Showing papers in "Helvetica Chimica Acta in 1968"
TL;DR: The intrinsic acidity of surface silanol groups was determined by coulometric titration of silicagel at 25° in solutions of the constant ionic strength 0,1M consisting primarily of sodium perchlorate.
Abstract: The intrinsic acidity constant Kint. of surface silanol groups has been determined by coulometric titration of silicagel at 25° in solutions of the constant ionic strength 0,1M consisting primarily of sodium perchlorate. The experimental data are consistent with
232 citations
TL;DR: In this article, the role of wall coated capillaries in gas chromatography is discussed with emphasis on glass caillaries, and theoretical knowledge concerning spreading of liquids on the glass surface is still very insufficient, empirical rules for producing coherent liquid films are proposed.
Abstract: The role of wall coated capillaries in gas chromatography is discussed with emphasis on glass capillaries. Since theoretical knowledge concerning the spreading of liquids on the glass surface is still very insufficient, empirical rules for producing coherent liquid films are proposed. Improvements for the carbonization procedure are reported. Since carbonized glass is suited for a restricted range of moderately polar liquids only, three new modifications of the glass surface opening a broader choice of wettability characteristics are described: (1) Bonding of various organic substituents to the silica frame work. (2) Polymerization and cross linking of butadiene on the glass surface, eventually with engrafted polar groups. (3) Polymerization of trifluorchlorethylene on the glass surface.
170 citations
TL;DR: This paper showed that the known thermal cyclisation of aryl propargyl ethers to chromenes involves a preliminary [3, 3]-sigmatropic rearrangement.
Abstract: 2, 6-Dimethylphenyl propargyl ether (10) and its derivatives 12–15 rearrange thermally to 1, 5-dimethyl-6-methylene-tricyclo [3.2.1.02,7]oct-3-en-8-one (9) and related compounds 16–19. The ethers undergo first an aromatic [3, 3]-sigmatropic rearrangement to ortho-allenyldienones 11, which then undergo ring closure to the tricyclic products by an electrocyclic reaction.
Only in the case of the γ-methylpropargyl ether 13, the ortho-dienone 11 is further rearranged in low yield to the para-butynylphenol 20, but the tricyclic ketone 17 is again the main product.
New data show that the known thermal cyclisation of aryl propargyl ethers to chromenes (e. g. 4 8) involves a preliminary [3, 3]-sigmatropic rearrangement.
136 citations
TL;DR: Spiro [4.4] nonane-1, 6-dione (III) has been synthesized in optically active form, and the chirality of the enantiomers determined by chemical correlation as discussed by the authors.
Abstract: Spiro [4.4] nonane-1, 6-dione (III) has been synthesized in optically active form, and the chirality of the enantiomers determined by chemical correlation.
132 citations
TL;DR: The application of the Nα-Dpoc group to solid-phase peptide synthesis permits the use of a new combination of protecting groups in which the side chains of trifunctional amino-acids are blocked by acid-labile residues that can be easily split in the final step of the synthesis.
Abstract: The 2-(p-diphenyl)-isopropyloxycarbonyl (Dpoc) residue has been chosen for the selective protection of α-amino groups in the synthesis of peptides containing additional acid-labile protecting residues. It is easily introduced into amino-acids by reacting either the mixed carbonate I or the azide III with esters or salts of amino-acids. It is split by dilute acetic acid and other weakly acidic reagents at rates which permit a selective cleavage in the presence of other acid-labile protecting groups, especially those derived from t-butanol
A number of peptide syntheses have been carried out with the new group either in the conventional manner or by the solid-phase method. No effects due to steric hindrance, as observed previously with the N-trityl residue, are encountered. The application of the Nα-Dpoc group to solid-phase peptide synthesis permits the use of a new combination of protecting groups in which the side chains of trifunctional amino-acids are blocked by acid-labile residues that can be easily split in the final step of the synthesis.
114 citations
TL;DR: The acidolytic cleavage of a series of new N-aralkyloxycarbonyl protecting groups has been found to proceed as a first-order reaction, the rate of cleavage being dependent on the stability of the corresponding aralkyl carbonium ions.
