Hexin Xie

Superresolution imaging of targeted proteins in fixed and living cells using photoactivatable organic fluorophores

Superresolution imaging techniques based on sequential imaging of sparse subsets of single molecules require fluorophores whose emission can be photoactivated or photoswitched. Because typical organic fluorophores can emit significantly more photons than average fluorescent proteins, organic fluorophores have a potential advantage in super-resolution imaging schemes, but targeting to specific cellular proteins must be provided. We report the design and application of HaloTag-based target-specific azido DCDHFs, a class of photoactivatable push-pull fluorogens which produce bright fluorescent labels suitable for single-molecule superresolution imaging in live bacterial and fixed mammalian cells.

Highly enantioselective aldol reactions catalyzed by a recyclable fluorous (S) pyrrolidine sulfonamide on water.

Fluorous (S) pyrrolidine sulfonamide serves as an efficient promoter for highly enantioselective aldol reactions of ketones and aldehydes with aromatic aldehydes on water. A notable feature of the organocatalyst is that it can be recovered from the reaction mixtures by simple fluorous solid-phase extraction and subsequently reused (up to seven cycles) without a significant loss of catalytic activity and stereoselectivity.

Catalytic asymmetric oxa-Michael-Michael cascade for facile construction of chiral chromans via an aminal intermediate.

An unprecedented highly enantioselective cascade oxa-Michael-Michael reaction has been developed. The simple and practical process, efficiently catalyzed by chiral diphenylprolinol TMS ether, affords a powerful access to highly functionalized synthetically useful chiral chromans. Moreover, notably a new activation mode involving an aminal is disclosed for the first time.

A Biocompatible Condensation Reaction for the Labeling of Terminal Cysteine Residues on Proteins

This communication describes a novel protein labeling method that uses a single amino-acid tag — N-terminal cysteine residue — and small-molecule probes carrying the cyanobenzothiazole unit for specific labeling of proteins in vitro and at the surface of live cells. This simple ligation reaction proceeds with a high degree of specificity in physiological conditions, and should offer an important alternative to currently available protein labeling methods.

Bifunctional Cinchona Alkaloid Thiourea Catalyzed Highly Efficient, Enantioselective Aza‐Henry Reaction of Cyclic Trifluoromethyl Ketimines: Synthesis of Anti‐HIV Drug DPC 083

The aza-Henry reaction is a powerful method for C C bond formation. Moreover, the versatile nitro group can be conveniently transformed into a variety of new functional groups, which are highly valuable in the preparation of related analogues in drug discovery. In recent years, significant efforts have been directed toward the development of catalytic asymmetric aza-Henry reactions. However, the electrophilic substrates have been largely confined to imines derived from aldehydes. The development of an efficient protocol for an enantioselective aza-Henry reaction of ketimines to generate a chiral quaternary center remains elusive because of the lower reactivity of ketimines and difficulties in enantiofacial discrimination. To our knowledge, to date, there has been only one report, by Feng and co-workers, of a chiral N,N’dioxide copper complex (20 mol %) catalyzed asymmetric aza-Henry reaction between acyclic ketimines and nitromethane; this reaction proceeded with good enantioselectivities (71–96 % ee) but generally in poor yields (21–70%). Dihydroquinazolinones, as an important class of heterocyclic compounds are characterized by their broad spectrum of intriguing biological properties, such as antiviral and antiobesity activities, and their use in the treatment of cardiovascular diseases and pain. Notably, among these compounds, drug candidates DPC 083 and DPC 961, bearing a chiral trifluoromethyl moiety, are potent HIV-1 nonnucleoside reverse transcriptase inhibitors (Scheme 1). It is believed that the trifluoromethyl motif plays a pivotal role in the bioactivity. Accordingly, efficient approaches to valuable chiral molecular architectures with sites for functional group diversification are of considerable synthetic and biological importance. Furthermore, catalytic enantioselective syntheses of DPC 083 and DPC 961 are needed. Given the important challenge of the construction of a functionalized quaternary stereogenic center in the dihydroquinazolinone scaffolds, significant efforts have been made but with limited success. The approaches reported to date mainly rely on chiral auxiliaries to control the stereoselectivity. An atom-economical catalytic enantioselective process is more attractive but, to our knowledge, such a method giving a useful level of enantioselectivity (> 90% ee) is an unmet synthetic issue. Herein, we report a novel highly efficient organocatalytic enantioselective aza-Henry reaction for the preparation of the enantioenriched trifluoromethyl dihydroquinazolinones. Notably, a highly efficient hydrogen-bond-mediated enantioselective addition of nitroalkanes to ketimines has been achieved for the first time under mild reaction conditions in high yields using as low as 1 mol% catalyst loading. Furthermore, we also observed that the trifluoromethyl group is critical for not only biological activity, but also for chemical reactivity. Finally, the highly enantioselective synthesis of DPC 083 has been achieved using the aza-Henry reaction as a key step. We envisioned that a catalytic enantioselective aza-Henry reaction could be realized by the reaction of 2(1H)-quinazolinones 1 with versatile nitroalkanes to generate chiral dihydroquinazolinones (Table 1). Accordingly, our investigation began with the model reaction between trifluoromethylquinazolin-2(1H)-one (1a ; 1.0 equiv) and nitromethane (2a ; 2.0 equiv) in the presence of quinine (4a ; 10 mol%) as the catalyst in CH2Cl2 at room temperature (Table 1, entry 1). [11]

