Showing papers in "Liebigs Annalen in 1997"
TL;DR: In this article, the authors focus on total syntheses from their own laboratories that employ the Horner-Wittig reaction and point out the importance of the Wittig reaction in organic synthesis.
Abstract: Originally reported in Liebigs Annalen in 1953 (then called Justus Liebigs Annalen der Chemie), the Wittig reaction has evolved to include the Horner–Wittig and Horner–Wadsworth–Emmons reactions and several other variations. Today, these reactions constitute some of the most powerful processes for the construction of carbon–carbon bond frameworks, facilitating the chemical synthesis of myriads of organic molecules both in research laboratories and industrial settings. The Wittig and related reactions were proven particularly useful in the field of total synthesis, where they enabled the construction of highly complex structures and had a significant impact in shaping the art to its present state of sophistication. In this article the authors focus, after a brief introduction, on total syntheses from their own laboratories that employ such processes. These examples, together with the numerous others adorning the chemical literature, illustrate amply the importance of the Wittig and related reactions and point to their continuing prominence in organic synthesis.
197 citations
TL;DR: In this paper, the synthesis, characterization and X-ray crystallographic structure determination are described for a stable thiazol-2-ylidene 2 and its corresponding dimer.
Abstract: The synthesis, characterization and X-ray crystallographic structure determination are described for a stable thiazol-2-ylidene 2. This thiazol-2-ylidene is the first example of a stable, crystalline carbene in which the singlet carbene center bears a sulfur substituent. The carbene 2 is the closest stable analog of the important thiamin (vitamin B1) carbene. Although the thiazol-2-ylidene 2 is sufficiently stable to isolate at room temperature in the absence of moisture and oxygen, it will dimerize to form a normal carbene–carbene dimer {2}2. With the isolation and structure determination of both 2 and {22}, this system is the first in which a stable carbene and its corresponding dimer have been isolated and characterized. Additionally reported is the “reduced” 2H-thiazoline 2 · H2 in which the former carbene center in 2 has been reduced to a methylene (CH2) group. Thiazol-2-ylidenes with smaller groups on nitrogen (e.g. mesityl or methyl) are too unstable to allow easy isolation. The dimer from 3,4,5-trimethylthiazol-2-ylidene possesses a very unusual geometry in which one of the former carbene centers is planar while the second is pyramidal.
190 citations
TL;DR: In this article, a total of 121 organic solvents were studied and classified by means of the solvatochromic comparison method of Kamlet and Taft, to give a solvent acidity scale for hydrogen-bond donor solVents based on the parameter SA.
Abstract: A total of 121 organic solvents were studied and classified by means of the solvatochromic comparison method of Kamlet and Taft, to give a solvent acidity scale for hydrogen-bond donor solvents based on the parameter SA. This parameter can readily be evaluated from the visible spectrum of a suitable basic probe (o-tert-butylstilbazolium betaine dye, TBSB) and its non-basic homomorph (o,o′-di-tert-butylstilbazolium betaine dye, DTBSB). The proposed scale has many advantages when compared to the more widely used acidity scales such as that based on Gutmann's acceptor number (AN) and the Taft-Kamlet α scale. Whilst data for the proposed scale are derived from electronic transitions only, they do provide an accurate description for solvent acidity effects in spectroscopy and other chemical areas, including thermodynamics and kinetics.
147 citations
TL;DR: A comprehensive discussion of the supposed biosynthetic pathways draws the conclusion that two polyketide synthases (PKSα, PKSβ) exist, forming two different series of pentaketides in parallel.
Abstract: New 10-membered lactones, named aspinolide A–C (3, 4, 5), and the aspinonene/aspyrone co-metabolites, named trienediol (6), isoaspinonene (7), dihydroaspyrone (8), and dienetriol 9 were discovered by chemical screening methods in the cultures of Aspergillus ochraceus (DSM-7428) under altered fermentation conditions. The structures were established by detailed spectroscopic analysis. The polyketide origin of the skeleton and the biogenesis of the oxygen atoms were verified by feeding sodium [1,2-13C2]acetate and [18O2] gas, respectively, to growing cultures of the fungus. A comprehensive discussion of the supposed biosynthetic pathways draws the conclusion that two polyketide synthases (PKSα, PKSβ) exist, forming two different series of pentaketides in parallel. One pathway leads to branched-chain metabolites by a Favorski-like rearrangement of an epoxide intermediate, while the other involves detachment of the pentaketides from the PKSβ by lactonisation without any rearrangement of the carbon skeleton. The former pathway is affected by an inhibitor of cytochrome P450 dependent monooxygenases.