Abstract: The acidolytic cleavage of a series of new N-aralkyloxycarbonyl protecting groups has been found to proceed as a first-order reaction, the rate of cleavage being dependent on the stability of the corresponding aralkyl carbonium ions. Some of the groups are cleaved at much the same rate as the N-trityl residue and up to 60000 times faster than the t-butyloxycarbonyl (Boc) group. The rate is also strongly influenced by the acidity of the reaction media. The relative rates at which aralkyloxycarbonyl and Boc groups are split off can be largely controlled by appropriate selection of the reaction conditions.
The implications of these findings for peptide syntheses are discussed and the use of the 2-(p-diphenyl)-isopropyloxycarbonyl residue as an especially suitable N-protecting group is proposed.
109 citations
TL;DR: Experimental evidence indicates that calcitonin D represents the antiparallel dimeer of calcitonins M, two highly active peptides isolated from C cell tumours, using chemical and enzymatic methods.
Abstract: Human calcitonin M and its dimer calcitonin D, two highly active peptides isolated from C cell tumours, were subjected to sequence studies using chemical and enzymatic methods. For calcitonin M, containing 32 amino acid residues, the following structure was derived:
Though the disulphide bridge between position 1 and 7, and the C-terminal proline amide of human calcitonin M are the same as in porcine α-thyrocalcitonin, many amino acid residues - 18 in all - are different throughout the molecule. Arginine and tryptophan are absent; on the other hand, lysine and isoleucine are to be found at position 18 and 27 respectively. Methionine changes its place from position 25 to 8 adjacent to the disulphide bridge. Experimental evidence indicates that calcitonin D represents the antiparallel dimeer of calcitonin M.
102 citations
TL;DR: Identity of the synthetic and the natural hormone was established by thin-layer chromatography, thin- layer electrophoresis and conversion to oxidation products, as well as by reference to the pattern of tryptic degradation and by comparing the biological activity of the two hormones.
Abstract: A preliminary account is giver of the synthesis of calcitonin M (I), isolated from human C-cell tumour tissue [2] [3]. Identity of the synthetic and the natural hormone was established by thin-layer chromatography, thin-layer electrophoresis and conversion to oxidation products, as well as by reference to the pattern of tryptic degradation and by comparing the biological activity of the two hormones. The findings also afforded additional confirmation of the results of structural elucidation [1].
In the synthesis of I, use was made of methods described previously [4] for the preparation of porcine α-thyrocalcitonin, and also of a new method [5] which easily permits the formation of cyclic cystine peptides.
99 citations
TL;DR: In this article, the formation constants of enniatins with sodium and potassium salts are given, as well as complex formation constants for different antibiotics with both salt and potassium, respectively.
Abstract: Crystalline 1: 1-complexes of enniatins with potassium salts are described. Complex formation constants of different antibiotics with sodium and potassium are given.
89 citations
TL;DR: In this article, the stereochemistry of the transition state in the CLAISEN rearrangement of crotyl propenyl ethers has been established by heating trans, cis-, cis, cis- and trans, trans-Crotyl Propenyl Ether at 142,5°, erythro and threo 2, 3-dimethylpent-4-en-1-al (3 and 4) were obtained.
Abstract: The stereochemistry of the transition state in the CLAISEN rearrangement of crotyl propenyl ethers (2) has been established. By heating trans, cis-, cis, cis- and trans, trans-crotyl propenyl ether at 142,5°, erythro and threo 2, 3-dimethylpent-4-en-1-al (3 and 4) were obtained. From the ratio 3/4 it was shown that the rearrangement of the three ethers largely involves (97–98%) a chair-like transition state.
84 citations
TL;DR: A simple procedure for the preparation of 7-aminocephalosporanic acid in high yield is described.
Abstract: 7-Aminocephalosporanic acid or esters thereof are prepared either by intramolecular aminolysis of esters of the antibiotic cephalosporin C or via imino-ethers derived from the latter.
