Showing papers in "Helvetica Chimica Acta in 1981"
TL;DR: In this paper, the Michael-additions of aliphatic, alicyclic, and arylsubstituted nitroolefins and enamines lead to γ-nitroketones 3 in good chemical and excellent (> 90%) diastereomeric yields.
Abstract: The Michael-additions of aliphatic, alicyclic, and arylsubstituted nitroolefins and enamines lead to γ-nitroketones 3 in good chemical and excellent (> 90%) diastereomeric yields (see Table 1).
The known threo-configuration of one type of adducts 3 (entries 8, 10, and 11 of Table 1) can be arrived at by assuming the approach 8 of the Michael-acceptor and -donor; the reaction follows a topological rule, which is formulated and which is applicable to such diverse reactions as the diene synthesis, cyclopropanations, carbonyl olefinations and methylenations, aldol- and nitroaldol-type additions, as well as additions of lithium, boron, and chromium derivatives to aldehydes (see 9, 10, 11, and Table 2).
339 citations
TL;DR: In this article, aqueous CdS dispersions loaded with Pt and RuO2 by visible light produces hydrogen and oxygen in stoichiometric proportion, and no degradation of the photocatalyst is noted after 60 h of irradiation time.
Abstract: Illumination of aqueous CdS dispersions loaded with Pt and RuO2 by visible light produces hydrogen and oxygen in stoichiometric proportion. No degradation of the photocatalyst is noted after 60 h of irradiation time. The RuO2 deposit on the particle surface greatly accelerates the transfer of holes from the semiconductor valence band to the aqueous solution thus inhibiting photocorrosion.
168 citations
TL;DR: The photophysical and electron transfer properties of the lowest excited state of nine ruthenium (polypyridine) complexes have been characterized in this article, where the results show that the Ru (bpy)2(DMCH)2+ complex is expected to be a more efficient mediator than Ru(bpy)-2+3 in the water splitting reaction by solar energy.
Abstract: The photophysical and electron transfer properties of the lowest excited state of nine ruthenium (polypyridine) complexes have been characterized. The complexes studied are Ru (bpy)3-n (LL)2+n, where n varies from 0 to 3, and LL is 4, 4′-di-t-butyl-2,2′-bipyridine (DTB-bpy), 3, 3′-dimethyl-2, 2′-bipyridine (DM-bpy), or a 2, 2′-diquinolyl derivative (DMCH). The results obtained show that the Ru (bpy)2(DMCH)2+ complex is expected to be a more efficient mediator than Ru (bpy)2+3 in the water-splitting reaction by solar energy.
167 citations
TL;DR: The structures of the crystalline lithium enolates derived from 3, 3-dimethyl-2-butanone and from cyclopentanone have been determined by X-ray analysis as mentioned in this paper.
Abstract: The structures of the crystalline lithium enolates derived from 3, 3-dimethyl-2-butanone and from cyclopentanone have been determined by X-ray analysis. Both structures are tetrameric aggregates based on a Li4O4-cube, each Li+-ion being additionally bonded to the O-atom of a tetrahydrofuran solvent molecule.
149 citations
TL;DR: In this paper, the overall enantioselective α-alkylation of chiral, non-racemic α-heterosubstituted carboxylic acids (4 → 6) is realized.
Abstract: Dioxolanones 7 and 8a and oxazolinones 9a derived from pivalaldehyde and lactic acid, mandelic acid, and proline, respectively, furnish chiral enolates of type 3 by deprotonation with LDA. Reactions of these enolates with alkyl halides, aldehydes, and ketones (→ 8b, 9b, 11–13) are highly diastereoselective. Thus, the overall enantioselective α-alkylation of chiral, non-racemic α-heterosubstituted carboxylic acids (4 → 6) is realized.
142 citations
TL;DR: In this article, the authors investigated the variation of the transient optical density as a function of the intensity of the exciting laser in homogeneous solutions (1-chloronaphthalene, 1-propanol) and in microemulsion.
