A cucurbit[6]uril analogue: host properties monitored by fluorescence spectroscopy

This paper describes the host properties of a new cucurbit[6]uril analogue, studied by fluorescence and 1H NMR spectroscopy. This host has an elongated cavity with oval-shaped portals. It is intrinsically fluorescent, and more importantly, this fluorescence is sensitive to guest encapsulation, allowing for the study of the inclusion of nonfluorescent guests by fluorescence spectroscopy. In the case of benzene as guest, significant enhancement of the cucurbit[6]uril analogue host fluorescence was observed upon addition of benzene; this allowed for the determination of the binding constant for 1:1 host-guest complexation, yielding a value of K = 6900 +/- 1100 M(-1). This complexation was also studied by 1H NMR, yielding a similar value of K = 8980 +/- 500 M(-1). The binding of a much larger guest, the dye Nile Red, was also studied, but in this case using guest fluorescence. Significant suppression of the Nile Red fluorescence was observed upon 1:1 complexation with the cucurbit[6]uril analogue, with an extremely large binding constant of 8.2 +/- 0.5 x 10(6) M(-1), indicating a very strong host-guest interaction and an excellent size and shape match. In both cases, binding was much stronger than in the case of the same guests with cucurbit[6]uril itself, and in the case of Nile Red, binding was also much stronger than with modified beta- or gamma-cyclodextrins. This is partly a result of the partial aromatic nature of the host walls, which allow for pi-pi interactions not possible in cucurbiturils or cyclodextrins. The ability to study its inclusion complexes using either host or guest fluorescence, and the very high binding constants observed, illustrates the versatility and potential usefulness of this new host compound.     

Novel C9 and C11 Hydrocarbons from the Brown Alga Cutleria multifida; Sigmatropic and Electrocyclic Reactions in Nature. Part VI

In addition to the known C₁₁H₁₆ hydrocarbons multifidene ( 4 ), aucantene ( 2 ), and ectocarpene ( 5 ), the marine brown alga Cutleria multifida produces trace amounts of the C₉H₁₂ hydrocarbon 7-melhylcycloocta-1,3,5-triene ( 8 ) and its valence tautomer 7-methylbicyclo[4.2.0]octa-2,4-diene, A second novel C₉H₁₂ hydrocarbon is 6-vinyicyclo-hepta-1,4-diene ( 9 ), a lower homologue of ectocarpene ( 5 ). Among the C₁₁H₁₆ hydrocarbons, 7-((1E/Z)-prop-l-enyl)cycloocta-1,4-diene ( 10 / 11 ) is found for the first time. The structure of all new products is confirmed by synthesis and spectroscopic data. The biosynthesis of the new hydrocarbons 8 – 11 is obviously linked to the pathways which lead to the major products giffordene ( 7 ), (6S)-ectocarpene ((6S)- 5 ), and (4R,5R)-aucantene ((4R,5R)- 2 ). Consecutive reactions of certain thermolabile primary products proceed via electrocyclic ring closure, 3,3-sigmatropic rearrangement, or a 1,7-sigmatropic H-shift.     

Efficient and flexible synthesis of chiral gamma- and delta-lactones

An efficient and highly flexible synthesis for chiral gamma- and delta-lactones with high enantiomeric purity is described (>99% ee and 57-87% overall yield). The protocol involves alkylation of chiral 1,2-oxiranes with terminally unsaturated Grignard reagents. Subsequent oxidative degradation (OsO(4)-Oxone) of the terminal double bond from chiral alk-1-en-5-ols and alk-1-en-6-ols affords 4- or 5-hydroxy acids and gamma- and delta-lactones after acidic workup. The flexibility and efficiency of the protocol is illustrated by the synthesis of several alkanolides and alkenolides, hydroxy fatty acids and dihydroisocoumarins.     