A simple procedure for the preparation of 7-aminocephalosporanic acid in high yield is described.
TL;DR: A re-evaluation of the electronic spectra of benzalaniline (II), 3, 3-dimethyl-2-phenyl-indolenine (X), mono-aryl substituted azomethines (XI, XIV, XV) as well as of their conjugate acids strongly supports the hypothesis of ISMAILSKI & SMIRNOV [14] and of EBARA [15].
Abstract: A re-evaluation of the electronic spectra of benzalaniline (II), 3, 3-dimethyl-2-phenyl-indolenine (X), mono-aryl substituted azomethines (XI, XIV, XV) and trimethylindolenine (XII) as well as of their conjugate acids strongly supports the hypothesis of ISMAILSKI & SMIRNOV [14] and of EBARA [15] that benzalaniline exists in a preferred peri-perpendicular conformation. The same is true for N-phenylazomethines. The n π* transition of a ‘planar benzalaniline’ has been located at 27 600 cm−1 (e ≈ 60).
TL;DR: In this paper, the ESR spectra of the radical anions of trimethylsilyl-substituted derivatives of p-benzoquinone, phenylketone, diimine, p-benequinone-diimide, aniline and p-phenylenediamine are reported.
Abstract: The ESR. spectra of the radical anions of trimethylsilyl-substituted derivatives of p-benzoquinone, phenylketone, diimine, p-benzoquinone-diimide, aniline and p-phenylenediamine are reported. In most cases the coupling constants of 29Si isotopes in natural abundance have been measured. The comparison of the ESR. data for the radical anions of the trimethylsilyl derivatives with those for the radical anions of the corresponding parent hydrocarbons and/or alkyl-substituted compounds provide additional evidence for the overall electron withdrawing effect of the trimethylsilyl substituent (SiCπ and Sinπ delocalization).
TL;DR: In this paper, the conversion of S-trityl-cysteine-containing protected peptides to cystine peptides by a reaction with iodine in methanol is described.
Abstract: A report is given on the conversion of S-trityl-cysteine-containing protected peptides to cystine peptides by a reaction with iodine in methanol. The new method permits the synthesis of symmetrical, open-chain unsymmetrical, and especially cyclic cystine peptides.
TL;DR: Two highly active calcitonin peptides, M with 32 amino acids, and D a dimer of M, were isolated from a large human mediastinal C cell tumour, which is very different from that of porcine α-thyrocalcitonin.
Abstract: Two highly active calcitonin peptides, M with 32 amino acids, and D a dimer of M, were isolated from a large human mediastinal C cell tumour. D can easily be transformed into M by the action of 1N ammonia; D and M afford two different sulphoxides, but all four peptides yield the same product upon oxidation with performic acid. Both D and M have a potency of about 120 MRC units/mg dry weight; their sulphoxides, by contrast, are almost inactive. Tryptic digestion of M produces an N-terminal octadecapeptide (TrI) and a C-terminal tetradecapeptide (TrII), the latter being also obtained from D. Amino acid analysis and other analytical data are presented. The structure of the human calcitonin peptides D and M is thus very different from that of porcine α-thyrocalcitonin.
TL;DR: The structure of ovalicin, a metabolite of the fungus Pseudeurotium ovalis with immunosuppressive activity, has been determined to be 11.5-methine-2,3,4,5-triphosphate, a major component of the polypeptide of E.coli A.
Abstract: The structure of ovalicin, a metabolite of the fungus Pseudeurotium ovalis with immunosuppressive activity, has been determined to be 11.
TL;DR: Pachybiose (1) is shown to be 4-O-(3-O-methyl-6-deoxy-β-D-allopyranosyl)-D-cymarose.
Abstract: Pachybiose (1) is shown to be 4-O-(3-O-methyl-6-deoxy-β-D-allopyranosyl)-D-oleandrose and asclepobiose (14) 4-O-(3-O-methyl-6-deoxy-β-D-allopyranosyl)-D-cymarose.
