Symmetry breaking of fluorophore binding to a G-quadruplex generates an RNA aptamer with picomolar KD.

Fluorogenic RNA aptamer tags with high affinity enable RNA purification and imaging. The G-quadruplex (G4) based Mango (M) series of aptamers were selected to bind a thiazole orange based (TO1-Biotin) ligand. Using a chemical biology and reselection approach, we have produced a MII.2 aptamer-ligand complex with a remarkable set of properties: Its unprecedented KD of 45 pM, formaldehyde resistance (8% v/v), temperature stability and ligand photo-recycling properties are all unusual to find simultaneously within a small RNA tag. Crystal structures demonstrate how MII.2, which differs from MII by a single A23U mutation, and modification of the TO1-Biotin ligand to TO1-6A-Biotin achieves these results. MII binds TO1-Biotin heterogeneously via a G4 surface that is surrounded by a stadium of five adenosines. Breaking this pseudo-rotational symmetry results in a highly cooperative and homogeneous ligand binding pocket: A22 of the G4 stadium stacks on the G4 binding surface while the TO1-6A-Biotin ligand completely fills the remaining three quadrants of the G4 ligand binding face. Similar optimization attempts with MIII.1, which already binds TO1-Biotin in a homogeneous manner, did not produce such marked improvements. We use the novel features of the MII.2 complex to demonstrate a powerful optically-based RNA purification system.

Artificial RNA tags that tightly bind fluorogenic ligands have many RNA imaging and RNA-protein biomolecular purification applications. Here, we report and structurally characterize a very small (20-nt) biologically compatible G-quadruplex based aptamer that can be inserted into commonly found GNRA tetraloops. This aptamer binds its fluorogenic ligand with an unprecedented picomolar binding affinity and is very stable against thermal and chemical insults. As the ligand can be modified to include biotin, this RNA tag can also be bound to streptavidin magnetic beads. After washing, tagged RNA can be cleanly eluted by exposing the beads to intense green light, which photobleaches the bound fluorogenic ligand, triggering the release of the bound RNA complex.

An α-chloroaldehyde-based formal synthesis of eribulin.

Eribulin (Halaven) is the most structurally complex non-peptidic drug made by total synthesis and has challenged preconceptions of synthetic feasibility in drug discovery and development. However, despite decades of research, the synthesis and manufacture of eribulin remains a daunting task. Here, we report syntheses of the most complex fragment of eribulin (C14-C35) used in two distinct industrial routes to this important anticancer drug. Our convergent strategy relies on a doubly diastereoselective Corey-Chaykovsky reaction to affect the union of two tetrahydrofuran-containing subunits. Notably, this process relies exclusively on enantiomerically enriched α-chloroaldehydes as building blocks for constructing the three densely functionalized oxygen heterocycles found in the C14-C35 fragment and all associated stereocenters. Overall, eribulin can now be produced in a total of 52 steps, which is a significant reduction from that reported in both academic and industrial syntheses.

N-Triflylphosphoramides: highly acidic catalysts for asymmetric transformations.

N-Triflylphosphoramides (NTPA), have become increasingly popular catalysts in the development of enantioselective transformations as they are stronger Brønsted acids than the corresponding phosphoric acids (PA). Their highly acidic, asymmetric active site can activate difficult, unreactive substrates. In this review, we present an account of asymmetric transformations using this type of catalyst that have been reported in the past ten years and we classify these reactions using the enantio-determining step as the key criterion. This compendium of NTPA-catalysed reactions is organised into the following categories: (1) cycloadditions, (2) electrocyclisations, polyene and related cyclisations, (3) addition reactions to imines, (4) electrophilic aromatic substitutions, (5) addition reactions to carbocations, (6) aldol and related reactions, (7) addition reactions to double bonds, and (8) rearrangements and desymmetrisations. We highlight the use of NTPA in total synthesis and suggest mnemonics which account for their enantioselectivity.

Calculation of VS,max and Its Use as a Descriptor for the Theoretical Calculation of pKa Values for Carboxylic Acids.

The theoretical calculation of pKa values for Brønsted acids is a challenging task that involves sophisticated and time-consuming methods. Therefore, heuristic approaches are efficient and appealing methodologies to approximate these values. Herein, we used the maximum surface electrostatic potential (VS,max) on the acidic hydrogen atoms of carboxylic acids to describe the H-bond interaction with water (the same descriptor that is used to characterize σ-bonded complexes) and correlate the results with experimental pKa values to obtain a predictive model for other carboxylic acids. We benchmarked six different methods, all including an implicit solvation model (water): Five density functionals and the Møller⁻Plesset second order perturbation theory in combination with six different basis sets for a total of thirty-six levels of theory. The ωB97X-D/cc-pVDZ level of theory stood out as the best one for consistently reproducing the reported pKa values, with a predictive power of 98% correlation in a test set of ten other carboxylic acids.

Reactivity of electrophilic chlorine atoms due to σ-holes: a mechanistic assessment of the chemical reduction of a trichloromethyl group by sulfur nucleophiles.

σ-Holes are shown to promote the electrophilic behavior of chlorine atoms in a trichloromethyl group when bound to an electron-withdrawing moiety. A halogen bond-type non-covalent interaction between a chlorine atom and a negatively charged sulfur atom takes place, causing the abstraction of such a chlorine atom while leaving a carbanion, subsequently driving the chemical reduction of the trichloromethyl group to a sulfide in a stepwise process. The mechanism for the model reaction of trichloromethyl pyrimidine 1 with thiophenolate and thiophenol to yield phenylsulfide 4 was followed through 1H-NMR and studied using DFT transition state calculations, and the energy profile for this transformation is fully discussed. MP2 calculations of the electrostatic potential were performed for a series of trichloromethyl compounds in order to assess the presence of σ-holes and quantify them by means of the maximum surface electrostatic potential. Such calculations showed that the chlorine atoms behave as electrophilic leaving groups toward a nucleophilic attack, opening a new possibility in the synthetic chemistry of the trichloromethyl group.