Synthetic Attempts Research Articles

Versuche zur Synthese von [2.2]- und [2.3]Cyclopentadienophanen

Synthetic Attempts towards [2.2]- and [2.3]Cyclopentadienophanes. The first [2.2]Cyclopentadienophane (=2,2,3,3,8,8,9,9-octamethyltricyclo[8.2.1.14,7]tetradecatetraene; 1b) has been synthesized (Scheme 5) by reductive coupling of 6,6-dimethylfulvene 3→5 (50%) followed by base-induced twofold condensation of 1,2-di(cyclopentadienyl)-1,2-dimethylbutane 5 with acetone to give difulvene 15 (95%). Reductive coupling of 15 gives a complex mixture of tautomers of 16, 17, and 1b, which contains ca. 50% of the target molecule 1b. Other synthetic attempts towards [2.2]cyclopentadienophanes 1a and 1b and [2.3]cyclopentadienophane 18 are discussed.

Characterization of the binding site of the histamine H3 receptor. 1. Various approaches to the synthesis of 2-(1H-imidazol-4-yl)cyclopropylamine and histaminergic activity of (1R,2R)- and (1S,2S)-2-(1H-imidazol-4-yl)-cyclopropylamine.

Various approaches to the synthesis of all four stereoisomers of 2-(1H-imidazol-4-yl)cyclopropylamine (cyclopropylhistamine) are described. The rapid and convenient synthesis and resolution of trans-cyclopropylhistamine is reported. The absolute configuration of its enantiomers was determined by single-crystal X-ray crystallographic analysis. The distinct trans-cyclopropylhistamine enantiomers were tested for their activity and affinity on the histamine H3 receptor. (1S,2S)-Cyclopropylhistamine (VUF 5297) acts as an agonist both on the rat cortex (pD2 = 7.1; alpha = 0.75) and on guinea pig jejunum (pD2 = 6.6; alpha = 0.75). Its enantiomer, (1R, 2R)-cyclopropylhistamine (VUF 5296), is about 1 order of magnitude less active. Both enantiomers show weak activity on H1 and H2 receptors. All synthetic attempts to cis-cyclopropylhistamine were unsuccessful. Nevertheless, the results of this study provide an ideal template for molecular modeling studies of histamine H3 receptor ligands.

Versuche zur Synthese von polymergebundenen `Push-Pull'-Acetylenen

In view of possible applications of `push-pull' acetylenes 1 and `push-pull' olefins 2 as potential non-linear optics materials, synthetic attempts towards polymers of type 4 and 6 (Scheme 2) have been undertaken. Two so far unknown `push-pull' acetylenes 1a,b with R=styrene, have been prepared under carefully controlled conditions which quite surprisingly fail to give polymers 4a,b by anionic or radical-induced polymerization. On the other hand, `push-pull' acetylenes may be reacted with commercially available poly(4-vinylphenol) to give polymers 6c,d with a 40 – 70% conversion of phenol units.

Mixed Chloride Phosphine Complexes of Dirhenium Cores. 1. New Reactions and Unprecedented Structures Involving Trimethylphosphine

The unusual 1,2,7-isomer of Re2Cl5(PMe3)3 (1) has been used as a starting material to prepare previously unknown dirhenium complexes. One-electron reduction of 1 by cobaltocene followed by nonredox substitution of the resulting anionic species with PMe3 led to the formation of a triply bonded 1,2,7,8-Re2Cl4(PMe3)4 (2). In the crystal structure of 2, phosphine ligands on both metal centers exhibit a cis arrangement with a P−Re−P angle of 93.3° in contrast to the well-known type of 1,3,6,8-isomers with a trans arrangement of the monodentate phosphines connected to each metal atom. Complex 2 represents the first example of an isomer of this type in the large M2X4(PR3)4 class of compounds (M = Re, Tc, W, Mo; X = Cl, Br, I; PR3 = monodentate phosphine). The one-electron oxidation product of 1 cocrystallized with one molecule of tetrabutylammonium chloride afforded Re2Cl6(PMe3)2 (3), for which all previous synthetic attempts had failed. This quadruply bonded complex exhibits an unusual 3-fold disorder of the Re2 unit with equal populations for all three orientations. We also report that when the “classic” reaction of octachlorodirhenate anion, Re2Cl82-, with trimethylphosphine is carried out in benzene at room temperature the reduction processes do not occur and the product is a novel paramagnetic complex, Re2Cl6(PMe3)4 (4), which does not have a metal−metal bond (the Re−Re separation is 3.8476(4) Å). Another interesting feature of compound 4 is that the PMe3 ligands have a cis disposition at each rhenium center and are located in the same plane as the metal atoms and bridging chlorine ligands. For such a ligand arrangement the molecule of 4 is the only example of a nonmetal−metal-bonded dinuclear compound with monodentate phosphine ligands.

