Enantioselective Addition of Allylic Trimethoxysilanes to Aldehydes Catalyzed by p-Tol-BINAP small middle dotAgF

Catalytic Enantioconvergent Conjugate Addition of Organosilanes via a Strategy of Fluorodesilylation

Fluorodesilylation is a widely used strategy to activate organosilanes as nucleophiles for the development of organic transformations. To date, highly enantioselective catalytic fluorodesilylations have been limited to the activation of silyl ethers, organosilanes bearing specific substituents such as trifluoromethyl and cyanide, allylsilanes, and acylsilanes. However, the catalytic enantioconvergent reaction of racemic organosilanes bearing variable substituents via fluorodesilylation has been rarely reported. We report an unprecedented enantioconvergent fluorodesilylation of racemic organosilanes bearing various substituents with a chiral ammonium fluoride. Notably, these results demonstrated that the fluorodesilylation could potentially be a general strategy for the development of catalytic asymmetric reactions of racemic organosilanes.

Conjugated ynones in catalytic enantioselective reactions

Conjugated ynones are easily accessible feedstock and the existence of an alkyne bond endows ynones with different attractive reactivities, thus making them unique substrates for catalytic asymmetric reactions. Their compatibility under organocatalytic, metal-catalyzed as well as cooperative catalytic conditions has resulted in numerous enantioselective transformations. Importantly, conjugated ynones can act as nucleophiles or electrophiles, and serve as easily accessed synthons for different cyclization pathways. This review summarizes the recent literature examples of the catalytic reactions of conjugated ynones and related compounds such as alkyne conjugated α-ketoesters, and classifies these reaction types alongside mechanistic insights whenever possible. We aim to trigger more intensive research in the future to render the asymmetric transformation of ynones as a common and reliable tool for asymmetric synthesis.

A new drug testing platform based on 3D tri-culture in lab-on-a-chip devices

Drug discovery has a 90% rate of failure because preclinical platforms for drug testing do not mimic the in vivo conditions. Doxorubicin (DOX) is a commonly used drug to treat breast cancer patients even though it has side effects. Lab-on-a-chip (LOC) devices provide spatial control at the micrometer scale and can thus emulate the cancer microenvironment. Here, using a multidisciplinary approach, a new drug testing platform based on 3D tri-culture in LOC devices was developed. Breast cancer cells alone or with normal mammary epithelial cells and macrophages were cultured in matrigel in LOC devices. The platform was used to test DOX and (R)-4'-methylklavuzon (KLA), which is a new anti-cancer drug candidate. Results showed that DOX and KLA were equally effective on breast cancer cells in 3D monoculture. KLA produced 26% less death for breast cancer cells than DOX in 3D tri-culture. More importantly, DOX was not selective between breast cancer cells and normal mammary epithelial cells in 3D tri- culture whereas KLA caused 56% less cell death than DOX for normal mammary epithelial cells. Results strongly recommend that 3D tri-culture in LOC devices be used for assessment of drug toxicity at the preclinical stage.

Antiproliferative activity of (R)-4'-methylklavuzon on hepatocellular carcinoma cells and EpCAM+/CD133+ cancer stem cells via SIRT1 and Exportin-1 (CRM1) inhibition

Cytotoxic effects of (R)-4'-methylklavuzon were investigated on hepatocellular carcinoma cells (HuH-7 and HepG2) and HuH-7 EpCAM+/CD133+ cancer stem cells. IC50 of (R)-4'-methylklavuzon was found as 1.25 μM for HuH-7 parental cells while it was found as 2.50 μM for HuH-7 EpCAM+/CD133+ cancer stem cells. (R)-4'-methylklavuzon tended to show more efficient in vitro cytotoxicity with its lower IC50 values on hepatocellular carcinoma cell lines compared to its lead molecule, goniothalamin and FDA-approved drugs, sorafenib and regorafenib. Cell-based Sirtuin/HDAC enzyme activity measurements revealed that endogenous Sirtuin/HDAC enzymes were reduced by 40% compared to control. SIRT1 protein levels were upregulated indicating triggered DNA repair mechanism. p53 was overexpressed in HepG2 cells. (R)-4'-methylklavuzon inhibited CRM1 protein providing increased retention of p53 and RIOK2 protein in the nucleus. HuH-7 parental and EpCAM+/CD133+ cancer stem cell spheroids lost intact morphology. 3D HepG2 spheroid viabilities were decreased in a correlation with upregulation in p53 protein levels.

