Showing papers in "Helvetica Chimica Acta in 1995"

Ein neuer Zugang zu 2′-O-Alkylribonucleosiden und Eigenschaften deren Oligonucleotide

Abstract: A New Access to 2′-O-Alkylated Ribonucleosides and Properties of 2′-O-Alkylated Oligoribonucleotides A general access to 2′-O-alkylated ribonucleosides using the key intermediate 5 is presented. The incorporation of 2′-O-‘ethyleneglycol’- and 2′-O-‘glycerol’-substituted (i.e., 2′-O-(2-hydroxyethyl)- and 2′-O-(2,3-dihydroxypropyl)-substituted) ribonucleosides into oligonucleotides affords a new generation of oligonucleotides with high affinity for RNA, high specificity, and increased nuclease resistance.

Two‐State Reactivity in Organometallic Gas‐Phase Ion Chemistry

Abstract: In contrast to organic reactions, which can almost always be described in terms of a single multiplicity, in organometallic systems, quite often more than one state may be involved. The phenomenon of two states of different multiplicities that determine the minimum-energy pathway of a reaction is classified as two-state reactivity (TSR). As an example, the ion/molecule reactions of ‘bare’ transition-metal-monoxide cations with dihydrogen and hydrocarbons have been analyzed in terms of the corresponding potential-energy hypersurfaces. It turns out that, besides classical factors, such as the barrier heights, the spin-orbit coupling factor is essential, since curve crossing between the high- and low-spin states constitutes a distinct mechanistic step along the reaction coordinates. Thus, TSR may evolve as a new paradigm for describing the chemistry of coordinatively unsaturated transition-metal complexes. This concept may contribute to the understanding of organometallic chemistry in general and for the development of oxidation catalysts in particular.

Enantioselective allylic substitution catalyzed by chiral [bis(dihydrooxazole)]palladium complexes - catalyst structure and possible mechanism of enantioselection

Abstract: Allylpalladium complexes with chiral bis(dihydrooxazole) ligands were studied as catalysts for the enantioselective allylic substitution reaction of rac-1,3-diphenylprop-2-enyl acetate (rac-5) with the anion of dimethyl malonate (Scheme 1). Using enantiomerically pure (S,E)-1-(4-tolyl)-3-phenylprop-2-enyl acatete ((S)-25) as substrate, the reaction was shown to proceed by a clean ‘syn’ displacement of acetate by dimethyl malonate (Scheme 6). The [Pd11(η3-allyl)] complex 18 and the analogous [Pd(η3-1,3-diphenylallyl)] complex 20, both containing the same bis(dihydrooxazole) ligand, were characterized by X-ray structure analysis and by NMR spectroscopy in solution. The structural data reveal that steric interactions of the allyl system with the chiral ligand result in selective electronic activation of one of the allylic termini. The higher reactivity of one allylic terminus toward nucleophilic attack is reflected in a significantly longer PdC bond and a shift of the corresponding 13C-NMR resonance to higher frequency.

Pyranosyl-rna ('p-rna') : base-pairing selectivity and potential to replicate

Abstract: A review and discussion, with 42 refs. Base pairing in p-RNA (b-D-ribopyranosyl-(4'->2')-oligonucleotides) is not only stronger than in DNA and RNA, but also more selective in the sense that it is strictly confined to the Watson-Crick mode. Homopurine sequences (tested up to decamers) exist as single strands under conditions where they undergo reverse-Hoogsteen self-pairing in homo-DNA or Hoogsteen self-pairing in DNA. This exceptional pairing selectivity is rationalized as hinging on 2 structural features of p-RNA: the large inclination between backbone axis and base-pair axes in p-RNA duplexes, and the higher rigidity of the p-RNA backbone compared with RNA, DNA, and homo-DNA. The most important consequence of the pairing selectivity refers to the potential of p-RNA to replicate. Replicative copying of sequence information by nonenzymic template-controlled ligation is not hampered by self-pairing of guanine-rich templates, as it is known to be the case in the RNA series. Two replicative cycles are demonstrated in which G-rich p-RNA-octamer templates induce sequence-selective ligation of tetramer-2'-phosphate derivs. to complementary C-rich octamer sequences, and in which the latter, with comparable efficiency, induce corresponding ligation reactions back to the original G-rich octamers. Ligation is most satisfactorily achieved after pre-activation of the 2'-phosphate groups as 2',3'-cyclophosphate derivs.; in this version, the process does not proceed as oligocondensation, but as a genuine oligomerization. This is of considerable promise for the search for potentially natural conditions under which homochiral p-RNA strands might self-assemble and self-replicate. [on SciFinder (R)]

