM [K+] solution, and this effect was rapidly reversible and blocked by BA1. These results indicate that, in CO2/HCO−3‐free solution, pHi regulation in hippocampal astrocytes was mediated by Na+−‐H+ exchange and by a BA1‐inhibitable proton pump. Because the proton pump's activity was influenced by membrane potential, this acid exporting mechanism could contribute to the depolarization‐induced alkalinization that is seen in astrocytes. Although v‐type H+−ATPase had been previously isolated from the brain, this is the first report indicating that it has a role in regulating pHi in brain cells. © 1993 Wiley‐Liss, Inc.
Untersuchungen über stereoisomere Salze des Leucin‐methylesters
Authors: Weil K. , Kuhn Werner
Published: December 2004

Es wurden die Eigenschaften, insbesondere die Löslichkeiten diastereomerer Salze von d(−)‐ und von l(+)‐Leucin‐methylester mit optisch aktiven Säuren untersucht. Aus solchen Löslichkeitsbestimmungen lassen sich Schlüsse ziehen auf die Verwendbarkeit der einzelnen Säuren für die Eliminierung des im natürlichen Leucin in grosser Menge vorhandenen häufigen und damit für eine Anreicherung des seltenen Antipoden.
Etudes sur les matières végétales volatiles. XLIII. Présence de nérolidol dans des huiles essentielles de papilionacées
Authors: Naves Y. R.
Published: December 2004

Le d‐nérolidol présent dans le baume du Pérou n'est pas ou n'est qu'en partie le fruit de processus pathologiques. Associé à une petite proportion de farnésol, il constitue plus de 80% des essences de bois de Myrocarpus fastigiatus et M. frondosus Allem. (légumineusespapilionacées).
Etudes sur les matières végétales volatiles. XLIV. Présence de nérolidol dans les huiles essentielles de papilionacées. (2ème communication)
Authors: Naves Y.‐R.
Published: December 2004

L'essence du bois rouge (vraisemblablement Myrospermum erythroxylon Fr. Allem.) possède une composition analogue à celle des huiles essentielles de bois d'autres sophorées, elle est constitée en majeure partie de d‐nérolidol.
Acetoxytropyliumbromid: Synthese, Strukturbeweis, Verhalten in Lösung
Authors: Engel Peter , Keller Urs Martin , Bigler Peter , Neuenschwander Markus
Published: December 2004

Acetoxytropyliumbromide: Synthesis, Structural Evidence, Behaviour in Solution.

Acetoxytropyliumbromide (9) is formed from tropone and acetylbromide. In solution the tropylium salt equilibrates with the educts, 9 being favored at low temperature in polar aprotic solvents. The structure of the title compound has been proved by X‐ray analysis, as well as by investigation of the 1H‐NMR. spectra of solutions of 9.
Über Pterinchemie. 56. Mitteilung. Die Kristallstruktur von Xanthopterin‐hydrochlorid
Authors: Bieri Jost H. , Hummel Willi‐Peter , Viscontini Max
Published: December 2004

The crystal structure of xanthopterine‐hydrochloride.

The crystal structure of the title compound, a pterine, has been determined by X‐ray analysis (direct methods) and refined with 1332 structure amplitudes to R = 0.027. The crystal system is monoclinic, space group P21/c, with unit cell dimensions a = 7.942, b = 8.417, c = 12.076 Å, β = 93.77°. The molecule is protonated at the N(1)‐position. The angle between the planes of the pyrimidine and pyrazine rings is 2.02°.
An ESR. and ENDOR. Study of the Radical Anions of Cyano‐ and Nitro‐Monosubstituted Naphthalenes
Authors: Fürderer Peter , Gerson Fabian
Published: December 2004

1H‐ and 14N‐coupling constants have been determined by ESR. and ENDOR. spectroscopy for the radical anions of 1‐ and 2‐cyano‐, and 1‐ and 2‐nitronaphthalene.
Synthesis with 1,2‐Oxazines. I. A Synthesis of 4,5‐Dihydro‐6H‐1,2‐Oxazin‐6‐one Derivatives
Authors: Hardegger Bruno , Shatzmiller Shimon
Published: December 2004

A synthesis of some 4,5‐dihydro‐6H‐1,2‐oxazin‐6‐one derivatives is described.
The Reaction of Singlet and Triplet Oxygen with 2‐Phenylnorbornene
Authors: Jefford Charles W. , Boschung André F. , Rimbault Christian G.
Published: December 2004

The reaction of singlet oxygen with 2‐phenylnorbornene (1) in aprotic solvents gives 3‐formylcyclopentyl phenyl ketone (2) (10%) and uncharacterized polymer (90%). When methanol is used as solvent, endo‐2‐phenyl‐exo‐2‐methoxy‐exo‐3‐hydroperoxynorbornane (4) and endo‐2‐(anti‐1′, 4′‐epidioxy‐5′,6′‐epoxycyclohex‐2′‐enyl)‐exo‐2,3‐epoxynorbornane (6 and 7) are obtained in addition to 2. Triplet oxygen with 1 gave 2, endo‐2‐phenyl‐exo‐2,3‐epoxynorbornane (8), and the trimer 9 or 10 of exo‐2,3‐epidioxy‐endo‐2‐phenylnorbornane. With protic solvents the amount of epoxide increased at the expense of trimer. The singlet and triplet oxygen reactions are discussed in the light of possible intermediates.
Herstellung und Reaktionen der valenzpolaromeren Verbindung (4,4‐Dimethyl‐2‐thiazolin‐5‐dimethyliminium)‐2‐thiolat ⇌ (1‐Dimethylthiocarbamoyl‐1‐methyl‐äthyl)‐isothiocyanat aus 3‐Dimethyl‐amino‐2,2‐dimethyl‐2H‐azirin und Schwefelkohlenstoff
Authors: Chaloupka Stanislav , Heimgartner Heinz , Schmid Hans , Link Helmut , Schönholzer Peter , Bernauer Karl
Published: December 2004

Synthesis and reactions of the valence polaromeric compound (4,4‐dimethyl‐2‐thiazoline‐5‐dimethyliminium)‐2‐thiolate ⇌ 1‐dimethylthiocarbamoyl‐1‐methyl‐ethyl isothiocyanate from 3‐dimethylamino‐2,2‐dimethyl‐2H‐azirine and carbon disulfide.