TL;DR: In this article, a new method for the synthesis of glycosides is described, which is based on the configuration at C-1 of the pyranose compound employed for the glycosidation.
Abstract: A new method for the synthesis of glycosides is described. Epipodophyllotoxin (4) reacts with 2,3,4,6-tetra-O-acetyl-β-D-glucopyranose in the presence of BF3-etherate at low temperature to yield tetra-O-acetyl-epipodophyllotoxin-β-D-glucopyranoside (6). This compound, which is sensitive to acid and base, can be converted into the free glucoside 8 by zinc acetate catalysed methanolysis. Glycosidation of podophyllotoxin (1) occurs under the same conditions but is associated with an inversion at C-1 of the aglycone moiety, leading also to the acetylated epi-glucoside 6. It is assumed that the glycosidation proceeds through a common carbonium ion intermediate (12), generated from 1 and 4 respectively by the action of BF3. The intermediate 12 is substituted by the pyranose component from the less hindered side, giving exclusively 1-epi derivatives (e.g.6). The glycosidation reaction is also highly stereoselective with respect to the glycosidic linkage. The stereochemistry of this bond is determined by the configuration at C-1 of the pyranose compound employed for the glycosidation. Further experimental evidence for the proposed mechanism consists in the glycosidation of diphenylmethanol (14) to the corresponding β-D-glucoside 15. Scope and limitation of the new glycosidiation method are briefly discussed.
TL;DR: In this article, the spectral properties of the antibiotic granaticin and its derivatives and degradation products were investigated using an X-ray structural analysis, and the spectroscopic properties agreed with the formula XX for the antibiotic.
Abstract: UV.-, IR.- and NMR.-spectra of the antibiotic granaticin and several of its derivatives and degradation products are discussed. The spectroscopic properties agree with the formula XX for the antibiotic, which was fully elucidated by an X-ray structural analysis (see following communication).
TL;DR: In this article, the second-order formation rate constant increases in the sequence Ni2+ < Co2+ > Mn2+> Zn2+< Cd2+ ˜ Cu2+ · Ca2+
Abstract: The kinetics of the reaction of murexide with different divalent metal ions of class A and B have been measured by the temperature-jump-relaxation method. The second-order formation rate constant increases in the sequence Ni2+ < Co2+ < Mn2+ < Zn2+ < Cd2+ ˜ Cu2+ ˜ Ca2+ < Sr2+ < Ba2+ < Pb2+.
Thermodynamic data obtained from kinetic and equilibrium studies, respectively, are in good agreement. The results are compared with the characteristic rate constants for H2O-exchange in the inner coordination sphere of these metal ions, which follow the same sequence. The rate constants of the reaction of murexide with various trivalent metal ions, including the lanthanides, are also discussed in terms of current ideas on metal complex formation.
TL;DR: In this paper, it was shown that 3,19-dioxo-17β-acetoxy-Δ4-androstene can be formed by a secondary radical reaction rather than a primary photochemical step, and the formation of compound 23 proceeds through the elimination of the formyl radical and incorporation of a hydrogen from the medium.
Abstract: Irradiation of 3,19-dioxo-17β-acetoxy-Δ4-androstene (2) at room temperature in either of its two absorption bands centered at about 245 and 315 nm, respectively, led to products 21, 22, and 23 (Chart 3). Compounds 21 and 22 result from rearrangements involving intramolecular formal 1,2- (21) and 1,3-shifts (22) of the angular formyl group, and the formation of compound 23 proceeds through the elimination of the formyl radical and the incorporation of a hydrogen from the medium. Evidence favors the latter process to be a secondary radical reaction rather than a primary photochemical step.
TL;DR: In this paper, the position of the tautomeric equilibrium is a function of the substituents present in the phenyl nuclei on N-1 and N-5.
Abstract: 15N-H-Coupling in 1, 3, 5-triphenylformazan derivatives demonstrates that these compounds are not resonance hybrids but tautomeric pairs. The position of the tautomeric equilibrium is a function of the substituents present in the phenyl nuclei on N-1 and N-5.
