Showing papers in "Helvetica Chimica Acta in 1986"

Photochemical and Electrochemical Reduction of Carbon Dioxide to Carbon Monoxide Mediated by (2,2′‐Bipyridine)tricarbonylchlororhenium(I) and Related Complexes as Homogeneous Catalysts

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.

Photogeneration of Carbon Monoxide and of Hydrogen via Simultaneous Photochemical Reduction of Carbon Dioxide and Water by Visible‐Light Irradiation of Organic Solutions Containing Tris(2,2′‐bipyridine)ruthenium(II) and Cobalt(II) Species as Homogeneous Catalysts

Abstract: CO and H2 are photogenerated simultaneously by visible-light irradiation of systems containing a photosensitizer, the [Ru(bpy)3]2+ complex, Co(II) species as homogeneous catalysts, which mediate CO2 and H2O reduction by intermediate formation of Co(I), a tertiary amine as electron donor, which provides the electrons for the reduction, and an organic solvent which also facilitates dissolution of CO2. The efficiency of (CO + H2) gas production and the selectivity CO/H2 markedly depend upon the composition of the medium, the nature of the tertiary amine, the solvent, and the ligand of the Co ions. 2,9-Dimethyl-1,10-phenanthroline is particularly effective in promoting CO and H2 formation, giving a quantum yield of 7.7% in (CO + H2) (1.2% for CO and 6.5% for H2). The process consists of two catalytic cycles: a photocatalytic cycle for the Ru complex and a double dark reaction pathway for the Co species; oxidative and/or reductive quenching of the excited state of the photosensitizer lead to the formation of Co(I) species which reduce either CO2 or H2O to CO or H2, respectively.

Anion Coreceptor Molecules. Linear Molecular Recognition in the Selective Binding of Dicarboxylate Substrates by Ditopic Polyammonium Macrocycles

Abstract: Three macrocyclic hexaamines 1, 2, and 4, and the acyclic tetraamine 5 and hexaamine 6 have been synthesized. The hexaamines 1, 2, and 4 are ditopic coreceptor molecules containing two triamine subunits which may bind anionic substrates when protonated. The stability constants of the complexes between the protonated forms of the macrocyclic polyamines and terminal dicarboxylates −O2C−(CH2)m- CO2− as well as amino-acid and dipeptide dicarboxylates have been determined by pH-metric measurements. Around neutral pH, 1 and 2 give mainly complexes of the fully protonated species 1·6H+ and 2·6H+, whereas 4 yields predominantly complexes of 4·5H+ and 4·4H+. The stability sequences of the complexes formed indicate preferential binding of the dianionic substrates whose length is compatible with the separation of the triammonium binding subunits in the protonated receptor molecules 1, 2, and 4. This selectivity pattern corresponds to a process of linear molecular recognition based on ditopic binding between the two ammonium subunits of the coreceptor and the terminal carboxylates of the substrate of complementary length. The complexes of the acyclic ligands 5 and 6 are much weaker and much less selective, indicating a marked macrocyclic effect on both stability and selectivity of binding, i.e. on recognition.

Anion Selectivity of Metalloporphyrins in Membranes

Abstract: Lipophilic Co(III) and Mn(III) complexes of 5,10,15,20-tetrakis[4-(hexyloxycarbonyl)phenyl]porphyrin act as positively charged carriers for anions and induce anion selectivities in membranes clearly deviating from the sequence of classical anion exchangers. Different anion selectivities are observed for the Co(III) and Mn(III) porphyrins.

Metal complexes with macrocyclic ligands. Part XXV. One‐step synthesis of mono‐N‐substituted azamacrocycles with a carboxylic group in the side‐chain and their complexes with Cu2+ and Ni2+

Abstract: Mono-N-substituted azamacrocycles 2–7, containing a carboxyalkyl or carboxyaryl side-chain, are obtained by reacting a five-fold excess of the macrocycle with 1 equiv. of a suitable halogenocarboxylic acid in alkaline aqueous EtOH. For halogenocarboxylic acids, which easily lactonize under alkaline conditions, a variant with the corresponding ester or nitrile as alkylating agent is also described. The salient point of this synthesis lies in the use of an excess of the macrocycle over the alkylating agent, thus reducing the amount of polyalkylation to a minimum, and in the easy separation of the excess of unreacted educt from the aminocarboxylic acid. These new ligands form Ni2+ and Cu2+ complexes, the spectral properties of which have been studied. In the case of the Cu2+ complexes with ligand 2, 3, and 6, a pH-dependent color change is observed. This is explained with an equilibrium between a species, in which the carboxylate group is bound to the metal, and one, in which it is protonated and non-coordinated. In the case of the Ni2+ complexes with the same ligands, only the species with a coordinated carboxylate was observed. In the Cu2+ and Ni2+ complexes with ligands 4 and 5, however, the carboxylate group does not coordinate at all, because of the length or the special structure of the chain.

