Two‐Dimensional Acetylenic Scaffolding: Extended Donor‐Substituted Perethynylated Dehydroannulenes

Starting from (Z)‐bis(N,N‐diisopropylanilino)‐substituted tetraethynylethene (TEE), perethynylated octadehydro[12]‐ and dodecadehydro[18]annulenes were prepared by oxidative Hay coupling. The dodecadehydro[18]annulene with six peripheral N,N‐diisopropylanilino substituents was characterized by X‐ray crystallography. Elongation of the Z‐bisdeprotected TEE by Cadiot–Chodkiewicz coupling with 1‐bromo‐2‐(triisopropylsilyl)ethyne provided a Z‐configured bis(butadiyne), which after alkyne deprotection afforded under Hay coupling conditions N,N‐diisopropylanilino‐substituted perethynylated hexadecadehydro[20]‐ and tetracosadehydro[30]an‐nulenes. The diisopropylanilino substituents enhance the properties of these unprecedented all‐carbon perimeters in several distinct ways. They ensure their solubility, increase their stability, and importantly, engage in strong intramolecular charge‐transfer interactions with the electron‐accepting all‐carbon cores, resulting in intense, bathochromically shifted charge‐transfer bands in the UV/Vis spectra. The charge‐transfer character of these bands was confirmed by protonation‐neutralization experiments. The redox properties of the new carbon‐rich chromophores were investigated by cyclic voltammetry and rotating disk voltammetry, which indicated different redox behavior for aromatic (4n+2 π electrons) and antiaromatic (4n π electrons) dehydroannulenes.     

The Unique Antithrombin III Binding Domain of Heparin: A Lead to New Synthetic Antithrombotics

Heparin, a sulfated glycosaminoglycan, is well known for its anticoagulant effect mediated by the serine protease inhibitor antithrombin III (AT III). Heparin has been used clinically for more than half a century for the prophylaxis and treatment of venous thrombosis and thromboembolism. Up until the 1980s it was assumed that the biological activity of heparin was mainly caused by its polyanionic character. However, this paradigm was contradicted when it was discovered that part of the heparin polysaccharides contains a well-defined pentasaccharide domain that specifically binds and activates AT III. The specificity of the interaction between the characteristic pentasaccharide and AT III has become more obvious after the synthesis and biological testing of various heparin analogues. This article reviews the synthesis of the heparin pentasaccharide, some closely related counterparts, and some highly modified analogues. With the aid of molecular modeling and through “tailored” molecular modifications of the pentasaccharide, much knowledge has been gained concerning structure-activity relationships. On this basis not only have more potent and simplified derivatives been developed, but also the recognition between heparin and AT III can now be understood in greater detail at the molecular level.     

Isolation and identification of the anti-isohumulones and the anti-acetylhumulinic acids

The anti-isohumulones [5-(3-methylbutanoyl)-2-(3-methylbut-2-enyl)-4-hydroxy-4-(4-methylpent-3-enoyl)-cyclopentane-1,3-diones] and the anti-acetylhumulinic acids [5-(3-methylbutanoyl)-2-(3-methylbut-2-enyl)-4-ethanoyl-4-hydroxy-cyclopentane 1,3-diones] have been isolated from an isomerisation reaction mixture of humulone [2-(3-methylbutanoyl)-4,6-di(3-methylbut-2-enyl)-6-hydroxy-cyclohexane-l,3,5-trione] by counter-current distribution and identified by spectroscopic techniques. The formation mechanism is presented and the stereochemical consequences are discussed. The anti-isohumulones are the most bitter hop compounds presently known.     

The alkaloidal profile of cocculus pendulus

(+)-Kohatine (5), (+)-kun-amine (7), (+)-norpenduline (11), and (+)-cheratamine (12), are four new bisbenzylisoquinolines obtained from Cocculus pendulus (Forsk) Diels (Menispermaceae). Ten known bisbenzylisoquinolines are also present. All fourteen alkaloids possess the S chirality at C-1'. The biogenetic sequence involves initial condensation of two coclaurine-type units to form a dinner such as 16. Species 16 may in turn lead to dimers 17 or 18 with two diaryl ether linkages. Only dimer 17 may undergo further oxidative coupling to bisbenzylisoquinolines with three diaryl ether bridges.     

