石川 紘輝

We developed a chiral symmetry breaking method for monoacylated meso diols. The X-ray crystal structure analysis of monoacylated 1,4-anhydroerythritols, meso cyclic diols with a cis configuration, revealed that the O-(p-anisoyl) derivative crystallized as a racemic conglomerate of the P212121 crystal system. It was confirmed that the substrate racemized by intramolecular transfer of the acyl group in the presence of a catalytic amount of base. Evaporating the solvent gradually from the solution or Viedma ripening to promote crystallization-induced deracemization efficiently led to enantiomer crystals. These results provide the first successful example of asymmetric expression and amplification by deracemization of sugar derivatives without an external chemical chiral source. Furthermore, we applied this methodology to acyclic meso-1,2-diols. Three O-monoacylated substrates were successfully deracemized to 99 % ee by Viedma ripening. We also developed asymmetric desymmetrization of meso-1,2-diols by combining acylation and crystallization-induced deracemization.

We developed a chiral symmetry breaking method for monoacylated meso diols. The X-ray crystal structure analysis of monoacylated 1,4-anhydroerythritols, meso cyclic diols with a cis configuration, revealed that the O-(p-anisoyl) derivative crystallized as a racemic conglomerate of the P2(1)2(1)2(1) crystal system. It was confirmed that the substrate racemized by intramolecular transfer of the acyl group in the presence of a catalytic amount of base. Evaporating the solvent gradually from the solution or Viedma ripening to promote crystallization-induced deracemization efficiently led to enantiomer crystals. These results provide the first successful example of asymmetric expression and amplification by deracemization of sugar derivatives without an external chemical chiral source. Furthermore, we applied this methodology to acyclic meso-1,2-diols. Three O-monoacylated substrates were successfully deracemized to 99 % ee by Viedma ripening. We also developed asymmetric desymmetrization of meso-1,2-diols by combining acylation and crystallization-induced deracemization.

Atroposelective resolution for axially chiral nicotinamides was achieved by dynamic chiral salt formation with L-DBTA using six types of nicotinamides that could not be optically resolved by the preferential crystallization method. Kinetic studies of their racemization indicated that the chiral conformation was retained for a significant period of time. Two methods of crystallization-induced asymmetric transformation were examined by dynamic diastereomeric salt formation: solvent evaporation from a supersaturated solution, and attrition-enhanced asymmetric transformation. The attrition method was more effective for asymmetric amplification of diastereomeric salts of axially chiral materials. Attrition of equimolar amount of the nicotinamide salts with L-DBTA converged to one diastereomer salts, and the corresponding enantiomers in 87-99 % ee were obtained after the chiral acid was removed. Changing the ratio of two of the nicotinamides with L-DBTA to 1 : 2 inverted the axial chirality.

Atroposelective resolution for axially chiral nicotinamides was achieved by dynamic chiral salt formation with L-DBTA using six types of nicotinamides that could not be optically resolved by the preferential crystallization method. Kinetic studies of their racemization indicated that the chiral conformation was retained for a significant period of time. Two methods of crystallization-induced asymmetric transformation were examined by dynamic diastereomeric salt formation: solvent evaporation from a supersaturated solution, and attrition-enhanced asymmetric transformation. The attrition method was more effective for asymmetric amplification of diastereomeric salts of axially chiral materials. Attrition of equimolar amount of the nicotinamide salts with L-DBTA converged to one diastereomer salts, and the corresponding enantiomers in 87–99 % ee were obtained after the chiral acid was removed. Changing the ratio of two of the nicotinamides with L-DBTA to 1 : 2 inverted the axial chirality.

Carbonization process of pyromellitic dianhydride (PMDA)-4,4'-diaminodiphenyl ether (ODA)-type polyimide has been studied for decades. However, various reaction mechanisms have been proposed and the detailed mechanisms are still controversial. It is essential to understand the carbonized structures of PMDA-ODA-type polyimide before analyzing the defect structures in graphite. In this work, the carbonization mechanisms of polyimide heated at 1273 K or lower were unveiled and the methodology to analyze carbon materials containing nitrogen, oxygen, pentagons, and heptagons using computational spectral analysis combined with molecular dynamics simulation (ReaxFF) are exhibited. For example, the formations of isoimide and cyclic ether were estimated by ReaxFF, and the presence was supported by experimental and calculated X-ray photoelectron spectroscopy (XPS) and carbon-13 nuclear magnetic resonance (C-13 NMR) spectra of polyimide heated between 813 and 873 K. The formations of pentagons and heptagon, suggested by ReaxFF, were also supported by Raman and C-13 NMR spectra of polyimide heated between 833 and 1273 K. This work also revealed that the previously reported structures were unstable and that amine in the basal plane was the most plausible structure in polyimide heated at 1073 K or higher, as clarified by XPS, ReaxFF, and the energy calculation. (C) 2021 Elsevier Ltd. All rights reserved.

