Total Synthesis of Bryostatin 1: A Short Route

A chemical legacy: Keck and his team have chemically pursued the bryostatin 1 structure with great vigor in recent years and, in late 2010, they finally completed their quest of developing a short and efficient total synthesis of this complex natural product (see structure). The present Highlight provides a brief but nevertheless detailed overview of the Keck synthesis and its chemical legacy in terms of new reactions.

From imagery to intention: A dual route model of imagined contact effects

Imagined intergroup contact (Crisp & R. Turner, 2009) is a new indirect contact strategy for promoting tolerance and more positive intergroup relations. In this chapter, we review existing research on imagined contact and propose two routes-cognitive and affective-through which it can exert a positive influence on contact-related attitudes and intentions. We first review research that has established the beneficial impacts of imagined contact on intergroup attitudes via reduced intergroup anxiety, supporting its efficacy as an intervention where there exists little or no opportunity for direct contact. We then review more recent research showing that imagined contact not only improves attitudes, but can also enhance intentions to engage in future contact. These studies suggest that contact imagery provides a behavioural script that forms the cognitive basis for subsequent judgements about future contact intentions. Collectively, the findings from this research programme support the idea that imagined contact can complement more direct forms of contact-providing a way of initially encouraging an interest in engaging positively with outgroups before introducing face-to-face encounters. We discuss the implications of these findings for future theory and research, and how they can inform prejudice-reduction interventions seeking to capitalise on the beneficial effects of mental imagery.

Degradation of 4-fluorobiphenyl by mycorrhizal fungi as determined by (19)F nuclear magnetic resonance spectroscopy and (14)C radiolabelling analysis

The pathways of biotransformation of 4-fluorobiphenyl (4FBP) by the ectomycorrhizal fungus Tylospora fibrilosa and several other mycorrhizal fungi were investigated by using (19)F nuclear magnetic resonance (NMR) spectroscopy in combination with (14)C radioisotope-detected high-performance liquid chromatography ((14)C-HPLC). Under the conditions used in this study T. fibrillosa and some other species degraded 4FBP. (14)C-HPLC profiles indicated that there were four major biotransformation products, whereas (19)F NMR showed that there were six major fluorine-containing products. We confirmed that 4-fluorobiphen-4'-ol and 4-fluorobiphen-3'-ol were two of the major products formed, but no other products were conclusively identified. There was no evidence for the expected biotransformation pathway (namely, meta cleavage of the less halogenated ring), as none of the expected products of this route were found. To the best of our knowledge, this is the first report describing intermediates formed during mycorrhizal degradation of halogenated biphenyls.

A New Stereocontrolled Synthetic Route to (-)-Echinosporin from D-Glucose via Padwa Allenylsulfone [3+2]-Anionic Cycloadditive Elimination

A new formal total synthesis of (-)-echinosporin has been developed based upon the Padwa [3 + 2]-cycloadditive elimination reaction of allenylsulfone 4 with the D-glucose-derived enone 14 which provides cycloadduct 12.

Exchange potential from the common energy denominator approximation for the Kohn-Sham Green's function: Application to (hyper)polarizabilities of molecular chains

An approximate Kohn-Sham (KS) exchange potential v(xsigma)(CEDA) is developed, based on the common energy denominator approximation (CEDA) for the static orbital Green's function, which preserves the essential structure of the density response function. v(xsigma)(CEDA) is an explicit functional of the occupied KS orbitals, which has the Slater v(Ssigma) and response v(respsigma)(CEDA) potentials as its components. The latter exhibits the characteristic step structure with "diagonal" contributions from the orbital densities \psi(isigma)\(2), as well as "off-diagonal" ones from the occupied-occupied orbital products psi(isigma)psi(j(not equal1)sigma). Comparison of the results of atomic and molecular ground-state CEDA calculations with those of the Krieger-Li-Iafrate (KLI), exact exchange (EXX), and Hartree-Fock (HF) methods show, that both KLI and CEDA potentials can be considered as very good analytical "closure approximations" to the exact KS exchange potential. The total CEDA and KLI energies nearly coincide with the EXX ones and the corresponding orbital energies epsilon(isigma) are rather close to each other for the light atoms and small molecules considered. The CEDA, KLI, EXX-epsilon(isigma) values provide the qualitatively correct order of ionizations and they give an estimate of VIPs comparable to that of the HF Koopmans' theorem. However, the additional off-diagonal orbital structure of v(xsigma)(CEDA) appears to be essential for the calculated response properties of molecular chains. KLI already considerably improves the calculated (hyper)polarizabilities of the prototype hydrogen chains H-n over local density approximation (LDA) and standard generalized gradient approximations (GGAs), while the CEDA results are definitely an improvement over the KLI ones. The reasons of this success are the specific orbital structures of the CEDA and KLI response potentials, which produce in an external field an ultranonlocal field-counteracting exchange potential. (C) 2002 American Institute of Physics.

Novel synthetic route to the C-nucleoside, 2-deoxy benzamide riboside

2-Deoxy-C-nucleosides are a subcategory of C-nucleosides that has not been explored extensively, largely because the synthesis is less facile. Flexible synthetic procedures giving access to 2-deoxy-C-nucleosides are therefore of interest. To exemplify the versatility and highlight the limitations of a synthetic route recently developed to that effect, the first synthesis of 2-deoxy benzamide riboside is reported. Biological properties of this novel C-nucleoside are also discussed. (c) 2012 Elsevier Ltd. All rights reserved.