Electrochemistry of Mono‐ through Hexakis‐adducts of C60

Abstract: The first systematic electrochemical study by cyclic voltammetry (CV) and rotating-disk electrode (RDE) of the changes in redox properties of covalent fullerene derivatives (2–11) as a function of increasing number of addends is reported. Dialkynylmethanofullerenes 2–4 undergo multiple, fullerene-centered reduction steps at slightly more negative potentials than C60 (1; see Table and Fig. 1). The two C-spheres in the dumbbell-shaped dimeric fullerene derivative 4 show independent, identical redox characteristics. This highlights the insulating character of the sp3-C-atoms in methanofullerenes which prevent through-bond communication of substituent effects from the methano bridge to the fullerene sphere. In the series of mono- through hexakis-adducts 5–11, formed by tether-directed remote functionalization, reductions become increasingly difficult and more irreversible with increasing number of addends (see Table and Fig. 2). Whereas, in 0.1M Bu4NPF6/CH2Cl2, the first reduction of mono-adduct 5 occurs reversibly at −1.06 V vs. the ferrocene/ferricinium couple (Fc/Fc+), hexakis-adduct 11 is reduced irreversibly only at − 1.87 V. Hence, with incremental functionalization of the fullerene, the LUMO of the remaining conjugated framework is raised in energy. Reduction potentials are also dependent on the relative spatial disposition of the addends on the surface of the fullerene sphere. Observed UV/VIS spectral changes and changes in the chemical reactivity along the series 5–11 are in accord with the results of electrochemical measurements. Further, with increasing number of addends, the oxidation of derivatives 5–11 becomes more reversible. Whereas oxidations are increasingly facilitated upon going from mono-adduct 5 (+1.22 V) to tris-adduct 7 (+0.90 V), they occur at nearly the same potential (+0.95 to +0.99 V) in the higher adducts 8–11. This indicates that the oxidations occur in these compounds at a common sub-structural element, for which a ‘cubic’ cyclophane is proposed (see Fig. 3). This sub-structure is fully developed in hexakis-adduct 11.

Tetraethynylethenes: Fully cross-conjugated π-electron chromophores and molecular scaffolds for all-carbon networks and carbon-rich nanomaterials

Abstract: The preparation of tetraethynylethene (3,4-diethynylhex-3-ene-1,5-diyne) 1 as well as of a great diversity of differentially mono-, di-, and triprotected derivatives by newly developed synthetic routes is described. These fully cross-conjugated molecules are versatile building blocks and precursors to two-dimensional all-C networks and novel C-rich nanoarchitecture with unusual structural and electronic properties, such as perethynylated expanded radialenes, or molecular wires and polymers with the novel polytriacetylene backbone. A key step in all of these routes was the Corey-Fuchs dibromoolefination of aldehydes and ketones. Dibromoolefination of silyl-protected penta-1,4-diyn-3-ones yielded the corresponding dibromomethylidene derivatives which, by twofold Pd-catalyzed alkyne coupling, were transformed into tetraethynylethene derivatives. In routes to tetraethynylethenes with free cis-or trans-enediyne moieties, dibromoolefination of aldehyde groups produced geminal dibromoethenes which, upon elimination/metallation with LDA followed by quenching with H+ or other electrophiles, yielded free or substituted ethynyl groups in high yields. Tetra- and triprotected tetraethynylethenes are rather stable compounds that could be isolated in pure form, whereas derivatives with two or more free CH termini were only stable in dilute solution and polymerized rapidly in pure form. A trans-bis(triisopropylsilyl)-protected derivative represented an exception and could be isolated as stable crystals. X-Ray analysis revealed that the two bulky (i-Pr)3Si groups isolate the reactive chromophores from one another in the crystal and prevent intermolecular reactions. The structures of several tetraethynylethenes were revealed in high-quality X-ray crystal structures.

Synthesis, structure, and antimalarial activity of some enantiomerically pure, cis-fused cyclopenteno-1,2,4-trioxanes

Abstract: Two pairs of enantiomerically pure cis-fused cyclopenteno-1,2,4-trioxanes (7, ent-7 and 8, ent-8) are prepared (Schemes 1-3). Their identities are established by dye-sensitized photo-oxygenation of ent-7 and 8, ent-8 to the allylichydroperxides, reduction to the corresponding alcohols, and conversion to the (1S)-camphanates (Scheme 4), the structures of which are determined by X-ray analysis. The dynamic properties of ent-7 are investigated by NMR spectroscopy and PM3 calculations. Evidence for an easily accessible twist-boat conformation is obtained. The in vitro and in vivo antimalarial activities of 7, ent-7,8, and ent-8 as well as those of the racemic mixtures are evaluated against Plasmodium falciparum, P. berghei, and P. yoelii. No correlation is observed between configuration and activity. Racemates and pure enantiomers have commensurate activities. The mode of action on the intraerythrocytic parasite is rationalized in terms of close docking by the twist-boat conformer of the trioxane on the surface of a molecule of heme, single-electron transfer to the O--O d* orbital, and scission to the acetal radical which then irreversibly isomerizes to a C-centered radical, the ultimate lethal agent (Scheme 5).