Organization of Carotenoid-Phospholipid Bilayer Systems. Incorporation of Zeaxanthin, Astaxanthin, and their C50 Homologues into Dimyristoylphosphatidylcholine Vesicles

Abstract: Four carotenoids (zeaxanthin 1, astaxanthin 2, and their C50 synthetic isoprene-homologues 3 und 4) have been incorporated into bilayer unilamellar vescles of dimyristoylphospatidylcholine as proved by coelution on a Sepharose column. The incorporation into the bilayer proper has been proved by the sensitivity to phase transition of UV/VIS spectra and circular dichroism. The UV/VIS absorptions of the carotenoids are typical of a lipidic environment. At the temperature of phase transition occur both intermolecular phenomena (aggregation of carotenoid molecules) and intramolecular changes (conformational change). The relative solubilities of the various carotenoids in this phospholipid membrane can be fitted with molecular parameters (length of the lipophilic carotenoid segment vs. lipid bilayer thickness).

Chiral metals? A chiral substrate for organic conductors and superconductors

Abstract: Chiral metals are so far practically unknown. The synthesis of a chiral substrate for possible organic conductors and superconductors is described.

Metal complexes of macrocyclic ligands. Part XXIII. Synthesis, properties, and structures of mononuclear complexes with 12‐ and 14‐membered tetraazamacrocycle‐N,N′,N″,N‴‐tetraacetic Acids

Abstract: The two tetraazamacrocycle-N,N′,N″,N‴-tetraacetic acids H4dota and H4teta form with Ni2+, Cu2+, and Zn2+ (M2+) mononuclear complexes MLH2 and M′[ML], M′ being an alkaline earth ion. The structures of Ni(H2dota) and Cu(H2dota) have been solved by X-ray structure analysis. The metal ions are in a distorted octahedral geometry coordinated by four amino N-atoms and two carboxylates. In the case of Cu2+, the distortions are more pronounced than for Ni2+ indicating that the Jahn-Teller effect is operating. Starting from these two structures, the coordination geometry of the other complexes is discussed using VIS and IR spectra.

Enantiomerenreine Pyrrolidin-Derivate aus trans-4-Hydroxy-L-prolin durch elektrochemische oxidative Decarboxylierung und Titantetrachlorid-vermittelte Umsetzung mit Nukleophilen

Abstract: Enantiomerically Pure Pyrrolidine Derivatives from trans-4-Hydroxy-L-proline by Electrochemical Oxidative Decarboxylation and Titanium-Tetrachloride-Mediated Reaction with Nucleophiles Preparative electrolysis of N-methoxycarbonyl-O-[(t-butyl)dimethylsilyl]hydroxyproline 4 in MeOH leads to substitution of the COOH by a MeO group (oxidative decarboxylation). The mixture 5 of the two diastereoisomers (ca 1:1) thus obtained was reacted in CH2Cl2 with nucleophilic silylated compounds (such as allylsilane, silyl cyanide and 1-phenyl-1-silyloxyethane) or with trimethyl phosphite in the presence of TiCl4 to give 2-allyl-, 2-cyano-, 2-(2-oxo-2-phenylethyl)- and 2-phosphono-substituted hydroxypyrrolidines, respectively, with high diastereoselectivities (≥ 90%, products 6-12). The configuration of two of the products (6/7 and 8/9) was shown to be cis.

Preparation of Enantiomerically Pure β-Silylcarboxyl Derivatives by Asymmetric 1,4-Addition to N-Enoyl-sultams. Preliminary Communication

Abstract: EtAlCl2-promoted additions of organocopper reagents to camphor-derived, conjugated N-enoyl-sultams gave saturated and olefinic β-silylcarboxyl derivatives with high diastereodifferentiation. Nondestructive removal of the chiral auxiliary followed by oxidative Si-C bond cleavage furnished enantiomerically pure acetate-derived aldols and propionate-derived ‘anti’ -aldols (via silyl-directed α-methylation).

