Azobenzene derivatives carrying a nitroxide radical

Several trans-azobenzene derivatives carrying a nitroxide (aminoxyl) radical (2a, 6a-12a) were prepared, and their photoisomerization reactions to the corresponding cis-isomers were investigated. Although no fruitful results could be obtained for the photoisomerizations of the derivatives with para-subsituents (9a-12a), the unsubstututed derivatives at the para-position (2a, 6a, 7a, 8a) were found to show photoisomerizations by irradiation to give the corresponding cis-isomers (2b, 6b, 7b, 8b), being isolated as relatively stable solid materials, and the change of the intermolecular magnetic interactions was apparently observed by the structural change for each photochromic couple.     

La2XB3 (X = Cl,Br): A Novel Layered Structure with Planar Boron Nets of B3, B6 and B8 Rings

Two new layered rare earth boride halides, La₂XB₃ (X = Cl, Br) have been synthesized. They crystallize in the space group (No. 174) with a = 7.872(1) Å, c = 8.219(2) Å, V = 441.1(1) ų for the chloride, and with a = 7.834(1) Å, c = 8.440(1) Å, V = 448.6(1) ų for the bromide compound, respectively. The crystals of La₂ClB₃ are twinned resulting in an apparent symmetry P6/mmm (No. 191). In the crystal structure of La₂XB₃, trigonal La₆ prisms are condensed into sheets in the a‐b plane, and the halogen atoms X sandwich the La double layers. The connection of B atoms which center the prisms and rectangular prism faces leads to B nets of B₃, B₆ and B₈ rings embedded between the La atom double layers. The chemical bonding is analyzed for the well ordered bromide, and the characteristic disorder in the chloride is discussed.     

Color‐Discriminating Retinal Configurations of Sensory Rhodopsin I by Photo‐Irradiation Solid‐State NMR Spectroscopy

SRI (sensory rhodopsin I) can discriminate multiple colors for the attractant and repellent phototaxis. Studies aimed at revealing the color‐dependent mechanism show that SRI is a challenging system not only in photobiology but also in photochemistry. During the photoreaction of SRI, an M‐intermediate (attractant) transforms into a P‐intermediate (repellent) by absorbing blue light. Consequently, SRI then cycles back to the G‐state. The photoreactions were monitored with the ¹³C NMR signals of [20‐¹³C]retnal‐SrSRI using in situ photo‐irradiation solid‐state NMR spectroscopy. The M‐intermediate was trapped at ?40 °C by illumination at 520 nm. It was transformed into the P‐intermediate by subsequent illumination at 365 nm. These results reveal that the G‐state could be directly transformed to the P‐intermediate by illumination at 365 nm. Thus, the stationary trapped M‐ and P‐intermediates are responsible for positive and negative phototaxis, respectively.     

Highly Efficient Chiral Resolution of dl‐Arginine by Cocrystal Formation Followed by Recrystallization under Preferential‐Enrichment Conditions

An excellent chiral symmetry‐breaking spontaneous enantiomeric resolution phenomenon, denoted preferential enrichment, was observed on recrystallization of the 1:1 cocrystal of dl ‐arginine and fumaric acid, which is classified as a racemic compound crystal with a high eutectic ee value (>95 %), under non‐equilibrium crystallization conditions. On the basis of temperature‐controlled video microscopy and in situ time‐resolved solid‐state ¹³C NMR spectroscopic studies on the crystallization process, a new mechanism of phase transition that can induce preferential enrichment is proposed.     

A facile synthesis of 2-[1-(dialkylamino)alkyl]-4H-3,1-benzoxazin-4-ones by the reaction of 1,1-dimethylethyl 2-isocyanobenzoates with N,N-dialkyliminium iodides

A convenient synthetic approach to 2-[1-(dialkylamino)alkyl]-4H-3,1-benzoxazin-4-ones has been developed. Thus, 1,1-dimethylethyl 2-isocyanobenzoates, which can be readily prepared from 2-nitrobenzoic acids by a simple four-step sequence, react with N,N-dialkyliminium iodides without using any catalysts under mild conditions to give the desired products in generally fair-to-good yields.     

Synthesis of modified pyrimidine nucleosides via Vorbrüggen-type N-glycosylation catalyzed by 2-methyl-5-phenylbenzoxazolium perchlorate

Several acidic azolium salts prepared from oxazole, thiazole, and imidazole derivatives were investigated as catalysts in N-glycosylation reaction using a silylated modified pyrimidine and an acylated ribose or glucose to afford the corresponding pyrimidine nucleoside. Among the salts, 2-methyl-5-phenylbenzoxazolium perchlorate showed the highest catalytic activity. This salt is a nonhygroscopic crystalline compound that shows higher activity than the frequently used trimethylsilyl triflate. Reactions with this salt can be conducted in gram scales.     

Effect of Brønsted acid co-catalyst in asymmetric conjugate addition of 3-aryloxindoles to maleimide under base-free phase-transfer conditions

Highly diastereo- and enantioselective conjugate addition of 3-aryloxindoles to maleimide was accomplished under base-free phase-transfer conditions. The effect of Brønsted acid co-catalyst was examined, and we found that the use of a phenol derivative as co-catalyst was effective for obtaining the target conjugate adduct in good yield with high stereoselectivity.