Philip Garner

Aziridine-mediated ligation and site-specific modification of unprotected peptides.

A synthesis of aziridine-containing peptides via the Cu(II)-promoted coupling of unprotected peptide thioacids and N-H aziridine-2-carbonyl peptides is reported. The unique reactivity of the resulting N-acylated aziridine-2-carbonyl peptides facilitates their subsequent regioselective and stereoselective nucleophilic ring-opening to give unprotected peptides that are specifically modified at the ligation site. The aziridine-mediated peptide ligation concept is exemplified using H(2)O as the nucleophile, producing a Xaa-Thr linkage (where Xaa can be an epimerizable and hindered amino acid). The overall process is compatible with a variety of unprotected amino acid functionality, most notably the N-terminal and Lys side chain amines.

Analogs of Tetrahydroisoquinoline Natural Products That Inhibit Cell Migration and Target Galectin-3 Outside of Its Carbohydrate-binding Site

Cell migration is central to a number of normal and disease processes. Small organic molecules that inhibit cell migration have potential as both research probes and therapeutic agents. We have identified two tetrahydroisoquinoline natural product analogs with antimigratory activities on Madin-Darby canine kidney epithelial cells: a semisynthetic derivative of quinocarmycin (also known as quinocarcin), DX-52-1, and a more complex synthetic molecule, HUK-921, related to the naphthyridinomycin family. It has been assumed that the cellular effects of reactive tetrahydroisoquinolines result from the alkylation of DNA. We have reported previously that the primary target of DX-52-1 relevant to cell migration appears to be the membrane-cytoskeleton linker protein radixin. Here we extend the analysis of the protein targets of DX-52-1, reporting that the multifunctional carbohydrate-binding protein galectin-3 is a secondary target of DX-52-1 that may also be relevant to the antimigratory effects of both DX-52-1 and HUK-921. All known inhibitors of galectin-3 target its β-galactoside-binding site in the carbohydrate recognition domain. However, we found that DX-52-1 and HUK-921 bind galectin-3 outside of its β-galactoside-binding site. Intriguingly HUK-921, although a less potent inhibitor of cell migration than DX-52-1, had far greater selectivity for galectin-3 over radixin, exhibiting little binding to radixin, both in vitro and in cells. Overexpression of galectin-3 in cells led to a dramatic increase in cell adhesion on different extracellular matrix substrata as well as changes in cell-cell adhesion and cell motility. Galectin-3-overexpressing cells had greatly reduced sensitivity to DX-52-1 and HUK-921, and these compounds caused a change in localization of the overexpressed galectin-3 and reversion of the cells to a more normal morphology. The converse manipulation, RNA interference-based silencing of galectin-3 expression, resulted in reduced cell-matrix adhesion and cell migration. In aggregate, the data suggest that DX-52-1 and HUK-921 inhibit a carbohydrate binding-independent function of galectin-3 that is involved in cell migration.

Rapid Formation of N-Glycopeptides via Cu(II)-Promoted Glycosylative Ligation

Herein is described the chemoselective Cu(II)-HOBt promoted chemical ligation of glycosylamines and peptide thioacids to give N-glycosylated peptides. The method is distinguished from other chemical approaches to peptide N-glycosylation in that (1) it can be employed in the presence of unprotected N-terminal and Lys side chain amines; (2) it is remarkably fast, going to completion in under 30 min; and (3) it produces glycopeptides without attendant aspartimide formation.