3‐Dimethylamino‐2,2‐dimethyl‐2H‐azirine (1) reacts with carbon disulfide to give crystals which have the dipolar structure 3a [(4,4‐dimethyl‐2‐thiazoline‐5‐dimethyliminium)‐2‐thiolate, Scheme 1]. In solution, the non‐dipolar (charge‐free) isomeric form 3b (1‐Dimethyl‐thiocarbamoyl‐1‐methyl‐ethyl isothiocyanate) is almost exclusively populated. Reaction products are derived from both forms: Derivatives of 3a are the hydrolysis product 6, the sodium borohydride reduction product 7 and the methylation products 9 and 10, respectively (Scheme 2). The isothiocyanate form 3b is responsible for the various reaction products with amines (Scheme 3). One of the reaction products with ammonia, namely 20, is also obtained by the reaction of 1 with thiocyanic acid.

Thermolysis of the azirine/carbon disulfide adduct 3 leads to 2‐dimethylamino‐4,4‐dimethyl‐2‐thiazoline‐5‐thione (17) in high yield. A possible mechanism is outlined in Scheme 4. The same compound is also formed by rearrangement of 3 under the catalytic influence of dimethylamine. Its structure has been established by X‐ray crystallography (section 4). Again a rearrangement is involved in the reductive (NaBH4) conversion of 17 to 7, the direct reduction product of the dipolar species 3a (Scheme 5).

The isothiocyanate form 3b is able to react with a second molecule of 3‐dimethylamino‐2,2‐dimethyl‐2H‐azirine (1) to yield compound 25, which in the crystalline or dissolved state appears to be almost entirely populated by the carbodiimide form with structure 25b (Scheme 7), though all reaction products of 25 (reduction with sodium borohydride, addition of water or hydrogen sulfide, Schemes 7 and 8) are derived from the dipolar form 25a, not detectable as such; here again therefore there is a dynamic equilibrium 25a ⇌ 25b.

The two forms of adduct 3, namely 3a and 3b, are obviously very easily interconverted at room temperature and therefore can be considered as valence polaromeric forms (section 5). A classification of the dipolar (zwitterionic) form is given, which allows a comparison of various dipolar species and gives as indication of charge stabilization by delocalization.

The versatile reactivity of the 3‐dimethylamino‐2,2‐dimethyl‐2H‐azirine/carbon disulfide adduct is demonstrated by the fact that with simple reagents approximately 25 derivatives have been obtained, most of them being new heterocyclic compounds.
Zur Reaktion zwischen Bis (dimethylamino)acetonitril und Phenylisocyanat. Vorläufige Mitteilung
Authors: Seckinger Karl
Published: December 2004

On the Reaction of Bis(dimethylamino)acetonitrile with Phenylisocyanate

Bis(dimethylamino)acetonitrile (1) reacts with phenylisocyanate to yield the imidazolidinedione 2, as confirmed by X‐ray structure analysis. The mechanism of formation of 2 is discussed.
Reaktion von 3‐Dimethylamino‐2,2‐dimethyl‐2H‐azirin mit Barbitursäure
Authors: Link Helmut , Bernauer Karl , Daly John J. , Chaloupka Stanislav , Heimgartner Heinz
Published: December 2004

Reaction of 3‐Dimethylamino‐2,2‐dimethyl‐2H‐azirine with Barbituric Acid

The reaction of 3‐dimethylamino‐2,2‐dimethyl‐2H‐azirine (1) with barbituric acid (4) in dimethyl formamide at room temperature yields a mixture of several compounds. The two main products 5 and 6 have been isolated in 40 and 10% yield, respectively, and their structures established by X‐ray analysis. In Schemes 4–6 reaction mechanisms for the formation of 5 and 6 are postulated, the first step beeing either a C‐ or an N‐alkylation of barbituric acid.

Reduction of 5 and 6 with NaBH4 in ethanol at room temperature yields 6,6‐dimethyl‐1,5,6,7‐tetrahydro‐pyrrolo[2,3‐d]pyrimidin‐2,4(3H)‐dione (7) and 3,3‐dimethyl‐2,3‐dihydro‐imidazo[1,2‐c]pyrimidin‐5,7(1H, 6H)‐dione (8) in 38 and 48% yield, respectively. Treatment of 6 with 3N aqueous NaOH at room temperature gives 3,3‐dimethyl‐imidazo[1,2‐c]pyrimidin‐2,5,7 (1H, 3H, 6H)‐trione (9) in 51% yield (Scheme 3).
Halogenierte Cyclobutanone als variationsreiche Vorläufer von Pyrethroiden: Synthesen durch [2+2]‐Cycloadditionen von Haloketenen an 1,1‐disubstituierte Äthylene
Authors: Martin Pierre , Greuter Hans , Belluš Daniel
Published: December 2004

Halogenated Cyclobutanones as Highly Versatile Precursors of Pyrethroids: Syntheses via [2+2]‐Cycloadditions of Haloketenes to 1,1‐Disubstituted Ethylenes

[2+2]‐Cycloaddition reactions of the new chloro‐(2,2,2‐trichloroethyl)ketene (9) with various 1,1‐disubstituted ethylenes as well as [2+2]‐cycloadditions of highly halogenated ketenes of type 32 are described. The halogenated cyclobutanones thus obtained represent valuable intermediates in a new synthesis of insecticidal pyrethroids (Scheme 1). Ketene 9, prepared in situ from readily available precursors, parallels, if not exceeds, dichloroketene in its reactivity. With isobutene, the cyclobutanone 10 (a precursor for cis‐3‐(2,2‐dichlorovinyl)‐2,2‐dimethylcyclopropane carboxylic acid) is obtained in 70% yield. The reaction of 9 with propene is stereospecific and leads to cis‐17, exclusively. This result may be explained in terms of the energetically most favourable approach of the reacting molecules in a [π2s + π2s] process.
Euphroside, A New Iridoid Glucoside from Euphrasia salisburgensis HOPPE
Authors: Sticher Otto , Salama Osama
Published: December 2004

