Chaxine and its analogues, isolated from a Chinese edible mushroom, are highly oxidized steroidal natural products. This paper describes our proposed biosynthetic routes of chaxines and an eight-step synthesis of chaxines from ergosterol, on the basis of our biosynthetic proposal, which includes photo-induced 6-π-electrocyclic reaction of ergosterol, site and regioselective Baeyer-Villiger oxidation of 2-ene-1,4-dione, and acyloxy migration of epoxide of the resulting enol ester as key steps. This biomimetic synthesis enabled us to revise the structures of chaxine B and its analogues proposed by the spectroscopic analysis.

Transition metal hydride species are important components in a variety of catalytic and stoichiometric reactions. Particularly, their importance in recent synthetic organic and inorganic chemistries cannot be overemphasized. Multimetallic polyhydride compounds are of remarkable interest as they may show synergistic effects through the cooperation of multiple M-H sites. However, studies on molecular multimetallic hydride clusters to date have been mainly based on late transition metals, while the chemistry of multinuclear early transition metal polyhydride clusters has remained relatively unexplored.

This article is intended to overview and highlight recent advances in synthesis of early transition metal hydride clusters composed of rare earth or group 4 transition metals and their unique reactions toward small molecules such as N₂, CO, CO₂, H₂, and aromatic compounds. A new family of d-f heteromultimetallic hydride clusters composed of both rare earth and d-block transition metals are also described. These studies are opening a new avenue of multimetallic early transition metal hydride chemistries.

In recent years, acyl fluorides have received a great deal of attention in organic chemistry. Among carboxylic acid derivatives, acyl fluorides display a good balance between stability and reactivity due to their moderate electrophilicity. They are easily prepared from the corresponding carboxylic acids and can be used without any special precautions against moisture. In this review, we describe the recent progress of transformation with acyl fluorides: Lewis base-assisted reactions and transition-metal-catalyzed reactions. The reactions with Lewis base proceeded well to provide a variety of acyl compounds such as lactones, and ketones, etc. In addition, transition-metal-catalyzed transformation gave the corresponding acyl compounds or decarbonylative compounds such as ketones and biaryls, etc.

C3-Nucleophilic substitution of indoles has been limited because the transformation requires an umpolung of the C3 position of indoles. Among the indole derivatives, indole-2,3-epoxides could be potentially be used as C3-electrophilic reagents due to the electrophilic nature of the C2 and C3 positions. However, their use as C3 electrophilic reagents has not been possible so far due to their instability.

We describe a novel and bench stable surrogate of indole-2,3-epoxide, 2-hydroxyindoline-3-triethylammonium bromide (HITAB), which was found to be a convenient reagent for formal C3-electrophilic reactions of indoles with nucleophiles. By taking advantage of the nucleophilic character of the oxygen of the 2-hydroxyindoline, interrupted retro-Claisen and interrupted Feist-Bénary reactions with 1,3-dicarbonyl compounds, one-pot formation of furodiindolines from 3-substituted indoles were achieved. Furthermore, we developed a novel cascade reaction of the indole-2,3-epoxide surrogate with γ-carbolines to access multiheterocyclic compounds containing both isotryptamines and pyrimido［1,6-a］indoles. This reaction utilizes the in situ formation of a bulky quaternary ammonium salt via ammonium exchange, which undergoes Hofmann elimination/vinylogous Mannich/retro-Mannich/cyclization cascade sequences. The synthetic potential of the ammonium salts was demonstrated by the short synthesis of cryptolepine, iheyamine A, racemocine B derivative, and neocryptolepine derivative.