Defining difference, forging unity: The co‐construction of race, ethnicity and nation in Belize

This article examines competing nationalist projects which compete to constitute a Belizean nation: pluralist nationalism constructs the nation as ethnically diverse; synthetic nationalism attempts to submerge ethnic or racial difference into a shared national identity; hegemonic nationalism works to attach preferentially a single racial identity to the nation and exclude other identities. Within these projects, the homogenizing processes which construct national sameness are integrally related to the individualizing processes which constitute subnational difference: both national sameness and subnational differences are constituted in terms of race, ethnicity, or a conflation of the two. The article explores how the essentialization of racial, ethnic and national identities facilitates their assimilation of one another.

Structural and Quantum Chemical Study of Bi(5)(3+) and Isoelectronic Main-Group Metal Clusters. The Crystal Structure of Pentabismuth(3+) Tetrachlorogallate(III) Refined from X-ray Powder Diffraction Data and Synthetic Attempts on Its Antimony Analogue.

Pentabismuth(3+) tetrachlorogallate(III), (Bi(5)(3+))(GaCl(4)(-))(3), has been synthesized by reducing a BiCl(3)-GaCl(3) melt with bismuth metal and the crystal structure refined from X-ray (Cu Kalpha(1)) powder diffraction data. The structure was found to belong to space group R-3c, with the lattice parameters a = 11.871(2) Å and c = 30.101(3) Å (Z = 6). It is isostructural with the previously characterized Bi(5)(AlCl(4))(3). An attempt to synthesise the antimony analogue Sb(5)(GaCl(4))(3) by reducing a SbCl(3)-GaCl(3) mixture with gallium metal produced a black solid phase. The gallium content of this phase is consistent with the stoichiometry Sb(5)(GaCl(4))(3), and the Raman spectrum of the phase dissolved in SbCl(3)-GaCl(3) comprises strong, low-frequency bands attributable to Sb-Sb stretch vibrations in Sb(5)(3+) or another reduced antimony species. Quantum chemical analyses have been performed for the isoelectronic, trigonal pyramidal closo-clusters Sn(5)(2-), Sb(5)(3+), Tl(5)(7-), Pb(5)(2-), and Bi(5)(3+), both with extended Hückel (eH) and Hartree-Fock (HF) methods. The HF calculations were performed with and without corrections for the local electron-electron correlation using second-order Møller-Plesset perturbation theory (MP2). All theoretical results are compared and evaluted with respect to experimental cluster structures and vibrational frequencies. The results from the calculations agree well with available experimental data for the solid-state structures and vibrational spectra of these cluster ions, except for the Tl(5)(7-) ion. Isolated Tl(5)(7-) is suggested to be electronically unstable because of the high charge density. The Sb(5)(3+) cluster ion is indicated to be stable. According to the calculations, Sn(5)(2-) and Pb(5)(2-) may be described in terms of edge-localized bonds without substantial electron density between the equatorial atoms, whereas Sb(5)(3+) and Bi(5)(3+) have electron density evenly distributed over all M-M vectors. Furthermore, the theoretical results give no support for a D(3h) --> C(4v) fluxionality of these clusters.

