General Light-Mediated, Highly Diastereoselective Piperidine Epimerization: From Most Accessible to Most Stable Stereoisomer.

We report a combined photocatalytic and hydrogen atom transfer (HAT) approach for the light-mediated epimerization of readily accessible piperidines to provide the more stable diastereomer with high selectivity. The generality of the transformation was explored for a large variety of di- to tetrasubstituted piperidines with aryl, alkyl, and carboxylic acid derivatives at multiple different sites. Piperidines without substitution on nitrogen as well as N-alkyl and aryl derivatives were effective epimerization substrates. The observed diastereoselectivities correlate with the calculated relative stabilities of the isomers. Demonstration of reaction reversibility, luminescence quenching, deuterium labeling studies, and quantum yield measurements provide information about the mechanism.

Highly Diastereoselective Functionalization of Piperidines by Photoredox-Catalyzed α-Amino C-H Arylation and Epimerization.

We report a photoredox-catalyzed α-amino C-H arylation reaction of highly substituted piperidine derivatives with electron-deficient cyano(hetero)arenes. The scope and limitations of the reaction were explored, with piperidines bearing multiple substitution patterns providing the arylated products in good yields and with high diastereoselectivity. To probe the mechanism of the overall transformation, optical and fluorescent spectroscopic methods were used to investigate the reaction. By employing flash-quench transient absorption spectroscopy, we were able to observe electron transfer processes associated with radical formation beyond the initial excited-state Ir(ppy)3 oxidation. Following the rapid and unselective C-H arylation reaction, a slower epimerization occurs to provide the high diastereomer ratio observed for a majority of the products. Several stereoisomerically pure products were resubjected to the reaction conditions, each of which converged to the experimentally observed diastereomer ratios. The observed distribution of diastereomers corresponds to a thermodynamic ratio of isomers based upon their calculated relative energies using density functional theory (DFT).

π-Facial Selectivities in Hydride Reductions of Hindered Endocyclic Iminium Ions.

The origins of π-facial selectivities in the borohydride reduction of endocyclic iminium ions have been elucidated by density functional theory calculations. In reductions of conjugated ("thermodynamic") iminium ions, the π-facial preference of the hydride attack was found to be due to torsional steering. Attack at the favored π-face leads to a lower-energy "half-chair"-like conformation of the tetrahydropyridine product, whereas attack at the other π-face results in an unfavorable "twist-boat" conformation. In reductions of nonconjugated ("kinetic") iminium ions, torsional distinction is small between the top- and bottom-face attacks, and the π-facial selectivity of the hydride approach is primarily due to steric hindrance.

Formation of Aminocyclopentadienes from Silyldihydropyridines: Ring Contractions Driven by Anion Stabilization.

Highly functionalized aminocyclopentadienes can be formed through the rearrangement of anions generated from readily prepared 6-silyl-1,2-dihydropyridines by desilylation with fluoride. The scope and generality of the reaction was defined, and diverse transformations were performed on the highly functionalized products. A mechanism and driving force for the rearrangement were identified from experiments and DFT calculations.

Enantioconvergent synthesis of functionalized γ-butyrolactones via (3 + 2)-annulation.

A dynamic kinetic resolution of ß-halo α-keto esters in an asymmetric homoenolate reaction is described. A chiral N-hetereocyclic carbene catalyzes the a(3) → d(3)-umpolung addition of α,ß-enals to racemic α-keto esters, forming γ-butyrolactones with three contiguous stereocenters. The addition occurs with high regio-, diastereo-, and enantiocontrol. This methodology constitutes an intermolecular DKR process to set three stereocenters during the key bond forming event.

Experiments probing the viability of donor-acceptor norbornenes for (5 + 2)-annulation.

