A short synthesis of the erythrina skeleton and of (+/-)-alpha-lycorane

[reaction: see text] A new nonchain 5-endo radical cyclization starting with xanthates was exploited in a short synthesis of (+/-)-alpha-lycorane and the erythrina ring system.     

Effects of palmitic acid and cholesterol on proton transport across black lipid membranes

We studied the effect of palmitic acid (PA) and cholesterol (approximately 17 wt.%) on proton translocation across asolectin (charged) and diphytanoylphosphatidylcholine (DPhPC, neutral) black lipid membranes (BLMs). Potential difference (PD), short circuit current (SCC), and conductance (G(total)) were measured with a digital electrometer. Membranes were exposed to pH gradients (0.4-2.0 units), followed by PA addition to bath (symmetrically, 40-65 microM). The membrane conductive pathway was subdivided into an unspecific and a proton-related routes. A computer program estimated the conductances (G(un) and G(H)) of the two pathways from the measured parameters. No significant differences in proton selectivity were found between DPhPC membranes and DPhPC/cholesterol membranes. By contrast, cholesterol incorporation into asolectin increases membranes selectivity to proton. Cholesterol dramatically reduced G(un) reflecting, probably, its ability of inducing order in lipid chains. In asolectin membranes, PA increases proton selectivity, probably by acting as a proton shuttle according to the model proposed by Kamp and et al. [Biochemistry 34 (1995) 11928]. Cholesterol incorporation into asolectin membranes eliminates the PA-induced increase in proton selectivity. In DPhPC and DPhPC/cholesterol membranes, PA does not affect proton selectivity. These results are discussed in terms of the presence of cardiolipin (CL) in asolectin, cholesterol/PA interactions, and cholesterol order-inducing effects on acyl-chains.     

Binuclear Cu2+ complex mediated discrimination between L-glutamate and L-aspartate in water

L-glutamate and L-aspartate selectivity is achieved by the action of two Cu2+ metal ions rightly disposed in a cyclophane-type macrocyclic framework; electrochemical sensing of glutamate has been achieved by adsorption of the copper complexes on graphite electrodes.     

Lewis acid catalysis in heterolysis reactions of glycol ether radicals mimicking diol dehydratase-catalyzed reactions

Zinc bromide-catalyzed heterolysis reactions of glycol ether radicals were studied by laser flash photolysis methods, which gave the binding constants and catalytic rate constants for fragmentation. The Lewis acid-catalyzed heterolysis reactions mimic a putative reaction pathway in diol dehydratase-catalyzed reactions and are potentially useful polar processes for incorporation into conventional radical chain reaction sequences. [reaction: see text]     

Stepwise cycloreversion of oxetane radical cations with initial C-O bond cleavage

2,4,6-Triaryl(thia)pyrylium salts have been used as electron-transfer photosensitizers for the cycloreversion of the oxetane ring system. The radical cation of 2,3-diphenyl-4-hydroxymethyloxetane (1) undergoes stepwise splitting via initial O-C2 cleavage. Spin and charge in the resulting intermediate are located in the oxygen and carbon atoms, respectively. Subsequent intramolecular nucleophilic attack produces 2,3-diphenyl-4-hydroxytetrahydrofuran (4a). Formation of this product occurs in the submicrosecond time scale, competing with C3-C4 cleavage to the detectable (lambdamax = 470 nm) trans-stilbene radical cation.     

Reductive PET cycloreversion of oxetanes: singlet multiplicity, regioselectivity, and detection of olefin radical anion

Cycloreversion of 2-(p-cyanophenyl)-4-methyl-3-phenyloxetane (1) is achieved using 1-methoxynaphthalene (2) as electron-transfer photosensitizer. The experimental results are consistent with the reaction taking place from the singlet excited state of the sensitizer. Ring splitting of the radical anion 1*- occurs with cleavage of O-C2 and C3-C4 bonds, leading to products (acetaldehyde and p-cyanostilbene) different from the reagents used in the Paterno-Büchi synthesis of 1. The olefin radical anion involved in the electron-transfer process has been detected by means of laser flash photolysis.     