Preparation of the PdCl2 Complex of TADDOP, the Bis(diphenylphosphinite) of TADDOL: Use in enantioselective 1,3-diphenylallylations of nucleophiles and discussion of the mechanism

Abstract: Reaction of the tartrate-derived diol (R,R)-α,α,α′,α′-tetraphenyl-2,2-dimethyl-1,3-dioxolane-4,5-dimethanol (TADDOL) with chlorodiphenylphosphane gives a new bis(diphenylphosphanyl) ligand (TADDOP). The complex 4 formed with PdCl2 has been crystallized and its structure determined by X-ray diffraction (Fig.1). The complex is used for Pd-catalyzed enantioselective 1,3-diphenylallylations of various nucleophiles which give products with enantiomer ratios of up to 88:12 (Scheme 2). Crystallization procedures lead to the enantiomerically pure (> 99:1) product 11 derived from dimethyl malonate. The structure of the TADDOP complex 4 is compared with those of other transition-metal complexes containing chelating bis(diphenylphosphanyl) ligands (Fig.2). A crystallographic data base search reveals that the structures of transition-metal complexes containing two Ph2P groups (superpositions in Fig.3) fall into one of two categories: one with approximate C2 symmetry and the other with C1 symmetry (20 and 19 examples, resp.). A mechanistic model is proposed which correlates the conformational chirality (δ or λ) of the four Ph groups' arrangement in such complexes with the topicity of nucleophile approach on Pd-bound trans,trans-1,3-diphenylallyl groups (Scheme 3 and Table).

Salmycin A–D, Antibiotika aus Streptomyces violaceus, DSM 8286, mit Siderophor‐Aminoglycosid‐Struktur

Abstract: Salmycin A–D, Antibiotics from Strep tomy ces violaceus, DSM 8286, Having a Siderophor-Aminoglycoside Structure Salmycin B (2) and C (3) were isolated under acid conditions, under which they are stable, from the culture broth of Streptomyces violaceus, DSM 8286. The acid- and alkaline-labile, native main component salmycin A (1), as well as salmycin D (4), were obtained under strictly neutral pH conditions. The compounds 1 (C41H70FeN7O21), 2 (C41H69FeN6O21), 3 (C40H67FeN6O21), and 4 (C40H68FeN7O21) are classified as sideromycins and are stable when dry. Mild alkaline hydrolysis of 1 and 2 yielded the known siderophor danoxamin (5; C27H46FeN5O11), and amino-disaccharides. The amino-glycoside 6 (C14H25NO11) of salmycin B was stabilized by hydrogenation and the structure of the corresponding peracetate 10 determined by 1H,1H- and 1H,13C-correlation NMR spectroscopy (Table 1). Compound 6 consists of a glucos-2-ulose unit which is linked to the 2-position of a 6-deoxy-6-(methylamino)heptopyranose. The danoxamin is bonded via the carboxy group by ester linkage to the primary alcohol of the glucos-2-ulose. Salmycin A (1) is a natural oxime of 2, it was synthesized from 2 with hydroxylamine. The salmycins and those derivatives which contain hexapyranos-2-ulose form stable ketone hydrates which can be identified by mass spectrometry. Although several recently identified features of the danomycins do not correspond with those of the salmycins, 13C-NMR spectra show that both groups of antibiotics are closely related. All salmycins, especially component 1, are highly active against Staphylococci and Streptococci, even against resistant strains of these pathogens.

Absolute Rate Constants for the Addition of Benzyl (PhĊH2) and Cumyl (PhĊMe2) Radicals to Alkenes in Solution

Abstract: Absolute rate constants and their temperature dependence were determined by time-resolved electron spin resonance for the addition of the radicals PhĊH2 and PhĊMe2 to a variety of alkenes in toluene solution. To vinyl monomers CH2=CXY, PhĊH2 adds at the unsubstituted C-atom with rate constants ranging from 14 M−1S−1 (ethoxyethene) to 6.7 · 103M−1S−1 (4-vinylpyridine) at 296 K, and the frequency factors are in the narrow range of log (A/M−1S−1) = 8.6 ± 0.3, whereas the activation energy varies with the substituents from ca. 51 kJ/mol to ca. 26 kJ/mol. The rate constants and the activation energies increase both with increasing exothermicity of the reaction and with increasing electron affinity of the alkenes and are mainly controlled by the reaction enthalpy, but are markedly influenced also by nucleophilic polar effects for electron-deficient substrates. For 1,2-disubstituted and trisubstituted alkenes, the rate constants are affected by additional steric substituent effects. To acrylate and styrenes, PhĊMe2 adds with rate constants similar to those of PhĊH2, and the reactivity is controlled by the same factors. A comparison with relative-rate data shows that reaction enthalpy and polar effects also dominate the copolymerization behavior of the styrene propagation radical.