This paper describes effects of a phenolic hydroxyl group of catalyst ligand desorbing proton depending on the pH in aqueous solution on catalytic properties, i.e., (i) electronic effect, (ii) pH-tuning of water-solubility, and (iii) pendant base effect. The oxyanion generated by deprotonation of hydroxyl group under basic conditions shows strong electron donation, which significantly enhances catalytic activity. On the basis of the proton-responsive catalysts, we developed highly efficient catalysts for CO₂ hydrogenation as H₂ storage, formic acid (FA) dehydrogenation as H₂ production, and transfer hydrogenation in aqueous solution. In CO₂ hydrogenation, we developed highly efficient catalysts which can produce formate from CO₂ under atmospheric pressure and room temperature. In addition, the phenanthroline-based catalyst was recovered after the reaction due to self-precipitation by change in the pH of the solution. The dehydrogenation of formic acid evolved hydrogen for 35 days with a TON of 10 million using the efficient and robust catalysts by continuous addition of formic acid. Furthermore, high-pressure hydrogen can be supplied by use of closed reaction system. The proton-responsive catalysts can be applied to transfer hydrogenation using formic acid as a hydrogen donor in water. Interestingly, the transfer deuterogenation via H^＋/D^＋ exchange of iridium catalysts provided deuterated compounds using HCO₂H as an electron source and D₂O as a deuterium source.

Green described a novel method for classifying covalent bonds in transition metals in 1995. Based on this definition, there are three types of ligands, the L-, X- and Z-types, which are categorized depending on the bonding functions for the ligating atom of a ligand. The X-ligand provides one electron to the metal forming the covalent bond (M-X) exemplified by a hydride, alkyl, and halogen. The L-ligand (e.g., P, N, CO, and olefin etc.) provides two-electrons for occupying an empty orbital on the metal center. The so-called Lewis acidic atom (B, Al, and Si etc.) is classified as a Z-type ligand. Although there is quite a wide variety of metal complexes bearing the X- and L-type ligands, the metal complexes featuring the Z-type ligand (M→Z inte-ractions) are still limited. Therefore, the catalytic reaction utilizing the M→Z interaction has been rarely reported. Recent effort in our laboratory focused on the synthesis of gold complex Au(DPB)X (DPB＝diphosphine-borane) featuring Z-type ligand, and its catalytic reaction. As a result, the gold catalysts showed the higher activity than the general catalysts (without Z-ligand) for the several cyclization reactions presumably due to the decrease of the electron density at the gold center by the electron-withdrawing effect of Z-ligand. This article describes the structure analysis of the Au→Z complex and the catalytic reactions based on the alkyne activation.

Lithium-sulfur batteries using sulfur as a cathode are one of the promising next-generation high-energy density rechargeable batteries. However, the short cycle life due to the gradual dissolution of the cathode as soluble lithium polysulfide intermediates should be improved for practical application. Utilization of organic-sulfur composites as cathode materials would be a possible solution for the problems. In this review, recent syntheses of sulfur-rich organic materials with elemental sulfur as a raw material are described.

Artificial metalloenzymes (ArMs) have been developed as a means to enable unique transformations not observed in nature. The most common ArM platform reported to date is the biotin-streptavidin assembly system because of the high binding affinity. Herein, development of rhodium(III)-bound tetrameric/monomeric streptavidins based on the biotin-streptavidin interaction and its use for catalytic asymmetric syntheses of dihydroisoquinolones and α,β-unsaturated-δ-lactams via C(sp²)-H bond activation are described.

The new class of degradable polymers that can be fully degraded to small molecules under mild conditions has attracted much attention because of their high potential for achieving sustainable development. This mini-review overviews approaches for the precise synthesis and complete degradation of polymers obtained by metathesis reactions. The incorporation of cleavable bonds such as hemiaminal ether, acetal, and silyl ether bonds in the polymer backbones was successfully achieved wsithout disturbing the well-defined metathesis polymerization system.

Neooxazolomycin is an antitumor agent isolated by Uemura in 1985. Due to the complexity of highly functionalized lactam-lacton core and the instability of polyene moiety to acid, base, and light, there are only few reports on its total synthesis. In this short review, the total syntheses of neooxazolomycin reported by three groups are described focusing on the construction of lactam-lactone skeleton.