bluuebunny/crossref_metadata_2025_split huggingface dataset:
Intramolekulare 1,3‐dipolare Cycloadditionen mit alkenyl‐substituierten 3,4‐Diarylsydnonen
Authors: Meier Hansruedi , Heimgartner Heinz 
Published: December 2004

Intramolecular 1,3‐Dipolar Cycloaddition Reactions of Alkenyl‐substituted 3,4‐DiarylsydnonesThe 3,4‐diarylsydnones1and12a–ewith an allyl or alkenyloxy substituent on the N(3)‐phenyl group have been synthesized by classical methods starting from 2‐allylaniline and the corresponding alkenyloxyanilines, respectively (Schemes 2and3). Whereas the allyl‐substituted sydnone1undergoes an intramolecular 1,3‐dipolar cycloaddition at room temperature in solution to yield13(Scheme 4), the alkenyloxy‐substituted sydnones12a–eare thermally stable. On irradiation of1and12a–d, formation of the fused dihydropyrazoles2and14a–d(Schemes 1and5) is observed. In the case of12d, the yield of14dis very low, and the symmetric 1,2,3‐triazole15ahas been isolated as the main product. A very likely reaction mechanism for the formation of the photoproducts involves decarboxylation of the sydnones to give a nitrile‐imine which undergoes an intramolecular 1,3‐dipolar cycloaddition onto the CC bond.
Pteridine. Teil LXXXII. Synthese und Reaktivität von Lumazin‐7‐sulfensäuren
Authors: Heckel Armin , Pfleiderer Wolfgang 
Published: December 2004

Synthesis and Reactivity of Lumazine‐7‐sulfenic AcidsThe chemical synthesis of the pteridine‐7‐sulfenic acids 13–16 is described (Scheme 1). The 1,2,3,4‐tetrahydro‐1,3,6‐trimethyl‐2,4‐dioxopteridine‐7‐sulfenic acid (14) was isolated as a stable crystalline solid. Its chemical reactivity was investigated and the physical properties determined. In the solid state, the S‐oxide form 14A predominates, but in protic solvents the S‐OH tautomer 14 exists most likely. In basic medium, the pteridine‐7‐sulfenate species is stable, whereas a low pH gives rise to a disproportionation to the disulfide 10 and the corresponding pteridine‐7‐sulfinic acid 37 (Scheme 4). Reaction of 14 with ethyl propiolate leads, under cis‐addition, to 36, oxidation with KMnO4 forms 1,2,3,4‐tetrahydro‐1,3,6‐trimethyl‐2,4‐dioxopteridine‐7‐sulfonic acid (38) and NaBH4 reduction the corresponding 7‐mercaptopteridine dione 18 (Scheme 4).
Selective Transport Membranes and their Applicability for Novel Sensors
Authors: Morf Werner E. , Huser Marin , Lindemann Bernhard , Schulthess Peter , Simon Wilhelm 
Published: December 2004

AbstractSelective transport of given substrates through artificial membranes can be achieved by incorporation of electrically neutral or charged carriers. Such mediated transport described for alkali and alkaline‐earth metal cations is reported here for the first time for certain anions using cobyrinate‐type ionophores, and it is also conceivable to be realized for nonionic substrates (e.g. glucose). The fundamental requirements for corresponding co‐transport and counter‐transport membrane systems are set forth in view of a design of novel substrate‐selective sensors for analytical‐chemical applications.
Methionin als Vorläufer zur enantioselektiven Synthese α‐verzweigter Vinylglycine und anderer Aminosäuren
Authors: Weber Theodor , Aeschimann Roland , Maetzke Thomas , Seebach Dieter 
Published: December 2004