Five iridoid glucosides have been isolated from the whole plant of Euphrasia salisburgensis. The structure of the new compound, named euphroside, and the identity of the others have been established by chemical transformations and spectral data.
Die absolute Konfiguration von Macrolin, einem Abbauprodukt des Alkaloides Villalstonin 179. Mitteilung über organische Naturstoffe
Authors: Neukomm Gisela , Kletzhäundler Erika , Hesse Manfred
Published: December 2004

The Absolute Configuration of Macroline, a Degradation Product of the Alkaloid Villalstonine

The absolute configuration of macroline (1), a degradration product of villalstonine (2), was determined. The chemical degradation of 1 gave a mixture of (–)‐(20S)‐20,21‐dihydro‐19‐desoxo‐macroline (9) and (–)‐(20R)‐20,21‐dihydro‐19‐desoxo‐macroline (10), which was related to the degradation products 9 of (+)‐ajmaline (3) and 10 of (+)‐isoajmaline (4) of known absolute configuration. Together with the complete relative stereochemistry of 2 the absolute configuration of 2 and of the second moiety of 2, (+)‐pleiocarpamine (20), could be derived. Since the absolute configuration of the (–)‐6, 7‐dihydroaspidospermidine moiety of (+)‐pycnanthinine has been known already [15], the structure by Gorman et al. [17] represents the absolute configuration of (+)‐pycnanthinine.
Electronic Absorption and Emission Spectra of 1, 3‐Diaryltriazenes
Authors: Páarkányi Cyril , Vernin Gaston , Julliard Michel , Metzger Jacques
Published: December 2004

The Pariser‐Parr‐Pople (PPP) type LCI‐SCF‐MO calculations were used to study the models of 1, 3‐diphenyltriazene (1), 1, 3‐bis(3‐pyridyl)triazene (2), 1, 3‐bis (2, 4‐dichlorophenyl)triazene (3), and 1, 3‐bis (4‐ethoxycarbonyl)triazene (4). The results of the calculations were compared with the experimental electronic absorption and emission (fluorescence, phosphorescence) spectra of these compounds.
Macrocyclische Spermidinalkaloide aus Pleurostylia africana LOES. 9. Mitteilung über Celastraceen‐Inhaltsstoffe
Authors: Wagner Hildebert , Burghart Josef
Published: December 2004

Macrocyclic Spermidine Alcaloids from Pleurostylia africana LOES.

The structure of pleurostyline, a new macrocyclic spermidine alkaloid from Pleurostylia africana (Celastraceae), has been elucidated mainly by 1H‐ and 13C‐NMR.‐spectroscopy. Pleurostyline represents a new structure type in which spermidine is incorporated in a 13‐membered lactam ring to which an additional cinnamoyl residue is fused to yield a 7‐membered ring. The same plant contains also the spermidine alkaloids celacinnine and cellallocinnine of which 13C‐ and high field 1H‐NMR.‐measurements have been carried out for the first time. Both alkaloids are interconvertible by UV.‐irradiation. In addition the existence of rotational isomerism of one amide group of these alkaloids has been demonstrated by NMR.‐spectroscopy.
Desorption/Chemical Ionization Mass Spectrometry of Naturally Occurring Glycosides
Authors: Hostettmann Kurt , Doumas Jacques , Hardy Michel
Published: December 2004

Desorption/chemical ionization mass spectrometry (D/CI.‐MS.) is a recently developed technique especially indicated for highly polar and non volatile compounds. Various naturally occurring glycosides such as saponins, iridoid and secoiridoid glycosides, cardenolides and flavone‐O‐glycosides have been investigated by this method. All the measurements were carried out on underivatized compounds. In addition to the structural informations generally furnished by field‐desorption mass spectrometry (molecular ion and sugar sequence), the molecular ion and pertinent fragments of the aglycone could also be obtained.
Computer Assisted Structural Interpretation of Mass Spectral Data
Authors: Gray Neil A. B. , Buchs Armand , Smith Dennis H. , Djerassi Carl
Published: December 2004

An interactive system of programs has been developed to assist in structure elucidation based on mass spectral data. The program relies on algorithms for generating all the structural isomers that constitute alternative explanations of the observed data and it associates relative plausibility values with the different isomers. The structure assembly part of the program allows for the use of overlapping substructural components, such as substructures inferred from the appearance of particular ion patterns in the spectrum of an unknown compound. Mass spectrum interpretation procedures used with this structure assembly process could exploit any form of spectrum‐substructure correlation scheme. In this work, the emphasis has been on the use of detailed and class specific spectrum‐substructure correlations. Applications of the program are illustrated by means of an example analysis of the mass spectra of a variety of marine sterols.
Preferential Solvation of the Sodium Cation in Binary Mixtures of Tetrahydrofuran with Polyamines, in Relation with the Chelate Effect
Authors: Delville Alfred , Detellier Christian , Gerstmans André , Laszlo Pierre
Published: December 2004

The problem of determining the chelate effect brushes against methodological snags: the choice of concentration units, and of the appropriate standard states. We avoid these pitfalls by defining the chelate effect from measurements on bidentate ligands alone, without recourse to comparison with the corresponding unidentate ligands. Quantitation of the parameters extracted from the data is effected by three independent and mutually consistent procedures. Solvation of the Na+‐cation by the polyamines follows the sequence: cadaverin < 1, 3‐diaminopropane ≪ ethylene diamine ≪ diethylenetriamine. Entry of the first and of the second diamine molecule into the sodium coordination shell are independent and equiprobable steps: K1 = K3 and K2 = K4, within the accuracy of the measurements. For ethylene diamine, the values of K1 and K3 are in the range 1.0–1.5 and those for K2 and K4 are in the range 83–102: attachment of the second N‐atom is considerably easier, by two orders of magnitude (chelate effect). The chelate effect is strongly reduced in cadaverin, with a longer hydrocarbon chain connecting the two amine functions.
Studies of molecular ions. IX. Detection of low‐lying ‘non‐Koopmans’ states in polyacetylene molecular cations
Authors: Haselbach Edwin , Klemm Urs , Buser Urs , Gschwind Rudolf , Jungen Martin , Kloster‐Jensen Else , Maier John P. , Marthaler Oscar , Christen Heniz , Baertschi Peter
Published: December 2004