TL;DR: In this article, the methylester of cyclodopa (5.6-dihydroxy-indoline-2-carboxylic acid) is achieved by oxidative conversion of dopa methyl ester to dopachrome methyl esters and its subsequent reduction at pH 8, using respectively potassium hexacyanoferrate(III) and sodium dithionite.
Abstract: The synthesis of the methylester of cyclodopa (5.6-dihydroxy-indoline-2-carboxylic acid) is achieved by oxidative conversion of dopa methyl ester to dopachrome methyl ester and its subsequent reduction at pH 8, using respectively potassium hexacyanoferrate(III) and sodium dithionite. The efficiency of the oxidation step was found to be dependent on the rather low concentration of the reactants and the very short reaction time involved. After its production cyclodopa methyl ester was stabilized by immediate protonation followed by acetylation leading in a high yield to O,O,N-triacetyl cyclodopa methyl ester. By partial hydrolysis this derivative gives an O,N-diacetyl cyclodopa methyl ester and further N-acetyl cyclodopa methyl ester. Cyclodopa methyl ester and cyclodopa are obtained by prolonged anaerobic hydrolysis. Micro-separation procedures, nmr-, uv-spectra and chiral-optical properties of cyclodopa and its derivatives are reported and discussed. It is shown by nmr evidence that under acidic conditions the aromatic proton on C(7) of cyclodopa slowly exchanges with deuterium. When an alcoholic solution of cyclodopa methyl ester and semicarbazide is allowed to oxidize in the air, the semicarbazone of dopachrome methyl ester forms.
TL;DR: It is concluded that the aromatic ring of dopa is cleaved and that re-cyclization involving the nitrogen generates the dihydropyridine moiety.
Abstract: During studies on the biogenesis of betalains (I) in cactus fruits (Opuntia sp.). DL-dopa-1-[14C] and -2-[14C] were incorporated into betanin (III) which was obtained radiopure after crystallization. The specific activity remained constant after conversion to betanidin (IV) and to a neobetanidin derivative (IX). Reaction of radiobetanin with proline afforded indicaxanthin (V) carrying more than 90% of the radioactivity. Dopa (VI) is thus an efficient precursor for betalamic acid (VIII) but not for cyclodopa (VII). Decarboxylation of radiobetanidin and radioindicaxanthin showed that the carboxyl group of dopa remained a carboxyl group in the biotransformation to betalamic acid. It is concluded that the aromatic ring of dopa is cleaved and that re-cyclization involving the nitrogen generates the dihydropyridine moiety. Under the same conditions mevalonic acid, aspartic acid and phenylalanine showed low incorporations. Studies with beet seedlings and DL-dopa-1-[14C], -2-[14C] and DL-tyrosine-1-[14C] afforded similar results but with low incorporations.
TL;DR: Several 7-aza-norbornadiene-derivatives have been synthesized and their UV-and NMR-data are reported as mentioned in this paper ; see Section 2.1.
Abstract: Several 7-aza-norbornadiene-derivatives have been synthesized. Their UV.- and NMR.-data are reported.
TL;DR: Two new antifungal compounds have been isolated from a strain of Streptomyces aureofaciens DUGGAR that gave on methanolysis the methyl glycoside of 2-deoxy-D-rhamnose and the corresponding 3-O-carbamate.
Abstract: The known antibiotic Venturicidin A and two new antifungal compounds, Venturicidin B and Botrycidin, have been isolated from a strain of Streptomyces aureofaciens DUGGAR. Venturicidin B gave on methanolysis the methyl glycoside of 2-deoxy-D-rhamnose, Venturicidin A the corresponding 3-O-carbamate.
TL;DR: In this article, it was shown that only the allyl group migrates and that this rearrangement is an intramolecular, one-step process, and that the interaction of the two parts is controlled by the symmetry of the highest occupied π-orbitals (ψ3 for toluene and ψ2 for the ally group) in agreement with the Woodward-Hoffmann rules.