70 citations
TL;DR: In this article, N-protected amino acid allylic esters with LDA at −78°C and subsequent addition of a metal salt presumably results in the formation of a chelated metal enolate that undergoes Claisen rearrangement upon warming up to room temperature, giving rise to γ, Δ-unsaturated amino acids.
Abstract: Deprotonation of N-protected amino acid allylic esters with LDA at −78°C and subsequent addition of a metal salt presumably results in the formation of a chelated metal enolate that undergoes Claisen rearrangement upon warming up to room temperature, giving rise to γ,δ-unsaturated amino acids. Many different metal salts can be used for chelation, but in general the best results are obtained with zinc chloride. Due to the fixed enolate geometry, as a result of chelate formation, and a strong preference for the chair-like transition state, the rearrangement proceeds with a high degree of diastereoselectivity. This method can be applied to acyclic as well as to cyclic substrates, and even to peptides, and allows the synthesis of amino acids containing quaternary carbon centers. Esters of chiral allylic alcohols produce optically active amino acids via chirality transfer from the ester moiety to the α-carbon center of the amino acid. Another possibility to introduce chirality is given by the rearrangement in the presence of chiral ligands. The best results are obtained if Al(OiPr)3 is used as the chelating metal salt and the Chinchona alkaloids as bidentate ligands. Quinine gives rise to (2R)-configured amino acids, whereas quinidine delivers the (2S) derivatives in very high yields and excellent enantioselectivities (80–93% ee). The ligands can easily be recovered nearly quantitatively from the reaction mixture.
68 citations
TL;DR: In this article, a design logic for the self-assembly of a molecular trefoil knot and of its isomeric trivial knot is described, which relies on the formation of a double-stranded supramolecular complex between an acyclic π-electron rich 1,5-di-oxynaphthalene-based polyether and a bipyridinium-based tetracation.
Abstract: A design logic for the self-assembly of a molecular trefoil knot and of its isomeric trivial knot is described. The approach relies on the formation of a double-stranded supramolecular complex between an acyclic π-electron rich 1,5-di-oxynaphthalene-based polyether and an acyclic π-electron deficient bipyridinium-based tetracation. The reversible complexation of the two acyclic complementary components in solution is followed by the irreversible formation of covalent bonds connecting the termini of these two precursors within the supramolecular complex, affording both a trefoil knot and a trivial knot, though only in extremely low yields. The isomers were separated by preparative high-performance liquid chromatography and characterized by liquid secondary ion mass spectrometry which revealed the binding of up to two ammonium ions in the case of only one of the two isomers – namely, the trivial knot. In addition, we have performed binding studies between some model acyclic and macrocyclic 1,5-dioxynaphthalene-based polyethers and the bipyridinium based precursor of the knots. One of the complexes was also characterized in the solid state by X-ray crystallography which revealed the formation of a [2]pseudorotaxane in the solid state.
67 citations
TL;DR: In this article, a review on the transformations of products obtained by VNS, particularly into heterocyclic compounds, is presented, and characteristic features of the VNS mechanism are briefly described.
Abstract: Vicarious Nucleophilic Substitution (VNS) provides the possibility of the replacement of hydrogen atoms in electrophilic arenes with α-functionalized alkyl substituents. This review focuses on the transformations of products obtained by VNS, particularly into heterocyclic compounds. In addition, some characteristic features of the VNS mechanism are briefly described.
66 citations
TL;DR: In this article, the preparation, present uses, and deprotection of amino sugar derivatives employing electron-withdrawing groups are described, including tetrachlorophthaloyl (TCP), dichlorophthoyl(DCPhth), dithiasuccinoyls (Dts), N-pent-4-enoyl and N,N-diacetyl functionalities.