Designing stable redox-active surfaces: chemical attachment of an osmium complex to glassy carbon electrodes prefunctionalized by electrochemical reduction of an in situ-generated aryldiazonium cation

The production of stable redox-active layers on electrode surfaces can lead to improvements in electronic device design. Enhanced stability can be achieved by pretreatment of electrode surfaces to provide surface chemical functional groups for covalent tethering of redox complexes. Herein, we describe pretreatment of glassy carbon electrodes to provide surface carboxylic acid groups by electro-reduction of an in situ-generated aryl diazonium salt from 3-(4-aminophenyl)propionic acid. This surface layer is characterized by attenuated total reflection infrared spectroscopy, atomic force microscopy, and electrochemical blocking studies. The surface carboxylic acid generated is then used to tether an osmium complex, [Os(2,2'-bipyridyl)2(4-aminomethylpyridine)Cl]PF6, to provide a covalently bound redox-active monolayer, E(0) ' of 0.29 V (vs Ag/AgCl in phosphate buffer, pH 7.4), on the pretreated glassy carbon electrode. The layer proves stable to pH, temperature, and storage conditions, retaining electroactivity for at least 6 months.     

Negative ion mode matrix-assisted laser desorption/ionisation time-of-flight mass spectrometric analysis of oligosaccharides using halide adducts and 9-aminoacridine matrix

9-Aminoacridine was established as a matrix for the detection of neutral oligosaccharides in negative ion mode matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) mass spectrometry. Sodium iodide proved to be a useful additive inducing formation of stable iodide adducts of the analytes, in particular for oligosaccharides with a degree of polymerisation (DP) of three and higher. Lower oligomers (DP <3) and monosaccharides show more stable adducts with chloride ions. After optimisation of the sample preparation procedure, limits of quantitation were determined for alpha-cyclodextrin and cellopentaose at 7 and 13 pmol, respectively, with a linear detector response over two concentration orders. The iodide additive could be successfully employed on MALDI-TOF mass spectrometers with vacuum and atmospheric pressure ion sources. The value of the new method to solve biological problems has been demonstrated by the analysis of a mixture of beta-glucane elicitors isolated from the cell walls of Phytophthora sojae.     

A conjugate-gradient optimization method invariant to nonlinear scaling

A conjugate-gradient optimization method which is invariant to nonlinear scaling of a quadratic form is introduced. The technique has the property that the search directions generated are identical to those produced by the classical Fletcher-Reeves algorithm applied to the quadratic form. The approach enables certain nonquadratic functions to be minimized in a finite number of steps. Several examples which illustrate the efficacy of the method are included.     

Stereospezifische Synthese cis/trans-disubstituierter Tetrahydrofurane

Multifidene 1 is a chemical messenger for male gametes of the brown alga Cutleria multifida. The synthesis of messenger analogues with a tetrahydrofurane ring will be reported. The general synthetic approach offers the possibility to prepare a great variety of stereospecific 2,3- and 3,4-cis/trans-disubstituted tetrahydrofurans.     

The numerical solution of fredholm integral equations with rapidly varying kernels

Approximating a solution to the Fredholm integral equation ø(x)=α(x) + ∫ ^b_aK(x, y)ø(y) dy by the Nyström method involves some numerical quadrature for approximating the integral, producing a linear system satisfied by approximate function values of ø. This paper discusses the use of generalized product-interpolatory formulas which model ø as one mth-degree polynomial on each subinterval and model K as a (possibly large) sequence of nth-degree polynomials. In cases where K is varying much more rapidly than ø this allows for ø to be sampled much less often than K. Since K is modeled as a sequence of polynomials, its frequent sampling does not require a prohibitive increase in the degree of the interpolating polynomials. Coefficient formulas and examples are given for the (m,n) cases (1,1), (1,2), (2,1) and (2,2).     

Product type quadrature formulas

This paper is concerned with the numerical approximation of integrals of the form _a ^b f(x)g(x)dx by means of a product type quadrature formula. In such a formula the functionf (x) is sampled at a set ofn+1 distinct points and the functiong(x) at a (possibly different) set ofm+1 distinct points. These formulas are a generalization of the classical (regular) numerical integration rules. A number of basic results for such formulas are stated and proved. The concept of a symmetric quadrature formula is defined and the connection between such rules and regular quadrature formulas is discussed. Expressions for the error term are developed. These are applied to a specific example.The work of the first author was supported in part by NIH Grant No. FRO 7129-01 and that of the second author in part by U.S. Army Ballistic Research Laboratories Contract DA-18-001-AMC-876 X.