Intramolecular Cyclopropanation of Active Methylene Derivatives Based on FeCl2 or FeCl3-Promoted Radical-Polar Crossover Reactions

Radical-polar crossover reactions were studied for the intramolecular cyclopropanation of active methylene derivatives. In the presence of FeCl3 as a stoichiometric oxidant and K2HPO4 as a base, the dehydrogenative cyclopropanation of active methylenes proceeded through the FeCl3-promoted oxidative radical cyclization followed by the ionic cyclization to give the bicyclic cyclopropanes. The use of α-chloro-active methylenes leads the subcatalytic cyclopropanation involving two redox pathways. In the presence of K2HPO4, the redox cyclopropanation proceeded by using FeCl2 (20 mol%) in combination with ligand (20 mol%).

Regiocontrol Using Fluoro Substituent on 3,6-Disubstituted Arynes

The effect of fluoro substituent on the regioselectivity of several reactions of 3,6-disubstituted arynes was studied. These arynes contained another inductively electron-withdrawing substituent other than fluorine. A reasonable degree of regiocontrol was achieved in the (3 + 2) cycloaddition reaction of 3,6-disubstituted aryne containing both fluorine and bromine atoms with benzyl azide. Furthermore, the insertion reaction of aryne into Sn-F σ-bonds and the three-component coupling reaction involving the insertion of aryne into C=O π-bonds also led to the high degree of regiocontrol.

Regiocontrol by Halogen Substituent on Arynes: Generation of 3-Haloarynes and Their Synthetic Reactions

The use of arynes as highly reactive intermediates has attracted substantial attention in organic synthesis. To enhance the utility of arynes, the regiocontrol in the reactions of unsymmetrically substituted arynes is an important task. The introduction of halogen substituent at 3-position of arynes leads to sufficient regiocontrol for various synthetic reactions. This short review highlights the utility of 3-halo­arynes in organic synthesis and discusses the distortion models used to explain regioselectivity, representative reactions of 3-haloarynes generated from polyhaloarenes, and the preparation and reactions of easily activatable aryne precursors.

1 Introduction

2 Distortion Models

3 Reaction of Precursors Activated by an Organometallic Reagent or Base

4 Preparation of Easily Activatable Precursors

5 Reactions of Easily Activatable Precursors

6 Concluding Remarks

Oxidation of α,β-Unsaturated Ketones by Organophotocatalysis Using Rhodamine 6G under Visible Light Irradiation: Insight into the Reaction Mechanism

The oxidative transformation of α,β-unsaturated ketones was investigated under visible-light-induced photocatalytic conditions using rhodamine 6G as an organophotocatalyst. In this organocatalysis, the mild co-oxidant bromotrichloromethane (BrCCl3) acts not only as a quencher toward the activated photocatalyst species, having reductant properties, but also as a brominating reagent for the intermediate radicals. This study shows that bromine atom transfer from BrCCl3 to intermediate radicals is a key step in the reaction mechanism of our oxidation method.

Aryne Precursors for Selective Generation of 3-Haloarynes: Preparation and Application to Synthetic Reactions

The synthesis and reaction of new 3-haloaryne precursors 2a-2h were studied. The ortho-(trimethylsilyl)aryl triflate precursors 2a-2h were prepared by a simple procedure involving O-trimethylsilylation and migration of a trimethylsilyl group followed by triflation. The remarkable feature of new precursors is the selective generation of 3-haloarynes by suppressing the competitive thia-Fries rearrangement, which is the problem in the reaction using the well-known 3-haloaryne precursors. The advantage of new precursor 2a over a typical precursor 1 was confirmed by the direct comparisons in several reactions. The application of precursors 2a-2h to the syntheses of heterocycles was also reported.

Stereoselective Organic Reactions in Heterogeneous Semiconductor Photocatalysis

The most significant feature of heterogeneous semiconductor photocatalysis is that both oxidation and reduction occur in a one-pot process. Thus, photocatalysis leads to unique redox organic reactions that cannot be achieved by conventional techniques using oxidants or reductants. Semiconductor photocatalysis is expected to be a new method for fine chemical syntheses of highly valuable molecules such as chiral medicines. However, the use of semiconductor photocatalysts in stereoselective reactions has been limited so far. This mini-review highlights recent progress in stereoselective organic reactions using semiconductor photocatalysts, briefly summarizing the enantio- and diastereoselective reactions based on the currently available literature.

Transition-Metal-Free Activation of Amide Bond by Arynes

Highly reactive arynes activate the N–C and C=O bonds of amide groups under transition metal-free conditions. This review highlights the insertion of arynes into the N–C and C=O bonds of the amide group. The insertion of arynes into the N–C bond gives the unstable four-membered ring intermediates, which are easily converted into ortho-disubstituted arenes. On the other hand, the selective insertion of arynes into the C=O bond is observed when the sterically less-hindered formamides are employed to give a reactive transient intermediate. Therefore, the trapping reactions of transient intermediates with a variety of reactants lead to the formation of oxygen atom-containing heterocycles. As relative functional groups are activated, the reactions of arynes with sulfinamides, phosphoryl amides, cyanamides, sulfonamides, thioureas, and vinylogous amides are also summarized.

