Self-assembly of multinuclear complexes with enantiomerically pure chiral binaphthoxy imine ligands: effect of the alkyl spacer connecting two binaphthyl units on the metal binding

Metal complexing behavior of enantiomerically pure alpha,omega-diiminoalkanes possessing the two terminal binaphthyl units (L1 and L2) was studied. The ligands L1 and L2 were prepared by the reaction of optically pure 2'-butoxy-3-formyl-2-hydroxy-1,1'-binaphthyl with propane and pentane diamines. Reactions of L1 and L2 with equimolar amount of Cu(OAc)2 afforded quantitatively multinuclear complexes 1. The structure of 1 was confirmed by MALDI-TOF MS spectroscopy, X-ray single-crystal-structure analysis, and UV/vis and CD spectroscopic analyses. The reaction of L1 having a 1,3-propanediyl spacer resulted in the formation of a self-assembled product, which was assigned as enantiopure trinuclear circular helicate 1a, while the ligand having a 1,5-pentanediyl spacer L2 gave a different self-assembled product, dinuclear side-by-side complex 1b. The circular dichroism (CD) spectrum of 1a in solution showed intense Cotton effects in both the pi-pi* transition of the naphthalene units and the LMCT region of the N,O-chelate moieties. The CD spectrum of 1b was completely different from that of 1a; in particular the Cotton effects in the LMCT region were very weak, contrary to those of 1a. These results suggest that 1a retains some chirality induced on the N,O-chelating moieties even in solution, while the induced chirality on the N,O-chelating moieties in 1b is not very significant, being consistent with the consequences of the X-ray single-crystal-structure studies.     

Insect peptides with improved protease-resistance protect mice against bacterial infection

At a time of the emergence of drug-resistant bacterial strains, the development of antimicrobial compounds with novel mechanisms of action is of considerable interest. Perhaps the most promising among these is a family of antibacterial peptides originally isolated from insects. These were shown to act in a stereospecific manner on an as-yet unidentified target bacterial protein. One of these peptides, drosocin, is inactive in vivo due to the rapid decomposition in mammalian sera. However, another family member, pyrrhocoricin, is significantly more stable, has increased in vitro efficacy against gram-negative bacterial strains, and if administered alone, as we show here, is devoid of in vitro or in vivo toxicity. At low doses, pyrrhocoricin protected mice against Escherichia coli infection, but at a higher dose augmented the infection of compromised animals. Analogs of pyrrhocoricin were, therefore, synthesized to further improve protease resistance and reduce toxicity. A linear derivative containing unnatural amino acids at both termini showed high potency and lack of toxicity in vivo and an expanded cyclic analog displayed broad activity spectrum in vitro. The bioactive conformation of native pyrrhocoricin was determined by nuclear magnetic resonance spectroscopy, and similar to drosocin, reverse turns were identified as pharmacologically important elements at the termini, bridged by an extended peptide domain. Knowledge of the primary and secondary structural requirements for in vivo activity of these peptides allows the design of novel antibacterial drug leads.     

Living enantiosymmetric and enantioasymmetric polymerization of methylthiirane in homogeneous phase

The polymerization of racemic methylthiirane in homogeneous phase, initiated by bis(isopropyl-S-cysteinato) cadmium is a living process. The resulting polymers are isotactic and optically active at partial conversion. The optical purity of the residual monomer may reach 27% at half conversion. The propagation occurs mainly on one valency of Cd, however oligomers grow slowly on the second valency. The stereoregularity of the polymer chain appears only when the length of the oligomer becomes high enough, making possible a bicoordination of the Cd counterions. The stereoregularity of the polymer is characterized by the molar fraction σ of isotactic diads which varies from 0.5 for atactic chains—formed at the beginning—to about one for isotactic segments formed for longer chains. The stereospecifictity also depends on temperature of propagation and on initiator concentration. The kinetics observed (zero order in monomer and one-half in Cd) are explained by monomer coordination before insertion and dimeric association of the thiolate end groups. The enantioasymmetric process observed results from an unbalance in the number of the two different types of active sites and possibly from a difference in their reactivities. Enantioasymmetry has been found to decrease significantly when the dielectric constant ε of the medium increases.     

Preparation and characterization of well defined d-erythro sphingomyelins

A simple semisynthetic procedure for the preparation of various d-erythro sphingomyelins (SPMs), differing in their acyl chains, is described. They were prepared by one-step condensation of the desired free fatty acid with sphingosyl phosphorylcholine (SPC) using dicyclohexylcarbodiimide. The d-erythro SPMs were obtained in high purity, high yields and resemble bovine brain SPM in their chromatographic behavior, infrared, circular dichroism (CD) and proton NMR (PMR) spectra as well as in their rate of hydrolysis by Staphylococcus aureus sphingomyelinase. Multilamellar vesicles can be prepared from the semisynthetic SPMs. Their thermotropic behavior is dependent mainly on the acyl chain though it is also affected by the heterogeneity of the sphingosine base composition. Intact sealed small unilamellar vesicles (SUV) cannot be prepared from a single semisynthetic saturated SPM but can be prepared from their mixtures. This acylation procedure can also be applied for preparing simple neutral glycosphingolipids. The sphingolipids prepared by this method can be used to study metabolism, enzymology and physicochemical properties of d-erythro well defined simple sphingolipids.     

Helix-sense-selective polymerization of diphenyl-2-pyridylmethyl methacrylate with chiral anionic initiators

Polymerization of diphenyl-2-pyridylmethyl methacrylate was carried out with the complexes of organolithium compounds with 22 chiral ligands. Helix-sense-selectivity of the polymerization was largely affected by a slight structural difference of chiral ligands. (+)-(S)-2-(1-Pyrrolidinylmethyl)pyrrolidine was the most effective ligand in producing a one-handed helical polymer with narrow molecular weight distribution.     

Stereospecific determination of the in vitro dissolution of modified release formulations of (±)-verapamil

The determination of the in vitro dissolution profiles of three different modified-release formulations of (±)-verapamil were determined and compared to a reference (Calan SR). The determination of (±)-verapamil utilized a microwave-facilitated derivatization reaction with an enantiomerically pure reagent followed by measurement of drug by reversed-phase high-performance liquid chromatography (HPLC). The dissolution profile of each of the four modified-release formulations was followed for the first hour in simulated gastric fluid (pH 1.2) and then in simulated intestinal fluid (pH 7.5) for up to a total of 12 h. The resulting dissolution profiles of each modified-release formulation suggested that only one of the three test formulations had a similar dissolution to the Calan SR reference. Interestingly, the (S:R)-enantiomer ratios of two of the test products were significantly different from unity, and the third test product was also significantly different from unity if 4 out of 5 outliers were omitted. It is suggested that dissolution testing of modified-release formulations containing chiral active ingredients must be stereospecific in order to discern whether a drug-excipient interaction occurs. Additionally, it may be suggested that dissolution techniques relying more upon diffusion of medium through the tablet matrix, rather than erosion of the tablet, may accentuate enantiomeric differences in release rates. © 1993 Wiley-Liss, Inc.