Enhanced catalytic activity on titanosilicate molecular sieves controlled by cation-π interactions

A new class of heterogeneous catalytic systems utilizing cation-guest interactions was designed based on microporous titanosilicate molecular sieves. Introducing heavier alkali metal cations on ion-exchange sites of the framework resulted in a significant enhancement of the catalytic activity for oxidation of cyclohexene and styrene, whereas such an enhancement was not observed in oxidation of cyclohexane without π systems. Distinct relationships between the catalytic activities and intermolecular interaction energies which were determined by IR spectroscopic and computational approaches clearly evidenced the predominance of the cation-π interaction in this catalytic system.     

Clarification of the structure of Persoons's periplanone-A, an artifact derived from Hauptmann's periplanone-A

Thermal rearrangement of Hauptmann's periplanone-A by GLC gave Persoons's periplanone-A, of which structure was determined through an X-ray crystallographic analysis of the corresponding alcohol.     

Stereochemical recognition of doubly functional aminotransferase in 2-deoxystreptamine biosynthesis

The doubly functional aminotransferase BtrS in the 2-deoxystreptamine (DOS) biosynthesis, in which two transaminations are involved, was characterized by a genetic as well as a chemical approach with the heterologously expressed enzyme. The gene disruption study clearly showed that BtrS is involved, in addition to the previously confirmed first transamination, in the second transamination as well. This dual function of BtrS for the DOS biosynthesis was further confirmed by the structural determination of the reverse reaction product from DOS. Enantiospecific formation of the reverse reaction product from DOS clearly showed that BtrS distinguishes the enantiotopic amino groups of DOS, but in contrast, both enantiomers of 2-deoxy-scyllo-inosose (DOI) were efficiently accepted by BtrS to give a racemic product. This unique stereochemical recognition of DOI chirality and DOS prochirality by BtrS is mechanistically explained by a specific hydrogen-bond donating force in the enzyme active site as a particular feature of this doubly functional enzyme.     

Enantioselective total synthesis of pteridic acid A

Pteridic acid A, a potent plant growth promoter with auxin-like activity, was synthesized enantioselectively by using the Evans asymmetric aldol reaction as the key C-C bond forming step.     

Purification of boron nitride nanotubes through polymer wrapping

An effective method was proposed to remove obstinate boron nitride phase impurities in boron nitride nanotubes (BNNTs). The method is based on strong interactions between BNNTs and a conjugated polymer wrapping them and significant weight and size difference between BNNTs and impurities. The as-grown samples and purified samples were compared through detailed characterization, using scanning electron microscopy, transmission electron microscopy, and Raman and Fourier transformed infrared spectroscopy. The results reveal that impurities are effectively removed and resultant BNNTs possess perfect crystallization.     

Synthetic study of a moss-produced oxylipin and its structural revision

The proposed structure for a moss-produced acetylenic oxylipin (10-keto-type structure) was synthesized from a known glycidol derivative by an 11-step sequence involving epoxide ring opening with a terminal acetylene and enzymatic hydrolysis of a methyl ester intermediate. The NMR spectra of the proposed structure was, however, different from those of the natural oxylipin, which prompted us to synthesize its 9-epi-isomer and two diastereomeric 12-keto-type positional isomers. Comparison of the NMR spectra of the three isomers with those of the natural oxylipin indicated that the natural oxylipin was actually a mixture of the two diastereomeric positional isomers.     

Synthesis of the N-Amykitanosyl Tetramic Acid Moiety of Amycolamicin

An improved five-step synthetic route from l -fucose to an N-glycosyl l -valine methyl ester has been developed. The new route involves glycosidation of l -fucose with phenol in a β-selective manner without protection/deprotection steps and one-pot stereochemical inversion of a secondary alcohol intermediate and is superior to our previous one both in the number of steps and in overall yield. An N-glycosyl l -valine benzyl ester, prepared from l -fucose in an analogous way, has been elaborated into an N-amykitanosyl tetramic acid derivative, Li's synthetic intermediate for amycolamicin, via a four-step sequence which features the utilization of Bestmann's ylide to stereoconvergently construct an N-glycosyl tetramic acid intermediate in a single step, opening of a cyclic carbonate ring with an amine to regioselectively install a carbamate functionality, and visible light-mediated oxidative debenzylation of an N,N-dibenzyl carbamate.     

Skeletal Ni Catalysts Prepared from Amorphous Ni–Zr Alloys: Enhanced Catalytic Performance for Hydrogen Generation from Ammonia Borane

Skeletal Ni catalysts were prepared from Ni–Zr alloys, which possess different chemical composition and atomic arrangements, by a combination of thermal treatment and treatment with aqueous HF. Hydrogen generation from ammonia borane over the skeletal Ni catalysts proceeded efficiently, whereas the amorphous Ni–Zr alloy was inactive. Skeletal Ni prepared from amorphous Ni₃₀Zr₇₀ alloy had a higher catalytic activity than that prepared from amorphous Ni₄₀Zr₆₀ and Ni₅₀Zr₅₀ alloys. The atomic arrangement of the Ni–Zr alloy also strongly affected the surface structure and catalytic activities. Thermal treatment of the amorphous Ni–Zr alloys at a temperature slightly lower than the crystallization temperature led to an increase of the number of surface‐exposed Ni atoms and an enhancement of the catalytic activities for hydrogen generation from ammonia borane. The skeletal Ni catalysts also showed excellent durability and recyclability.