Application of high-field NMR and cryogenic probe technologies in the structural elucidation of poecillastrin a,a new antitumor macrolide lactam from the sponge poecillastra species

Poecillastrin A (1), a new polyketide-derived macrolide lactam, was isolated from a deep-water collection of the marine sponge Poecillastra species. The structure of poecillastrin A (1) was assigned using NMR data acquired at 500 MHz with an inverse-detection cryogenic probe and at 800 MHz with a room-temperature probe.     

Near-IR luminescence of monolayer-protected metal clusters

Visible-near-IR luminescence spectra of gold MPCs that are similar, irrespective of the number of core atoms (all <2 nm diameter) and different monolayers, are reported. The luminescence can be quantitatively invoked by introducing polar ligands into nonpolar MPC monolayers and by galvanic exchange of metal atoms on the MPC core surface with different metals. The observed emissions are believed to result from surface-localized states that depend on both the core metal of the nanoparticle and the ligands attached to the metal surface.     

Morphology and oxygen sensor response of luminescent Ir-labeled poly(dimethylsiloxane)/polystyrene polymer blend films

Polymer films consisting of a linear poly(dimethylsiloxane) end-functionalized with a luminescent Ir(III) complex (Ir-PDMS), blended with polystyrene (PS), function as optical oxygen sensors. The sensor response arises by quenching of the luminescence from the Ir(III) chromophore by oxygen that permeates into the polymer film. The morphology and luminescence oxygen sensor properties of blend films consisting of Ir-PDMS and PS have been characterized by fluorescence microscopy, atomic force microscopy, and scanning electron microscopy. The investigations demonstrate that microscale phase segregation occurs in the films. In blends that contain a relatively small amount of Ir-PDMS in PS (ca. 10 wt %), the Ir-PDMS exists as circular domains, with diameters ranging from 2 to 5 mum, surrounded by the majority PS phase. For larger weight fractions of Ir-PDMS in the blends, the film morphology becomes bicontinuous. A novel epifluorescence microscopy method is applied that allows the construction of Stern-Volmer quenching images that quantify the oxygen sensor response of the blend films with micrometer spatial resolution. These images provide a map of the oxygen permeability of the polymer blend films with a spatial resolution of ca. 1 mum. The results of this investigation show that the micrometer-sized Ir-PMDS domains display a 2-3-fold higher oxygen sensor response compared to the surrounding PS matrix. This result is consistent with the fact that PDMS is considerably more gas permeable compared to PS. The relationship of the microscale morphology of the blends to their performance as macroscale optical oxygen sensors is discussed.     

Chrysomelidial by chromyl chloride oxidation: A revised structure for gastrolactone

Non-conjugated $\text{gem}$-dialkylated alkenes were oxidized to aldehydes in the presence of α,β-unsaturated carbonyl functional groups, providing a new synthesis of $\text{1}$ and a synthesis of $\text{2}$ that led to a revised structure for gastrolactone.     

Entropy-driven hydrogen bonding: stereodynamics of a protonated, N,N-chiral "proton sponge"

The C2-symmetric ("[DL]") and achiral ("[meso]") diastereoisomers of the hydrogen iodide salt of 1,8-bis-(N-benzyl-N-methylamino)naphthalene ([2H]-[I] ) interconvert in solution. Direct interconversion of the diastereoisomers of [2H]+ must involve hydrogen bond fission (to give "[nonHB-2H+]") and rotation-inversion of the non-protonated nitrogen centre. The global activation parameters (deltaH++ and deltaS++) for diastereoisomer interconversion in [D7]DMF have been determined from rate data obtained by temperature-drop and magnetisation-transfer 13C NMR spectroscopy over a temperature range of 170 degrees C. The process is found to have a high entropy of activation in both directions (deltaS++=163(+/-4) and 169(+/-4) JK(-1)mol(-1)) and this is suggested to arise through hydrogen bonding of the ammonium centre in [nonHB-2H+] with the solvent ([D7]DMF). Comparison of the enthalpy of activation (deltaH++) with that earlier found for diastereoisomer interconversion of the free-base form 2 suggests that the intramolecular hydrogen bond in [2H]+ is roughly equal in enthalpic strength (deltaH) with that made with the solvent ([D7]DMF) in the non-hydrogen-bonded intermediate [nonHB-2H+]. As such, the hydrogen bonding in [2H]+ may be considered as predominantly an entropically driven process, without any unusual enthalpic strength.     

Anti-HIV cyclotides

The cyclotides are a recently discovered, structurally unique family of bioactive plant peptides. Their discovery spawned a series of structural analyses, synthetic efforts, and studies to define the biosynthesis and biological properties of these novel peptide metabolites. Cyclotides have a head-to-tail cyclized amino acid backbone and a conserved cystine knot motif that provides an extremely stable structural framework. They all share a common global fold and are highly resistant to denaturation and to cleavage by proteolytic enzymes. However, these macrocyclic peptides are quite permissive to amino acid substitutions or additions in several peripheral loop regions, since changes in these loops do not alter the core cyclotide structure. These features make cyclotides attractive templates for incorporating desired amino acid sequences and then delivering these peptide sequences in a well defined, highly stable framework. Cyclotides likely function in a defensive role in the source plants since they exhibit a broad spectrum of antimicrobial activity and are detrimental to the growth and survival of herbivorous insects. Cyclotides are gene-encoded polypeptides that are cleaved from larger precursor proteins and then cyclized. This review summarizes research done on a subset of cyclotides that were discovered due to their HIV inhibitory properties. It details the isolation and characterization of these compounds and describes this work in the context of our current state of knowledge of the entire cyclotide family.     