Abstract: Triplet state formation (Φisc) and properties (eT-T, τT) of phthalocyanine (HPC) and zinc phthalocyanine (ZnPC) have been characterized in homogeneous solutions (1-chloronaphthalene, 1-propanol) and in microemulsion by investigating the variation of the transient optical density as a function of the intensity of the exciting laser. Experimental results follow the theoretically predicted dependence only for very low intensities of the exciting pulse. At higher intensities, a more complicated scheme of primary reactions has to be taken into account, implicating qualitative restrictions in the application of saturation experiments. The observed transients at high-intensity excitation are interpreted as being perturbed by aggregational phenomena.
117 citations
TL;DR: The enantiomerically pure synthetic building blocks with four C-Atoms and two or three functional groups from β-hydroxy-butanoic, Malic, and Tartaric acid were introduced in this article.
Abstract: Enantiomerically Pure Synthetic Building Blocks with Four C-Atoms and Two or Three Functional Groups from β-Hydroxy-butanoic, Malic, and Tartaric Acid
The pool of chiral, non-racemic electrophilic building blocks, which are available from simple natural products in both enantiomeric forms is enlarged by the epoxides 3, 5, and 10, by the tosylate 12a, and by the aldehydes 18 (cf. symbols A-D, 14, and Scheme 1). Key steps of the conversions leading from hydroxyacids to the building blocks are: epoxide-opening by triethylborohydride (1 → 2a) and tosylate reduction (12a → 12b); the Mitsunobu inversion (2a → 4a); the reduction of (R, R)-tartaric ester to (R)-malic ester by NBS (N-bromosuccinimide) opening of the benzaldehyde acetal 8 and tin hydride reduction (6c → 7c); the enantiomer enrichment of optically active ethyl β-hydroxy-butanoate through the crystalline dinitrobenzoate 21b. Detailed procedures are given for large scale preparations of the key intermediates. The enantiomeric purities of the building blocks are secured by correlations.
108 citations
TL;DR: Starting from the enantiomerically pure monoterpenes (+)-pulegone (3), (+)-limonene (7), (−)-β-pinene (9), (+-) and (−)-camphor (13) or (+)-cholestenone (11) the chiral alcohols 4, 5, 6, 8, 10, 12, 14, 15, 16, 17 et 18 were prepared; their acrylates II underwent a TiCl4-promoted Diels-Alder addition to cyclopentadiene (Sche
Abstract: Starting from the enantiomerically pure monoterpenes (+)-pulegone (3), (+)-limonene (7), (−)-β-pinene (9), (+)- and (−)-camphor (13) or (+)-cholestenone (11) the chiral alcohols 4, 5, 6, 8, 10, 12, 14, 15, 16, 17 et 18 were prepared; their acrylates II underwent a TiCl4-promoted Diels-Alder addition to cyclopentadiene (Scheme 3, Table) giving in a predictable manner either the (2R)- or the (2S)-adducts III with 63 to 88% asymmetric induction.
104 citations
TL;DR: The configurational isomers of astaxanthin from the flesh of salmon (Salmo salar and Oncorhynchus) were isolated and analyzed as (−)-camphanic acid diesters by means of HPLC as discussed by the authors.
Abstract: The configurational isomers of astaxanthin (3,3′-dihydroxy-β,β-carotene-4,4′-dione) from the flesh of salmon (Salmo salar and Oncorhynchus) caught at different places in Europe and Canada were isolated and analyzed as (−)-camphanic acid diesters by means of HPLC. The biological variation in the composition of the configurational isomers in seven fish was surprisingly similar: 78 to 85% of (3S, 3′S)-astaxanthin, 12 to 17% (3R, 3′R)-astaxanthin and 2 to 6% meso-astaxanthin.