Abstract: The dienol-benzene rearrangement of syn and anti-4-allyl-4-methylcyclohexa-2,5-dien-1-ol (syn and anti 15) occurs by formation of a benzonium ion intermediate in p-toluene-sulphonic acid in ether below 0° and leads to a mixture of 2-, 3- and 4-allyltoluenes in the ratio 54:10:36. By the introduction of 14C-, D- and methyl labelled dienols it is shown that only the allyl group migrates and that this rearrangement is an intramolecular, one-step process. The formation of 2-allyltoluene occurs with retention, whereas the 3- and 4-allyltoluenes are formed by inversion of the carbon skeleton of the migrating allyl group. These rearrangements can be therefore classified as suprafacial, aromatic sigmatropic reactions of the order [1,2], [3,3] and [3,4]. The transition state can be postulated as representing a positively charged complex consisting of interacting allyl and tolyl radicals. The interaction of the two parts is controlled by the symmetry of the highest occupied π-orbitals (ψ3 for toluene and ψ2 for the allyl group) in agreement with the Woodward-Hoffmann rules. The better “distribution” of the charge in the transition state of these reactions in comparison to the ground state is chiefly responsible for the CoPE-like [3,3] sigmatropic reaction occurring at low temperatures. In general, sigmatropic reactions in charged systems are faster.
The rearrangement of syn and anti 2-allyl-2-methylcyclohexa-3,5-dien-1-ol (syn and anti 28) gives results similar to those obtained with the para-allyldienols.
The thermal rearrangement of 15 and 28 gives 3-allyltoluene by a [3,3] sigmatropic Cope rearrangement followed by elimination of water.
TL;DR: A synthesis of the dotriacontapeptide sequence I, reported by Neher et al. for porcine α-thyrocalcitonin (α-TC), is described, and is shown to be identical with α-TC by thin layer chromatography and electrophoresis, by the pattern of trypsin degradation, and by its biological activity.
Abstract: A synthesis of the dotriacontapeptide sequence I, reported by Neher et al. [1] for porcine α-thyrocalcitonin (α-TC), is described. The synthetic peptide is shown to be identical with α-TC by thin layer chromatography and electrophoresis, by the pattern of trypsin degradation, and by its biological activity. Oxidation with hydrogen peroxide gave the methionine25 sulfoxide derivative, which was identical with α-TC-sulfoxide (formerly called β-thyrocalcitonin) from pig thyroids.
TL;DR: In this article, it was shown that a charge transfer complex between the chlorine atoms as electron acceptors and benzene molecules as electron donors can be detected within less than 0.3 μs after the irradiation by a high-energy electron pulse.
Abstract: In a solution of benzene in carbon tetrachloride a transient absorption in the visible part of the spectrum could be detected. It appears within less than 0.3 μs after the irradiation by a high-energy electron pulse, and it can be shown to be due to the charge-transfer complex between the chlorine atoms as electron acceptors and benzene molecules as electron donors. A variety of aromatic hydrocarbons also yield similar absorption bands in the visible. They show a linear correlation between the absorption energy and the ionisation potential of the aromatic molecules, which is typical for charge-transfer complexes. A minimum value for the equilibrium constant of complex formation is given. The equilibrium is almost fully shifted to the complex side. An estimated G value for the charge-transfer complex indicates that the complex is actually part of a main reaction in the radiation-induced mechanism. The decay of the charge-transfer complex is mostly pseudo-first order with a half life of a few microseconds.
TL;DR: The highly active peptide hormones isolated from pig thyroids – i.e. α-thyrocalcitonin and its sulfoxide, containing 32 amino acid residues – were subjected to sequence studies using chemical and enzymatic methods and the following structure was derived.
Abstract: The highly active peptide hormones isolated from pig thyroids – i.e. α-thyrocalcitonin and its sulfoxide, containing 32 amino acid residues – were subjected to sequence studies using chemical and enzymatic methods. On the basis of these studies the following structure was derived for α-thyrocalcitonin:
In the α-thyrocalcitonin sulfoxide (designated formerly as β-thyrocalcitonin) the methionine residue is replaced by methionine sulfoxide.