Abstract: Recent synthetic efforts in the construction of oligosaccharides, glycoproteins, glycolipids and other glycoconjugates containing a variety of 2-amino-2-deoxy sugars have led to the development of new methods for nitrogen protection. These new nitrogen-protecting groups have been removed from substrates using mild and often chemoselective conditions. In this microreview, the preparation, present uses, and deprotection of amino sugar derivatives employing these new agents are described. Applications of phthalimides with electron-withdrawing groups including the tetrachlorophthaloyl (TCP), dichlorophthaloyl (DCPhth), dithiasuccinoyl (Dts), N-pent-4-enoyl and N,N-diacetyl functionalities are discussed.
65 citations
TL;DR: In this article, the stereochemical features and synthetic potentials of the [1, 2]-Wittig rearrangement were reviewed and compared with the [3, 4] and [5] rearrangements.
Abstract: The stereochemical features and synthetic potentials of the [1,2]-Wittig rearrangement are reviewed.
64 citations
TL;DR: Combinatorial approaches to molecular recognition are discussed, i.e. screening of libraries of ligands with synthetic receptors and libraries of synthetic receptors with the ligands for which selective binding is desired.
Abstract: Combinatorial libraries offer a new powerful tool for gathering empirical information about host/guest interactions and for developing catalysts for organic reactions. Combinatorial approaches to molecular recognition are discussed, i.e. screening of libraries of ligands with synthetic receptors and libraries of synthetic receptors with the ligands for which selective binding is desired. Recent achievements in the development of new catalysts for organic synthesis using combinatorial approaches are also reviewed.
55 citations
TL;DR: Ozonolyses of O-methyl oximes of acyclic ketones (7, 10, 19) and of cyclic ketones (16, 24, 27, 31) in the presence of cyclically-ketone-free oximes gave the corresponding tetrasubstituted cross-ozonides (13, 21, 29, 32).
Abstract: Ozonolyses of O-methyl oximes of acyclic ketones (7, 10, 19) and of cyclic ketones (16, 24, 27, 31) in the presence of acyclic ketones (8, 9, 17) or of cyclic ketones (18, 25, 26, 31) gave the corresponding tetrasubstituted cross-ozonides (13, 21, 29, 32). Ozonolyses of O-methylated monooximes of 1,4-, 1,5-, and 1,6-dicarbonyl compounds (35b–f) gave the corresponding bicyclic ozonides 36b–f.
TL;DR: In this paper, an efficient method for the asymmetric epoxidation of α-enones with oxygen in the presence of diethylzinc and a chiral amino alcohol is presented.
Abstract: A new efficient method for the asymmetric epoxidation of α-enones with oxygen in the presence of diethylzinc and a chiral amino alcohol is presented. The epoxidation proceeds in moderate to excellent yields, complete diastereoselectivity and enantiomeric excesses of up to >99% by using enantiomerically pure (1R,2R)-N-methylpseudoephedrine 7 as a chiral additive. The amino alcohol auxiliary 7 employed can be almost completely recovered with unchanged enantiomeric purity.
TL;DR: Hino et al. as discussed by the authors extracted peptides from a submerse culture of the fungus Hypocrea muroiana and identified two major groups of peptides, designated hypomurocin (HM) A and B, belonging to the peptaibol family.
