Two new 5-deoxyflavones from Albizia odoratissima

Two new 5-deoxyflavones, 7,8-dimethoxy-3',4'-methylenedioxyflavone (1) and 7,2',4'-trimethoxyflavone (2) together with a known flavone, 7,4'-dimethoxy-3'-hydroxyflavone (3) were isolated from the rootbark of Albizia odoratissima. The structures of these new compounds were elucidated by electrospray ionization mass spectrometry (ESI-MS) and 1D and 2D-NMR spectral studies including (1)H-(1)H correlation spectroscopy (COSY), heteronuclear single quantum coherence (HSQC), heteronuclear multiple bond connectivity (HMBC) and nuclear Overhauser enhancement spectroscopy (NOESY).     

“Dibutylmagnesium”, a convenient reagent for the synthesis of useful organic magnesium reagents MgA2 including cyclopentadienyls,aryloxides, and amides. Preparation of Zr(C5H5)Cl3. X-ray structure of [Mg{μ-N(SiMe)3C6H4N}(SiMe3)-o(OEt2)]2

n-Heptane-soluble “di-butylmagnesium” (I) (a commercially available material, prepared by addition of LiBu^s to MgBuⁿCl, and subsequent addition of ca. 5% MgOct₂ⁿ) has been found to be a useful starting material for obtaining numerous organic magnesium compounds. This is illustrated by its reaction with a number of protic compounds HA to give in good yields Mg(C₅H₅)₂, Mg(C₅H₄Me)₂, or the new compounds MgA₂: IV (A = C₅H₄SiMe₃), V [A = C₅H₃(SiMe₃)₂], VII (A = OC₆H₂Bu₂^t-2,6-Me-4), and X [A₂ = N(SiMe₃)C₆H₄N(SiMe₃)-o(OEt₂)]. The value of such compounds MgA₂ as mild ligand transfer reagents is illustrated by the synthesis of Zr(C₅H₃X₂)Cl₃ (X = H or SiMe₃). Compound X was isolated from OEt₂ solution as the crystalline dimer

with two o-$\text{N}$(SiMe₃)C₆H₄$\text{N}$(SiMe₃) ligands bridging two magnesium atoms and a terminal OEt₂ ligand completing a distorted tetrahedral environment around each Mg. Some key parameters are: MgN_t 1.997(7), MgN_b 2.083(8), MgO 2.041(7) Å; OMgN_t 112.1(3), OMgN_b 119.7(3), and N_tMgN_b 118.5(3)°.     

Advanced approaches for the characterization of a de novo designed antiparallel coiled coil peptide

We report here an advanced approach for the characterization of the folding pattern of a de novo designed antiparallel coiled coil peptide by high-resolution methods. Incorporation of two fluorescence labels at the C- and N-terminus of the peptide chain as well as modification of two hydrophobic core positions by Phe/[15N,13C]Leu enable the study of the folding characteristics and of distinct amino acid side chain interactions by fluorescence resonance energy transfer (FRET) and NMR spectroscopy. Results of both experiments reveal the antiparallel alignment of the helices and thus prove the design concept. This finding is also supported by molecular dynamics simulations. Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) in combination with NMR experiments was used for verification of the oligomerization equilibria of the coiled coil peptide.     

Comment on "First principles electrochemistry: electrons and protons reacting as independent ions" [J. Chem. Phys. 117, 10193 (2002)

The entropy of the electron gas used by Llano and Eriksson (LE) in the definition they used for an absolute half-cell potential (AHCP) is indeterminate. This AHCP is therefore not based on a thermodynamically well-defined process and cannot easily be extended to other thermodynamic functions such as entropy. An alternative approach to the AHCP avoids these difficulties. The present comment also corrects statements and concepts presented by LE about the temperature coefficient of the Fermi level and surface charge of the electrolyte solution.     

Enantioselective epoxidation of alkenes catalyzed by 2-fluoro-N-carbethoxytropinone and related tropinone derivatives

Several alpha-substituted N-carbethoxytropinones have been evaluated as catalysts for asymmetric epoxidation of alkenes with Oxone, via a dioxirane intermediate. alpha-Fluoro-N-carbethoxytropinone (2) has been studied in detail and is an efficient catalyst which does not suffer from Baeyer-Villiger decomposition and can be used in relatively low loadings. This ketone was prepared in enantiomerically pure form using chiral base desymmetrization of N-carbethoxytropinone. Asymmetric epoxidation catalyzed by 2 affords epoxides with up to 83% ee. Among other derivatives tested, the alpha-acetoxy derivative 7 affords the highest enantioselectivities.     

New structural motifs in the aggregation of neutral gold(I) complexes: structures and luminescence from (alkyl isocyanide)AuCN

The preparation and X-ray crystal structures of (CyNC)Au(I)CN, (n-BuNC)Au(I)CN, and (i-PrNC)Au(I)CN.0.5CH(2)Cl(2) are reported and compared with those of (MeNC)Au(I)CN and (t-BuNC)Au(I)CN, which were previously described. These linear molecules are all organized through aurophilic interactions into three structural classes: simple chains ((CyNC)Au(I)CN and (t-BuNC)Au(I)CN), side-by-side chains in which two strands make Au...Au contact with each other ((n-BuNC)Au(I)CN), and nets in which multiple aurophilic interactions produce layers of gold(I) centers ((i-PrNC)Au(I)CN and (MeNC)Au(I)CN). All of these five solids dissolve to produce colorless, nonluminescent solutions with similar UV/vis spectra. However, each of the solids displays a unique luminescence with emission maxima occurring in the range 371-430 nm.     