Comparison of the photoelectron spectra of the title compounds with the electronic absorption spectra of the corresponding radical cations led to the detection of low lying ‘Non‐Koopmans’‐states in the ionic species.
1‐Aryl‐ und 1‐Alkyl‐2‐phthalimido‐diazen‐1‐oxide, diacylierte Vertreter von trisubstituierten Triazen‐1‐oxiden: Bildung, Eigenschaften, Stereoisomerisierung und Fragmentierung
Authors: Hoesch Lienhard , Köppel Bruno
Published: December 2004

1‐Aryl‐ and 1‐Alkyl‐2‐phathalimido‐diazene‐1‐oxides, Diacylated Examples of Trisubstituted Triazene‐1‐oxides: Formation, Properties, Stereoisomerization and Fragmentation1)

Oxidatively generated phthalimido‐nitrene (1) reacts with nitrosobenzene (8a), p‐nitrosotoluene (8b), o‐nitrosotoluene (8c), p‐dimethylamino‐nitrosobenzene (8d), p‐methoxycarbonyl‐nitrosobenzene (8e), 1, 1‐dimethyl‐1‐nitrosoethane (8f) and nitrosocyclohexane (8g) to give the respective 1‐substituted (Z)‐2‐phthalimidodiazene‐1‐oxides 14a‐g.

The constitution of 2‐imidodiazene‐1‐oxides 14 is deduced from their spectroscopic properties. The UV. spectra of 14a‐e are similar with those of the corresponding nitrobenzenes 18, thus supporting the concept of comparibility of the phthalimido‐N group with an O‐atom and indicating that the phthalimido group is not in conjugation with the diazene‐oxide function.

This is in contrast to the situation in the non‐acylated trisubstituted triazene‐1‐oxides 12, where conjugation is extended over the chain of all three N‐atoms as exhibited by the UV. spectra of (Z)‐1, 3, 3‐triphenyl‐, (Z)‐3, 3‐dimethyl‐1‐phenyl‐, (Z‐3‐methyl‐1, 3‐diphenyl‐, (Z)‐1‐(1, 1‐dimethylethyl)‐3, 3‐diphenyl‐, and (Z)‐1‐(1, 1‐dimethylethyl)‐3, 3‐dimethyltriazene‐1‐oxide (12a‐e). These triazene‐1‐oxides are formed by the reaction of 1, 1‐disubstituted hydrazines 11 with three mol‐equiv. of nitroso compounds 8 or from equimolar amounts of 11 and 8 in the presence of mercury oxide. Over 90% of 1, 2‐diphenyldiazeneoxide (19, R = C6H5) are isolated by the reaction 11 + 3 8a. Possible mechanisms of the reaction of 11 with 8, and of the formation of 12 via triazanols D and of 19 (R = C6H5) via N‐Phenylhydroxylamine (20, R = C6H5) are given in Scheme 6 (in the latter case with experimental evidence). An independent synthesis of 12c confirms the constitutional assignment for 12.
Ein neuer Zugang zum tetracyclischen Grundgerüst des Cyclohepta (def)fluorens
Authors: Neidlein Richard , Kramer Walter
Published: December 2004

A New Synthetic Way to the Tetracyclic Skeleton of Cyclohepta (def)fluorene

The synthesis of 2‐Methyl‐4, 5, 6, 8, 9, 10‐hexahydrocyclohepta (def)fluorene 2 is described starting with the reduction of the ketone 3 by a (1:1)‐mixture of LiAlH4/AlCl3 to the hydrocarbon 4. After metallation with butyllithium 4 was allowed to react with bromoacetic acid to yield 5. The cyclization of this compound was performed with p‐toluenesulfonic acid to give the tetracyclic ketone 6 which was converted to the tetracyclic hydrocarbon 2 by reduction with LiAlH4/AlCl3.
Steroids and Sexhormones. Part 265. Limonin, Part VI. Synthesis of a model compound incorporating rings D and E of 14,15‐deepoxylimonin
Authors: Emmer Gerhard , Graf Walter
Published: December 2004

Starting from the known formyl ketene thioacetal 6, model compound 11 was synthesized. The key intermediates, the epimeric furylmethanols 7a and 7b, were converted into the same dithioortholactone 8b (Scheme 1) and further elaborated into the model compound 11 (Scheme 2), a versatile compound in the synthesis of limonin (1). The acid catalyzed conversion of the epimers 7a and 7b into 8b may probably involve a hydride‐transfer reaction with inversion of configuration at C(17) of alcohol 7a (Scheme 4, row b).
Aufbau der Ligandsysteme des C, D‐Tetradehydrocorrins und Isobakteriochlorins durch Sulfidkontraktion. Kurzmitteilung
Authors: Ofner Silvio , Rasetti Vittorio , Zehnder Beat , Eschenmoser Albert
Published: December 2004

Synthesis of C, D‐Tetradehydrocorrins and Isobacteriochlorins via Sulfide Contraction

The reaction sequences shown in Schemes 4 and 5 illustrate the use of the sulfide contraction method for the synthesis of isobacteriochlorins and (metal free) C, D‐tetradehydrocorrins. The tetradehydrocorrin synthesis fills a gap in a spectrum of corrinoid (A → D) ring closures that ranges from A.W. Johnson's oxidative (A → D)‐cyclizations to octadehydrocorrinoids on the one side to the photochemical 1, 19‐secocorrin → corrincycloisomerization on the other (Scheme 7). Such a multiplicity of reaction paths towards corrinoids is of interest in connection with the problem of the origin of the corrin structure.
Etude de la fragmentation par impact électronique en ions positifs et négatifs de dérivés nitrés de l'amino‐2‐benzothiazole
Authors: Claude Saturnin , Tabacchi Raffaele , Duc Laurent , Marrel Jean‐François
Published: December 2004

Study of Positive and Negative Ions Electron Impact Fragmentation of 2‐Amino‐benzothiazole Nitro Derivatives