Methionine as Precursor for the Enantioselective Synthesis of α‐Branched Vinylglycines and of Other Amino AcidsMethionine is converted by previously published methods into the diastereoisomerically pure 3‐thiabutyl‐substituted oxazolidinone (7) and imidazolidinones 5 and 6. An X‐ray crystal structure determination of cis‐3‐benzoyl‐2‐(tert‐butyl)‐4‐(3‐thiabutyl)oxazolidin‐5‐one (7) confirms the configurational assignments made by NOE‐NMR measurements. Oxidation to sulfoxides and pyrolytic elimination produce vinyl‐substituted heterocycles (see 19, 21). Diastereoselective alkylations of the enolate 14 from the imidazolidinone 5 and of the dienolate 23 from the vinyl derivative 19 give geminally alkyl‐ and/or vinyl‐substituted heterocycles. Some of these products were hydrolyzed to free amino acids, such as (R)‐2‐methyl‐ (25a) and (R)‐2‐ethyl‐2‐vinylglycine (25b) (R)‐2‐methylhomoserine (27). Raney‐Ni desulfurization of 5 and oxidative degradation of 19 lead to enantiomerically pure derivatives of α‐aminobutyric acid (see 28) and of glycine (see 31), respectively.
Synthese von racemischen Aminozuckersäure‐lactonen: xylo‐ und lyxo‐2,3‐Diacetylamino‐5‐acetoxypentan‐4‐olid und ‐2,3,5‐Triacetylaminopentan‐4‐olid
Authors: Egli Markus , Dreiding Andre S. 
Published: December 2004

Synthesis of Racemic Aminosugar Lactones: xylo‐ and lyxo‐2,3‐Diacetylamino‐5‐acetoxypentan‐4‐olide and ‐2,3,5‐Triacetylaminopentan‐4‐olideStarting with 5‐hydroxy‐2‐penten‐4‐olide (1), the tricyclic intermediate 4 was prepared via the chloride 2, the acyl azide 3, and an intramolecular nitrene addition (Scheme 3). Azide ion opened the aziridine ring in 4 at C(α) to give 5, which was transformed via 7 into one of the title compounds, the triacetylated diamino‐hydroxy‐lactone 13 (Scheme 4). An alternative conversion of 4 into 13 involved the synthesis of the N‐acetylaziridine 10, the opening of the 3‐ring of 10 with N to form 12, and a final reductive acetylation (Scheme 5). The third N‐substituent was introduced at C(δ) of 13 by the following sequence: hydrolysis of the AcO group (→14), mesylation (→15), substitution by N (→ 16), and reductive acetylation to yield the other title compound, the triacetylated triaminolactone 17 (Scheme 6). Since the ring opening of aziridines by nucleophiles occurs by inversion, the primary products 5a and 12a of the N reactions as well as the substances derived from them, i.e. 6a, 7a, and 13a‐17a, have the xylo‐configuration (a‐series). Under some of the reaction conditions, the primary xylo‐products suffered a partial epimerization at C(α) to yield mixtures containing the corresponding lyxo‐products (b‐series): The equilibrium between the xylo‐ and lyxo‐isomers was estimated for 5a/5b=1:3, 12a/12b=5:2, 13a/13b=3:1, and 16a/16b=2:1. Since the stereoisomers of the a‐ and the b‐series were always separable, the other lyxo‐products, i.e. 6b, 7b, could be prepared from 5b and 12b.
Synthese und Strukturaufklärung von Merucathinon und Synthese von Cathinon. Inhaltsstoffe von Catha edulis FORSK
Authors: Wolf Jean‐Pierre , Pfander Hanspeter 
Published: December 2004

Synthesis and Structure Elucidation of Merucathinone and Synthesis of Cathinone. Constituents of Catha edulis FORSK.Starting from L‐alanine, two constituents of Catha edulis FORSK., (S)‐4‐amino‐1‐phenylpent‐1‐en‐3‐on ((S)‐3; merucathinone) and (S)‐2‐amino‐1‐phenylpropan‐1‐on ((S)‐1; cathinone) were synthesized.
Synthese von Pentafulvalen durch oxidative Kupplung von Cyclopentadienid mittels Kupfer(II)‐chlorid
Authors: Escher André , Rutsch Werner , Neuenschwander Markus 
Published: December 2004