98 citations
TL;DR: In this paper, the trans-8-phenyl-menthyl enoates were added to the cis-crotonate to obtain enantiomerically pure β-substituted alkanoic acids.
Abstract: 1,4-Additions of PhCu · BF3, n-Bu · BF3 and MeCu · BF3 to the trans-8-phenyl-menthyl enoates 1 proceeded with high chiral induction. Saponification of the resulting esters 2 gave the corresponding enantiomerically pure β-substituted alkanoic acids 3 and the recovered (−)-8-phenylmenthol in good overall yields. Analogous additions to the cis-crotonate 1 led preferentially to the acids 3 enantiomeric to those obtained from the trans-crotonate 1, although with lower selectivity. A stereochemical model is proposed consistent with the observed results (Scheme 2, Table).
89 citations
TL;DR: The tetrameric cubic structure established for lithium enolates in the solid state is used as a model to discuss several facets of enolate chemistry: solubility, effects of cosolvents, addition to aldehyde and acid chloride, influence of mixed aggregates on diastereo- and enantioselectivities.
Abstract: The tetrameric cubic structure established for lithium enolates in the solid state is used as a model to discuss several facets of enolate chemistry: solubility, effects of cosolvents, addition to aldehyde and acid chloride, influence of mixed aggregates on diastereo- and enantioselectivities.
TL;DR: In this paper, Lysergic acid was synthesized from 4-hydroxymethyl-1-tosylindole by a sequence of 9 steps and the crucial thermolysis involved the in situ generation of the transient diene III which undergoes an intramolecular cycloaddition to a C, N-double bond at 200° and at low stationary concentration of III.
Abstract: (±)-Lysergic acid (1) has been synthesized from 4-hydroxymethyl-1-tosylindole (2) by a sequence of 9 steps. The crucial thermolysis 9 10 involves the in situ-generation of the transient diene III which undergoes an intramolecular cycloaddition to a C, N-double bond at 200° and at low stationary concentration of III.
TL;DR: In this article, an efficient and simple procedure was developed for the preparation of the threo- and erythro-3, 4-epoxy-1, 2-butanediol derivatives from tartaric acid.
Abstract: Chiral Electrophilic Synthetic Building Blocks with Four Different Functional Groups from Tartaric Acid, 2, 3- and 3, 4-Epoxy-butanediol Derivatives in All Four Stereoisomeric Forms
Efficient and simple procedures have been developed for the preparation of the threo- and erythro-3, 4-epoxy-1, 2-butanediol derivatives 3 and 8 and of the cis- and trans-2, 3-epoxy-1, 4-butanediol derivatives 7 and 9 in both enantiomeric forms from tartaric acid (see Scheme 2). These epoxides should prove versatile as alkylating reagents in syntheses of enantiomerically pure natural products and of physiologically active compounds.
TL;DR: The racemate and enantiomers of patchouli alcohol have been synthesized by stereocontrolled routes as mentioned in this paper, and the olfactive properties of the patches are reported.
Abstract: The racemate and both enantiomers of patchouli alcohol have been synthesized by stereocontrolled routes. The olfactive properties of the patchouli alcohols prepared are reported.
TL;DR: In this article, it was shown that some heterosubstituted organotitanium compounds are more stable thermally than their lithium counterparts, and that with the chiral (S)-2-methyl-l-butoxy group an enantioselective addition can be achieved.
Abstract: Solutions of the title compounds R-Ti(OR′)3 (1) are generally available from organolithium (or magnesium) derivatives according to equation 1. It is shown (Table 1) that some heterosubstituted organotitanium compounds are more stable thermally than their lithium counterparts. The reagents 1 are highly selective carbonylophiles (Tables 1 and 2), their reactivity can be modified by variation of the R′O-group (Table 3) and with the chiral (S)-2-methyl-l-butoxy group an enantioselective addition can be achieved [equ. 2].