Abstract: From a submerse culture of the fungus Hypocrea muroiana Hino et Katsumoto (Ascomycetes: Hypocreales) two major groups of peptides, designated hypomurocin (HM) A and B, belonging to the peptaibol family, could be characterized. Both groups showed antibiotic activity (Bacillus subtilis) and caused hemolysis of rat erythrocytes, HM A being less active than HM B. Peptides were isolated from the culture broth by chromatography on XAD-2 adsorber resin, octyl silica, and Sephadex LH 20. HM A and B were separated by preparative TLC, and individual peptides from each microheterogeneous group were isolated by preparative HPLC. Amino acid analysis and sequence determination by fast atom bombardment and electrospray tandem mass spectrometry revealed the composition and structures of six 11-mer peptides of the HM A group and of six 18-mer peptides of the HM B group. Positions of isomeric amino acids Leu/Ile and Val/Iva (present in some of the peptides) were determined by methanolytic cleavage of the pure peptides, followed by trifluoroacetylation of the dipeptide methyl esters released and assignment of their structures by gas chromatography-selected ion monitoring mass spectrometry. As examples, two sequences of HM A and HM B are presented (exchange positions in parentheses). – HM A-1(2): Ac-Aib1(D-Iva1)-Gln-Val-Val-Aib-Pro-Leu-Leu-Aib-Pro-Leuol11; HMB-1(2): Ac-Aib1-Ser-Ala-Leu-Aib-Gln-Aib-Val-Aib-Gly-Aib-Pro-Leu-Aib-Aib-Gln-Valol18 (Leuol18), (Ac = acetyl, Aib = α-aminoisobutyric acid, Iva = D-isovaline, Leuol = L-leucinol, Valol = L-valinol).
TL;DR: The myxobacterial metabolites chivosazole A and its variants were discovered in Sorangium cellulosum, strain So ce12, which simultaneously provides the broad spectrum antibiotic sorangicin as well as the sorangiolides and disorazoles.
Abstract: The myxobacterial metabolites chivosazole A (1) and its variants (2–6) were discovered in Sorangium cellulosum, strain So ce12, which simultaneously provides the broad spectrum antibiotic sorangicin as well as the sorangiolides and disorazoles. The antifungal and cytotoxic chivosazoles (1–6) are novel glycosides of 6-deoxyglucopyranose derivatives and an aglycon that includes an oxazole in its 31-membered macrolide ring. The aglycon itself, chivosazole F (7), was formed by strain So ce885 and showed similar activity antibiotic and cytotoxic.
The biogenetic origin of the structural elements in chivosazole F (7) was studied by feeding experiments with [1-13C]-, [1,2-13C]acetate, [methyl-13C]methionine and [1-13C]serine. Accordingly, the aglycon 7 is a polyketide assembled by condensation of nine acetate units, one serine unit and a further seven acetate units. While C-1 of serine is part of the macrolide ring, the amido to hydroxyl part of the serine together with C-1 of the adjacent N-acetyl unit form the 1,3-oxazole ring. C- and O-methyl groups are derived from methionine.
TL;DR: Tetraethynylethene (3,4-diethynylhex-3-ene-1,5-diyne, C10H4) and its derivatives have been applied to the preparation of perethynylated dehydroannulenes, expanded radialenes, poly(triacetylenes), organometallic σ-acetylide species, and donor/acceptor functionalized chromophores with interesting nonlinear optical properties as discussed by the authors.
Abstract: Tetraethynylethene (3,4-diethynylhex-3-ene-1,5-diyne, C10H4) and its differentially protected derivatives are versatile building blocks for carbon-rich molecular scaffolding. They have been applied to the preparation of perethynylated dehydroannulenes, expanded radialenes, poly(triacetylenes), organometallic σ-acetylide species, and donor/acceptor-functionalized chromophores with interesting nonlinear optical properties. This article gives an account of the synthesis of tetraethynylethene and its derivatives as well as their elaboration into novel linearly or cross-conjugated chromophores and functional carbon-rich nanomaterials.
TL;DR: The most recent successes achieved in organic synthesis by use of high pressure technique are reviewed in this article, where the authors present a review of the most recent success achieved by using high pressure techniques.
Abstract: The most recent successes achieved in organic synthesis by use of high-pressure technique are reviewed.
TL;DR: Carbamate- and ureasubstituted calix (26,27) can be obtained via isocyanatocalix[4]arenes using a one-pot synthesis starting from the corresponding aminocalix(4)arenes in moderate to good yields.