Synthesis of Pentafulvalene by Oxidative Coupling of Cyclopentadienide with Copper(II) ChlorideStarting with a nearly quantitative coupling of cyclononatetraenide 7 to 1, 1′‐dihydrononafulvalene 8 by means of AgBF4, a simple general synthetic concept for fulvalenes is outlined (Scheme 2), consisting in an oxidative coupling of ‘Hückel anions’ like 2 and 7 to 1, 1′‐dihydrofulvalenes 10 with Ag(I) or Cu(II) salts, followed by deprotonation (→11) and oxidation (→12); it has been realised in the case of pentafulvalene (1; overall yield 61%; Scheme 3) and 1,2:5,6‐dibenzopentafulvalene (18; overall yield 66%, Scheme 4). NMR‐spectroscopic investigations show that 1 is a non‐aromatic compound with strongly alternating bond‐lengths, its π‐system being even more localised than that of simple pentafulvenes. In fact, 1 is extremely reactive in concentrated solutions above −50°. Besides of polymerisations, Diels‐Alder dimerisation 1→19 followed by a rearrangement 19→20 takes place (Scheme 5).
Stoffwechselprodukte von mikroorganismen. 238. Mitteilung. Isolierung und Strukturaufklärung von Differolid
Authors: Keller‐Schierlein Walter , Bahnmüller Urs , Dobler Max , Bielecki Jacek , Stümpfel Joachim , Zähner Hans 
Published: December 2004

Isolation and Structure Elucidation of DifferolideFrom cultures of a new actinomycete, Streptomyces aurantiogriseus (PREOBRAZHENSKAYA 1975), strain Tü 3149, a compound C12H12O4 was isolated, which enhances the formation of aerial mycelium and spores of Streptomyces glaucescens. The structure determined by X‐ray diffraction studies was that of racemic dilactone 1 ((±)‐differolide).
1,3‐Butadienyl‐thiocyanate in der Diels‐Alder‐Reaktion mit anschliessender [3,3]‐sigmatroper Umlagerung
Authors: Huber Stefan , Stamouli Peristera , Jenny Titus , Neier Reinhard 
Published: December 2004

1,3‐Butadienyl Thiocyanates in the Diels‐Alder Reaction Followed by a [3,3]‐Sigmatropic Shift(E)‐ and (Z)‐1,3‐Butadienyl thiocyanates 3, 4, and 12–15 have been synthesized selectively. Their use as dienes for Diels‐Alder reactions followed by a [3,3]‐sigmatropic shift to obtain an isomeric isothiocyanate has been studied. The butadienyl thiocyanates are, unfortunately, not very reactive in Diels‐Alder reactions. This disadvantage can be overcome, if a trapping reaction with EtOH is added to the two‐step sequence. This sequence allows to get good yields of the O‐ethyl thiocarbamates 18–23, even if the first two reactions have not favorable equilibrium constants.
Enantioselective Syntheses of α‐Amino Acids from 10‐(Aminosulfonyl)‐2‐bornyl Esters and Di(tert‐butyl) Azodicarboxylate. Preliminary Communication
Authors: Oppolzer Wolfgang , Moretti Robert 
Published: December 2004

AbstractSuccesive treatment of chiral esters 1 with LiN(i‐Pr)2/Me3SiCl and di(tert‐butyl) azodicarboxylate/TiCl4/Ti(i‐PrO)4 gave N,N′ ‐di[(tert‐butoxy)carbonyl]hydrazino esters 9 which on deacylation, hydrogenolysis, transesterification, and acidic hydrolysis furnished (2S)‐α‐amino acids 6 in high enantiomeric purity with efficient recovery of the auxiliary alcohol 7.
Photochemical and Electrochemical Reduction of Carbon Dioxide to Carbon Monoxide Mediated by (2,2′‐Bipyridine)tricarbonylchlororhenium(I) and Related Complexes as Homogeneous Catalysts
Authors: Hawecker Jeannot , Lehn Jean‐Marie , Ziessel Raymond 
Published: December 2004