TL;DR: In this article, the 3-trialkylsilyloxy-1,4-diene 3a and subsequent electrophilic substitution of the non-isolated 3 trialkyl silyoxypentadienyllithium 4 gives the α- and γ-products 8 and/or 6 in good yields.
Abstract: Deprotonation of the 3-trialkylsilyloxy-1,4-diene 3a and subsequent electrophilic substitution of the non-isolated 3-trialkylsilyoxypentadienyllithium 4 gives the α- and γ-products 8 and/or 6 in good yields. Whereas alkylation of 4 proceeds with variable regioselectivity (Table 1) aldehydes and ketones attack preferentially the γ-position of 4(Table 2). The desired γ-products 6 may be directly subjected to inter- and intramolecular [4 + 2]-additions as demonstrated by the reactions 5a ( 6d) → 7 and 6h → 19(Schemes 4 and 12). Alternatively, smooth fluoride-promoted silylether-cleavage 6 → 11(Scheme 8) provides a convenient approach to substituted vinyl ketones such as to the natural products 11f(Table 3). The stereoselective conversion 6k → 23(Scheme 13) implies an endo-selective intramolecular Diels-Alder addition (26 → 23) and exemplifies the use of 4 as an equivalent of the hypothetical anion IV. Furthermore, some electrophilic substitutions of the hexadienyllithium 15 have been studied (Scheme 10).
TL;DR: In this article, a general strategy for the synthesis of spherical macrotricyclic ligands has been developed, and four spherical cryptands, SC-24, SC25, SC26 and SC-27 have been obtained by this route.
Abstract: Spherical Cryptates. Synthesis and Inclusion-Complexes of Spherical Macrotricyclic Ligands
A general strategy for the synthesis of spherical macrotricyclic ligands has been developed. Four spherical cryptands, SC-24, SC-25, SC-26 and SC-27 have been obtained by this route. The synthesis and cation-complexing properties of these compounds are described in detail. Stability constants and cation exchange rates of the spherical cryptates obtained with alkali and alkaline-earth cations have been determined. Highly stable complexes are formed by SC-24; the Rb+ and Cs+ cryptates of SC-24 are the most stable complexes of these cations known to date. The size of the intramolecular cavity affects the complexation selectivity. The cation exchange rates are very slow, and the corresponding free energies of activation are even larger than, for macrobicyclic cryptates of similar stability. Both the high complex stabilities and the high activation energies required for cation exchange indicate a marked ‘spherical cryptate effect’ resulting from the highly connected nature of the molecular architecture of spherical macrotricyclic ligands.
TL;DR: In this paper, a sensitive gas chromatographic method was developed to determine the diastereoisomeric and enantiomeric purity of α-tocopherol samples as the methyl ethers.
Abstract: All eight stereoisomers of α-tocopheryl acetate have been synthesized in a state of high chemical and stereoisomeric purity. Key chiral side-chain intermediates were prepared from (+)-(S)-3-hydroxy-2-methylpropanoic acid. New routes to (2R, 4′ RS, 8′ RS)-α-tocopheryl acetate, a mixture of four diastereoisomers, were also developed. A sensitive gas chromatographic method was developed to determine the diastereoisomeric and enantiomeric purity of α-tocopherol samples as the methyl ethers. It was established for the first time that naturally occurring α-tocopherol is essentially a single enantiomer (2 R, 4′ R, 8′ R), synthetic all-rac-α-tocopherol an equimolar mixture of four racemates, and that natural (E)-(7 R, 11 R)-phytol is diastereoisomerically and enantiomerically homogeneous.
TL;DR: The structure of elaiophylin (azalomycin B) was elucidated by extended spectroscopic investigations and chemical degradation as discussed by the authors, and the synthesis of 7-acetoxy-6-ethyl-3-octanone (14), the acetyl derivative of an important degradation product was described.