The positive and negative ions mass spectra of 2‐amino‐4,6‐dinitrobenzothiazole (1), 2‐amino‐4‐nitrobenzothiazole (2), 2‐amino‐6‐nitrobenzothiazole (3) are reported and discussed. These compounds give an intense molecular ion and show interesting fragmentations in both positive and negative ions spectra. Specific 15N‐labelling was used in order to confirm the fragmentation patterns.
Chemical Selectivities Disguised by Mass Diffusion. VIII. Influence of pH‐gradients on the product distribution in mixing‐disguised azo coupling reactions. 9th Communication on the selectivity of chemical processes
Authors: Belevi Hasan , Bourne John R. , Rys Paul
Published: December 2004

In mixing‐disguised azo coupling reactions the protons which are released during the electrophilic substitution steps cause a pH‐gradient in the reaction zone. Since, owing to the acid‐base pre‐equilibria, these pH‐gradients also determine the local concentrations of the reactants in the reaction zone, they also have an influence on the measured product distribution of such fast reactions. In the present work, the product distribution of the azo coupling of 1‐naphthol with 4‐sulfophenyldiazonium ion was experimentally measured and the results were compared with the distributions predicted from our mixing‐reaction model developed previously [1]. Furthermore, some experimental evidence for the existence of isoselectivity points are presented, at which a change of the initial pH‐value has no influence on the product distribution.
Chemical Degradation of Lasalocid: (A) The Mannich reaction (B) Baeyer‐Villiger oxidation of the retro‐aldol ketone
Authors: Coffen David L. , Katonak David A.
Published: December 2004

Under Mannich reaction conditions (diethylamine and formaldehyde in toluene under reflux) lasalocid (1) undergoes a unique transformation in which the carboxyl group is replaced by a diethylaminomethyl group. The resulting Mannich base 2 was converted back to lasalocid, proving that none of the other chemical and stereochemical features of the molecule were disturbed. Like other phenolic Mannich bases, the one derived from lasalocid readily alkylated mercaptans.

The known thermal and base‐induced retro‐aldol degradations of lasalocid both produce a ketone fragment 9 containing the cyclic ether units. Baeyer‐Villiger oxidation of this ketone afforded a carboxylic acid fragment which still contained these ether units. The normal regiochemistry involved in oxidizing this ketone (RCH2COCHR′R″ type) was cleanly reversed by first converting it into the hydroxymethylidene derivative 10.
Structure Determination of Brominated Morphinan‐6‐ones by 13C‐NMR. Spectroscopy: A novel closure of the oxygen bridge using 4‐acetoxymorphinan‐6‐ones
Authors: Brossi Arnold , Hsu Fu‐Lian , Rice Kenner C. , Rozwadowska Maria D. , Schmidhammer Helmut , Hufford Charles D. , Chiang Chian Chian , Karle Isabella L.
Published: December 2004

Bromination of (−)‐4‐hydroxy‐N‐methylmorphinan‐6‐one (3), prepared from natural morphine, with 1 mol of bromine in acetic acid, afforded the 1‐bromo ketone 5. The structure of 5 was assigned by 13C‐NMR.spectroscopy, and confirmed by X‐ray diffraction analysis of its hydrobromide salt. It is suggested that monobromination of synthetic (±)‐2,4‐dihydroxy‐N‐formylmorphinan‐6‐one (7) takes in principle a similar course, although the 13C‐NMR.spectrum of the primary reaction product 9 could not be measured because of insolubility in commonly used solvents. Monobromination of (−)‐4‐acetoxy‐N‐formylmorphinan‐6‐one (12) of the natural series, and of (±)‐2,4‐diacetoxy‐N‐formylmorphinan‐6‐one (8) of the synthetic series, followed by treatment of the monobrominated ketones with potassium carbonate in methanol resulted in closure of the O‐bridge, and afforded after acid hydrolysis, the corresponding 4,5‐epoxy‐morphinan‐6‐ones (−)‐16 and (±)‐17 respectively. This variation of the ring closure reaction represents a novel and convenient method to convert 4‐hydroxymorphinan‐6‐ones into their corresponding 4,5‐epoxymorphinan‐6‐ones, without involving aromatic bromination and with only 1 mol of bromine.
Structure Elucidation and Synthesis of Flavonol Acylglycosides. III.. The synthesis of tiliroside
Authors: Vermes Barbara , Chari Vedanta Mohan , Wagner Hildebert
Published: December 2004

Naturally occurring kaempferol 3‐O‐β‐D‐(6″‐O‐coumaroyl)glucopyranoside (tiliroside) has been synthesized thereby confirming its structure. This is the first acylated flavonoid glycoside to be synthesized.
Ionization Energies of Methyl‐substituted[2.2]Paracyclophanes
Authors: Zhong‐zhi Yang , Kovač Branka , Heilbronner Edgar , Eltamany Sayed , Hopf Henning
Published: December 2004

The photoelectron (PE.) spectra of thirteen methyl‐substituted [2.2]paracyclophanes have been recorded and analyzed, to assess the influence of methyl‐substitution on their ionization energies. It is shown that this influence is qualitatively and quantitatively the same as for benzenes and other π‐systems. Comparison with the previous results obtained for the [2r]cyclophanes (r=2 to 6) strongly suggests that the hyperconjugative model for alkyl‐group/π‐system interactions is more appropriate than the inductive one.
Carbon Participation in the Solvolysis of 6‐endo‐Substituted 2‐exo‐Norbornyl Toluenesulfonates. Norbornanes part 6
Authors: Grob Cyril A. , Günther Bettina , Hanreich Reinhard
Published: December 2004

The solvolysis rate constants k for the 6‐endo‐substituted 2‐exo‐norbornyl toluenesulfonates 7 have been determined. Values of logk correlate well with the respective inductive constants of the substitutents except when the latter are nucleophilic and therefore lead to endo‐cyclization, or when they are n‐electron donors and cause concerted fragmentation. In general 6‐endo‐substituted tosylates 7 react somewhat more slowly than their 6‐exo‐epimers.