Enantioselective Synthesis of D-erythro-Sphingosine and of Ceramide

Abstract: The enynol 2 was transformed into D-erythro-sphingosine 11 (7 steps, 46%) and into ceramide 1 (8 steps, 41% overall yield). The key steps were the mono-epoxidation of the enynol 5 (Ti(t-BuO)4, (−)-D-diethyl tartrate, t-BuOOH) to 6 (86%, ≥ 98% ee), the regioselective intramolecular opening of the oxirane 6via the benzylurethane 7, and the reductive tranformation of the acetylene 9 into the oxazolidinone 10 (Li, EtNH2, 88%).

Cation-response mechanism of neutral carrier based ion-selective electrode membranes

Abstract: Results of voltammetric, potentiometric, chronoamperometric, ion transport, and extraction studies on neutral carrier based, plasticized poly(vinyl chloride) membranes are summarized. They unambiguously confirm that such bulk membranes dispose of immobile anionic sites. These fixed sites lead to a Donnan exclusion of other anions from the membrane and thus to a permselectivity for cations. The results are in perfect agreement with the predictions of earlier membrane models. A rigorous Poisson-Boltzmann analysis of macroscopic liquid membranes clearly indicates that space-charge at the membrane/solution interfaces does not influence the electrochemical properties and the ion-selectivity behavior at steady state.

Kupplung von Peptiden mit C‐terminalen α,α‐disubstituierten α ‐ Aminosäuren via Oxazol‐5(4H)‐one

Abstract: Peptide-Bond Formation with C-Terminal α,α-Disubstituted α - Amino Acids via Intermediate Oxazol-5(4H)-ones The formation of peptide bonds between dipeptides 4 containing a C-terminalα,α-disubstituted α-amino acid and ethyl p-aminobenzoate (5) using DCC as coupling reagent proceeds via 4,4-disubstituted oxazol-5(4H)-ones 7 as intermediates (Scheme 3). The reaction yielding tripeptides 6 (Table 2) is catalyzed efficiently by camphor-10-sulfonic acid (Table 1). The main problem of this coupling reaction is the epimerization of the nonterminal amino acid in 4via a mechanism shown in Scheme 1. CSA catalysis at 0° suppresses completely this troublesome side reaction. For the coupling of Z-Val-Aib-OH (11) and Fmoc-Pro-Aib-OH (14) with H-Gly-OBu1 (12) and H-Ala-Aib-NMe2 (15), respectively, the best results have been obtained using DCC in the presence of ZnCl2 (Table 3).

Diastereoselectivity and Reactivity in the Diels‐Alder Reactions of α‐Chloronitroso Ethers

Abstract: The structure of the α-chloronitroso ether 1, obtained from the hydroximolactone 2 and tert-butyl hypochlorite (89%), was established by X-ray crystallographic analysis. The [4 + 2] cycloadditions of 1 with the dienes 3 and 8–11 led to the N-unsubstituted 3,6-dihydro-2H-1,2-oxazines 6 and 12–16 in high enantiomeric excess (Table 1). Due to the additional α-alkoxy group, the reactivity of 2 is much superior to the one of known α-chloronitrosoalkanes. The reactive conformation of 1 was deduced from the X-ray analysis as well as the high diastereoselectivity of the cycloadditions. The importance of the α-alkoxy group was evidenced from the similar reactivity of the racemic α-chloronitroso ethers 25–27 which were prepared from the hydroximo ethers 28–30 and tert-butyl hypochlorite.

Enantioselective Syntheses of α‐Amino Acids from 10‐(Aminosulfonyl)‐2‐bornyl Esters and Di(tert‐butyl) Azodicarboxylate. Preliminary Communication

Abstract: Succesive 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.

1,3‐Dioxanone Derivatives from β‐Hydroxy‐carboxylic Acids and Pivalaldehyde. Versatile Building Blocks for Syntheses of Enantiomerically Pure Compounds. A Chiral Acetoacetic Acid Derivative Preliminary Communication

Abstract: (R)-3-Hydroxybutyric acid (from the biopolymer PHB) and pivalaldehyde give the crystalline cis - or (R,R)-2-(tert-butyl)-6-methyl-1,3-dioxan-4-one (1a), the enolate of which is stable at low temperature in THF solution and can be alkylated diastereoselectively (3, 4, 5, and 7). Phenylselenation and subsequent elimination give an enantiomerically pure enol acetal 10 of aceto-acetic acid. Some reactions of 10 have been carried out, such as Michael addition (11), alkylation on the CH3 substituent (13), hydrogenation of the CC bond (1a) and photochemical cycloaddition (16). The overall reactions are substitutions on the one stereogenic center of the starting β-hydroxy acid without racemization and without using a chiral auxiliary.