Multiple Cyclopropanations of C70. Synthesis and characterization of bis‐, tris‐, and tetrakis‐adducts and chiroptical properties of bis‐adducts with chiral addends, including a recommendation for the configurational description of fullerene derivatives with a chiral addition pattern

Abstract: The regioselectivity of multiple cyclopropanations of C70 with 2-bromopropanedioates in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as base (Bingel reaction) was investigated in a systematic study. Bisadduct formation occurred preferentially at the 66 bonds formed by the most pyramidalized sp2-C-atoms at the two opposite poles of the fullerene and, in the reaction with achiral bis[(ethoxycarbonyl)methyl]2-bromopropanedioate (13a), yielded three constitutionally isomeric bis(methano)fullerenes (Scheme 2). Two of them, C2-symmetrical (±)-1 and (±)-2, are chiral; a fact which had not been considered in previous investigations. Formation of the third, C2v-symmetrical isomer 3 was observed for the first time. Configurational descriptions for fullerene derivatives which possess a chiral chromophore as a result of specific functionalization patterns are proposed. Cyclopropanations of C70 with optically active bis[(S)-1-phenylbutyl] 2-bromopropanedioate (13b) yielded five optically active, C2-symmetrical bis-adducts 7–11 which could be separated by preparative HPLC and fully characterized (Scheme 3, Fig.4). Compounds 7/8 and 9/10 represent two constitutionally isomeric pairs of diastereoisomers, and their circular dichroism (CD) spectra show pronounced Cotton effects mainly due to strong chiroptical contributions from the chirally functionalized fullerene chromophores (Fig.7). Since the addition patterns on the fullerene surface in each pair of diastereoisomers have an enantiomeric relationship, their CD spectra closely resemble those expected for two enantiomers. In the third constitutional isomer 11, the addition pattern on the fullerene surface is C2v-symmetrical, and optical activity only results from the chiral addends. Its CD spectrum shows weak Cotton effects mainly from induced circular dichroism originating from the perturbation of the achiral fullerene chromophore by the attached chiral addends. Addition of diethyl 2-bromopropanedioate (2 equiv.) to the C2-symmetrical racemic bis-adduct (±)-2 yielded a mixture of tris-adducts and one major, C2-symmetrical tetrakis-adduct (±)-4 which was isolated in pure form (Scheme 4). Starting from the achiral C2v-symmetrical bis-adduct 3, one single Cs-symmetrical tris-(5) and one C2v-symmetrical tetrakis-adduct (6) were obtained as major products which were isolated and fully characterized (Scheme 5). The regioselectivity for introduction of a second addend in the same hemisphere of C70 is high and resembles the preferred pattern of bis-addition seen in the functionalization of C60.

Dendrophanes: Water‐Soluble Dendritic Receptors. Preliminary communication

Abstract: The dendritic cyclophanes (dendrophanes )1–3 containing a [6.1.6.1]paracyclophane as the initiator core embedded in dendritic poly(ether-amide) shells of first (1), second (2), and third (3) generation were prepared and characterized. The X-ray crystal-structure analyses of esters 7 and 4, derivatives of cyclophane core 9 and first-generation dendrophane 1, respectively, displayed open cavity binding sites suitable for the inclusion complexation of aromatic substrates. With their carboxylate surface groups, dendrophanes 1–3 were readily soluble in aqueous phosphate buffer (pH 8.0), and the complexation of naphthalene derivatives was investigated by 1H-NMR and fluorescence titrations. The binding studies demonstrated that the cyclophane cavity remains open and accessible to appropriate substrates even at higher dendritic generations. The 1:1 complexes formed in aqueous buffer were of similar stability to those formed by the unbranched core 9 (Ka between 1000 and 10000 1 mol−1, 300 K). Investigations with the fluorescent probe 6-(p-toluidino)naphthalene-2-sulfonate (12) showed that the micropolarity at the dendrophane core decreases with increasing generation number.

Duplex Stabilization of DNA: Oligonucleotides containing 7‐substituted 7‐deazaadenines

Abstract: The oligonucleotide building blocks 4b–d derived from 7-bromo-, 7-chloro-, and 7-methyl-substituted 7-deaza-2′-deoxyadenosines 3b–d were prepared. They were employed in the solid-phase synthesis of the oligonucleotides 7–25. The dA residues of the homomer d(A12), the alternating d[(A-T)6], and the palindromic d(G-T-A-G-A-A-T-T-C-T-A-C) were replaced by 3b–d as well as by the parent 7-deaza-2′-deoxyadenosine (3a). The melting profiles and CD spectra of oligonucleotide duplexes, showing this major groove modification, were measured, and the Tm values as well as the thermodynamic data were determined. It was found that small substituents such as Br, Cl, or Me introduced in the 7-position of a 7-deazaadenine residue increase the duplex stability compared to oligonucleotides containing adenine.

Carbenoid Reactions in Rhodium(II)‐Catalyzed Decomposition of Iodonium Ylides

Abstract: The intermediacy of metallocarbenes in decomposition reactions of iodonium ylides with [Rh2(OAc)4] was established by comparison with reactions of the corresponding diazo compounds. The sensitivity of the RhII-catalyzed intermolecular cyclopropane formation from substituted styrenes and bis(methoxycarbonyl)(phenyliodono)methanide (1a) or dimethyl diazomalonate (1b) is identical. The Hammett plot (with σ+) has a slope of −0.47. Iodonium ylides and diazo compounds afford the same products in [Rh2(OAc)4]-catalyzed cyclopropane formations, cycloadditions, and intramolecular CH insertions, and exhibit the identical selectivity in intramolecular competitions for cyclopropane formation and insertion. The intramolecular CH insertion of the ylide 20c, when carried out in the presence of a chiral catalyst ([Rh2{(−)-(S)-ptpa}4]), results in formation of 21a having an ee of 67%, identical to the ee obtained with the diazo compound 20b.