Abstract[fac‐Re(bpy) (CO)3Cl] (bpy = 2,2′‐bipyridine) is an efficient homogeneous catalyst for the selective and sustained photochemical or electrochemical reduction of CO2 to CO. A quantum yield of 14% and a faradic efficiency of 98% were measured in the presence of excess Cl− ions. The photochemical process took place under visible‐light irradiation and consumed a tertiary amine as electron donor. A formato‐rhenium complex was isolated in the absence of excess Cl− ions. Substitution by Cl− ion generated free formate, but no CO was detected. Luminescence measurements showed that the tertiary amine quenches the metal‐to‐ligand charge‐transfer excited state of the rhenium complex via a reductive mechanism, with a rate constant of 3.4 × 107M−1S−1. The 19e‐complex [Re(bpy) (CO)3X]− produced either photochemically or electrochemically appears to be the active precursor in the CO‐generation process. Detailed spectroscopic studies on 13C‐enriched carbonyl‐rhenium and formato‐rhenium complexes derived from 13C‐enriched CO2 were performed in order to confirm the origin of the products and to study the exchange of the ligands. A mechanism for the present CO2 photoreduction process is presented; it involves separate pathways for CO and formate generation, in which the [Re(bpy) (CO)3X] complex plays the role of both the photoactive and the catalytic center.
New Sesquiterpenoids from Cabreuva Oil
Authors: Maurer Bruno , Hauser Arnold , Ohloff GÜNther 
Published: December 2004

AbstractSix sesquiterpenoids 1–6, formally derived from (+)‐(S)‐nerolidol by oxidative cyclization, have been isolated for the first time from commercial cabreuva oil. Whereas the two tetrahydrofurans 5 and 6 have already been described, the four bicyclic ethers (cabreuva oxides A–D) 1–4 are new. The structures of 1–4 are confirmed by synthesis and their absolute configurations are shown to be 3S. The organoleptic properties of the synthetic cabreuva oxides are discussed.
Further Saponins from Phytolacca dodecandra L′HERIT
Authors: Dorsaz Anne‐Christine , Hostettmann Kurt 
Published: December 2004

AbstractThree new bidesmosidic saponins 1–3 have been isolated from the berries of Phytolacca dodecandra L′HERIT (Phytolaccaeae). The structures have been established by spectroscopic (1H‐NMR, 13C‐NMR, FAB‐MS, DCI‐MS, and GC/MS) and chemical methods (acid, basic, and enzymatic hydrolysis). Saponins 1 and 3 are tetra‐ and pentaglycosides of oleanolic acid, whereas 2 is a tetraglycoside of bayogenin. The corresponding prosapogenins 1a‐3a, obtained by cleavage of a glucose unit esterified at C(28), exhibited strong molluscicidal activity against schistosomiasis‐transmitting snails.
The Thermal Reactivity of 5,8α‐Epidioxy‐5α‐cholestan‐3β‐yl Acetate in Acetic Acid
Authors: Fuhrer Hermann , Rihs Grety , Lorenc Ljubinka , Bondarenko Lidija , Kalvoda Jaroslav , Mihailović Mihailo Lj. 
Published: December 2004

AbstractThe thermolysis of the steroidal 5α,8β‐peroxide 1, under acidic conditions afforded in addition to the expected 5,10:8,9‐diseco derivative 2, the rearranged 5α,8β‐epoxide 3, the structure of which was determined by NMR spectroscopy and X‐ray analysis.
Zwei weitere Iridoid‐glycoside und ein Monoterpen‐glycosid aus Sambucus ebulus L. (Caprifloiaceae). 3. Mitteilung über die Inhaltsstoffe der Zwergholunderwurzel
Authors: Gross Gian‐Andrea , Sticher Otto , Anklin Clemens 
Published: December 2004