The acid catalysed rearrangement of a diterpenoid epoxide

A study has been made of the principal rearrangement products resulting from formic acid treatment of 19-hydroxy-ent-beyerene epoxide (3). In concentrated solutions 3 has been found to undergo a deep seated rearrangement to the allylic alcohol (14). A mechanism for the formation of 14 is proposed involving a novel 1,4-hydride shift in the bicyclo[3:2:1]octane C/D ring system following cleavage of the C₁₅O bond. Supporting evidence has been obtained from a study of the specifically labelled epoxide (4), the deuterium in 14 appearing exclusively at C₁₂. Four products (7 and 11–13) emanating from the known beyerane → kaurane interconversion have been identified.     

Photochemistry and photophysics of Cr(III) macrocyclic complexes

The phosphorescence and photochemical behavior of the macrocyclic complexes (1,4,7,10,13,16-hexaazacyclooctadecane)chromium(III) (Cr([18]-aneN(6))(3+); 1) and (4,4',4'-ethylidynetris(3-azabutan-1-amine)) chromium(III) (Cr(sen)(3+); 2) have been compared to each other and to the complex Cr(en)(3)(3+). For both macrocyclic complexes, phosphorescence from room temperature aqueous solutions is too weak to be observed, contrasting with Cr(en)(3)(3+), though both had somewhat longer 77 K lifetimes than Cr(en)(3)(3+). Phosphorescence lifetimes for these macrocyclics decreased with increasing temperature much faster than for Cr(en)(3)(3+) and a conventional extrapolation based on a fit of reciprocal lifetimes (corrected for the low-temperature contribution) to the Arrhenius equation gave estimated room temperature phosphorescence lifetimes of a few nanoseconds, consistent with the failure to observe room temperature emission. Fitting of the nonlinearity of the data seen in these plots suggested that two high-temperature processes were occurring with estimated activation parameters (E in kJ mol(-1) and A in s(-1)) for Cr([18]-aneN(6))(3+): E(1) = 40, A(1) = 1 x 10(16); E(2) = 24, A(2) = 1 x 10(14): Cr(sen)(3+); E(1) = 45, A(1) = 2 x 10(15); E(2) = 29, A(2) = 7 x 10(11). Cr([18]-aneN(6))(3+) was photochemically inert on irradiation. On irradiation into the lowest quartet ligand field absorption band, Cr(sen)(3+) photolyzes with a quantum yield of 0.098 +/- 0.001 at room temperature. Laser flash photolysis with conductivity detection showed that this photoreaction occurred faster than protonation of the liberated amine ligand at all practical proton concentrations. The quantum yield for irradiation directly into the doublet absorption band of Cr(sen)(3+) was 0.077 +/- 0.003. Photoaquation of Delta-Cr(sen)(3+) led to loss of optical activity and product analysis by capillary electrophoresis showed that both racemic and Delta-Cr(sen)(3+) photoaquate to a single main product, trans-Cr(sen-NH)(H(2)O)(4+). The product stereochemistry is shown to be consistent with predictions based on the angular overlap model for Cr(III) photochemistry, recognizing the additional constraints imposed by the ligand. The abnormally short room temperature solution lifetime of the doublet state is a result of a radiation-less process that competes with other processes depleting the doublet state. However, this doublet-state deactivation process does not lead to photoaquation but competes with BISC and photoaquation via the quartet state, resulting in an unprecedented reduction in photoaquation quantum yield on direct irradiation into the doublet state.     

Temperature and humidity aging of poly(p-phenylene-2,6-benzobisoxazole) fibers: Chemical and physical characterization

In recent years, poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers have become prominent in high strength applications such as body armor, ropes and cables, and recreational equipment. The objectives of this study were to expose woven PBO body armor panels to elevated temperature and moisture, and to analyze the chemical, morphological and mechanical changes in PBO yarns extracted from the panels. A 30% decrease in yarn tensile strength, which was correlated to changes in the infrared peak absorbance of key functional groups in the PBO structure, was observed during the 26 week elevated temperature/elevated moisture aging period. Substantial changes in chemical structure were observed via infrared spectroscopy, as well as changes in polymer morphology using microscopy and neutron scattering. When the panels were removed to an ultra-dry environment for storage for 47 weeks, no further decreases in tensile strength degradation were observed. In a follow-on study, fibers were sealed in argon-filled glass tubes and exposed to elevated temperature; less than a 4% decrease in tensile strength was observed after 30 weeks, demonstrating that moisture is a key factor in the degradation of these fibers.     

Cross-metathesis assisted by microwave irradiation

Microwave irradiation effectively accelerates cross-coupling metathesis reactions between deactivated olefins. Reactions have been carried out with the phosphine-free Hoveyda-Grubbs catalyst and the "second generation Grubbs' catalyst." While there have been reports that a "microwave effect" is observed in various transformations, the accelerations we observe are due to the efficient and rapid heating and increased pressure in the microwave apparatus.