Molecular Clefts Derived from 9,9′‐spirobi[9H‐fluorene] for enantioselective complexation of pyranosides and dicarboxylic acids

Abstract: The molecular clefts (R)- and (S)-3, incorporating 9,9′-spirobi[9H-fluorene] as a spacer and two N-(5,7-dimethyl-1,8-naphthyridin-2-yl)carboxamide (CONH(naphthy)) units as H-bonding sites were prepared via the bis(succinimid-N-yl esters) of (R)-and (S)-9,9′-spirobi[9H-fluorene]-2,2′-dicarboxylic acid (5). Derivative 6, with one CONH(naphthy) unit and one succinimid-N-yl ester residue allowed easy access to spirobifluorene clefts with two different H-bonding sites, as exemplified by the synthesis of 4. Binding studies with (R)- and (S)-3 and optically active dicarboxylic acids in CDCl3 exhibited differences in free energy of the formed diastereoisomeric complexes (Δ(ΔGo)) between 0.5 and 1.6 kcal mol−1 (T 300 K). Similar enantioselectivities were observed with the spirobifluorene clefts (R)- and (S)-1, bearing two N-(6-methylpyridin-2-yl)carboxamide (CONH(py)) H-bonding sites. The thermodynamic quantities ΔHo and ΔSo for the recognition processes with (R)- and (S)-1 were determined by variable-temperature 1H-NMR titrations and compared to those with (R)- and (S)-2, which have two CONH(py) moieties attached to the 6,6′-positions of a conformationally more flexible 1,1′-binaphthyl cleft. All association processes showed high enthalpic driving forces which are partially compensated by unfavorable changes in entropy. Pyranosides bind to the optically active clefts 1 and 3 in CDCl3 with −ΔGo = 3.0–4.3 kcal mol−1. Diastereoisomeric selectivities up to 1.2 kcal mol−1 and enantioselectivities up to 0.4 kcal mol−1 were observed. Cleft 4 and N-(5,7-dimethyl-1,8-naphthyridin-2-yl)acetamide (25) complexed pyranosides 22–24 as effectively as 3 indicating that only one CONH(naphthy) site in 3 associates strongly with the sugar derivatives. Based on the X-ray crystal structure of 3, a computer model for the complex between (S)-3 and pyranoside 22 was constructed. Molecular-dynamics (MD) simulations showed that differential geometrical constraints are at the origin of the high enantioselectivity in the complexation of dicarboxylic acid (S)-7 by (R)- and (S)-1 and (R)- and (S)-3.

L-Phenylalanine Cyclohexylamide: A Simple and Convenient Auxiliary for the Synthesis of Optically Pure α,α-Disubstituted (R)- and (S)-Amino Acids.

Abstract: This work describes L-phenylalanine cyclohexylamide (5c) as a simple, cheap, and powerful chiral auxiliary for the synthesis of a series of optically pure α,α-disubstituted (R)- and (S)-amino acids of type 1, such as (R)- and (S)-2-methyl-phenylalanine (1a), (R)- and (S)-2-methyl-2-phenylglycine (1b), and (R)- and (S)-2-methylvaline (1c; Scheme 3). These amino acids were efficiently transformed into the suitably protected and activated amino acid building blocks (R)- and (S)-12b and (R)- and (S)-12c (Scheme 4) which are ready for incorporation into peptides by solution or solid-phase techniques. Based on the crystal structures of 6b, 6c, and 7a belonging to the diastereoisomeric peptides series 6 and 7, the absolute configurations of each member of the series were determined. β-Turn geometries of type II′ and I were observed for 6b and 7a, respectively, whereas 6c crystallized in an extended conformation. The impacts of side-chain variation on conformation and crystal packing of these triamides are discussed.

High-Yield Syntheses of Sodium, Potassium, and Thallium Hydrotris[3,5-bis(trifluoromethyl)pyrazolyl]borates and the X-ray crystal structure of {hydrotris[3,5-bis(trifluoromethyl)pyrazolyl]borato} thallium(I)

Abstract: An improved synthesis of 3,5-bis(trifluoromethyl)pyrazole (1) is described. This compound was used for the high-yield syntheses of the tris(pyrazoly1)borates Nd[HB(3,5-(CF3),-pz)3] (2a) and the corresponding potassium salt, 2b, starting from 1 and NaBH4 and KBH4, respectively. A convenient route to the corresponding thallium(I) salt, 2c, using thallium(I) acetate and either 2a or 2b in CHCI3, is also described. The sodium (3a), potassium (3b), and thallium (3c) salts of bis(pyrazolyl)borate [H2B(3,5-(CF3)2-pz)2]− were also prepared. The above pyrazolylborates were characterized by 1H-, 13C-, 19F-, and 11B-NMR spectroscopy. The X-ray crystal structure of the thallium derivative 2c was determined. The compound crystallizes in the monoclinic space group P21/m with a = 8.248(9) A, b = 15.034(12) A c = 9.243(8) A, β = 100.10(7)°, Z = 2. The Tl-atom adopts a pyramidal geometry with respect to the three N-atoms. However, two TI–N distances (2.725(7) A) are longer than the third (2.675(10) A).

