Engineered Phenylalanine Ammonia-Lyases for the Enantioselective Synthesis of Aspartic Acid Derivatives.

Biocatalytic hydroamination of alkenes is an efficient and selective method to synthesize natural and unnatural amino acids. Phenylalanine ammonia-lyases (PALs) have been previously engineered to access a range of substituted phenylalanines and heteroarylalanines, but their substrate scope remains limited, typically including only arylacrylic acids. Moreover, the enantioselectivity in the hydroamination of electron-deficient substrates is often poor. Here, we report the structure-based engineering of PAL from Planctomyces brasiliensis (PbPAL), enabling preparative-scale enantioselective hydroaminations of previously inaccessible yet synthetically useful substrates, such as amide- and ester-containing fumaric acid derivatives. Through the elucidation of cryo-electron microscopy (cryo-EM) PbPAL structure and screening of the structure-based mutagenesis library, we identified the key active site residue L205 as pivotal for dramatically enhancing the enantioselectivity of hydroamination reactions involving electron-deficient substrates. Our engineered PALs demonstrated exclusive α-regioselectivity, high enantioselectivity, and broad substrate scope. The potential utility of the developed biocatalysts was further demonstrated by a preparative-scale hydroamination yielding tert-butyl protected l-aspartic acid, widely used as intermediate in peptide solid-phase synthesis.

Palladium-peptide oxidative addition complexes for bioconjugation.

The synthesis of palladium oxidative addition complexes derived from unprotected peptides is described. Incorporation of 4-halophenylalanine into a peptide during solid phase peptide synthesis allows for subsequent oxidative addition at this position upon treatment with a palladium precursor and suitable ligand. The resulting palladium-peptide complexes are solid, storable, water-soluble, and easily purified via high-performance liquid chromatography. These complexes react with thiols in aqueous buffer, offering an efficient method for bioconjugation. Using this strategy, peptides can be functionalized with small molecules to prepare modified aryl thioether side-chains at low micromolar concentrations. Additionally, peptide-peptide and peptide-protein ligations are demonstrated under dilute aqueous conditions.

Structural Organization of Dibromodiazadienes in the Crystal and Identification of Br···O Halogen Bonding Involving the Nitro Group.

Nitro functionalized dibromodiazadiene dyes were prepared and fully characterized including X-ray single crystal analysis. Electron deficient dibromodiazadienes were found to be able to act as donors of halogen bonding (XB), while the nitro group acted as an acceptor of the XB. Depending on the substituents, the Br···O XB competed with other weak interactions, and for some of the dyes, they even outcompeted the XB involving the nitro group. However, the nitro functionalized dibromoalkenes 6a and 10a, which had only the nitro moiety as the most plausible acceptor of the XB, reliably formed 1D chains via Br⋯O XB. Experimental work was supported by the DFT calculations and topological analysis of the electron density distribution within the framework of Bader's theory (QTAIM method).

Palladium-Protein Oxidative Addition Complexes by Amine-Selective Acylation.

Palladium oxidative addition complexes (OACs) are traditionally accessed by treating an aryl halide-containing substrate with a palladium(0) source. Here, a new strategy to selectively prepare stable OACs from amino groups on native proteins is presented. The approach relies on an amine-selective acylation reaction that occurs without modification of a preformed palladium(II)-aryl group. Once transferred onto a protein substrate, the palladium(II)-aryl group facilitates conjugation by undergoing reaction with a second, cysteine-containing protein. This operationally simple method is applicable to native, nonengineered enzymes as well as antibodies and is carried out in an aqueous setting and open to air. The resulting Pd-protein OACs are stable, storable reagents that retain biological activity and can be used to achieve protein-protein cross-coupling at nanomolar concentrations within hours.

Mechanistic Investigations of Nickamine- catalyzed Hydrosilylation of Alkenes: Nickel Nanoparticles Are the Active Species.