On the regular heptagon

The geometry of rings with 5, 6, 7, 8 and 9 links, particularly those with seven links such as occur in cyclo‐heptane and analogous molecules* is re‐considered with the aid of distance geometry, now more practicable with modern mathematical software. Use is made of a quadric surface which may be fitted to nine points. *In admiration for the author I have ventured to appropriate the title of Archimedes’ work “On the regular heptagon”, which came to light only in 1926 [C. Schoy, Graeco‐Arabische Studien, Isis 8 (1926) 21–40]. “Regular”, as applied to rings in space admits of various definitions. This revised version was published online in July 2006 with corrections to the Cover Date.     

A simple procedure for the chromatographic analysis of nanoliter samples

A simple method for the analysis of nanoliter droplets is proposed, which is profitable when larger samples cannot be collected as, for instance, in the case of several biological fluids and particularly in clinical chemistry. A glass capillary associated to a micromanipulator is used to collect submicroliter volumes which are partially transferred into transparent polymeric tubings with known internal diameters (120–178 μm), where the volumes sampled are measured by meniscus collimations with a collimator microscope at suitable magnification. Both ends of these tubings are preliminarily equipped with ferrules and fittings, so as to make them suitable for connection as the loop to a conventional high-pressure injection valve. The reliability of this procedure has been tested for the analysis of Na⁺, K⁺ and Ca²⁺ present in minute synthetic standard samples (10–200 nL) by a conventional ion-chromatographic instrumentation. Relative standard deviations in peak area measurements (5–6%) are discussed in terms of the whole approximation affecting volume measurements, which depends on both the inconstancy of the inner diameter of the polymeric tubings employed and the uncertainty characterizing meniscus collimations. The proposed procedure can be easily extended to the determination of any organic or inorganic species present in very small samples, provided that their detection can be achieved by any chromatographic approach or, more generally, by flow injection analysis. Received: 16 June 1997 / Revised: 12 August 1997 / Accepted: 14 August 1997     

Studies in polyphenol chemistry and bioactivity. 4.(1) Synthesis of trimeric,tetrameric, pentameric,and higher oligomeric epicatechin-derived procyanidins having all-4beta,8-interflavan connectivity and their inhibition of cancer cell growth through cell cycle arrest

We report an improved synthesis of bis(5,7,3',4'-tetra-O-benzyl)epicatechin 4beta,8-dimer (3) from 5,7,3',4'-tetra-O-benzylepicatechin (1) and 5,7,3',4'-tetra-O-benzyl-4-(2-hydroxyethoxy)epicatechin (2) by replacing the previously employed Lewis acid, titanium tetrachloride, with the clay mineral Bentonite K-10. Under the same conditions, the benzyl-protected all-4beta,8-trimer, -tetramer, and -pentamer were obtained regioselectively from their lower homologues, albeit in rapidly decreasing yields. Reaction of 2 with an organoaluminum thiolate generated from 2-mercaptobenzothiazole and trimethylaluminum followed by acetylation produced 3-O-acetyl-4-[(2-benzothiazolyl)thio]-5,7,3',4'-tetra-O-benzylepicatechin (12). Medium-sized protected oligomers with 4beta,8-interflavan linkages are obtained in improved yields by using this compound as the electrophile and silver tetrafluoroborate as activator and are isolated by reversed-phase HPLC. Their deprotection by ester saponification followed by hydrogenolysis yielded the free procyanidins, which were characterized as their peracetates. The synthetic procyanidins are identical by normal-phase HPLC with fractions isolated from cocoa. The principle of chain extension by two members was demonstrated using a dimeric electrophile obtained by self-condensation of compound 12. Both the synthetic and natural pentamer 32 inhibit the growth of several breast cancer cell lines. Using the MDA MB 231 line, it was established that this outcome is based on the induction of cell cycle arrest in the G0/G1 phase. Subsequent cell death is more likely necrotic rather than apoptotic. Control experiments demonstrate that the polyphenol itself, rather than hydrogen peroxide potentially formed by its autoxidation, is the causative agent.     

Reactions of alkynes with bis(triphenylphosphine)platinum-ethylene: A P-{H} NMR study

Internally consistent assignments of the ³¹P-{¹H} NMR parameters of the complexes [Pt(RCCR′)(PPh₃)₂] are proposed, based on the premise that the magnitude of ¹J(PtP) depends mainly on the nature of the moiety CR trans to P. For a given R, ²J(PP) correlates with ¹J(PtP) for thebond trans to CR. The alkynes PhCCSnEt₃, PhCCSnPh₃, Me₃SiCCCl, Me₃SiCCBr, Et₃SiCCI and MeCCI undergo oxidative addition reactions with [Pt(C₂H₄)(PPh₃)₂]; the intermediate alkyne complex was detected for PhCCSnEt₃, Me₃SiCCCl and Me₃CCBr. The triyne Me(CC)₃Me forms platinum(0) complexes by coordination with the central or terminal CC bond and appears also to give a platinum(II) complex by oxidative addition.