Identical or different mixtures were obtained from the C(6)‐epimers 7 and 1 depending on whether the substituent was an electron donor or acceptor. It is concluded that donor substituents at C(6) enhance 1,3‐bridging in the intermediate epimeric cations and lead to their rapid and complete equilibration, and that electron acceptors reduce bridging and hence their equilibration rates.
Enantioselective Addition of Chiral Organotitanium Derivatives to Aldehydes
Authors: Olivero Alan G. , Weidmann Beat , Seebach Dieter
Published: December 2004

Alkoxy‐ and aryloxy‐organotitanium compounds 2–4 derived from (S)‐2‐methyl‐1‐butanol, (R)‐2‐butanol, (‐)‐menthol, quinine, cinchonine, and (S)‐1.1′‐binaphthol are added to aromatic aldehydes to give optically active alcohols 5–10 in enantioselectivities of up to 88% e. e., with nucleophilic transfer of methyl, phenyl, and 1‐naphthyl groups. The Tables 1–3 list the effects of varying the reagents, the substrates, and the reaction conditions of the new asymmetric synthesis.
Synthese und Chiralität der enantiomeren 6‐Hydroxy‐α‐ionone sowie von cis‐ und trans‐5,6‐Dihydroxy‐5,6‐dihydro‐β‐iononen
Authors: Eschenmoser Walter , Uebelhart Peter , Eugster Conrad Hans
Published: December 2004

Synthesis and Chirality of the Enantiomeric 6‐Hydroxy‐α‐ionones and of cis‐ and trans‐5,6‐Dihydroxy‐5,6‐dihydro‐β‐ionones

We describe the synthesis of (−)‐(5R,6S)‐5,6‐dihydroxy‐5,6‐dihydro‐β‐ionone (10) and (+)‐(5S,6S)‐5,6‐dihydroxy‐5,6‐dihydro‐β‐ionone (12). The absolute configuration of these compounds has been established by correlation with (R)‐4‐hydroxy‐β‐ionone (1).

Therefore, by comparison, the 5,6‐dihydroxy‐5,6‐dihydro‐β‐ionone recently isolated from Rehmannia glutinosa var. purpurea by Endo et al.[4] has the (5R,6R)‐configuration ((−)−12). The same holds for the 5‐β‐D‐glucopyranoside (15) isolated from Aeginetia indica var. gracilis.
Interaction between Exocyclic s‐cis‐Butadiene and Homoconjugated Functions. Preparation and Diels‐Alder Reactivity of Remotely Substituted 2,3‐Dimethylidenebicyclo[2.2.2]octanes
Authors: Avenati Marco , Pilet Olivier , Carrupt Pierre‐Alain , Vogel Pierre
Published: December 2004

The preparations of 5,6‐dimethylidene‐2exo‐bicyclo[2.2.2]octanol (8), its endo isomer 9, 5,6‐dimethylidene‐2‐bicyclo[2.2.2]octanone (10) and 2 exo, 3 exo‐epoxy‐5,6dimethylidenebicyclo[2.2.2]octane (11) are described. The kinetics of their cycloaddition to tetracyanoethylene has been measured in toluene at 25° together with those of 2,3‐dimethylidenebicyclo[2.2.2]octane (7) and 5,6‐dimethylidenebicyclo[2.2.2]oct‐2‐ene (12). The effects of remote substitution on the Diels‐Alder reactivity of 2,3‐dimethyl idenebicyclo[2.2.2]octanes are compared with those observed in the 2,3‐dimethylidenenorbornane series (1–6).
Photochemistry of Aliphatic Imines. The Photochemical Behaviour of Fluorinated N‐Isopropylidenecyclohexylamines
Authors: Margaretha Paul
Published: December 2004

The cyclohexanimines of 1, 3‐difluoro‐2‐propanone and of 1, 1, 1‐trifluoro‐2‐propanone have been synthesized. Their behaviour in acetone‐photosensitized reactions is compared to that of the non‐fluorinated parent compound.
Synthese von [16,16,16,16′,16′,16′‐2H6] Lycopin
Authors: Hofer Arnold , Eugster Conrad Hans
Published: December 2004

Synthesis of [16,16,16,16′,16′,16′ 2H6]Lycopene

Lycopene having at both ends the deuteriated methyl groups trans to the chain has been synthesized for further biochemical experiments aimed to elucidate the stereochemistry of the cyclization step.

Incidentally a gas‐chromatographic separation of the isotopisomers 1 and 2g has been observed.
The Stereochemical Outcomes – the Isoborneol/Borneol Ratios – in Lithium, Sodium and Potassium/Ammonia Reductions without a Proton Source of the Enantiomers and the Racemate of Camphor Differ Strongly
Authors: Rautenstrauch Valentin
Published: December 2004

The stereochemical outcomes [ratios (1 R, 2 R, 4 R)‐isoborneol/(l R, 2 S, 4 R) isborneol or (1R, 2 S, 4 S)‐isoborneol(l S, 2 R, 4 S)‐borneol and (1 RS, 2 RS, 4 RS)‐isoborneol/(l RS, 2 SR, 4 RS)‐borneol] in lithium, sodium and potassium/ammonia reductions without a proton source of the enantiomeric [(1 R, 4 R) or (1 S, 4 S)] and racemic (1RS, 4RS) forms of camphor under the same conditions differ strongly. This explains contradictory reports in the literature and, in the case of the reductions with potassium in which pinacol coupling does not compete, proves that the reductions involve bimolecular reactions between two ketyls, or between the ketone and the ketyl or the ketone dianion.
Stereochemical Aspects of Acid‐Catalyzed Cyclopropane Ring‐Opening Reactions. A Stereospecific Pathway to Crinosterol and Brassicasterol
Authors: Lang Robert W. , Djerassi Carl
Published: December 2004

It is shown that the acid‐catalyzed ring‐opening of the two diastereoisomeric 23:24‐methylenecholesterols 3 and 5 on treatment with gaseous HCl in acetic acid leads stereospecifically to the naturally occurring crinosterol (4) and brassicasterol (6), respectively (Scheme 1). This isomerization can be viewed as a biomimetic model of an in vivo methylation process of the type already known in plant sterol metabolism (cf. cycloeucalenol → obtusifoliol, 1 → 2). The synthetic application of this method provides a convenient labelling of sterol side chains for tracer experiments. The mechanistic features of the reaction with respect to its particular stereospecificity are discussed.
Macrocyclische Spermidinalkaloide aus Maytenus tnossambicensis (KLOTZSCH) BLAKELOCK 12. Mitteilung über Celastraceen‐Inhaltsstoffe
Authors: Wagner Hildebert , Burghart Joseph
Published: December 2004