Synthetic Attempts towards Polymers with Pentafulvene Structural Units

AbstractSynthetic attempts towards fully conjugated polymers 9 with pentafulvene‐diyl structural units are described. Cationic polymerization of pentafulvenes 1a (R = X = Me) and 1b (R = X = MeS) nearly quantitatively gives polymers 8a and 8b with typical Mn and Mw values of 38800 and 53750, respectively, for 8a, and 12000 and 35900, respectively, for 8b. Key step of the conversion 8a → 9a (Scheme 6) is a quantitative bromination 8a → 32a, the structure of 32a being confirmed by analytical data as well as by spectroscopic comparison with model compound 23. Best results in view of two‐fold the HBr elimination 32a → 9a are obtained with Et3N, but so far elimination has not been complete. Synthetic sequences are optimized with model compound 21 (Scheme 4). Here again, bromination 21 → 23 is quantitative, while two‐fold HBr elimination 23 → 22 with Et3N proceeds in 51% yield. Dibromide 23 easily undergoes HBr elimination followed by a Br shift to give bromide 29. Contrary to cationic polymerization, anionic polymerization of simple pentafulvenes 1 to 2 (which would be attractive in view of the conjugated polymers 3) is not successful: For pentafulvene 1b (R = X = MeS), the main reaction is Diels‐Alder‐type dimerization 1b → 15b (Scheme 2), even under anionic conditions.

Synthetic Attempts towards Polymers with Pentafulvene Structural Units

Pentafulvenes with weakly electron-donating substituents at C(6) (X = SMe) may be submitted to cationic polymerization which had been reported before only for 6-alkyl-, 6-aryl-, and 6,6-dialkylpentafulvenes. This procedure does not work for pentafulvenes like 1c with strong electron-releasing substituents at C(6) (X = Me2N). Based on bromination-elimination experiments with model compound 6, it has been shown that direct bromination of polyfulvene 2a → 10a is very efficient. This result is important in view of the synthesis of polymers 3a with pentafulvene structural elements. So far, only partial HBr elimination of polymer 10a has been achieved, while I2 treatment of 10a gives a insoluble charcoal-like black polymer absorbing in the whole UV/VIS range.

Synthetic Attempts towards ‘Aromatic’ Nonafulvenes

AbstractAccording to their spectroscopic behavior, four classes of nonafulvenes may be distinguished, but, so far, only three classes have been identified. Type‐A nonafulvenes (including parent 1a) are typically olefinic molecules with strongly alternating bond lengths and a nonplanar nine‐membered ring. Type‐B nonafulvenes are characterized by four pairs of equivalent ring H‐atoms and ring C‐atoms. Spectra of both Type‐A and Type‐B nonafulvenes are not dependent on temperature and solvent polarity. However, spectra of Type‐C nonafulvenes (including prototype 1d with R1 = R2 = NMe2) are strongly influenced by temperature and solvent polarity due to an equilibrium 1⇄1± between the nonpolar olefinic 1 and dipolar planarized 1±. So far, Type‐D nonafulvenes occurring exclusively in the dipolar form 1± were unknown. Synthetic attempts towards nonafulvenes of Type D are described and problems encountered in nonafulvene syntheses are discussed. Several new cyclononatetraenes and four new nonafulvenes (or nonafulvalenes) 31, 1n, 3, and 5 have been synthesized. Spectroscopic evidence shows that 11,12‐bis(diethylamino)nonatriafulvalene 5 is the first Type‐D nonafulvene.

Sulfur Containing Conjugated Polymers with Interesting Electronic Properties.