Asymmetric [C + NC + CC] Coupling Entry to the Naphthyridinomycin Natural Product Family: Formal Total Synthesis of Cyanocycline A and Bioxalomycin β2

A full account of our [C + NC + CC] coupling approach to the naphthyridinomycin family of natural products is presented, culminating in formal total syntheses of cyanocycline A and bioxalomycin β2. The key complexity-building reaction in the synthesis involves the Ag(I)-catalyzed endo-selective [C + NC + CC] coupling of aldehyde 7, (S)-glycyl sultam 8, and methyl acrylate (9) to provide the highly functionalized pyrrolidine 6, which was carried forward to an advanced intermediate (compound 33) in Fukuyamas synthesis of cyanocycline A. Since cyanocycline A has been converted to bioxalomycin β2, this constitutes a formal synthesis of the latter natural product as well. The multicomponent reaction-based strategy reduces the number of steps previously needed to assemble these complex molecular targets by one-third. This work highlights the utility of the asymmetric [C + NC + CC] coupling reaction in the context of a complex pyrrolidine-containing target and provides an illustrative guide for its application to other synthesis problems. The synthesis also fueled collaborative biological and biochemical research that identified a unique small molecule inhibitor of cell migration (compound 30).

[C+NC+CC] Coupling-Enabled Synthesis of Influenza Neuramidase Inhibitor A-315675

An efficient synthesis of the neuramidase inhibitor A-315675 is reported. The fully functionalized pyrrolidine core of the target is assembled in one pot via an exo-selective asymmetric [C+NC+CC] coupling reaction.

Diastereoselection in an aqueous Diels–Alder reaction: a formal total synthesis of the Inhoffen–Lythgoe diol

A formal synthesis of the Inhoffen–Lythgoe diol (1) featuring a novel intermolecular Diels–Alder strategy wherein an intact C(20) stereocentre as part of a diene unit is used to elaborate directly the stereocentres at C(13) and C(17) of the hydrindan ring system of (1), is reported.

The Structure of (−)-Kaitocephalin Bound to the Ligand Binding Domain of the (S)-α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA)/Glutamate Receptor, GluA2

Background: The natural product (−)-kaitocephalin is a potential scaffold for development of subtype-specific glutamate receptor antagonists. Results: The crystal structure of (−)-kaitocephalin bound to an AMPA receptor ligand binding domain was determined. Conclusion: The orientation of (−)-kaitocephalin in the binding site can explain the subtype selectivity of the parent compound. Significance: Comparisons with the structure of other glutamate receptors provides avenues for development of new subtype-specific antagonists. Glutamate receptors mediate the majority of excitatory synaptic transmission in the central nervous system, and excessive stimulation of these receptors is involved in a variety of neurological disorders and neuronal damage from stroke. The development of new subtype-specific antagonists would be of considerable therapeutic interest. Natural products can provide important new lead compounds for drug discovery. The only natural product known to inhibit glutamate receptors competitively is (−)-kaitocephalin, which was isolated from the fungus Eupenicillium shearii and found to protect CNS neurons from excitotoxicity. Previous work has shown that it is a potent antagonist of some subtypes of glutamate receptors (AMPA and NMDA, but not kainate). The structure of kaitocephalin bound to the ligand binding domain of the AMPA receptor subtype, GluA2, is reported here. The structure suggests how kaitocephalin can be used as a scaffold to develop more selective and high affinity antagonists for glutamate receptors.

Site-Specific Incorporation of a Thioester Containing Amino Acid into Proteins

Here, we report the site-specific incorporation of a thioester containing noncanonical amino acid (ncAA) into recombinantly expressed proteins. Specifically, we genetically encoded a thioester-activated aspartic acid (ThioD) in bacteria in good yield and with high fidelity using an orthogonal nonsense suppressor tRNA/aminoacyl-tRNA synthetase (aaRS) pair. To demonstrate the utility of ThioD, we used native chemical ligation to label green fluorescent protein with a fluorophore in good yield.

Direct observation of the effect of transannular interaction on the single bond lengths of tricyclo[2.1.0.02,5]pentane derivatives

X-Ray structure analysis of 1′,5′-bis(acetoxymethyl)spiro(1,3-dioxolane-2,3′-tricyclo[2.1.0.02,5]pentane)(1) reveals that the bond distance (1.455 A) between the bridgehead carbon atoms is elongated in comparison with the same bond in the ketone 1,5-bis(acetoxymethyl)tricyclo[2.1.0.02,5]pentan-3-one (2), thereby providing direct evidence of an unusual transannular interaction between the carbonyl group and the bridging bond.