Rapid point-of-care detection of the tuberculosis pathogen using a BlaC-specific fluorogenic probe

Early diagnosis of tuberculosis can dramatically reduce both its transmission and the associated death rate. The extremely slow growth rate of the causative pathogen, Mycobacterium tuberculosis (Mtb), however, makes this challenging at the point of care, particularly in resource-limited settings. Here we report the use of BlaC (an enzyme naturally expressed/secreted by tubercle bacilli) as a marker and the design of BlaC-specific fluorogenic substrates as probes for Mtb detection. These probes showed an enhancement by 100-200 times in fluorescence emission on BlaC activation and a greater than 1,000-fold selectivity for BlaC over TEM-1 β-lactamase, an important factor in reducing false-positive diagnoses. Insight into the BlaC specificity was revealed by successful co-crystallization of the probe/enzyme mutant complex. A refined green fluorescent probe (CDG-OMe) enabled the successful detection of live pathogen in less than ten minutes, even in unprocessed human sputum. This system offers the opportunity for the rapid, accurate detection of very low numbers of Mtb for the clinical diagnosis of tuberculosis in sputum and other specimens.

Bioluminescent nanosensors for protease detection based upon gold nanoparticle–luciferase conjugates

This communication reports the use of click chemistry to site-specifically conjugate bioluminescent Renilla luciferase proteins to gold nanoparticles (Au NPs) for sensing protease activity. The bioluminescent emission from luciferase was efficiently quenched by Au NPs, but significantly recovered after the proteolytic cleavage.

Proline-Catalyzed Direct Inverse Electron Demand Diels–Alder Reactions of Ketones with 1,2,4,5-Tetrazines

An organocatalytic direct inverse electron demand Diels-Alder reaction of ketones with 1,2,4,5-tetrazines has been developed. The process is efficiently catalyzed by proline to give Diels-Alder adducts pyridazines in high yields.

Highly enantio- and diastereoselective organocatalytic cascade aza-Michael–Michael reactions: a direct method for the synthesis of trisubstituted chiral pyrrolidines

An unprecedented highly enantio- and diastereoselective cascade aza-Michael-Michael reaction of alpha,beta-unsaturated aldehydes with trans-gamma-Ts protected amino alpha,beta-unsaturated ester has been developed; the simple and practical process, efficiently catalyzed by chiral diphenylprolinol TMS ether, serves as a powerful access to highly functionalized trisubstituted chiral pyrrolidines.

A Quinine-Squaramide Catalyzed Enantioselective Aza-Friedel–Crafts Reaction of Cyclic Trifluoromethyl Ketimines with Naphthols and Electron-Rich Phenols

A highly enantioselective aza-Friedel-Crafts (aza-F-C) reaction of cyclic trifluoromethyl ketimines and naphthols/phenols was developed with fluorenyl-substituted quinine-squaramide as the catalyst. This protocol enables direct access to biologically important chiral trifluoromethyl dihydroquinazolinones with up to 99% yields and up to 99% ees.