Characterization of a peptide affinity support that binds selectively to staphylococcal enterotoxin B

The influences of mass transfer and adsorption-desorption kinetics on the binding of staphylococcal enterotoxin B (SEB) to an affinity resin with the peptide ligand, Tyr-Tyr-Trp-Leu-His-His (YYWLHH) have been studied. The bed and particle porosities, the axial dispersion coefficient and the pore diffusivity were measured using pulse experiments under unretained conditions. Adsorption isotherms for SEB on YYWLHH resins with peptide densities in the range from 6 to 220 micromol/g were measured and fitted to a bi-Langmuir equation. At peptide densities below 9 micromol/g and above 50 micromol/g, dissociation constants were lower (2 x 10(-3) to 7 x 10(-3) mol/m3), and binding capacities were larger (43-47 mg SEB/g). In the range from 9 to 50 micromol/g dissociation constants were larger (13 x 10(-3) to 24 x 10(-3) mol/m3) and capacities were lower (33-37 mg SEB/g). These observations are consistent with a transition from single point attachment of the protein to the ligand at low peptide densities to multipoint attachment at high peptide densities. The general rate (GR) model of chromatography was used to fit experimental breakthrough curves under retained conditions to determine the intrinsic rate constants for adsorption, which varied from 0.13 to 0.50 m3 mol(-1) s(-1), and exhibited no clear trend with increasing peptide density. An analysis of the number of transfer units for the various mass transfer steps in the column indicated that film mass transfer, pore diffusion (POR) and the kinetics of adsorption can all play an important role in the overall rate of adsorption, with the intrinsic adsorption step apparently being the rate determining step at peptide densities below 50 micromol/g.     

Carboxylate-substituted radicals from phenylselenide derivatives. Designs on models for coenzyme B12-dependent enzyme-catalyzed rearrangements

Laser flash photolysis (266 nm) of alpha- and beta-phenylselenyl esters, carboxylic acids, and carboxylates in aqueous acetonitrile media gave the corresponding radicals by homolytic cleavage of the phenylselenyl groups. In the beta-substituted systems, acid and carboxylate radicals reacted in intramolecular reporter reactions with approximately equal rate constants. For the alpha-substituted systems, an ester- and carboxylic acid-substituted radical reacted in an intramolecular reporter reaction with the same rate constants, but the analogous alpha-carboxylate radical, a radical anion, reacted an order of magnitude less rapidly and with an activation energy that is 3 kcal/mol greater than that found for analogues. A kinetic titration of the equilibrating alpha-acid and alpha-carboxylate radicals gave pKa = 4.6. The results indicate that alpha-ester and alpha-carboxylic acid radicals are unlikely to be appropriate models for alpha-carboxylate radicals, the intermediates formed in a large subset of coenzyme B12-dependent enzyme-catalyzed reactions.     

SPOCC-194, a new high functional group density PEG-based resin for solid-phase organic synthesis

A novel polymer matrix for solid-phase synthesis, SPOCC(194) resin (1), was designed featuring a backbone of homogeneous tetraethylene glycol (TEG(194)) macromonomer linked by quaternary carbon junctions and terminating in primary alcohol functionality. Beaded SPOCC(194) resin was effectively prepared by suspension polymerization of oxetanylated TEG macromonomer 5 in stirred silicon oil. Mechanically stable and inert to a diverse range of reaction conditions, SPOCC(194) possessed a high hydroxyl group loading (0.9-1.2 mmol/g) for substrate attachment and swelled effectively ( approximately 2-4 mL/g) in a variety of organic and aqueous solvents. Developed for solid-phase synthesis at high reactant concentrations for driving organic and aqueous reactions to completion, SPOCC(194) exhibited high functional group density (mmol/mL) similar to that of low-loaded aminomethylated polystyrene-divinylbenzene copolymer (PS-1%DVB) yet significantly higher than that of PEGA(1900), SPOCC(1500), and TentaGel S. High-resolution MAS NMR spectra of Fmoc-derivatized SPOCC(194) indicate that monitoring of functional group transformation is possible. Moreover, by employment of a nonaromatic resin-linker combination, electrophilic chemistry, such as Lewis acid catalyzed glycosylation and Friedel-Crafts acylation, was selectively performed on substrate bound to SPOCC(194) resin. Such properties make SPOCC(194) resin a promising new polymer matrix for the support-bound construction of small organic molecules by parallel and combinatorial synthesis and the scavenging of solution-phase reactants or byproducts.