Macrocyclic Spermidine Alkaloids from Maytenus mossambicensis (KLOTZSCH) BLAKELOCK

In addition to celabenzine (1), three new macrocyclic spermidine alkaloids cyclocelabenzine (2), isocyclocelabenzine (3) and hydroxyisocyclocelabenzine (4) have been isolated from Maytenus mossambicensis (klotzsch) blakelock, var. mossambicensis. The three alkaloids show the 13‐membered lactam ring of celabenzine being linked to the benzoyl residue within the spermidine unit. Hydroxyisocyclocelabenzine (4) is the first known spermidine alkaloid with a hydroxy function at the macrocyclus. The structural elucidation was based mainly on spectroscopic methods, specially 1H‐ and 13C‐NMR. spectroscopy.
Herstellung und Eigenschaften einiger hydrocorphinoider Nickel(II)‐Komplexe
Authors: Fässler Alexander , Pfaltz Andreas , Michael Müller Peter , Farooq Saleem , Kratky Christoph , Kräutler Bernhard , Eschenmoser Albert
Published: December 2004

Preparation and Properties of some Hydrocorphinoid Nickel(II)‐Complexes

Corphin derivatives synthesized earlier in our laboratory have been used for the preparation of tetrahydro‐ and hexahydrocorphinoid nickel(II)‐complexes containing novel chromophore systems. The UV./VIS. and 1H‐ and 13C‐NMR. spectral data of these complexes were relevant to the structure determination of Factor F430 described in the following paper.
Synthese von Glycerylätherphosphatiden 1. Mitteilung Herstellung von 1‐O‐Octadecyl‐2‐O‐acetyl‐sn‐glyceryl‐3‐phosphorylcholin1 (‘Platelet Activating Factor’), des Enantiomeren sowie einiger analoger Verbindungen
Authors: Hirth Georges , Barner Richard
Published: December 2004

Synthesis of Glyceryletherphosphatides, 1st Communication, Preparation of 1‐O‐Octadecyl‐2‐O‐acetyl‐sn‐glyceryl‐3‐phosphorylcholine (‘Platelet Activating Factor’), of its Enantiomer and of Some Analogous Compounds

Several synthetic sequences for the preparation of ‘Platelet Activating Factor’ (1a), for the corresponding enantiomeric compound (1′a) as well as for the ‘Lyso‐compounds’ (1b and 1′b) are described. The use of glycerolacetonide 2′a (from D‐Mannitol) for the preparation of 1′a and la is presented together with the synthesis of some analogues of 1′a and 1a. Structural assignment and optical purity of the compounds prepared are confirmed.
Über Pterinchemie 81. Mitteilung Die Konfiguration an C(6) von natürlichem 5,6,7,8‐Tetrahydro‐L‐biopterin und seinem Pentaacetylderivat
Authors: Prewo Roland , Bieri Jost H. , Ganguly Subhendu Narayan , Viscontini Max
Published: December 2004

The Configuration at C(6) of Natural 5,6,7,8‐Tetrahydro‐L‐biopterin and of its Pentaacetate

The structure of (6.R)‐pentaacetyl‐5,6,7,8‐tetrahydro‐L‐biopterin, one of two diastereoisomers obtained by catalytic hydrogenation and subsequent acetylation of L‐biopterin, has been determined by X‐ray diffraction analysis. The space group is P212121, a=8,053(l), b=14,955(3), c= 21,502 (4) Å. The asymmetric unit contains one molecule of the biopterin derivative and one of ethyl acetate. The R‐configuration can be assigned to C(6) by reference to the known configurations of the other asymmetric C‐atoms. As hydrolysis of this diastereoisomer yields the natural 5, 6,7,8‐tetrahydro‐L‐biopterin, the latter also possesses the (6 R)‐configuration.
Electrofugal Fragmentation of Alkylcobalamin Derivatives Using Cob(I)Alamin and Heptamethyl Cob (I)yrinate as Catalysts
Authors: Fischli Albert
Published: December 2004

The cob (I)alamin‐ (1(I)) and the heptamethyl cob(I)ynnate‐ (2(I)) catalyzed transformation of an epoxide to the corresponding saturated hydrocarbon 3→4→5 is examined (see Schemes 1 and 3–5). Under the reaction conditions, the epoxyalkyl acetate 3 is opened by the catalysts with formation of appropriate (b̃‐hydroxyalkyl)‐corrinoid derivatives (13, 14, 17, 18, see Schemes 12 and 14). Triggered by a transfer of electrons to the Co‐corrin‐π system, the Co, C‐bond of the intermediates is broken, generating the alkenyl acetate 4 (cf. Schemes 12 and 14) following an electrofugal fragmentation (cf. Schemes 2 and 12). The double bond of 4 is also attacked by the catalysts, leading to the corresponding alkylcorrinoids (15, 19, see Schemes 12 and 14) which in turn are reduced by electrons from metallic zinc, the electron source in the system, inducing a reductive cleavage of the Co, C‐bond with production of the saturated monoacetate 5 (see Schemes 2, 5 and 12). In the cascade of steps involved, the transfer of electrons to the intermediate alkylcorrinoids (13–15, 17–19, see Schemes 12 and 14) is shown to be rate‐limiting. Comparing the two catalytic species 1(I) and 2(I), it is shown that the ribonucleotide loop protects intermediate alkylcobalamins to some extent from an attack by electrons. The protective function of the ribonucleotide side‐chain is shown to be present in alkylcobalamins existing in the base‐on form (cf. Chap. 4 and see Scheme 14).
Note on the Synthesis of 1, 1‐Dichloro‐2, 3‐divinylcyclopropane and Other Functionalized Dichlorocyclopropanes
Authors: Müller Paul , Rey Michel
Published: December 2004