Synthesis of 2′‐Deoxyribofuranosides of 8‐Aza‐7‐deazaguanine and Related Pyrazolo[3,4‐d]pyrimidines

Abstract: The N(1)- and N(2)-(2′-deoxyribofuranosides) 1 and 2, respectively, of 8-aza-7-deazaguanine were prepared via phase-transfer glycosylation in the presence or absence of Bu4NHSO4 as catalyst of 6-amino-4-methoxy-lH-pyrazolo[3,4-d]pyrimidine (7c) with 2-deoxy-3,5-di-O-(p-toluoyl)-α-D-erythro-pentofuranosyl chloride (10). On a similar route, but without catalyst and employing THF as organic phase, the 6-amino-4-chloronucleosides 11b and 12b were synthesized from 7a and converted into the N(1)-and N(2)-substituted 4-thioxo analogues 17a and 18a, respectively. The ratio of N(1)- to N(2)-glycosylation was 2:1 for 7c and 1:1 for 7a, viz. depending on the nucleobase structure. The rate of the H+-catalyzed N-glycosyl hydrolysis was strongly decreased for the N(2)-(β-D-2′-deoxyribofuranosides) as compared to the N(1)-compounds. However, the N(1)-nucleoside 1, which is an isostere of 2′-deoxyguanosine, is sufficiently stable to be employed later in solid-phase oligonucleotide synthesis.

Membrane Structure of Substance P. I. Prediction of Preferred Conformation, Orientation, and Accumulation of Substance P on Lipid Membranes†

Abstract: Preferred conformation, orientation, and accumulation of substance P on a neutral hydrophilic-hydrophobic interface was estimated and extrapolated to interactions with neutral and anionic lipid bilayer membranes according to our general procedure. Nine residues at the C-terminus were predicted to be transferred to the hydrophobic phase as an α-helical domain, oriented quite perpendicularly on the membrane surface. The N-terminal residues remained in the aqueous phase with their charges exposed to H2O. The molecular amphiphilic moment vector was strong (338 arbitrary units) and pointed its hydrophilic end towards the N-terminus, only 15° away from the helix axis. The molecular electric dipole moment vector was also strong (124 debye) and pointed its positive end towards the N-terminus, only 9° away from the helix axis. Thus, it reinforced the effect of the amphiphilic moment of a peptide intruding into the membrane dipole layer. The estimated dissociation constant for the equilibrium between membrane-bound and free substance P was Kd ≈ 46 mM for neutral membranes, and Kd ≈0.43 mM for anionic membranes with a Gouy-Chapman surface potential of −40 mV. Thus, substance P behaved similarly to dynorphin A and adrenocorticotropin peptides which insert their N-terminal message segments as perpendicularly oriented helical domains into membranes, whereas their C-terminal address segments remain in the aqueous phase as random coils. Substance P is the first instance of a neuropeptide which is expected to insert a C-terminal message into lipid membranes.

Metal complexes of macrocyclic ligands. Part XXIV†. Binuclear complexes with tetraazamacrocycle-N,N′,N″,N‴-tetraacetic acids

Abstract: The three ligands H4dota, H4teta, and H4heta give binuclear complexes with Cu2+ and Ni2+, the spectral properties of which have been studied. The structures of Cu2(dota)·5H2O and Cu2(teta)·6H2O have been established by X-ray diffraction analysis.

Asymmetric and ‘anti’‐Selective Aldolisations of Acetates and Propionates. Preliminary Communication

Abstract: Starting from acetates 1 and propionates 6, TiCl4-mediated addition of their silyketene acetals 2 and 7 to aldehydes gave aldols 4 and 9, respectively, with high π-face and ‘anti’ differentiation (Schemes, and Tables 1 and 2). Alternation of the (E/Z)-enolate geometry led to reversed α- and β-inductions (7 → 9b, 8 → 10b). Non-destructive removal of the auxiliary yielded enantiomerically pure β -hydroxycarboxylic acids 13.