New Iridoid Glycosides and a Monoterpene Glycoside from Sambucus ebulus L. (Caprifloiaceae)From the roots of Sambucus ebulus L., two novel valeriana‐type ester iridoid glycosides, 6′‐O‐apiosylebuloside (1) and 7,7‐O‐dihydroebuloside (3), along with the open‐chain monoterpene glycoside 5 were isolated. Their structure elucidation is based mainly on one‐ and two‐dimensional NMR methods. 1H,1H‐COSY experiments permitted complete assignment of signals arising from the disaccharide unit in 1. Biogenetically, 3 seems to represent the equivalent of loganin in the valeriana‐type series of iridoid glycosides.
Formation of 2,3‐Dihydrothiophene‐3‐carboxylates from 2(5H)‐Thiophenones via Sequential Cyclization and Ring‐Opening Reactions of 3‐Thiahexa‐1,5‐dienyl Radicals. Preliminary Communication
Authors: Kiesewetter René , Margaretha Paul 
Published: December 2004

AbstractMethyl 4‐mercapto‐2‐alkenoates 1, obtained by irradiation of 2(5H)‐thiophenones 2 in MeOH, undergo light‐induced S–H bond homolysis to give thio radicals 3 which can be trapped with 2‐butyne to afford 3‐thiahexa‐1,5‐dienyl radicals 12. Intermediates 12 undergo selective 1,5‐ring closure to allylcarbinyl radicals 18, which cyclize to cyclopropylcarbinyl radicals 19. A novel rearrangement of 19 to 23, the direct precursors of title compounds 15 and 17, occurs via two additional ring‐opening and one 1,5‐ring‐closure step.
Photochemical Rearrangement of N‐Substituted 2‐Methyl‐5‐Nitro‐1H‐Imidazoles in the Presence of Water
Authors: Pfoertner Karl‐Heinz , Daly John J. 
Published: December 2004

AbstractThe photochemical behaviour of 2‐methyl‐5‐nitro‐1H‐imidazoles 1 in water‐containing solutions has been studied. In a first reaction step, the photorearrangement of 1 yields 2‐methyl‐2‐imidazoline‐4,5‐dione‐4‐oximes 2. Hydrolysis of 2 and subsequent elimination of water from a supposed intermediate 5 leads to 5‐methyl‐1,2,4‐oxadiazole‐3‐carboxamides 4. The basic structure of 4 was determined by X‐ray analysis of the derivative 4e. Further irradiation destroys the products 4 in part by light‐induced hydrolysis.
Intramolecular Cyclizations of Allenic Acylureas and ‐Amides
Authors: Trifonov Latchezar S. , Orahovats Alexander S. 
Published: December 2004

AbstractAllenic acids are found to add to dicyclohexylcarbodiimide affording, in the presence of Et2NH, the 4H‐1,3‐oxazin‐4‐ones 5 via 4. Under neutral conditions, they add to diaryl‐ or pyridyl(cyclohexyl)carbodiimides and triphenylketene imine to give the corresponding tricyclo[5.2.2.01,5]undeca‐4,8,10‐trien‐3‐ones 7, 8, 9, and 12. The allenic phenyl ester 13a dimerises, on heating in a [2+2] head‐to‐head fashion, to 14 but fails to undergo intramolecular Diels‐Alder cyclization, to 15.
Functionalisation of saturated hydrocarbons. Part 9. Oxidation of patchouli alcohol by the ‘gif system’: Isolation and organoleptic properties of three new ketonic derivatives
Authors: Barton Derek H. R. , Belœil Jean‐Claude , Billion Annick , Boivion Jean , Lallemand Jean‐Yves , Mergui Simone 
Published: December 2004

AbstractOxidation of patchouli alcohol (1) using the ‘Gif system’ afforded as major isolated products three new ketonic derivatives 16–18. The structures of these compounds were established by spectral techniques including 2D‐NMR. Ketones 16–18 display interesting organoleptic properties.
3‐(Dimethylamino)‐2,2‐dimethyl‐2H‐azirin als α‐Aminoisobuttersäure(Aib)‐Äquivalent: Cyclische Depsipeptide durch direkte Amid‐Cyclisierung
Authors: Obrecht Daniel , Heimgartner Heinz 
Published: December 2004