Chiral α-hydroxy acid-coadsorbed TiO2 photocatalysts for asymmetric induction in hydrogenation of aromatic ketones

In enantioselective photohydrogenation of aromatic ketones on TiO₂, the enantioselectivity is strongly affected by not only chiral reagents but also the crystalline phase, surface structure, and morphology of TiO₂.

Aqueous-Medium Selective Modification of Cysteine and Related Thiols with Tricyclic Oxygen-Heterocycles

The utility of tricyclic oxygen-heterocycles as a reagent for the thiol-selective modification toward bioconjugation was demonstrated by the use of l-cysteine, homocysteine, captopril, and glutathione as a nucleophile having a thiol group. These trapping reactions proceeded under the mild and aqueous reaction conditions.

Aqueous-Medium Carbon-Carbon Bond-Forming Radical Reactions Catalyzed by Excited Rhodamine B as a Metal-Free Organic Dye under Visible Light Irradiation

The utility of rhodamine B as a water-soluble organic photocatalyst was studied in the cascade radical addition-cyclization-trapping reactions under visible light irradiation. In the presence of (i-Pr)2NEt, the electron transfer from the excited rhodamine B to perfluoroalkyl iodides proceeded smoothly to promote the carbon-carbon bond-forming radical reactions in aqueous media. When i-C3F7I was employed as a radical precursor, the aqueous-medium radical reactions proceeded even in the absence of (i-Pr)2NEt. In these reactions, the direct electron transfer from the excited singlet state of rhodamine B would take place. Furthermore, the cleavage of the C-I bond in less reactive i-PrI could be achieved by the reductive electron transfer from the excited rhodamine B, which was confirmed by the fluorescence quenching of rhodamine B with the addition of i-PrI.

Cascade radical reaction of substrates with a carbon-carbon triple bond as a radical acceptor

The limitation of hydroxamate ester as a chiral Lewis acid coordination moiety was first shown in an intermolecular reaction involving a radical addition and sequential allylation processes. Next, the effect of hydroxamate ester was studied in the cascade addition–cyclization–trapping reaction of substrates with a carbon–carbon triple bond as a radical acceptor. When substrates with a methacryloyl moiety and a carbon–carbon triple bond as two polarity-different radical acceptors were employed, the cascade reaction proceeded effectively. A high level of enantioselectivity was also obtained by a proper combination of chiral Lewis acid and these substrates.

Polarity-mismatched addition of electrophilic carbon radicals to an electron-deficient acceptor: cascade radical addition-cyclization-trapping reaction

The polarity-mismatched perfluoroalkyl radical addition to electron-deficient alkenes was studied. For this study, several substrates having two polarity-different radical acceptors were employed to investigate the regiochemical courses of cascade reaction. In the case of substrate 1 having a methacryloyl moiety, we found polarity-mismatched perfluoroalkylation giving 15a-e as a major course over the polarity-matched perfluoroalkylation giving 16a-e. Moreover, in the case of substrates 2-7, perfluoroalkyl radicals selectively added to an electron-deficient alkene moiety of 2-7, to give polarity-mismatched perfluoroalkylation products without the formation of regioisomers. Next, the control of enantioselectivity was studied. In the case of substrates 1 and 3, the reaction proceeded with good enantioselectivities by employing a chiral Lewis acid, prepared from chiral box ligand 24 and Zn(OTf)(2). For direct comparison, we also studied the reaction with other carbon radicals, derived from ICH(2)CO(2)Et, ICH(2)CN, BrC(CO(2)Et)(2)Me, and CCl(3)Br, which have electrophilic character.

Lewis acid-mediated radical cyclization: stereocontrol in cascade radical addition-cyclization-trapping reactions

An efficient approach for achieving radical cyclizations by using hydroxamate ester as a coordination tether with Lewis acid was studied. The chiral Lewis acid-mediated cascade radical addition-cyclization-trapping reaction proceeded smoothly with good enantio- and diastereoselectivities, providing various chiral γ-lactams.

Expression and function of PD-1 in human γδ T cells that recognize phosphoantigens

Programmed cell death-1 (PD-1) is an inhibitory receptor and plays an important role in the regulation of αβ T cells. Little is known, however, about the role of PD-1 in γδ T cells. In this study, we investigated the expression and function of PD-1 in human γδ T cells. Expression of PD-1 was rapidly induced in primary γδ T cells following antigenic stimulation, and the PD-1(+) γδ T cells produced IL-2. When PD-1(+) γδ T cells were stimulated with Daudi cells with and without programmed cell death ligand-1 (PD-L1) expression, the levels of IFN-γ production and cytotoxicity in response to PD-L1(+) Daudi cells were diminished compared to the levels seen in response to PD-L1(-) Daudi cells. The attenuated effector functions were reversed by anti-PD-L1 mAb. When PD-1(+) γδ T cells were challenged by PD-L1(+) tumors pretreated with zoledronate (Zol), which induced γδ TCR-mediated signaling, the resulting reduction in cytokine production was only slight to moderate compared to the reduction seen when PD-1(+) γδ T cells were challenged by PD-L1(-) tumors. In addition, cytotoxic activity of PD-1(+) γδ T cells against Zol-treated PD-L1(+) tumors was comparable to that against Zol-treated PD-L1(-) tumors. These results suggest that TCR triggering may partially overcome the inhibitory effect of PD-1 in γδ T cells.