ABSTRACTThe electronic properties of the isomerie (C2S2)n conjugated sulfur containing polymers 1, 2, and 3 were characterized using ab initio and PRDDO Molecular orbital calculations, as well as EH band structure calculations. It is anticipated that such structures might exhibit enhanced electrical conductivities when doped owing to the possibility of the peripheral sulfur atoms extending orbital interactions transverse to the chain axis direction. Such increased dimensionality is demonstrated for the structurally similar superconductive Molecular TTF and ET charge-transfer salts. Of the three polymeric structures, indications are that polymer 2 is highest in energy. Polymer 3 can exist in both “chair” and “boat” conformations, with the chair form slightly more stable. Both the chair and boat forms of 3 were found to be of lower energy than 1. Calculated band gaps and band widths are reported along with preliminary synthetic attempts to prepare these polymers and observed properties.

New compounds related to podophyllotoxin and congeners: synthesis, structure elucidation and biological testing.

4-Azido, 4-amino, 4-amido and 4-alkoxy compounds related to the lignans podophyllotoxin and 4'-demethylepipodophyllotoxin have been synthesized, and their structures elucidated. The Ritter reaction was shown to be useful in the preparation of the 4-amido compounds with the required stereochemistry. A preparative method for 4-chloro-4-deoxypicrophyllotoxin, for which all earlier synthetic attempts resulted in the two dehydrated compounds, alpha- and beta-apopicropodophyllotoxin, was developed. Supplementary preliminary studies of the biological activities of some of the compounds were performed. All compounds had pronounced inhibitory effect on the in vitro growth of human cervical cancer cells and TC-mouse cells with 4-amino-4-deoxypodophyllotoxin and 4-azido-4-deoxypodophyllotoxin showing the highest activity. Alkaline elution studies indicate that the toxicity of the 4'-demethoxy derivatives is due to protein-mediated DNA nicking. None of the compounds were found to have antiviral effect against herpes simplex type 2 (HSV-2), human immunodeficiency (HIV), and cytomegalovirus (CMV) in doses not toxic to the cells.

Synthesis of 6′-substituted dobutamine analogues

Two 6′-substituted dobutamine analogues (1, 2) were synthesized from the coupling reaction of dopamine with the corresponding ketones. The ketone (8) was obtained from 4-nitrosalicylaldehyde via 6 synthetic steps while the ketone (12) was prepared from 2-methyl-5-nitrobenzoic acidvia 7 synthetic steps. Another synthetic attempts were also reported.

Synthesis of rhodium-containing heterobimetallic hydride complexes

The reduction chemistry of heterobimetallic dihydrides are of much interest. Three heterobimetallic monohydride complexes containing Rh bound to either Re or Ta were isolated during synthetic attempts at preparing heterobimetallic dihydrides. The mode of synthesis, characterization, and reactivity of these three heterobimetallic compounds are discussed herein. 19 refs.

Valence orbital energetics as a tool for organometallic synthesis: On the scarcity of two-legged piano-stool complexes

The stability of 18-electron two-legged piano-stool complexes of the type (η n -C n H n ,)ML 2( n = 4, 5 or 6; L = CO or NO; M = first-row transition metal) is correlated to the energetic separation of the HOMO from both the LUMO and the SHOMO (second-highest occupied molecular orbital) of the complexes calculated by using the Fenske-Hall molecular orbital method. Complex stability is observed to increase with increasing HOMO/LUMO and decreasing HOMO/SHOMO separations. This correlation is reflected in an examination of known stable complexes and unsuccessful synthetic attempts at others. On the basis of these findings, predictions are made as to the stability of a number of unknown 18-electron two-legged piano-stool complexes for which syntheses have not yet been attempted.