Abstract: Structure Elucidation of Elaiophylin: Spectroscopy and Chemical Degradation
The structure of the antibiotic elaiophylin (azalomycin B) was elucidated by extended spectroscopic investigations and chemical degradation. Elaiophylin (26) is a macrodiolide with a 16membered dilactone ring. The synthesis of 7-acetoxy-6-ethyl-3-octanone (14), the acetyl derivative of an important degradation product, is described.
TL;DR: In this article, a variety of protected vicinal nitroalcohols are described, as well as an efficient method for their reduction to the corresponding vicinal aminoalcohols 2, 4 and 6.
Abstract: Summary Regioselective and flexible procedures are described for the preparation of a variety of protected vicinal nitroalcohols 1, 3 and 5 (see Scheme 5), as is an efficient method for their reduction to the corresponding vicinal aminoalcohols 2, 4 and 6 Vicinal aminoalcohols have broad significance in organic chemistry Their synthetic importance can be seen in the Tcfeneau-Demjanov and related deam- inative semipinacol rearrangements [ 11, and their biological relevance in the structures of adrenalin and related mediators of the sympathetic nervous system [2] [3] All of the main synthetic routes to this functional class possess significant limitations to their use [3], particularly in terms of the attainable degree of regio- selectivity For example, the orientation of oxirane ring-opening by a N-nucleophile depends upon both the substitution pattern of the oxirane and the reaction conditions employed The modified osmylation method of
TL;DR: In this paper, disubstituted acetylenes are oxidized with PhIO in presence of Ru-catalysts to afford α-diketones in 65-85% yield.
Abstract: Disubstituted acetylenes are oxidized with PhIO in presence of Ru-catalysts to afford α-diketones in 65–85% yield. Under the same conditions terminal acetylenes are cleaved to carboxylic acids.
TL;DR: In this article, a large number of 5′-O-tritylated thymidine-3′-phosphotriesters with two different phosphate protecting groups have been prepared, characterized, and studied according to their chemical stability and use-fullness for oligonucleotide syntheses.
Abstract: XIV. Substituted β-Phenyl-ethyl Groups. New Blocking Groups for Oligonucleotide Syntheses by the Phosphotriester Approach
Various o- and p-substituted β-phenylethanols (2–10) have been synthesized and investigated as blocking groups in the phosphotriester approach. A large number of 5′-O-tritylated thymidine-3′-phosphotriesters (13–36) with two different phosphate protecting groups have been prepared, characterized, and studied according to their chemical stability and usefullness for oligonucleotide syntheses. The combination of a 5′-O-monomethoxytrityl- and a 3′-(2,5-dichlorophenyl, p-nitrophenylethyl)-phosphate function as in 18 turned out to possess optimal properties as a monomeric nucleotide building block due to the fact that these blocking groups can be quantitatively and selectively be removed without harming each other by trifluoroacetic acid in chloroform to 41, by oximate to 42, and by DBU to 43. The base-catalyzed removal of the monosubstituted phenylethyl groups by DBU or DBN respectively as well as the disubstituted phenylethyl groups by triethylamine in aprotic solvents is a β-elimination process leading to phosphodiesters without attack on the P-center.
TL;DR: In this paper, the NO group with dienes gives 3, 6-dihydrooxazine derivatives which isomerise to epoxyepimines in most cases of β-substituted nitrosoalkenes.
Abstract: Some α- and β-halonitrosoalkenes 1 have been synthesized and characterized. The halogen atoms of the oxime precursors 2 can be substituted by alkoxy groups. Two kinds of cycloaddition reaction of 1 have been observed: (i) reaction of the NO group with dienes gives 3, 6-dihydrooxazine derivatives 6 which isomerise to epoxyepimines 7 in most cases of β-substituted nitrosoalkenes; (ii) if 4, 5-dihydrooxazines 22 are obtained, the cycloaddition of the nitrosoalkenes as 4π-component is presumed.