Synthese von Pentafulvalen durch oxidative Kupplung von Cyclopentadienid mittels Kupfer(II)-chlorid

Abstract: Synthesis of Pentafulvalene by Oxidative Coupling of Cyclopentadienide with Copper(II) Chloride Starting 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 ‘Huckel 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 119 followed by a rearrangement 1920 takes place (Scheme 5).

An ESR Study of the Radical Cations of Tetrathiafulvalene (TTF) and Electron Donors Containing the TTF Moiety

Abstract: Hyperfine data and g factors are reported for the radical cations of tetrathiafulvalene (TTF; 1) and of its derivatives 2–13. From the intense satellite spectra of 1+–13+ not only the coupling constants of the 33S isotopes in the TTF moiety could be determined, but also, in favourable cases, those of the 13C isotopes in the central double bond. The former values range from 0.370 (8+) to 0.470 mT (4+) and the latter from 0.255 (8+) to 0.360 mT (4+) in the radical cations of bis(ethylenedithio)-TTF (8+) and tetracyano-TTF (4+). The radical cation of TTF (1+) exhibits intermediate values, 0.425 for the 33S and 0.285 mT for the 13C isotopes. The spin population in 1+–13+ resides, to a large extent, in the central S2C CS2 part of the π-system. It tends to increase (decrease) by substitution with electron-accepting (donating) groups in the 2,3,6,7-positions of TTF.

Methionin als Vorläufer zur enantioselektiven Synthese α-verzweigter Vinylglycine und anderer Aminosäuren

Abstract: Methionine as Precursor for the Enantioselective Synthesis of α-Branched Vinylglycines and of Other Amino Acids Methionine 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.

Deoxy‐nitrosugars 15th communication Synthesis of N‐acetylneuraminic acid and N‐acetyl‐4‐epineuraminic Acid

Abstract: A synthesis of N-acetylneuraminic acid (1) and of N-acetyl-4-epineuraminic acid (2, R = H) from 2-acetamido-4,6-O-benzylidene-1,2-dideoxy-1-nitro-D-mannopyranose (3) and 2-acetamido-1,2-dideoxy-4,6-O-isopropylidene-1-nitro-D-mannopyranose (4), respectively, is described. Michael addition of 3 and 4 to tert-butyl 2-(bromomethyl)prop-2-enoate (5) and subsequent hydrolytic removal of the NO2 group gave the 4-nonulosonate tautomers 6/7 and 8/9, respectively (Scheme). Stereoselective reduction of 6/7 and 8/9 with NaBH4/AcOH in dioxane/H2O yielded 12/13 (94:6) and 14/15 (92:8), respectively. Reduction of 6/7 and 8/9 in the absence of AcOH or in EtOH gave 12/13 (15:85) and 14/15 (15:85), respectively. Ozonolysis of 12 and 13 followed by hydrolysis gave tert-butyl neuraminate 22 and tert-butyl 4-epineuraminate 24, respectively. Ozonolysis of 14/15, separation of the products 20 and 21, and hydrolytic removal of the isopropylidene groups gave 22 and 24, respectively. The tert-butyl ester 22 was saponified to give 1, which was further characterized as the methyl ester 23. Saponification of 24 gave the crude 4-epimer of 1, which was converted into the stable Na salt 2 and also into the methyl ester 25.

Synthesis and Biological Properties of N‐Acetyl‐4‐deoxy‐D‐neuraminic Acid

Abstract: N-Acetylneuraminic acid (1) can be transformed into the methyl α-D-ketoside 2 which, by reaction with methanesulfonyl chloride, yields the corresponding 4-O-mesylate 3 and the 4,7-di-O-mesylate 4 as a by-product. Compound 3 reacts with Nal giving the 4-deoxy-4-iodo compound 5 with equatorial orientation of the I-atom. As second product, the dihydrooxazole 6 is produced. Catalytic hydrogenation of 5 is followed by ester cleavage and removal of the isopropylidene group yielding the methyl α-D-ketoside 8 which affords the title compound, N-acetyl-4-deoxyneuraminic acid (9), by reaction with fowl plague virus sialidase. Further biochemical activities of 8 and 9 are reported.