Hydrosilylation is an important chemical process for the synthesis of organosilanes and for the production of silicone polymers. The wide variety of catalysts developed for this reaction generally follow a Chalk-Harrod, or a sigma-bond metathesis mechanism. Recently, our group developed a nickel pincer complex, Nickamine, for highly selective hydrosilylation of alkenes. Preliminary mechanistic studies had suggested a pathway that deviates from both Chalk-Harrod and sigma-bond metathesis cycles. Here we used in situ NMR to monitor the hydrosilylation reaction. The observed induction period indicated that the species previously believed to be the resting state is merely a precatalyst. Via a combination of Transmission Electron Microscopy, mercury poisoning test, and competition reactions we show that the true catalyst is not a molecular nickel species, but rather nickel nanoparticles.

Protein-Protein Cross-Coupling via Palladium-Protein Oxidative Addition Complexes from Cysteine Residues.

Few chemical methods exist for the covalent conjugation of two proteins. We report the preparation of site-specific protein-protein conjugates that arise from the sequential cross-coupling of cysteine residues on two different proteins. The method involves the synthesis of stable palladium-protein oxidative addition complexes (Pd-protein OACs), a process that converts nucleophilic cysteine residues into an electrophilic S-aryl-Pd-X unit by taking advantage of an intramolecular oxidative addition strategy. This process is demonstrated on proteins up to 83 kDa in size and can be conveniently carried out in water and open to air. The resulting Pd-protein OACs can cross-couple with other thiol-containing proteins to arrive at homogeneous protein-protein bioconjugates.

Preclinical Development of PQR514, a Highly Potent PI3K Inhibitor Bearing a Difluoromethyl-Pyrimidine Moiety.

The phosphoinositide 3-kinase (PI3K)/mechanistic target of rapamycin (mTOR) pathway is a critical regulator of cell growth and is frequently hyperactivated in cancer. Therefore, PI3K inhibitors represent a valuable asset in cancer therapy. Herein we have developed a novel anticancer agent, the potent pan-PI3K inhibitor PQR514 (4), which is a follow-up compound for the phase-II clinical compound PQR309 (1). Compound 4 has an improved potency both in vitro and in cellular assays with respect to its predecessor compounds. It shows superiority in the suppression of cancer cell proliferation and demonstrates significant antitumor activity in an OVCAR-3 xenograft model at concentrations approximately eight times lower than PQR309 (1). The favorable pharmacokinetic profile and a minimal brain penetration promote PQR514 (4) as an optimized candidate for the treatment of systemic tumors.

An Easily Accessed Nickel Nanoparticle Catalyst for Alkene Hydrosilylation with Tertiary Silanes.

The first efficient and non-precious nanoparticle catalyst for alkene hydrosilylation with commercially relevant tertiary silanes has been developed. The nickel nanoparticle catalyst was prepared in situ from a simple nickel alkoxide precatalyst Ni(O(t) Bu)2 ⋅x KCl. The catalyst exhibits high activity for anti-Markovnikov hydrosilylation of unactivated terminal alkenes and isomerizing hydrosilylation of internal alkenes. The catalyst can be applied to synthesize a single terminal alkyl silane from a mixture of internal and terminal alkene isomers, and to remotely functionalize an internal alkene derived from a fatty acid.

Alkoxy Hydrosilanes As Surrogates of Gaseous Silanes for Hydrosilylation of Alkenes.

Me2SiH2, MeSiH3, and SiH4 are gaseous and flammable silanes that are inconvenient to use in chemical reactions. Catalytic amounts of a nickel pincer complex and NaO(t)Bu are reported to allow the synthesis of alkyl hydrosilanes from alkenes and alkoxy hydrosilanes, leading to the replacement of Me2SiH2, MeSiH3, and SiH4 by Me2(MeO)SiH, Me(EtO)2SiH, and (MeO)3SiH in hydrosilylation reactions of alkenes. The scope and mechanism of the reactions are also described.

Chemoselective Alkene Hydrosilylation Catalyzed by Nickel Pincer Complexes.

Chemoselective hydrosilylation of functionalized alkenes is difficult to achieve using base-metal catalysts. Reported herein is that well-defined bis(amino)amide nickel pincer complexes are efficient catalysts for anti-Markovnikov hydrosilylation of terminal alkenes with turnover frequencies of up to 83,000 per hour and turnover numbers of up to 10,000. Alkenes containing amino, ester, amido, ketone, and formyl groups are selectively hydrosilylated. A slight modification of reaction conditions allows tandem isomerization/hydrosilylation reactions of internal alkenes using these nickel catalysts.