Takeshi Kambara

Highly enantioselective homogeneous catalysis of chiral rare earth phosphates in the hetero-Diels–Alder reaction

Abstract Two types of novel rare earth (RE) complexes were synthesized and used as a chiral Lewis acid catalyst for the hetero-Diels–Alder reaction of carbonyl compounds with the Danishefskys diene under homogeneous conditions. The Y[( R )-H 8 -BNP] 3 ( 3 -Y )-catalyzed reaction of aromatic aldehydes and the Yb[( R )-BNP] 3 ( 1 -Yb )-catalyzed reaction of phenylglyoxylates afforded the corresponding cycloadducts with excellent optical purities (up to 99% ee) in high yields at room temperature. The successful recycling uses of the scandium catalyst ( 3 -Sc ) are also described.

A factor affecting enantioselective reaction of a ternary complex of lithium ester enolate with imine

Abstract Systematic investigation of the varying size of the alkoxy moiety of the lithium ester enolates in the chiral ligand mediated reaction with imines led to the improvement of the enantioselectivity of β-lactam products in up to 93%. The asymmetric induction step was indicated to be the addition step, not the cyclization step.

Role of proline residue in the channel-forming and catecholamine-releasing activities of the peptaibol, trichosporin-B-VIa

Trichosporin-B-VIa (TS-B-VIa) has a Pro14-kinked helical structure which is considered to be important for the formation of peptaibol-type ion-channels in lipid bilayer membranes. TS-B-VIa and its analog [Aib14]TS-B-VIa with Pro-->Aib substitution at position 14, resulting in a straight helical structure, were tested for ion-channel-forming activity in planar lipid bilayer membranes and for ability to induce catecholamine secretion from cultured bovine adrenal chromaffin cells. Voltage-dependent multi-channel conductance, which is characteristic of TS-B-VIa, was also observed for [Aib14]TS-B-VIa. In single-channel measurements, current fluctuations induced by [Aib14]TS-B-VIa had a shorter life-time and showed fewer substates than those induced by TS-B-VIa. Catecholamine secretion induced by these peptides at low concentrations is completely Ca(2+)-dependent. At high concentrations, TS-B-VIa-induced secretion was partly independent of external Ca2+, but this was not the case for the analog. The differences of behavior can be explained in terms of the differences of hydrophobicity, and magnitude of dipole moment due to the conformational changes around position 14 and the C-terminal domain caused by the Pro-->Aib substitution.

Catalytic asymmetric reaction of lithium ester enolates with imines

An external chiral tridentate amino diether ligand 7 catalyzed the condensation reaction of a lithium ester enolate 2 with an imine 3 to give the enantiomerically enriched β-lactam 4 in high yield.

Role of Gln7 in the ion-channel-forming properties of the peptaibol trichosporin-B-VIa

Replacement of the Gln7 residue, which is well conserved among ion-channel-forming peptaibols, by an Ala in trichosporin-B-Vla (TS-B-Vla = AcUAUAUUQUIUGLUPVUUQQPheol; U =α-aminoisobutyric acid, Pheol = phenylalaninol) alters neither the multilevel nature of the TS-B-Via channel nor its conductance, but abolished the rectification of the membrane current characteristic of the TS-B-Vla channel.

ION-CHANNEL-FORMING AND CATECHOLAMINE-RELEASING ACTIVITIES OF ELONGATED AND TRUNCATED ANALOGUES OF TRICHOSPORIN-B

Examination of newly synthesized elongated and truncated analogues of the ion-channel-forming peptide, trichosporin-B-VIa (20 residues), for ion-channel-forming activity in lipid bilayer membranes and catecholamine secretion-inducing activity from adrenal chromaffin cells revealed that the natural compound has the optimal molecular length for both activities.