TL;DR: The structure and absolute configuration of pseurotin A, a new metabolite, isolated from culture filtrates of Pseudeurotium ovalis STOLK(Ascomycetes) has been shown to be (1, 2, 5S, 8S, 9R, 3′Z)-8-benzoyl-2 (1′, 2-dihydroxy-3′-hexenyl)-9-hydroxy-8-methoxy-3-methyl-1-oxa-7-azaspiro [4
Abstract: The structure and absolute configuration of pseurotin A, a new metabolite, isolated from culture filtrates of Pseudeurotium ovalis STOLK(Ascomycetes) has been shown to be (1′S, 2′S, 5S, 8S, 9R, 3′Z)-8-benzoyl-2 (1′,2-dihydroxy-3′-hexenyl)-9-hydroxy-8-methoxy-3-methyl-1-oxa-7-azaspiro [4,4]non-2-ene-4,6-dione (1).
TL;DR: In this paper, Batiment sci phys,inst cristallog,ch-1015 lausanne dorigny,switzerland. Reference LGSA-ARTICLE-1981-007
Abstract: Note: Batiment sci phys,inst cristallog,ch-1015 lausanne dorigny,switzerland. Reference LGSA-ARTICLE-1981-007doi:10.1002/hlca.19810640613 Record created on 2005-11-09, modified on 2017-05-12
TL;DR: In contrast to their titanium analogues, vinylzirconium reagents are stable enough to be employed. as discussed by the authors showed that aldehyde and ketone carbonylophiles of exceedingly low basicity can be solved from aldehydes and aldeheeds using trialkoxyzircimiconium chloride.
Abstract: Solutions of the title compounds are accessible from organolithium reagents and trialkoxyzirconium chloride (equation 2). In contrast to their titanium analogues, vinylzirconium reagents are stable enough to be employed. Generally, organozirconium reagents are highly selective aldehyde and ketone carbonylophiles of exceedingly low basicity (Tables 1, 2, 3 and typical procedure).
TL;DR: In this paper, a correlation of the coordination shifts with the reactivity of the coordinated naphthalene is proposed, and the coordinated ring 13C-resonance is found to undergo a smooth transition to higher field with increasing donor character of the coligands L.
Abstract: The 13C-NMR. spectra of the series of complexes ?6-naphthalene · CrL3 (L--CO (1), PF3 (2), PF2OMe (6), P(OMe)3 (3), C10H8 (= 3 L) (4) and PMe3 (5)) are reported. Definite assignments of the 13C-NMR. resonances were made through the synthesis of [2, 3, 6, 7-2H4]-naphthalene complexes. The coordinated ring 13C-resonance are found to undergo a smooth transition to higher field with increasing donor character of the coligands L. A correlation of the coordination shifts with the reactivity of the coordinated naphthalene is proposed. In complexes containing strong acceptor ligands the naphthalene is activated to attack by nucleophiles. Sequential treatment of complexes 1-4, 6 and [C10H8FeC5H5]+[PF6]- (7) with stabilized carbanions and I2 or Ce(IV)-salt yields a-substituted naphthalenes in the case of 1, 2, 6 and 7 but not in the case of 3 and 4. Treatment of 3 with an excess of HBF4 results not in the expected metal protonation but in a novel ligand transformation to yield 6.
TL;DR: In this article, the molecular ion and pertinent fragments of the aglycone were obtained from field-desorption mass spectrometry (D/CI-MS) on underivatized compounds.
Abstract: 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.
TL;DR: A chiral macrocyclic crown ether exhibits an enantiomer-selectivity of 2.6 for α-phenylethylammonium ion when incorporated in solvent polymeric membranes as discussed by the authors.
Abstract: A chiral macrocyclic crown ether exhibits an enantiomer-selectivity of 2.6 for α-phenylethylammonium ion when incorporated in solvent polymeric membranes. The sequence of selectivity of these membranes clearly differs from that of lipophilicity for the different biogenic ammonium ions studied, indicating a significant structural contribution.