Stereoselective synthesis of the C14-C23 fragment of biselyngbyolide A and B enabled by transition metal catalysis

Transition met al catalysis has enabled the highly stereoselective and protecting group-free synthesis of the C14-C23 fragment of the apoptosis-inducing natural products biselyngbyolide A and B. A Pd-catalyzed Stille reaction between a vinyl stannane and a crotyl carbonate formed the skipped diene with complete control of the the trisubstituted bond and excellent control over the branched/linear products. A Cu-catalyzed Stahl oxidation was used to form the requisite aldehyde needed for a Ag-catalyzed asymmetric allylation. The latter provided the final fragment with excellent stereochemical control.

Ir-Catalyzed Asymmetric and Regioselective Hydrogenation of Cyclic Allylsilanes and Generation of Quaternary Stereocenters via the Hosomi-Sakurai Allylation

A number of cyclic dienes containing the allylsilane moiety were prepared by a Birch reduction and subjected to iridium-catalyzed regioselective and asymmetric hydrogenation, which provided chiral allylsilanes in high conversion and enantiomeric excess (up to 99 % ee). The compounds were successively used in the Hosomi-Sakurai allylation with various aldehydes employing TiCl₄ as Lewis acid, providing adducts with two additional stereogenic centers in excellent diastereoselectivity.

Synthesis and Topoisomerase I inhibitory properties of klavuzon derivatives

Klavuzon is a naphthalen-1-yl substituted α,β-unsaturated δ-lactone derivative, and is one of the anti-proliferative members of this class of compounds. Asymmetric and racemic syntheses of novel α,β-unsaturated δ-lactone derivatives are important to investigate their potential for the treatment of cancer. In this study, asymmetric and racemic syntheses of heteroatom-substituted klavuzon derivatives are reported. The syntheses were completed by a well-known three-step procedure. Anti-proliferative activity of seven novel racemic klavuzon derivatives were reported against MCF-7, PC3, HCT116 p53+/+ and HCT116 p53-/- cancer cell lines. Topoisomerase I inhibitory properties of 5,6-dihydro-2H-pyran-2-one derivatives were also studied.

Iridium-catalyzed intermolecular directed dehydrogenative ortho C–H silylation

An efficient method for iridium-catalyzed direct silylation of C(sp²)–H and C(sp³)–H bonds using HSiMe(OSiMe₃)₂ as the silylating reagent is described.

Synthetic Strategies Employed for the Construction of Fostriecin and Related Natural Products

Fostriecin and related natural products present a significant challenge for synthetic chemists due to their structural complexity and chemical sensitivity. This review will chronicle the successful efforts of synthetic chemists in the construction of these biologically active molecules. Key carbon-carbon bond forming reactions will be highlighted, as well as the methods used to install the numerous stereocenters present in this class of compounds.

Enantioselective Silver-Catalyzed Transformations

This review collects the major progress in the field of enantioselective transformations promoted by chiral silver catalysts, covering the literature since 2008, well illustrating the power of these especially mild Lewis acid catalysts to provide novel asymmetric reactions.

Acridine based (S,N,S) pincer ligand: designing silver(i) complexes for the efficient activation of A3 (aldehyde, alkyne and amine) coupling

The catalytic efficiency for the A³ reaction with complexes 1 and 2, which have been newly synthesized and structurally characterized, is highest for the pincer complex 2.

Silver-catalyzed reaction of enynals with alkenes: a tandem 1,3-dipolar cycloaddition/cyclopropanation

A silver-catalyzed reaction of enynals with alkenes to synthesize a series of polycyclic compounds has been developed. The reaction occurred smoothly using enynals with electron-deficient alkynes or terminal alkynes as substrates. The reaction was proposed to proceed through a tandem 1,3-dipolar cycloaddition/cyclopropanation process. The preliminary investigations of the asymmetric catalytic version revealed that the biphosphine ligand could be used as the potential efficient chiral ligand.

Iridium-catalyzed regioselective silylation of aromatic and benzylic C-H bonds directed by a secondary amine

Reported herein is an iridium-catalyzed, regioselective silylation of the aromatic C-H bonds of benzylamines and the benzylic C-H bonds of 2,N-dialkylanilines. In this process, (hydrido)silyl amines, generated in situ by dehydrogenative coupling of benzylamine or aniline with diethylsilane, undergo selective silylation at the C-H bond γ to the amino group. The products of this silylation are suitable for subsequent oxidation, halogenation, and cross-coupling reactions to deliver benzylamine and arylamine derivatives.

