Enantioselective Total Synthesis of the Neurotoxin Caramboxin

Herein, we report the first and efficient asymmetric total synthesis of the neurotoxin (-)-caramboxin. The key to success is the creation of a stereogenic center by using enantioselective catalytic phase-transfer α-alkylation of glycine imines, affording this unusual α-amino acid in good yields and up to 99% ee. This work validates the S configuration of the natural product.

Metal-free synthesis of γ-ketosulfones through Brønsted acid-promoted conjugate addition of sulfinamides

A straightforward and general metal-free method has been developed to add sufinamide-derived sulfone units on Michael acceptors under mild conditions. This reaction enables the preparation of a large variety of original γ-ketosulfones, of which only a few synthetic methods have been reported. The mild reaction conditions used tolerate a wide diversity of functional groups and empower the implementation of a late-stage functionalisation strategy.

Carbon Nucleophile-Initiated Rauhut-Currier Reaction: An Atom-Economical Synthesis of Highly Functionalized Carbocycles

A Rauhut-Currier reaction cascade is achieved in the presence of carbon nucleophiles under mild conditions. This original atom-economical transformation enables an efficient one-pot synthesis of densely substituted carbocycles from readily accessible substrates. The key promoter role of the cesium cation in the cascade process was demonstrated.

The dual reactivity of Weinreb amides applied to the late-stage divergent functionalisation of meso pyrrolidines

The dual reactivity of Weinreb amides was exploited to prepare diversified symmetrical and dissymmetrical 2,5-disubstituted pyrrolidines from simple building blocks.

Identification of a pre-active conformation of a pentameric channel receptor

Pentameric ligand-gated ion channels (pLGICs) mediate fast chemical signaling through global allosteric transitions. Despite the existence of several high-resolution structures of pLGICs, their dynamical properties remain elusive. Using the proton-gated channel GLIC, we engineered multiple fluorescent reporters, each incorporating a bimane and a tryptophan/tyrosine, whose close distance causes fluorescence quenching. We show that proton application causes a global compaction of the extracellular subunit interface, coupled to an outward motion of the M2-M3 loop near the channel gate. These movements are highly similar in lipid vesicles and detergent micelles. These reorganizations are essentially completed within 2 ms and occur without channel opening at low proton concentration, indicating that they report a pre-active intermediate state in the transition pathway toward activation. This provides a template to investigate the gating of eukaryotic neurotransmitter receptors, for which intermediate states also participate in activation.

DOI:http://dx.doi.org/10.7554/eLife.23955.001

In the nervous system, proteins of the pLGIC family are found in the membrane that surrounds each neuron. These proteins have channels that can allow ions to pass through the membrane and are responsible for transmitting electrical signals from one neuron to the next. Small molecules called neurotransmitters interact with the pLGICs to open or close the ion channel. If the ability of the pLGIC channels to open is altered, it can lead to behavioral changes like addiction, or diseases such as schizophrenia or epilepsy.

For a pLGIC channel to switch between the “open” and “closed” states, specific parts of the protein need to move in relation to each other. However, to study these transitions researchers have previously relied on comparing the three-dimensional structures of open and closed pLGICs extracted out of the cell membrane. Different techniques are needed to directly follow these movements within membranes.

Bacteria also have proteins belonging to the pLGIC family, and Menny et al. have now investigated one such bacterial protein to understand how pLGICs open. First, a small fluorescent molecule that glows differently if the environment around it changes was attached to various parts of the bacterial channel. These fluorescent markers revealed how several parts of the protein move and they also made it possible to measure how quickly these movements take place. Some of these movements happen before the channel opens, suggesting that the activation of this pLGIC protein happens in stages and involves the protein adopting a temporary intermediate state.

The next step will be to better understand the structure of the intermediate state, which could help us to understand how pLGICs work in the nervous systems of animals. In future this may aid the design of new drugs that can modify the activity of these channels in patients with neurological conditions or addictions.

DOI:http://dx.doi.org/10.7554/eLife.23955.002

Barbiturates Bind in the GLIC Ion Channel Pore and Cause Inhibition by Stabilizing a Closed State

Barbiturates induce anesthesia by modulating the activity of anionic and cationic pentameric ligand-gated ion channels (pLGICs). Despite more than a century of use in clinical practice, the prototypic binding site for this class of drugs within pLGICs is yet to be described. In this study, we present the first X-ray structures of barbiturates bound to GLIC, a cationic prokaryotic pLGIC with excellent structural homology to other relevant channels sensitive to general anesthetics and, as shown here, to barbiturates, at clinically relevant concentrations. Several derivatives of barbiturates containing anomalous scatterers were synthesized, and these derivatives helped us unambiguously identify a unique barbiturate binding site within the central ion channel pore in a closed conformation. In addition, docking calculations around the observed binding site for all three states of the receptor, including a model of the desensitized state, showed that barbiturates preferentially stabilize the closed state. The identification of this pore binding site sheds light on the mechanism of barbiturate inhibition of cationic pLGICs and allows the rationalization of several structural and functional features previously observed for barbiturates.

Two-Component Domino Reactions Initiated from Ketenes: Serendipitous Synthesis of Quinolizidinones Analogous to Chelated Lobeline's Conformation

An original and efficient synthesis of quinolizidinones through a one-pot two-component cascade reaction of norlobelanine with in situ generated ketenes is reported. Functionalized fused azabicyclic scaffolds bearing multiple stereogenic centers were prepared with excellent diastereoselectivities. Mild optimized conditions involving a key "shuttle base" deprotonation strategy was applied to the synthesis, in a short sequence, of a constrained mimetic of the privileged H-bonded conformation of (-)-lobeline.

Microwave-assisted telescoped cross metathesis-ring closing aza-Michael reaction sequence: step-economical access to nicotine–lobeline hybrid analogues

Step-economical access to nicotinic acetylcholine receptor ligands hybrids through an efficient telescoped cross-metathesis/cyclizing aza-Michael addition involving N-heteroaromatic olefinic derivatives.

Switchable stereocontrolled divergent synthesis induced by aza-Michael addition of deactivated primary amines under acid catalysis

Switchable tandem intramolecular aza-Michael/Michael and double aza-Michael reactions allow the oriented synthesis of highly functionalised cyclic skeletons. Conjugate addition of deactivated anilines triggers chemo- and stereo-divergent ring-closure reaction pathways with a striking selectivity depending on reaction conditions.

Thermodynamic epimeric equilibration and crystallisation-induced dynamic resolution of lobelanine, norlobelanine and related analogues

The step-economical synthesis of lobelanine involving a ring closing double aza-Michael (RCDAM) reaction is revisited and successfully extended to the synthesis of various configurationally more stable analogues.

Step-economical access to valuable Weinreb amide 2,5-disubstituted pyrrolidines by a sequential one-pot two-directional cross-metathesis/cyclizing aza-Michael process

Double cross-metathesis of 1,5-hexadiene with a variety of electron-deficient alkenes including the reluctant Weinreb acrylamide has been successfully accomplished. It was found that the process is quite general, and microwave irradiation effectively accelerates cross-coupling metathesis. This promotes a very versatile and high yielding methodology for the synthesis of symmetric Michael acceptors, which can be transformed into 2,5-disubstituted pyrrolidines through a sequential one-pot two-directional cross-metathesis/ring-closing double aza-Michael process.