Interaction of Double-Helical Polynuclear Copper(I) complexes with double-stranded DNA

Abstract: Double-helical metal complexes, the helicates H2–H5, were found to bind to double-helical DNA by spectroscopic, DNA-melting, and electrophoretic-mobility measurements. The helicates also inhibited the cleavage of DNA by two restriction enzymes, Sspl and EcoRV, a property which agrees with the binding occurring in the major groove of the double helix. Visible-light irradiation of solutions containing a helicate and the pBR322 plasmid led to single-strand cleavage, indicating that these complexes could be of interest as potential probes for nucleic-acid structure.

Acyclic Tetraethynylethene Molecular Scaffolding: Multinanometer-sized linearly conjugated rods with the poly(triacetylene) backbone and cross-conjugated expanded dendralenes

Abstract: Derivatives of fully cross-conjugated tetraethynylethene (3,4-diethynylhex-3-ene-1,5-diyne) 1 are versatile precursors to multinanometer-sized molecular rods with all-C-backbones. Oxidative polymerization (CuCl, N,N,N′,N′-tetramethylethylenthylenediamine (TMEDA), O2) of the trans-bis-deprotected trans-bis(triisopropylsilyl)-protected tetraethynylethene 2 yielded, after end-capping with phenylacetylene, the remarkably stable, soluble oligomers 3–7 with a persilylethynylated poly(triacetylene) (PTA) backbone [(CCCRCRCC)n] and a length of 19.4 (3), 26.8 (4), 34.3 (5), 41.8 (6), and 49.2 (7) A (Scheme 1). These compounds underwent facile one-electron reductions with the number of reversible reduction steps being equal to the number of tetraethynylethene moieties in each molecular rod. Oxidative Eglinton-Glaser homo-coupling of tetraethynylethenes 8–10 with a single free ethynyl group provided the fully silyl-protected 3,4,9,10-tetraethynyl-substituted dodeca-3,9-diene-1,5,7,11-tetraynes 11–13 (Scheme 2) and, after alkyne deprotection, the novel hydrocarbon 14, a C20H6 isomer, and its partially silyl-protected derivative 15. Oxidative hetero-coupling between two different tetraethynylethene derivatives, one with a single and the other with two free terminal ethynyl groups, yielded the extended chromophores 16–21 composed of 3 or 4 tetraethynylethene moieties (Scheme 3). The linearly conjugated oligomers 16 and 17 with the PTA backbone are isomeric to 19 and 20, respectively, which are members of the cross-conjugated expanded dendralenes, i.e., dendralenes with butadiynediyl fragments inserted between each pair of double bonds [(CCC(CR2)CC)n]. The electronic absorption spectra of these compounds were compared and analyzed in terms of the competition between linear and cross-conjugation in determining the extent of π-electron delocalization. Although steric factors on π-electron conjugation remain to be clarified, this analysis strongly suggests that cross-conjugation is not an efficient mechanism for π-electron delocalization. All extended acetylenic-olefinic chromophores considered in this study exhibited remarkably high stability and did not decompose when exposed to laboratory air and light for months. In agreement with this observation, electrochemical studies demonstrated that the compounds are difficult to oxidize with the oxidation potentials in THF (0.1M(Bu4N)PF6) being higher than 1.0 V (vs. the ferrocene/ferrocenium couple).

7‐Substituted 7‐Deaza‐2′‐deoxyguanosines: Regioselective Halogenation of Pyrrolo[2,3‐d]pyrimidine Nucleosides

Abstract: The synthesis of 7-chloro-, 7-bromo-, and 7-iodo-substituted 7-deaza-2′-deoxyguanosine derivatives 2b–d is described. The regioselective 7-halogenation with N-halogenosuccinimides was accomplished using 7-[2-deoxy-3,5-O-di(2-methylpropanoyl)-β-D-erythro- pentofuranosyl]-2-(formylamino)-4-methoxy-7H-pyrrolo[2,3-d]- pyrimidine (4) as the common precursor. A one-pot reaction (2N aq. NaOH) of the halogenated intermediates 5a–c furnished the desired compounds. Also the 7-hexynyl derivative 2e of 7-deaza-2′-deoxyguanosine is described.

Short, Enantiospecific Syntheses of Indolizidines 209B and 209D, and Piclavine A from Diethyl‐L‐Glutamate

Abstract: The 1H-pyrrole derivative obtained from diethyl L-glutamate hydrochloride and tetrahydro-2,5-dimethoxyfuran was cyclized with BBr3 to ethyl (5S)-5,6,7,8-tetrahydro-8-oxoindolizine-5-carboxylate (18). Catalytic hydrogenation of 18 over Pd/C in AcOH gave ethyl (5S,8aR)-octahydroindolizine-5-carboxylate (21), whereas hydrogenation over Rh/Al2O3 in EtOH/AcOH 99:1 afforded mainly ethyl (5S,8S,8aS)-octahydro-8-hydroxyindolizine-5-carboxylate (22). By functional-group interconversions, 21 was transformed into piclavine A (1) and indolizidine 209D (2). Similarly, (5R,8R,8aS)-octahydro-5-pentylindolizine-8-methanol (37), the final relay for indolizidine 209B (3), was obtained from 22.