Abstract: Aminocalix[4]arenes 7-12 react with triphosgene and thiophosgene to the corresponding iso(thio)cyanate-functionalized calix[4]arenes 13-20. Carbamate- (21-25) and ureasubstituted calix[4]arenes (26,27) can be obtained via isocyanatocalix[4]arenes using a one-pot synthesis starting from the corresponding aminocalix[4]arenes in moderate to good yields. Isothiocyanate-functionalized calix[4]arenes 16, 18 react selectively with amines to the corresponding thioureas 28-31 in 59-83% yield. Reaction of isothiocyanate-functionalized calix[4]arenes 16, 19, 20 with cavitands 34, 38 gives 2:2 products 42-45 in 35-42% yield.
TL;DR: In this article, the bimolecular rate constants kq for quenching of singlet oxygen (1Δg state) by 26 different natural and novel synthetic carotenoids were determined at 37 °C in a mixture of chloroform and ethanol.
Abstract: The bimolecular rate constants kq for quenching of singlet oxygen (1Δg state) by 26 different natural and novel synthetic carotenoids were determined at 37 °C in a mixture of chloroform and ethanol The steady-state technique used involves the generation of 1O2 by thermal decomposition of disodium 3,3′-naphtalene-1,4-diyl-dipropionate endoperoxide (NDPO2) and the detection of its luminescence intensity at 1270 nm Excitation energies (π,π*, 11Ag → 11Bu) and absorption maxima (430–590 nm) vary in the broadest range Deeply coloured blue carotenoids are also included in the studies for the first time An empirical correlation between the π,π* (11Ag → 11Bu) excitation energy and carotenoid structure (effective chain length Neff) was found: E(S) = 12642 cm−1 + 92027 cm−1 × 1/Neff The quenching ability of the investigated carotenoids depends on the excitation energy of their transition at long wavelengths in a characteristic way showing as limiting factors either the thermal Arrhenius activation or the diffusion-controlled rate This dependence and the suspected relationship between singlet E(S) and triplet E(T) energies, respectively, are discussed
TL;DR: In this paper, the preparation of 3-heteroatom-substituted oxetanes by the Paterno-Buchi reaction, and their application in synthesis are reviewed.
Abstract: The preparation of 3-heteroatom-substituted oxetanes by the Paterno-Buchi reaction, and their application in synthesis are reviewed. 3-Oxetanols and 3-aminooxetanes are the two most important oxetanes in this respect. By tuning the electronic properties of the enol and enamine substrates, a successful photocycloaddition to carbonyl compounds, with high yield, is possible. Since the oxetane formation proceeds stereoselectively, diastereomerically pure products are readily accessible, which can then be used in further transformations. To this end, regioselective ring-opening reactions have been developed, some of which will be discussed in this account.
TL;DR: In this paper, the authors give an account of the design of molecular bowls and capsules bearing an endohedral functionality and the stabilization of highly reactive species by covalent fixation in the interior of such molecules.
Abstract: Endohedral phases of molecular bowls and capsules have great potential for serving as a reaction environment for the internal functionality as well as a complexing site for the guest molecule. The reactivity of a functional group covalently fixed in the interior of such molecules can be regulated in a unique fashion by the surrounding framework. This article gives an account of the design of novel molecular bowls and capsules bearing an endohedral functionality and the stabilization of highly reactive species by covalent fixation in the interior of such molecules. The target species include a sulfenic acid, a selenenic acid, and a monosubstituted simple enol, which have been recognized as important but elusive intermediates in organic and enzymatic reactions.
TL;DR: In this paper, the authors describe β-turns or 310 helices (depending on the length of the oligopeptide) of alternating screw sense (M and P) in a head to tail alignment.
Abstract: X-Ray diffraction analyses of the fully protected peptides Boc-[(S)-Iva]n-OMe (n = 3, 4, 6) reveal two independent molecules in the asymmetric unit. The structures of these can be described as β-turns or 310 helices (depending on the length of the oligopeptide) of alternating screw sense (M and P) in a head to tail alignment. This structure is stabilized by hydrogen bonds between the NH(1) of the (M)-helix and the OC(ω-1) of the (P)-helix and the NH(2) (M) and the ester carbonyl group (P). Low temperature 1H-NMR spectra of the hexamer in CD2Cl2 solution show two interchanging species in a ratio of 4:1; NOESY experiments prove that these are the two helical conformers found in the crystal (P:M, 4:1). The NOESY spectrum at −90°C indicates the pairing of (P) and (M) helices. Thermodynamic and kinetic parameters for the helix transformation P ⇌ M (unfolding/folding) are presented.