Attrition-enhanced deracemization of two axially chiral nicotinamides, crystallizing as a conglomerate of a P2(1) crystal system, was performed. N,N-Dialkylnicotinamides with substituents on the 2- and 4-positions of the pyridine ring exhibited stable axial chirality due to rotationally restricted scaffolds for the Ar-C(=O)N bond. Crystallization of the racemic mixtures from the melt or attrition-enhanced deracemization led to the chiral breaking of symmetry to give 95-96 % ee of enantiomorphic crystals.

Dynamic enantioselective crystallization enabled the chiral symmetry breaking of two spiropyrans and one spirooxazine. The three spiro compounds afforded racemic conglomerate crystals, and easily racemized in alcoholic solution without irradiation. Optically pure enantiomorphic crystals were obtained by vapor-diffusion crystallization or attrition-enhanced deracemization (Viedma ripening). Their absolute configurations were determined by single-crystal X-ray analysis and each enantiomorphic crystal was correlated with its solid-state circular dichroism (CD) spectrum.

A new class of flavonoids bearing cyclic polyethers involving a phenyl ring was conveniently provided by the intramolecular photochemical dimerization of 2-chromonecarboxylic esters. Irradiation of 2-chromonecarboxylate with a polyether tethered at both ends promoted intramolecular [2 + 2] cyclobutane formation leading to 14- to 27-membered cyclic polyethers. The efficiency depended on the substituted position of the phenyl ring, with ortho- and meta-substituted derivatives giving cycloadducts in good chemical yields and quantum efficiencies, whereas the para-derivatives were inert toward photolysis. X-ray crystallographic analysis revealed that the stereochemistry of the macrocyclic cycloadducts exhibited C-2-symmetry. (C) 2019 Elsevier Ltd. All rights reserved.

Asymmetric Diels-Alder reaction was achieved under achiral conditions. Reaction of prochiral 2-methylfuran and N-phenylmaleimide in heptane or hexane solution at 80 degrees C efficiently gave a conglomerate crystal of exo-type Diels-Alder adduct selectively, and continuous suspension of the reaction mixture with glass beads promoted attrition-enhanced deracemization, leading to an optically active exo-adduct in 90% ee.

Photodimerization of 3-arylindenones in solution and in the solid state was examined. Irradiation of 3-arylindenones in benzene exclusively gave C-2-symmetric antiHH cyclobutane dimers in good yields. In contrast, photolysis in the solid state afforded syn-HH cyclobutane dimers efficiently, which was considerably influenced by the molecular arrangement in the crystal lattice.

C-2-Symmetric cyclic polyethers were conveniently provided by the photochemical dimerization of covalently-linked 2-chromonecarboxylic esters. Irradiation of polyethers possessing 2-chromonecarboxylate groups at both ends efficiently promoted intramolecular [2+2] cyclobutane formation leading to 14- to 23-membered cyclic polyethers in good chemical yields and quantum efficiencies. X-ray crystallographic analysis revealed that the stereochemistry of the cyclobutane was the C-2-symmetric structure.

<p>The reaction of <italic>N</italic>-cinnamoylthioureas with DBU proceeded in an unprecedented “umpolung” cyclization fashion to give five-membered iminothiazolidinones and/or thiohydantoins.</p>

Asymmetric synthesis involving photochemical dimerization of a prochiral flavonoid derivative in solution without any chiral source was achieved. Irradiation of ethyl 6-bromochromonecarboxylate in solution efficiently gave a C-2-chiral anti-head-to-head dimer in excellent chemical yield with good quantum efficiency (phi(365) = 0.15). X-ray crystallographic analysis revealed that the dimer crystallized as a conglomerate of C2 space group. The crystalline dimer precipitated upon irradiation of the monomer in solution, and indirect racemization of the dimer through a reversible photoreaction in solution and selective crystallization simultaneously occurred to give the C-2-chiral dimer in optically active form with up to 80%ee. Optically active photoproducts could be obtained by simply irradiating achiral materials in solution without an external chiral source.

Irradiation of prochiral (E)-aroylacrylamide with sunlight gave single-handed pyrrolinone quantitatively and with an enantiomeric excess (ee) of over 99% under absolutely achiral conditions. The phenomenon was explained by photoisomerization and reversible cyclization followed by dynamic crystallization involving deracemization.

<p>Unprecedented absolute asymmetric synthesis was carried out using frozen chirality derived from chiral crystals of achiral coumarin-3-carboxamide and carbenoids. An enantioselective cycloaddition reaction with a sulfur ylide gave an optically active cyclopropane up to 97% ee. Furthermore, a cycloaddition reaction with dichlorocarbene gave dihydrofuran in 93% ee. Nucleophilic addition of ylide or carbene occurs from the side of the carbonyl oxygen atom to avoid steric hindrance of substituents on the nitrogen atom.</p>