Metal Complexes with Macrocyclic Ligands. Part XXXIX. Mono‐ and binuclear copper(II) complexes of a bridging bis[1, 4, 7‐triazacyclononane]

Abstract: The new bis-macrocycle 1, 1′-[(1H-pyrazol-3], 5-diyl)bis(methylene)bis[1, 4, 7-triazacyclononane] (1) was synthesized and its complexation with Cu2+ studied. Potentiometric and spectrophotometric titrations indicate that, in addition to the mononuclear species [Cu(LH2)]4+, [Cu(LH)]3+, [CuL]2+, and [Cu(LH−1)]+, binuclear complexes such as [Cu2L]4+, [Cu2(LH−1)]3+, and [Cu2(LH-2)]2+ are also formed in solution. The stability constants and spectral properties of these are reported. The binuclear species [Cu2(LH−1)]3+ specifically reacts with an azide ion to give a ternary complex [Cu2(LH−1)(N3)]2+, the stability and structure of which were determined spectrophotometrically and by X-ray diffraction, respectively. The two Cu2+ ions are in a square-pyramidal coordination geometry. The axial ligand is one of the N-atoms of the 1, 4, 7-triazacyclononane ring, whereas at the base of the square pyramid, one finds the other two N-atoms of the macrocycle, one N-atom of the pyrazolide and one of the azide, both of which are bridging the two metal centres. In [Cu2(LH−1)(N3)]2+, a strong antiferromagnetic coupling is present, thus resulting in a species with a low magnetic moment of 1.36 B.M. at room temperature.

Macrocyclic Tetraethynylethene Molecular Scaffolding: Perethynylated aromatic dodecadehydro[18]annulenes, antiaromatic octadehydro[12]annulenes, and expanded radialenes

Abstract: Tetraethynylethene (3,4-diethynylhex-3-ene-1,5-diyne) molecular scaffolding provided access to novel macrocyclic nanometer-sized C-rich molecules with unusual structural and electronic properties. Starting from cis-bis-deprotected cis-bis(trialkylsilyl)protected tetraethynylethenes, the per(silylethynyl)ated octadehydro[12]annulenes 1 and 2 and the corresponding dodecadehydro[18]annulenes 4 and 5 were prepared by oxidative Hay coupling. X-Ray crystal-structure analyses of (i-Pr)3Si-protected 2 and Me3Si-protected 4 showed that both annulene perimeters are perfectly planar. Electronic absorption spectral comparisons provided strong evidence that the macro rings in the deep-purple-colored 1 and 2 are antiaromatic (4n π-electrons), whereas those in yellow 4 and 5 are aromatic ((4n + 2) π-electrons). Although unstable in solution, the antiaromatic compound 2 gave high-melting crystals in which the individual octadehydro[12]annulene chromophores are isolated and stabilized in a matrix-type environment formed by the bulky (i-Pr)3Si groups. Electrochemical studies demonstrated that the antiaromatic octadehydro[12]annulene 2 undergoes two stepwise one-electron reductions more readily that the aromatic chromophore 5. This redox behavior is best explained by the formation of an aromatic (4n + 2) π-electron dianion from 2, whereas 5 loses its aromaticity upon reduction. The Me3Si derivative 4 was deprotected with borax in MeOH/THF to give the highly unstable hexaethynyl-dodecadehydro[18]annulene 6, a C30H6 isomer and macrocyclic precursor to a two-dimensional all-C-network. Deprotection of 2 did not give isolable amounts of tetraethynyl-octadehydro[12]annulene 3 due to the extreme instability of the latter. Starting from dimeric and trimeric acyclic tetraethynylethene oligomers, a series of expanded radialenes were obtained. They possess large C-cores with silylethynyl-protected peripheral valences and can be viewed as persilylated C40 (7), C50 (8), and C60 (9) isomers. These expanded C-sheets are high-melting, highly stable, soluble materials which were readily characterized by laser-desorption time-of-flight (LD-TOF) mass spectrometry. Due to inefficient macrocyclic cross-conjugation and/or non-planarity, the extent of π-electron delocalization in 7–9 is limited to the longest linearly conjugated π-electron fragment. In agreement with these properties, all three expanded radialenes exhibited similar redox behavior; they are difficult to oxidize but undergo several reversible one-electron reductions in similar potential ranges. Presumably, the reduced π-electron delocalization is also at the origin of the particularly high stability of 7–9.