A highly stereoselective route to substituted bis-tetrahydrofurans from 1,5-dienes.

Bis-tetrahydrofurans $\text{1a}$ and $\text{1b}$, containing four chiral centers, are obtained with a 90 % isomeric purity from geranyl and neryl chlorides in four steps including two stereoselective cyclizations.     

The gas enhanced negative ion mass spectra of polychloroanisoles

Methane or a methane–oxygen mixture was used as an enhancement gas to obtain negative ion mass spectra of polychloroanisoles. Dichloroanisoles did not react with oxygen but the more highly chlorinated anisoles did. Compounds with hydrogen ortho to the methoxy group had [M? 1]^? ions, while others gave . The fragment arose through loss of an ortho chlorine and amethyl hydrogen. The loss of HCl followed by oxygen displacement of a remaining ortho or para chlorine produced [M? 55]^? ions; the para position was the preferred site of displacement. Another ion-molecule reaction with oxygen leads to [M? CH₂Cl]^?. The fragmentations resemble those of chlorinated aromatics such as the polychlorodibenzodioxins.     

Position of protonation of the cresols: Semi-empirical and ab initio calculations

STO-3G and INDO calculations on the isomeric cresols and the protonated cresols indicated that protonation occurs most readily para to the hydroxy substituent, as is observed commonly in experimental work.     

Consequences of acceptor effects on the polarization of oxo and imine bonds in acyl compounds: Part II. Semicarbazones CH3-C(=N-NH-CO-NH2)X and R-C(=N-NH-CO-NH2)-COOH. analysis of acid-base equilibria antecedent to reduction at the dropping mercury electrode

It is usually assumed that in α ketonic acids, recombination currents at the DME pertain to the carboxylate group. A comparison of the cathode half-wave potentials and the ratio ΔE_1/2/Δ pH obtained for semicarbazones R-C(=N-NH-CO-NH₂)-COOH leads to a different conclusion. If R=H (glyoxylate) or C₆H₅ (phenylglyoxylate), the results are in agreement with the usual hypothesis. On the contrary, if R=CH₃ (pyruvate), as the pH increases, the cessation of antecedent protonation occurs first at the carboxylate, and then at the imine group.     

Complexation du chlorure de vinyle par le tetrahydrofuranne et le n-butyraldéhyde: Étude par résonance magnétique nucléaire haute résolution

Examination of the HR-NMR spectra of vinyl chloride in the presence of tetrahydrofuran and n-butyraldehyde suggests the formation of donor-acceptor type complexes. The equilibrium quotients and NMR constants were estimated by using the Benesi-Hildebrand method. The existence of these complexes would explain the anomalies observed: first, in the anionic polymerization of vinyl chloride initiated by tert-butylmagnesium chloride is tetrahydrofuran. The initiation step seems to be governed by this complexation phenomenon; second, in the radical polymerization of the same monomer in n-butylaldehyde the obtained poly(vinyl chloride) is more syndiotactic than the radical polymer prepared in bulk by the usual methods.     

Colorimetric determination of chromium in titanium alloys

Chromium-titanium alloys are becoming increasingly important because of their strength and corrosion, resistance. Therefore, accurate methods for the determination of chromium in this type of alloy are needed. A colorimetric procedure for the determination of chromium in titanium alloys is proposed. In this method the chromium is oxidised to the chromate state by means of ammonium persulfate and potassium permanganate in the presence of silver nitrate as a catalyst. Diphenylcarbazide is then added to an aliquot of the solution, and the violet color read in 10 to 40 minutes at 580 millimicrons. None of the elements found in commercial titanium alloys, including iron, vanadium, molybdenum, and tungsten, interferes with the method. The proposed method is superior to the volumetric method for the determination of small amounts of chromium. The method is designed for titanium. alloys containing 0.02 to 4 per cent chromium.