Iridium-catalyzed regioselective silylation of secondary alkyl C-H bonds for the synthesis of 1,3-diols

We report Ir-catalyzed intramolecular silylation of secondary alkyl C-H bonds. (Hydrido)silyl ethers, generated in situ by dehydrogenative coupling of a tertiary or conformationally restricted secondary alcohol with diethylsilane, undergo regioselective silylation at a secondary C-H bond γ to the hydroxyl group. Oxidation of the resulting oxasilolanes in the same vessel generates 1,3-diols. This method provides a strategy to synthesize 1,3-diols through a hydroxyl-directed, functionalization of secondary alkyl C-H bonds. Mechanistic studies suggest that the C-H bond cleavage is the turnover-limiting step of the catalytic cycle. This silylation of secondary C-H bonds is only 40-50 times slower than the analogous silylation of primary C-H bonds.

Pd(OAc)2/o-chloranil/M(OTf)n: a catalyst for the direct C-H arylation of polycyclic aromatic hydrocarbons with boryl-, silyl-, and unfunctionalized arenes

Pd(OAc)(2)/o-chloranil/M(OTf)(n) can effectively promote the C-H arylation of fluoranthene with arylboron compounds or arylsilanes. The reaction takes place with high regioselectivity at the C3 position of fluoranthene. Moreover, the new catalytic system allows the use of unfunctionalized arenes as coupling partners in the arylation of polycyclic aromatic hydrocarbons.

Chiral silver amides as effective catalysts for enantioselective [3+2] cycloaddition reactions

Asymmetric [3+2] cycloaddition of α-aminoester Schiff bases with substituted olefins is one of the most efficient methods for the preparation of chiral pyrrolidine derivatives in optically pure form. In spite of its potential utility, applicable substrates for this method have been limited to Schiff bases that bear relatively acidic α-hydrogen atoms. Here we report a chiral silver amide complex for asymmetric [3+2] cycloaddition reactions. A silver complex prepared from silver bis(trimethylsilyl)amide (AgHMDS) and (R)-DTBM-SEGPHOS worked well in asymmetric [3+2] cycloaddition reactions of α-aminoester Schiff bases with several olefins to afford the corresponding pyrrolidine derivatives in high yields with remarkable exo- and enantioselectivities. Furthermore, α-aminophosphonate Schiff bases, which have less acidic α-hydrogen atoms, also reacted with olefins with high exo- and enantioselectivities. The stereoselectivities of the [3+2] cycloadditions with maleate and fumarate suggested that the reaction proceeded by means of a concerted mechanism. An NMR spectroscopic study indicated that complexation of AgHMDS with the bisphosphine ligand was not complete, and that free AgHMDS, which did not show any significant catalytic activity, existed in the catalyst solution. This means that significant ligand acceleration occurred in the current reaction system.

Diastereo- and enantioselective ruthenium-catalyzed hydrohydroxyalkylation of 2-silyl-butadienes: carbonyl syn-crotylation from the alcohol oxidation level

Exposure of alcohols 2a-2j to 2-silyl-butadienes in the presence of ruthenium complexes modified by (R)-SEGPHOS or (R)-DM-SEGPHOS results in redox-triggered generation of allylruthenium-aldehyde pairs, which combine to form products of carbonyl crotylation 4a-4j in the absence of stoichiometric byproducts and with high levels of syn-diastereo- and enantioselectivity. In the presence of isopropanol under otherwise identical conditions, aldehydes 3a-3j are converted to an equivalent set of adducts 4a-4j. Whereas reactions conducted using conventional heating require 48 h, microwave irradiation enables full conversion in only 4 h. Finally, as illustrated in the conversion of adduct 4a to compounds 6a and 6b, diastereoselective hydroboration-Suzuki cross-coupling with aryl and vinyl halides followed by Fleming-Tamao oxidation enables generation of anti,syn-stereotriads found in numerous polyketide natural products.