The Directionality of d‐Orbitals and Molecular‐Mechanics Calculations of Octahedral Transition‐Metal Compounds

Abstract: A novel approach to modeling the angular geometry about the metal centre in transition-metal complexes, using a variation of classical molecular-mechanics calculations, is presented. The approach is based on the combination of 1,3-nonbonded interactions around the metal centre and a harmonic sine function with a ligandfield-dependent force constant for the L–M–L′ terms. Force-field parameters for four-, five-, and six-coordinated first-row transition-metal coordination centres and a variety of ligands containing N-, S-, and O-donor sets are given. The new ‘electronically doped’ force field is shown to generally lead to computed structures with higher accuracy than those obtained when the coordination geometries are modeled with 1,3-nonbonded interactions alone.

Structural and photophysical properties of pseudo-tricapped trigonal prismatic lanthanide building blocks controlled by zinc(II) in heterodinuclear d-f complexes

Abstract: An efficient combination of electrospray mass spectrometry (ES-MS), spectrophotometric and 1H-NMR titrations in solution is used to characterize the assembly of the segmental ligand 2-{6-[1-(3,5-dimethoxybenzyl)-1H-benzimidazol-2-yl]pyridin-2-yl}-1, 1′-dimethyl-5,5′-methylene-2′-(5-methylpyridin-2-yl)bis[1H- benzimidazole] (L2) with ZnII and 4f metal ions, LnIII Ligand L2 reacts with Zn(ClO4)2 in MeCN to give successively [Zn(L2)2]2+, where the metal ion is coordinated by the tridentate binding units of the ligands, and the double-helical head-to-head complex [Zn2(L2)2]4+ When L2 reacts with Ln(ClO4)3 (Ln = La, Eu, Lu), LaIII only leads to a well-defined cylindrical C1-symmetrical homodinuclear head-to-tail complex [La2(L2)3]6+ in solution, while chemical-exchange processes prevent the 1H-NMR characterization of [Eu2(L2)3]6+, and LuIII gives complicated mixtures of complexes However, stoichiometric amounts of LnIII (Ln = La, Ce, Pr, Nd, Sm, Eu, Tb, Y, Lu), ZnII, and L2 in a 1:1:3 ratio lead to the selective formation of the C3-symmetrical heterodinuclear complexes [LnZn(L2)3]5+ under thermodynamic control Detailed NOE studies show that the ligands are wrapped about the C3 axis defined by the metal ions, and the separation of dipolar and contact contributions to the 1H-NMR paramagnetic shifts of the axial complexes [LnZn(L2)3]5+ (Ln = Ce, Pr, Nd, Sm, Eu) in MeCN establishes that ZnII occupies the pseudo-octahedral capping coordination site defined by the three bidentate binding units, while LnIII lies in the resulting ‘facial’ pseudo-tricapped trigonal prismatic site produced by the three remaining tridentate units Photophysical measurements show that [LnZn(L2)3]5+ (Ln = Eu, Tb) are only weakly luminescent because of quenching processes associated with the C3-cylindrical structure of the complexes The use of 3d metal ions to control and design isomerically pure ‘facial’ tricapped trigonal prismatic lanthanide building blocks is discussed together with the calculation of a new nephelauxetic parameter associated with heterocyclic N-atoms coordinated to LnIII

Substituent Reactivity and Tautomerism of Isoguanosine and Related Nucleosides

Abstract: The substituent reactivity and tautomerism of isoguanine nucleosides is studied. Benzoylation or tosylation of isoguanine nucleosides (pyridine, room temperature) yields the 2-benzoyl derivatives 7c, 11, and 12 or the 2-tosyl compounds 13 and 14. The isobutyrylation of the 6-amino group which did not occur under these conditions was induced in the presence of Me3SiCl. In the absence of Me3SiCl, the reactivity of isoguanine substituents decreases in the order from 2-oxo 5′-OH 3′-OH 6-NH2. From isoguanine nucleosides, the N1-(2b), N3-(17), N6-(15a,b), and 2-O-alkylated (3b) derivatives were prepared. Their pKa values were determined and the UV and 13C-NMR spectra compared with regard to the alkylation position. Also the tautomeric forms of isoguanine nucleosides were determined UV-spectrophotometrically in aqueous and nonaqueous solution. Isoguanosine (1a), its 2′-deoxy analogue 1b as well as the N6-methyl- and 8-substituted derivatives form lactam tautomers in aqueous solution, whereas the lactim form is present in dioxane.

The Decomposition of cis‐Fused Cyclopenteno‐1,2,4‐Trioxanes induced by Ferrous Salts and some oxophilic reagents

Abstract: Two cis-fused cyclopenteno-1,2,4-trioxanes, 1a and 1b, were subjected to Zn in AcOH or FeCl2 · 4H2O in MeCN. In the first case, the main course was deoxygenation to give cyclopentanone (18) and the 1,4-diphenyl- or 1,4-bis(4-fluorophenyl)cyclopent-3-ene-1,2-diol 10 (Scheme 5). In the second case, isomerization chiefly occurred resulting in the formation of a dimer 9 of the respective 3,5-diaryl-5-hydroxycyclopent-2-enyl 5-hydroxypentanoates 8 (Scheme 3).