Membrane Structure of Substance P. II. Secondary Structure of Substance P, [9‐Leucine]substance P, and Shorter Segments in 2,2,2‐Trifluoroethanol, Methanol, and on Liposomes Studied by Circular Dichroism

Abstract: Comparative CD studies with substance P (1), [Leu9]substance P ([Leu9]-1), and their shorter peptide segments supported the membrane structures predicted for substance P and [Leu9]substance P. They indicated that the C-terminal segments (from residue 3 or 4 onward) can adopt α-helical conformations in hydrophobic environments and on lipid membranes. The N-terminal segment, (residues 1–4) had a poly(proline)-like conformation in aqueous and hydrophobic surroundings. Residues 3 and 4 (Lys-Pro) appeared to belong to both domains and bring about the transition between the two. The estimated free energies of transfer for 1 and [Leu9]-1 from their random conformations in H2O to their partially helical conformations on an aqueous-hydrophobic interface are too small to allow detectable interaction with neutral lipid membranes at low concentrations. The two peptides should, however, interact detectably with anionic membranes because of favourable Boltzmann distribution factors. This prediction was shown to be correct for liposomes prepared from 1,2-dioleoyl-sn-glycero-3-phosphocholine (neutral) and phosphatidylserine (anionic).

Enantioselective Synthesis of β‐Necrodol and of 1‐Epi‐β‐necrodol via Asymmetric 1,4‐Addition and Magnesium‐Ene Cyclization. Preliminary Communication

Abstract: Enantiomerically pure β-necrodol (1) and its 1-epimer 16 have been synthesized starting from aldehyde 5. The two key steps are an asymmetric conjugate addtion/Mannich reaction tandem (10→12) and a type-II-magnesiumene cyclization/oxidation sequence (14→1 + 16).

4‐Methylumbelliferyl 5‐Acetamido‐3,4,5‐trideoxy‐α‐D‐manno‐2‐nonulopyranosidonic Acid: Synthesis and Resistance to Bacterial Sialidases

Abstract: The synthesis of 4-methylumbelliferyl α-D-glycoside 13 of N-acetyl-4-deoxyneuraminic acid and its behaviour towards bacterial sialidases is described. N-Acetyl-4-deoxyneuraminic acid (1) was transformed into its methyl ester 2 and then acetylated to give the anomeric pentaacetates 3 and 4 of methyl 4-deoxyneuraminate and the enolacetate 5 (Scheme). A mixture 3/4 was treated with HCl/AcCl to give the glycosyl chloride, which was directly converted into the 4-methylumbelliferyl α-D-glycoside 9 of methyl 7-O,8-O,9-O,N-tetraacetylneuraminate and into the 2,3-dehydrosialic acid 11. The ketoside 9 was de-O-acetylated to 12 with NaOMe in MeOH. Saponification (NaOH) of the methyl ester 12 followed by acidification gave the free 13, which was also converted into the sodium salt 14 by passage through Dowex 50 (Na+). The 4-deoxy α-D-glycoside 13 is not hydrolyzed at significant rates by Vibrio cholerae and Arthrobacter ureafaciens sialidase. Neither the free N-acetyl-4-deoxyneuraminic acid (1), nor the α-D-glycoside 13 inhibit the activity of these sialidases.

Generation and Reactions of Lithiated tert-Butyl and 2,6-Di(tert-butyl)-4-methylphenyl Cyclopropanecarboxylates

Abstract: tert-Butyl and 2,6-di(tert-butyl)-4-methylphenyl (BHT) cyclopropanecarboxylates (4, 6, 24, 25) are lithiated with LiN(i-Pr)2 and t-BuLi, respectively. Reactions with alkyl halides, aldehydes, acyl chlorides, and heteroelectrophiles give α-substituted BHT esters which can be cleaved (t-BuOK/H2O/THF) to the corresponding carboxylic acids or reduced (LiAlH4/THF) to the cyclopropanemethanols.

Synthese von 4,4‐disubstituierten 1,3‐Thiazol‐5(4H)‐thionen

Abstract: Synthesis of 4,4-Disubstituted 1,3-Thiazol-5(4H)-thiones An easy synthesis for the 1,3-thiazol-5(4H)-thiones 5, a class of heterocycles which have hitherto only been available with difficulty, is described. Reaction of 3-amino-2H-azirines 25 with thiocarboxylic acids at 0° yields monothiodiamides of type 20 (Scheme 6) which, on treatment with Lawesson reagent at 100°, undergo thiation and cyclization to give 5 in good yield.