The Pyridyldiisopropylsilyl Group: A Masked Functionality and Directing Group for Monoselective ortho-Acyloxylation and ortho-Halogenation Reactions of Arenes

A novel, easily removable and modifiable silicon-tethered pyridyldiisopropylsilyl directing group for C-H functionalizations of arenes has been developed. The installation of the pyridyldiisopropylsilyl group can efficiently be achieved via two complementary routes using easily available 2-(diisopropylsilyl)pyridine (5). The first strategy features a nucleophilic hydride substitution at the silicon atom in 5 with aryllithium reagents generated in situ from the corresponding aryl bromides or iodides. The second milder route exploits a highly efficient room-temperature rhodium(I)-catalyzed cross-coupling reaction between 5 and aryl iodides. The latter approach can be applied to the preparation of a wide range of pyridyldiisopropylsilyl-substituted arenes possessing a variety of functional groups, including those incompatible with organometallic reagents. The pyridyldiisopropylsilyl directing group allows for a highly efficient, regioselective palladium(II)-catalyzed mono-ortho-acyloxylation and ortho-halogenation of various aromatic compounds. Most impor-tantly, the silicon-tethered directing group in both acyloxylated and halogenated products can easily be removed or efficiently converted into an array of other valuable functionalities. These transformations include protio-, deuterio-, halo-, boro-, and alkynyldesilylations, as well as a conversion of the directing group into the hydroxy functionality. In addition, the construction of aryl-aryl bonds via the Hiyama-Denmark cross-coupling reaction is feasible for the acetoxylated products. Moreover, the ortho-halogenated pyridyldiisopropylsilylarenes, bearing both nucleophilic pyridyldiisopropylsilyl and electrophilic aryl halide moieties, represent synthetically attractive 1,2-ambiphiles. A unique reactivity of these ambiphiles has been demonstrated in efficient syntheses of arylenediyne and benzosilole derivatives, as well as in a facile generation of benzyne. In addition, preliminary mechanistic studies of the acyloxylation and halogenation reactions have been performed. A trinuclear palladacycle intermediate has been isolated from a stoichiometric reaction between diisopropyl-(phenyl)pyrid-2-ylsilane (3a) and palladium acetate. Furthermore, both C-H functionalization reactions exhibited equally high values of the intramolecular primary kinetic isotope effect (kH/kD = 6.7). Based on these observations, a general mechanism involving the formation of a palladacycle via a C-H activation process as the rate-determining step has been proposed.

PyDipSi: a general and easily modifiable/traceless Si-tethered directing group for C-H acyloxylation of arenes

A new general and easily installable silicon-tethered pyridyl-containing directing group (PyDipSi) that allows for highly efficient and regioselective Pd-catalyzed ortho C-H acyloxylation of arenes has been developed. It has also been demonstrated that this directing group can efficiently be removed as well as converted into a variety of other valuable functional groups. In addition, the installation of the PyDipSi directing group along with pivaloxylation and quantitative conversion of the PyDipSi group into a halogen functionality represents a formal three-step ortho oxygenation of haloarenes.

C-F Bond Formation for the Synthesis of Aryl Fluorides

A selection of carbon-fluorine bond-forming reactions is presented with particular focus on transition metal-mediated fluorination. A brief summary of conventional fluorination reactions is followed by a discussion of fluorination reactions mediated by palladium and silver. Investigations into the mechanism as well as the conceptual difficulty associated with transition metal-mediated carbon-fluorine bond formation are presented.

anti-Diastereo- and enantioselective carbonyl crotylation from the alcohol or aldehyde oxidation level employing a cyclometallated iridium catalyst: alpha-methyl allyl acetate as a surrogate to preformed crotylmetal reagents

Under the conditions of transfer hydrogenation employing an ortho-cyclometallated iridium catalyst generated in situ from [Ir(cod)Cl](2), 4-cyano-3-nitrobenzoic acid and the chiral phosphine ligand (S)-SEGPHOS, alpha-methyl allyl acetate couples to alcohols 1a-1j with complete levels of branched regioselectivity to furnish products of carbonyl crotylation 3a-3j, which are formed with good levels of anti-diastereoselectivity and exceptional levels of enantioselectivity. An identical set of optically enriched carbonyl crotylation products 3a-3j is accessible from the corresponding aldehydes 2a-2j under the same conditions, but employing isopropanol as the terminal reductant. Experiments aimed at probing the origins of stereoselection establish a matched mode of ionization for the (R)-acetate and the iridium catalyst modified by (S)-SEGPHOS, as well as reversible ionization of the allylic acetate with rapid pi-facial interconversion of the resulting pi-crotyl intermediate in advance of C-C bond formation. Additionally, rapid alcohol-aldehyde redox equilibration in advance of carbonyl addition is demonstrated. Thus, anti-diastereo- and enantioselective carbonyl crotylation from the alcohol or aldehyde oxidation level is achieved in the absence of any stoichiometric metallic reagents or stoichiometric metallic byproducts.