TL;DR: In this article, a mechanistic model was proposed to search for stereoselective protonating agents for endocyclic keto-enolates with high diastereoselectivities.
Abstract: Endocyclic keto-enolates of type 2 can be protonated under reagent control with high diastereoselectivities when chelating proton donors with a salicylate structure are used. The stereochemical course of these protonations is hardly affected by the ring size, the substitution pattern, and the presence of additional stereogenic centers in the enolate. The substrates can be prepared by conjugate cuprate addition as well as deprotonation; in the latter case, good diastereoselectivities are obtained by removal of competing proton sources and the inclusion of transmetalation steps. The chelate complex A serves as a mechanistic model that makes the usual trial and error search for stereoselective protonating agents unnecessary. The method is applied to stereoselective syntheses of a precursor for methyl epijasmonate and of the insect pheromone (2S,3S)-diprionyl acetate.
TL;DR: In this article, the synthesis of a bifluorophoric perylene dye (4a) is described, in which the two chromophores are linked by a chiral binaphthyl unit.
Abstract: The synthesis of a bifluorophoric perylene dye (4a) is described, in which the two chromophores are linked by a chiral binaphthyl unit. A strong exciton coupling of the two chromophores, a bathochromically shifted fluorescence and strong CD effects are observed. The structure of 4 is compared to the photosynthesis reaction center.
TL;DR: Several C-terminally modified peptides were synthesized with chain elongation being monitored by UV as it is standard practice in peptide synthesis in the usual direction, and particular attention was paid to racemization that may possibly occur.
Abstract: The possibility of peptide assembly in the N-C direction by using HOBt salts of the amino acid 9-fluorenylmethyl esters has been investigated. The first amino acid derivative has to be coupled with the N-terminus to the polymer-bound trityl linker. Determination of the loading with the first amino acid derivative could be carried out according to the same method as for Fmoc-amino acids on the resin. Kinetic measurements were made of both the coupling of the amino acid derivatives during chain elongation, and the cleavage of the peptide from the resin. Several C-terminally modified peptides were synthesized with chain elongation being monitored by UV as it is standard practice in peptide synthesis in the usual direction. Particular attention was paid to racemization that may possibly occur.
TL;DR: In this paper, an alternative approach involving the reaction of excess quinone with appropriate anthracene derivatives in acetic acid has been developed, which affords the target compounds in a single synthetic step in good yields and with high purity.
Abstract: The synthesis of appropriate porphyrin-quinone electron-transfer complexes as model compounds for photosynthesis requires access to functionalized triptycene quinone aldehydes with different redox potentials. A synthetic strategy involving a classical Diels-Alder reaction, tautomerization and oxidation was only of limited utility for preparing the desired compounds. Thus, an alternative approach was required and for this purpose a strategy involving the reaction of excess quinone with appropriate anthracene derivatives in acetic acid has been developed. This method affords the target compounds in a single synthetic step in good yields and with high purity. The aldehyde functionality required for the subsequent porphyrin synthesis can be incorporated into either the anthracene or the quinone starting component. Fifteen different triptycene quinones have been synthesized, eight of which have been studied in detail by single crystal X-ray crystallography to provide insight into their structural chemistry and solid-state aggregation properties. Additionally, the synthesis of two porphyrin-quinone target compounds is described.
TL;DR: In this paper, the synthesis of calixarenes and other macrocyclic compounds via stepwise procedures and fragment condensation is reviewed and properties such as conformational barriers, pKa values or the ability to complex metal ions are discussed.
