Faculty Bibliography

The total synthesis of the diterpenoids sandresolide B and amphilectolide from a common furan building block is presented. Key steps include palladium-mediated carbonylation, lanthanide catalyzed ring closure, Myers alkylation, intramolecular Friedel-Crafts acylation, photooxygenation, and a Kornblum-DeLaMare rearrangement.

Ansalactam A is a recently isolated ansa macrolide containing a spiro-gamma-lactam functionality that is structurally distinct from other members of the ansa macrolide family. Herein, we describe synthetic studies toward ansalactam A. A route has been developed for the synthesis of a model system featuring a xanthate, which constitutes a direct precursor for the projected radical cyclization step aimed at the formation of the spiro-gamma-lactam moiety of ansalactam A. In this context, a new practical method for the removal of an Evans auxiliary attached to sterically encumbered substrates was developed. The utilization of beta-lactones as an acylation reagent, an alternative to commonly used amide coupling reactions, is described. (C) 2013 Elsevier Ltd. All rights reserved.

gamma-Aminobutyric acid type A receptors (GABAA receptors) are chloride ion channels composed of five subunits, mediating fast synaptic and tonic inhibition in the mammalian brain. These receptors show near five-fold symmetry that is most pronounced in the second trans-membrane domain M2 lining the Cl- ion channel. To take advantage of this inherent symmetry, we screened a variety of aromatic anions with matched symmetry and found an inhibitor, pentacyanocyclopentdienyl anion (PCCP-) that exhibited all characteristics of an open channel blocker. Inhibition was strongly dependent on the membrane potential. Through mutagenesis and covalent modification, we identified the region alpha1V256-alpha1T261 in the rat recombinant GABAA receptor to be important for PCCP- action. Introduction of positive charges into M2 increased the affinity for PCCP- while PCCP- prevented the access of a positively charged molecule into M2. Interestingly, other anion selective cys-loop receptors were also inhibited by PCCP-, among them the Drosophila RDL GABAA receptor carrying an insecticide resistance mutation, suggesting that PCCP- could serve as an insecticide.

Neurological glutamate receptors bind a variety of artificial ligands, both agonistic and antagonistic, in addition to glutamate. Studying their small molecule binding properties increases our understanding of the central nervous system and a variety of associated pathologies. The large, oligomeric multidomain membrane protein contains a large and flexible ligand binding domains which undergoes large conformational changes upon binding different ligands. A recent application of glutamate receptors is their activation or inhibition via photo-switchable ligands, making them key systems in the emerging field of optochemical genetics. In this work, we present a theoretical study on the binding mode and complex stability of a novel photo-switchable ligand, ATA-3, which reversibly binds to glutamate receptors ligand binding domains (LBDs). We propose two possible binding modes for this ligand based on flexible ligand docking calculations and show one of them to be analogues to the binding mode of a similar ligand, 2-BnTetAMPA. In long MD simulations, it was observed that transitions between both binding poses involve breaking and reforming the T686-E402 protein hydrogen bond. Simulating the ligand photo-isomerization process shows that the two possible configurations of the ligand azo-group have markedly different complex stabilities and equilibrium binding modes. A strong but slow protein response is observed after ligand configuration changes. This provides a microscopic foundation for the observed difference in ligand activity upon light-switching.

Incorporation of the azobenzene derivative gluazo, a synthetic photochromic ligand, into a kainate receptor allows for the optical control of neuronal activity. The crystal structure of gluazo bound to a dimeric GluK2 ligand-binding domain reveals one monomer in a closed conformation, occupied by gluazo, and the other in an open conformation, with a bound buffer molecule. The glutamate group of gluazo interacts like the natural glutamate ligand, while its trans-azobenzene moiety protrudes into a tunnel. This elongated cavity presumably cannot accommodate a cis-azobenzene, which explains the reversible activation of the receptor upon photoisomerization.

A novel optoelectroactive system based on an oriented periodic mesoporous organosilica (PMO) film has been developed. A tetra-substituted porphyrin silsesquioxane was designed as a precursor, and the porphyrin macrocycles were covalently incorporated into the organosilica framework without adding additional silica sources, using an evaporation-induced self-assembly process. The synthesized PMO film has a face-centered orthorhombic porous structure with a 15 nm pore diameter. This large pore size enables the inclusion of electron-conducting species such as [6,6]-phenyl C61 butyric acid methyl ester in the periodic mesopores. Optoelectronic measurements on the resulting interpenetrating donor-acceptor systems demonstrate the light-induced charge generation capability and hole-conducting property of the novel porphyrin-based PMO film, indicating the potential of PMO materials as a basis for optoelectroactive systems.

The use of azobenzene photoswitches has become a dependable method for rapid and exact modulation of biological processes and material science systems. The requirement of ultraviolet light for azobenzene isomerization is not ideal for biological systems due to poor tissue penetration and potentially damaging effects. While modified azobenzene cores with a red-shifted cis-to-trans isomerization have been previously described, they have not yet been incorporated into a powerful method to control protein function: the photoswitchable tethered ligand (PTL) approach. We report the synthesis and characterization of a red-shifted PTL, L-MAG0460, for the light-gated ionotropic glutamate receptor LiGluR. In cultured mammalian cells, the LiGluR+L-MAG0460 system is activated rapidly by illumination with 400-520 nm light to generate a large ionic current. The current rapidly turns off in the dark as the PTL relaxes thermally back to the trans configuration. The visible light excitation and single-wavelength behavior considerably simplify use and should improve utilization in tissue.

The undecose nucleoside antibiotics herbicidin C and aureonuclemycin are biologically highly active and represent challenging targets for total synthesis. Herein, the gradual evolution of our synthetic strategy toward these natural products is described in detail. The initial route encompasses metalate addition chemistry but suffers from poor stereochemical control. In contrast, the ultimately successful strategy benefits from a variety of reagent-controlled stereoselective transformations, including a surprisingly facile and highly diastereoselective N-glycosylation process. The presented work also describes new building blocks that might find further application in carbohydrate chemistry.