This Note details experiments that probe the mechanism by which donor-acceptor norbornene systems epimerize. A number of mechanistic studies indicate that epimerization in these systems occurs via a Lewis acid catalyzed retro-Diels-Alder/Diels-Alder sequence, rather than bond rotation in an intimate ion pair. These results suggest that, under the reaction conditions examined, the ring strain present in norbornene is inadequate to induce zwitterion formation analogous to that observed with donor-acceptor cyclopropanes.

Targeting AML-associated FLT3 mutations with a type I kinase inhibitor.

Tyrosine kinase domain (TKD) mutations contribute to acquired resistance to FMS-like tyrosine kinase 3 (FLT3) inhibitors used to treat FLT3-mutant acute myeloid leukemia (AML). We report a cocrystal structure of FLT3 with a type I inhibitor, NCGC1481, that retained potent binding and activity against FLT3 TKD and gatekeeper mutations. Relative to the current generation of advanced FLT3 inhibitors, NCGC1481 exhibited superior antileukemic activity against the common, clinically relevant FLT3-mutant AML cells in vitro and in vivo.

Overcoming adaptive therapy resistance in AML by targeting immune response pathways.

Targeted inhibitors to oncogenic kinases demonstrate encouraging clinical responses early in the treatment course; however, most patients will relapse because of target-dependent mechanisms that mitigate enzyme-inhibitor binding or through target-independent mechanisms, such as alternate activation of survival and proliferation pathways, known as adaptive resistance. Here, we describe mechanisms of adaptive resistance in FMS-like receptor tyrosine kinase (FLT3)-mutant acute myeloid leukemia (AML) by examining integrative in-cell kinase and gene regulatory network responses after oncogenic signaling blockade by FLT3 inhibitors (FLT3i). We identified activation of innate immune stress response pathways after treatment of FLT3-mutant AML cells with FLT3i and showed that innate immune pathway activation via the interleukin-1 receptor-associated kinase 1 and 4 (IRAK1/4) complex contributes to adaptive resistance in FLT3-mutant AML cells. To overcome this adaptive resistance mechanism, we developed a small molecule that simultaneously inhibits FLT3 and IRAK1/4 kinases. The multikinase FLT3-IRAK1/4 inhibitor eliminated adaptively resistant FLT3-mutant AML cells in vitro and in vivo and displayed superior efficacy as compared to current targeted FLT3 therapies. These findings uncover a polypharmacologic strategy for overcoming adaptive resistance to therapy in AML by targeting immune stress response pathways.

The Bacteriophage T4 MotB Protein, a DNA-Binding Protein, Improves Phage Fitness.

The lytic bacteriophage T4 employs multiple phage-encoded early proteins to takeover the Escherichia coli host. However, the functions of many of these proteins are not known. In this study, we have characterized the T4 early gene motB, located in a dispensable region of the T4 genome. We show that heterologous production of MotB is highly toxic to E. coli, resulting in cell death or growth arrest depending on the strain and that the presence of motB increases T4 burst size 2-fold. Previous work suggested that motB affects middle gene expression, but our transcriptome analyses of T4 motBam vs. T4 wt infections reveal that only a few late genes are mildly impaired at 5 min post-infection, and expression of early and middle genes is unaffected. We find that MotB is a DNA-binding protein that binds both unmodified host and T4 modified [(glucosylated, hydroxymethylated-5 cytosine, (GHme-C)] DNA with no detectable sequence specificity. Interestingly, MotB copurifies with the host histone-like proteins, H-NS and StpA, either directly or through cobinding to DNA. We show that H-NS also binds modified T4 DNA and speculate that MotB may alter how H-NS interacts with T4 DNA, host DNA, or both, thereby improving the growth of the phage.

Asymmetric synthesis of indole homo-Michael adducts via dynamic kinetic Friedel-Crafts alkylation with cyclopropanes.

An enantioconvergent Friedel-Crafts alkylation of indoles with donor-acceptor cyclopropanes is described. The reaction is catalyzed by pybox•MgI2 and proceeds via a type I dynamic kinetic asymmetric transformation (DyKAT).