Abstract: The synthesis of special calixarenes and calixarene-like macrocyclic compounds via stepwise procedures and fragment condensation is reviewed. Among the compounds available are exo- and endo-calixarenes consisting of different phenolic units, having bridges other than methylene or showing inherent chirality (Cn-symmetry). Macrobi- and -tricyclic molecules such as bridged calixarenes, double calixarenes, bicyclocalixarenes and annelated calixarenes are also described. Single crystal X-ray structures are reported for several compounds. Some properties like conformational barriers, pKa values or the ability to complex metal ions are discussed, showing the potential of these compounds which may (only) be produced by directed synthesis.
TL;DR: In this paper, the synthesis and physical properties of cyclic tetrathiafulvalenes (TTF) are reviewed and the ring closure is performed in one of two different ways: (i) coupling of 1,3-dithiol derivatives thereby forming the central fulvene bond, or (ii) cyclization of a preformed TTF derivative.
Abstract: The syntheses and some physical properties (redox potentials) of cyclic tetrathiafulvalenes (TTF) are reviewed. The ring closure is generally performed in one of two different ways: (i) by coupling of 1,3-dithiol derivatives thereby forming the central fulvene bond, or (ii) by cyclization of a preformed TTF derivative. The second route is very efficient when using the cyanoethyl protection/deprotection method and has made possible the preparation of a number of two- and three-dimensional macrocycles, e.g. mono-, bis- and tetramacrocycles, oligomeric macrocycles and cage-type molecules. The use of tetrathiafulvalenophanes in supramolecular chemistry is also exemplified in this microreview.
TL;DR: In this article, it was shown that redox catalysts based on transition-metal complexes using 1,10-phenanthroline-5,6-dione as a ligand or based on N-methylated 1, 10-phenathroline-1,6dione acting via hydride ion abstraction are superior to alternative methods for the redox catalyst for the in situ NAD(P)+ regeneration.
Abstract: In comparative studies, we have been able to demonstrate that redox catalysts based on transition-metal complexes using 1,10-phenanthroline-5,6-dione as a ligand or based on N-methylated 1,10-phenathroline-5,6-dione acting via hydride ion abstraction are superior to alternative methods for the redox catalytic aerobic or indirect electrochemical in situ NAD(P)+ regeneration in enzymatic syntheses using alcohol dehydrogenases as production enzymes. Under preparative conditions in the gram scale we were able to obtain turnover frequencies of up to 130 turnovers per hour with respect to the redox catalyst. These are far larger than those of the presently most popular regeneration system.
TL;DR: In this article, the synthesis of telechelic biphenyls with one of the functional groups, B(OH)2, TMS, Br and I, at each ring is presented.
Abstract: The synthesis of a number of telechelic biphenyls with one of the functional groups, B(OH)2, TMS, Br and I, at each ring is presented. These biphenyls constitute a construction kit for modular chemistry using the Suzuki cross-coupling protocol.
TL;DR: In this article, the electrochemical cyanation of N-alkyl-substituted cyclic six-membered amines, including N-phenylpiperidine derivatives, was investigated at a graphite felt anode percolated with a methanolic solution of the amine.
Abstract: The electrochemical cyanation of N-alkyl-substituted cyclic six-membered amines including N-alkyl-1,2,3,4-tetrahydroquinoline and N-phenylpiperidine derivatives has been investigated. The reaction gives the corresponding α-aminonitriles and was carried out at a graphite felt anode percolated with a methanolic solution of the amine, containing six equivalents of sodium cyanide per mol of substrate, and lithium acetate as the supporting electrolyte. In the case of N-substituted tetrahydroquinolines 1a–f, cyanation occurs regioselectively at the ring and the position of substitution depends on the nature of the N-alkyl substituent. The stereoselectivity of the cyanide addition was investigated using various 3-methyltetrahydroquinolines 8a–c as substrates. In all cases, the major stereoisomers 9a–c possess a cis configuration in which the 2-cyano group has an axial disposition and the 3-methyl group is equatorial. In a similar fashion, various N-phenylpiperidines substituted at the ring by either one or two methyl groups were oxidized. In all the selected amines the introduction of the cyanide group occurs stereospecifically in the position α to the nitrogen atom. The cis or trans relative configuration of the cyanated compounds is discussed separately for each substrate. It is noteworthy that in all cases the cyano group was in an axial position.