3‐(Dimethylamino)‐2,2‐dimethyl‐2H,‐azirine as an α‐Aminoisobutyric‐Acid (Aib) Equivalent: Cyclic Depsipeptides via Direct Amid CyclizationIn MeCN at room temperature, 3‐(dimethylamino)‐2,2‐dimethyl‐2H‐azirine (1) and α‐hydroxycarboxylic acids react to give diamides of type 8 (Scheme 3). Selective cleavage of the terminal N,N‐dimethylcarboxamide group in MeCN/H2O leads to the corresponding carboxylic acids 13 (Scheme 4). In toluene/PhSH, phenyl thioesters of type 11 are formed (see also Scheme 5). Starting with diamides 8, the formation of morpholin‐2,5‐ diones 10 has been achieved either by direct amide cyclization via intermediate 1,3‐oxazol‐5(4H)‐ones 9 or via base‐catalyzed cyclization of the phenyl thioesters 11 (Scheme 3). Reaction of carboxylic acids with 1, followed by selective amide hydrolysis, has been used for the construction of peptides from α‐hydroxy carboxylic acids and repetitive α‐aminoisobutyric‐acid (Aib) units (Scheme 4). Cyclization of 14a, 17a, and 20a with HCI in toluene at 100° gave the 9‐, 12‐, and 15‐membered cyclic depsipeptides 15, 18, and 21, respectively.
Triaziridine 7. Mitteilung. Stabile pyramidale Konfigurationen an den Stickstoff‐Atomen von Dialkyl‐und Trialkyl‐triaziridinen
Authors: Hilpert Hans , Hoesch Lienhard , Dreiding André S. 
Published: December 2004

Stable Pyramidal configurations at the Nitrogen Atoms of Dialkyl‐and Trialkyl‐triaziridinesStereochemical features of the recently synthesized nine samples of di‐ and trialkyl‐triaziridines, namely the 1,3‐cyclopentylen‐(series a) and the two stereoisomers of the diisopropyl derivatives (series b and c), containing as the third substituent an H‐atom (2), a CH3 group (3)or a CH2OH group (4), were elaborated on the basis of the 1H‐, 13C‐, and 15N‐NMR spectra. The three N‐atoms of the saturated N3‐homocycle were found to be stable to pyramidal inversion in all cases. According to their NMR spectra, 2–4 of the series a and b possess twofold symmetry (Cs), while 2–4 of series c are asymmetric. Thus, series c has the trans‐configuration at N(2)/N(3) and, consequently, the cis‐configuration at N(1)/N(2), while series a and b have the cis‐configuration at N(2)/N(3) and ‐since the all‐cis‐arrangement is excluded‐the trans‐configuration at N(1)/N(2). The asymmetry of the trans‐configurated 2c turned into twofold symmetry (C2), when a little CF3COOH was added. The 1H‐ and 13C‐NMR data of series b and c of our alkyl‐triaziridines exhibit a shielding effect, according to which there are two types of i‐Pr groups, i‐Pr(a) and i‐Pr(b). They differ in the NMR signals of the H‐ and the C‐atoms of their CH groups: the H‐atoms of i‐Pr(a) are more deshielded by 0.75–1.111 ppm and its C‐atoms are more shielded by 10.0–160.0 ppm as compared to the corresponding atoms of i‐Pr(b). i‐Pr(a) is cis (on the N3‐homocycle) to a large substituent (such as i‐Pr, Me, CH2OH) and to a lone pair, while i‐Pr(b)is cis only to a small (H) or to no substituent and to one or two lone pairs. An analogous effect appears in the NMR signals of the CH3 and CH2OH groups at N(1) of 3 and 4 in the series b and c.
Studies on the Biosynthesis of Tabtoxin (Wildfire Toxin). Origin of the Carbonyl C‐Atom of the β‐Lactam Moiety from the C1‐Pool
Authors: Müller Barbara , Hädener Alfons , Tamm Christoph 
Published: December 2004