The title compound 1 was synthesized via addition of dichlorocarbene to dimethyl trars‐3‐hexenedioate (5), using o‐nitrophenylselenic acid‐elimination to form the double bonds. Reaction of dichlorocyclopropane 8 with ozone on silica gel furnished the monoketone 10; no diketone 11 could be isolated upon further exposure of 10 to O3/SiO2. When the bis(p‐toluenesulfonate) 13b was treated with t‐BuOK, 16 was obtained in low yield as the only isolable product. It is believed to arise from Cope‐rearrangement of an intermediate cis‐divinylcyclopropane 14 to cycloheptadiene 15, which undergoes a subsequent allylic rearrangement to 16.
Herstellung von Methylestern durch Umesterung funktionalisierter Substrate mit Titalsäureestern als Katalysatoren
Authors: Schnurrenberger Peter , Züger Max F. , Seebach Dietrich
Published: December 2004

Synthesis of Methyl Esters by Titanate‐catalyzed Transesterifications

The low solubility of tetramethyl titanate has so far prevented the laboratory use of titanate‐catalyzed transesterifications with formation of methyl esters. Two procedures are described here, which allow the application of this exceedingly mild method to the synthesis of methyl esters: one uses methyl propionate/Ti(OEt)4, the other one uses methanol/Gly‐Ti, a new type of catalyst, prepared from Ti(OEt)4/ethylene glycol 2:1.
Steric Effects on Reaction Rates – III. Application of Force‐Field Calculations to Chromic Acid Oxidation of Alcohols
Authors: Müller Paul , Blanc Jacky , Lenoir Dieter
Published: December 2004

The rates of oxidation with chromic acid of 15 bi‐ and polycyclic secondary alcohols have been measured and correlated with strain changes calculated by the MM1‐program between the alcohols and the corresponding ketones. A correlation of the same quality is obtained upon representation of OH‐strain by CH3‐strain. The significance of the correlations with respect to the oxidation mechanism as well as the limitations of the applicability of force‐field calculations to reactivity problems are discussed.
17O‐NMR. of Enriched Acetic Acid, Glycine, Glutamic Acid and Aspartic Acid in Aqueous Solution. I. Chemical Shift Studies
Authors: Gerothanassis Ioannis P. , Hunston Roger , Lauterwein Jürgen
Published: December 2004

The 17O‐NMR. chemical shifts of the enriched amino acids glycine, aspartic acid and glutamic acid were measured in aqueous solution as a function of pH. High magnetic fields are necessary to resolve the α, β‐ and α, γ‐carboxyl resonances of aspartic acid and glutamic acid, respectively. The chemical shifts of acetic acid were measured for comparative reasons. Ionization constants and titration shifts were obtained by nonlinear least‐squares fits to one‐proton titration curves. The average excitation energy approximation is discussed in terms of the observed changes in 17O‐shielding on deprotonation. No intramolecular association between the α‐amino group and the α‐carboxyl group in the zwitterionic form is required to explain the high‐frequency shift of the carboxylate ion. Also no indication of an intramolecular association between the α‐amino group and the side‐chain carboxyl groups of aspartic acid or glutamic acid was found.
Biosynthesis of the Indole Alkaloids. Cell‐free Systems from Catharanthus roseus Plants
Authors: Kutney James P. , Choi Lewis S. L. , Honda Toshio , Lewis Norman G. , Sato Toshitsugu , Stuart Kenneth L. , Worth Brian R.
Published: December 2004

Cell‐free systems from Catharanthus roseus plants are utilized for various studies relating to the biosynthesis of indole alkaloids. Tryptamine (5) and secologanin (6), two fundamental building units, are shown to be incorporated into the alkaloid vindoline (7). In another study, catharanthine (18) and vindoline (7) are utilized by this enzyme system and coupled to the important bisindole biointermediate 3′,4′‐anhydrovinblastine

The previously [20] used name for 17, 3′, 4′‐dehydrovinblastine, is incorrect.

(17). The latter substance is, in turn, incorporated and converted to the natural alkaloids leurosine (8), Catharine (9) and vinblastine (10), thereby providing information about the biosynthesis of these complex molecules. High pressure liquid chromatography assay of the enzymic mixture sheds light on the enzymes involved in the coupling of 18 and 7.
Synthese makrocyclischer Lactame aus Ketonen durch Ringerweiterung. Vorläufige Mitteilung
Authors: Wälchli Rudolf , Hesse Manfred
Published: December 2004

Synthesis of Macrocyclic Lactams from Ketones by Ring Enlargement Reaction

2‐Nitroalkanon 1 was converted to the N‐substituted amide 3 by condensation with acrylaldehyde followed by reductive amination. Under the analogous reaction conditions, 2‐nitrocyclododecanon (4) led to 12‐nitro‐15‐pentadecanlactam (6).
Photochemical Reduction of 2‐Bromo‐4,4‐dimethyl‐2‐cyclohexenone
Authors: Hombrecher Hermann , Margaretha Paul
Published: December 2004

The photochemical behaviour of 2‐Bromo‐4,4‐dimethyl‐2‐cyclohexenone (1) was studied in 2‐propanol and cyclohexane. In both solvents (n‐π*)‐excitation followed by intersystem crossing leads to population of a low‐lying triplet (T1) state, which can be quenched by 1,3‐cyclohexadiene but does not undergo chemical transformation efficiently. (π‐π*)‐Excitation affords 4,4‐dimethyl‐2‐cyclohexenone (2) as the only product. While in 2‐propanol 2 is formed in 60% from the S2‐state and in 40% from the T2‐state, in cyclohexane reduction occurs exclusively from this upper triplet state. The T2‐state can also be populated via energy transfer using acetone or benzene as sensitizer. The mechanistic dissimilarities for the reduction of excited 1 in either 2‐propanol or cyclohexane are discussed.
Nucleosides and Nucleotides. Part 19. On Detritylation with Zinc Bromide in Oligonucleotide Synthesis
Authors: Waldmeier Felix , De Bernardini Silvio , Leach Colin A. , Tamm Christoph
Published: December 2004

Zinc bromide has been shown by several groups of workers to be a useful reagent in the removal of trityl protecting groups from nucleotides. An attempt is made here to establish optimum conditions for the reaction, a strange observation is reported, and a novel workup via a soluble, lipophilic zinc complex is described.