Sila-retinol: das erste silasubstituierte vitamin

Synthesis of sila-vitamin A was achieved for the first time by treating sila-β-ionone in five steps; (i) with P-cyanomethyl-diethylphosphonate/NaH (Horner/Wittig reaction), (ii) reduction with diisobutylaluminum hydride, (iii) condensation with the ethyl ester of 3,3-dimethylacrylic acid/KNH 2/liquid NH 3 (Knoevenagel condensation), (iv) hydrolysis (KOH) and finally, (v) LiAlH 4-reduction (Sumitomo process). Other attempts to synthesize sila-vitamin A according to experiences in the silicon-free system with trimethylsilacyclohexanone (I) as starting material (Attenburrows concepts) only resulted in the isolation of 1-(1,1,3-trimethyl-1-sila-2-cyclohexen-2-yl)-3-methyl-1,3,5-octatrien-2-one (“sila-C 18-ketone”) and, again beginning with sila-β-ionone, in isolating (1-(1,1,3-trimethyl-1-sila-2-cyclohexen-2-yl)-3,7-dimethyl-9-methoxy-1,3,5,7-nonatetraen-9-one (methyl ester of “sila-vitamin A acid”). The organosolicon compounds prepared in the steps of the different synthetic attempts had very similar general properties to the silicon-free compounds, but did not follow the same chemistry in all cases. Trimethylsilacyclohexanone was also transformed into 1,1,3-trimethyl-1-silacyclohexan-2-ol, 2-cyano-1,1,3-trimethyl-2-trimethylsiloxy-1-sila-cyclohexane and 2-ethynyl-1,1,3-trimethyl-1-silacyclohexan-2-ol.

Synthesis and characterization of stoichiometric CdPS 3

Compounds from the system CdPS 3 xSe x (where x = 1, 2, and 3) have been synthesized by a variety of synthetic techniques and characterized by X-ray diffraction. The X-ray diffraction lines obtained from CdPSe 3 and from CdPSSe 2 could be indexed using the reported powder pattern for CdPSe 3. However, despite different synthetic attempts and reannealing, powder pattern data for the CdPS 3 samples, although reproducible from sample to sample, are not consistent with the C2/m structure previously reported. Luminescence data indicate the presence of neutral Cd atoms, Cd vacancies, and S vacancies in crystals annealed under selected conditions. Electron paramagnetic resonance of Mn(II) doped CdPS 3 and 113Cd solid state nuclear magnetic resonance of CdPS 3 spectrum are consistent with a single cadmium environment.

Solvent effects on the formation of iron oxide chloride intercalates with metallocene derivatives

The lamellar compound FeOCl has been shown to form, under suitable reaction conditions, intercalation compounds with ring-substituted ferrocenes as well as with (C 5H 5) 2Ni and (C 5H 5) 2Ru, for which previous synthetic attempts had been unsuccessful. Successful intercalation depends on the appropriate choice of solvents. Co-intercalation of solvent molecules occurs in some cases.

Piaget and Analysis: A Synthetic Attempt

Piaget and Analysis: A Synthetic Attempt

Les hormones œstrogènes, de leur découverte à leur synthèse totale

After the discovery of the œstrogenic hormones in 1929 and subsequent years, and the determination of their constitution, their synthesis was performed in two stages. Following many preliminary synthetic attempts (Bardhan, Butenandt, Robinson and others), in 1940 equilenine was synthesized byBachmann and, at almost the same time,Inhoffen succeeded in the partial synthesis of œstrone from cholesterol. In 1942 the preparation of a nearly inactive isomer of œstrone was disclosed byBachmann. Of decisive importance, however, was the elucidation since 1944 of the doisynolic acids deriving from the œstrogenic ketones and showing themselves high œstrogenic effectiveness, as well as the discovery of important sterical relationships between ketones and acids. The total synthesis of the doisynolic acids in 1947, which was conducted via sterically uniform ketoesters of the octahydrophenanthrene series, simultaneously cleared the way to the natural œstrone itself (1948). Now all genuine œstrogenes (œstrone, œstradiol, œstriol) are accessible to total synthesis. Attempts to obtain œstrone by means of a dienesynthesis have so far been unsuccessful. On the other hand, it was possible, by this method, to build up a sterically uniform but inactive monodehydro-œstrone and the corresponding highly active monodehydro-doisynolic acid. Attention is drawn to the new conception of the “constitutional field of activity”.

1,2,3,4-Dibenzphenanthrene and its Derivatives. II. Some New Synthetic Attempts1

1,2,3,4-Dibenzphenanthrene and its Derivatives. II. Some New Synthetic Attempts¹