AbstractRe‐isolation of Pseudomonas tabaci strain NCPPB 2730 from its host, the tobacco plant, led to an activation of the bacteria in order to produce the β‐lactam dipeptide tabtoxin (Wildfire toxin, 1). Incorporation of several 14C‐labelled amino acids as well as L‐[methyl‐13C]methionine, L‐[1,2‐13C2]‐ and L‐[3,4‐13C2]aspartate, rac ‐[1,2‐13C2]glycerol, and [1,2‐13C2]acetate into isotabtoxion (2) demonstrated that the building blocks of tabtoxin (1) are L‐threonine, L‐aspartate, the Me group of L‐methionine and a C2‐unit derived from the C3‐pool (Fig. 3). The Me group of L‐methionine provides the carbonyl C‐atom of the β‐lactam moiety. These findings represent a novel pathway in β‐lactam biosynthesis. Mechanistic aspects with respect to the β‐lactam ring formation are discussed. A biradical 16 is proposed as an intermediate during the cyclization of a N‐formyl‐α‐amino ketone 15.
Optisch aktive Alkohole aus 1,3‐Dioxan‐4‐onen: eine praktikable Variante der enantioselektiven Synthese unter nucleophiler Substitution an Acetal‐Zentren
Authors: Seebach Dieter , Imwinkelried René , Stucky Gerhard 
Published: December 2004

Optically Active Alcohols from 1,3‐Dioxan‐4‐ones. A Practical Version of Enantioselective Synthesis with Nucleophilic Substitution at Acetal CentersSecondary alcohols in enantiomeric excesses above 90% are accessible from 2‐substituted 6‐methyl‐1,3‐dioxan‐4‐ones (Scheme 4). The dioxanones are prepared from aldehydes and readily available (R)‐or (S)‐3‐hydroxybutanoic acid. Treatment of the dioxanones with silyl nucleophiles or triisopropoxy(methyl)titanium in the presence of [Cl3TiX] yields the corresponding 3‐alkoxy acids in diastereoselectivities ≥ 95%. The ‘chiral auxiliary’ is removed from the alkoxy acids by treatment with LiN(i‐Pr)2 to give the secondary alcohols with ≤ 90% ee. cis/trans‐Mixtures (9:1) of the dioxanones furnish products of the same configurational purity as those obtained from pure cis‐isomers. In comparison with other variants of enantioselective synthese with nucleophilic substitution at acetal centers, the following advantages of the dioxanone method are noteworthy: (i) (R)‐ and (S)‐3‐hydroxybutanoic acids are both readily available; (ii) reactions are not sensitive to changes in conditions; (iii) the ‘chiral auxiliary’ is removed simply by base elimination, no oxidation is required; (iv) no chromatographic purification steps are necessary. The overall reaction described here is an enantioselective nucleophilic addition to aldehydes with concomitant dehydration of enantiomerically pure 3‐hydroxybutanoic to crotonic acid.
Sequencing of Tentoxin by Using Fast‐Atom‐Bombardment (FAB)/High‐Resolution (HR)/Tandem Mass Spectrometry (MSMS). Scope and Limitation of a Novel Strategy
Authors: Eckart Klaus , Schwarz Helmut 
Published: December 2004

AbstractThe recently developed FAB/MSMS methodology (i.e.) ionization of an underivatized peptide by using fast‐atom‐bombardment (FAB) combined with tandem mass spectrometry (MSMS) is applied for the sequencing of the cyclotetrapeptide tentoxin (12). The scope and limitation of the strategy is discussed in detail. Possible resolutions to overcome problems related to (i) the resolution of isobaric fragment ions and (ii) the distinction of sequencevs. retro‐sequence are investigated. The novel strategy is compared with conventional techniques. Significant improvement of the presently used FAB/MSMS methodology can be achieved by combining this approach with accurate mass measurements.