Encapsulation with the Protrusion of Cruciform 9,10‐Bis(arylethynyl)anthracene Derivatives in a Self‐Assembled Boronic Ester Cavitand Capsule: Photochemical and Photophysical Properties

The self‐assembled boronic ester cavitand capsule 3 quantitatively and tightly encapsulates 2,6‐diacetoxy‐9,10‐bis(arylethynyl)anthracene derivatives 4 a – 4 c as highly fluorescent cruciform guests to form complexes 4 a @ 3 , 4 b @ 3 , and 4 c @( 3 )₂. The structural features of capsule 3 , which possesses two polar bowl‐shaped aromatic cavity ends and four large equatorial windows connected by dynamic boronic ester bonds, made it possible to encapsulate cruciform 4 with protection of the reactive anthracene core inside the capsule and with two protruding arylethynyl groups, the π‐conjugated arms of compound 4 , through two of the equatorial windows of the capsule. Thus, complexes 4 a @ 3 , 4 b @ 3 , and 4 c @( 3 )₂ show greater resistance to photochemical reactions in solution and fluorescence quenching in the powder state compared to free guests 4 . In addition to the improved photostability, restriction of the free rotation of the arylethynyl groups of guests 4 upon encapsulation results in sharpening of the UV/Vis absorption peaks with a red‐shift and a significant increase in some of the two‐photon‐absorption peaks of complexes 4 a @ 3 , 4 b @ 3 , and 4 c @( 3 )₂ compared with free guests 4 .     

Enantiodivergent syntheses of (−)- and (+)-dysibetaine CPa and N-desmethyl analog

The first total syntheses of (−)-dysibetaine CPa and the antipode have been achieved using enantioselective solvolysis of meso-cyclic anhydride mediated by quinine derivative as an organocatalyst. The synthesis features a demonstration of an enantiodivergent organic synthesis of both enantiomers of dysibetaine CPa whereby the absolute configurations of natural product were elucidated as (3R,4R). Application of the present methodology to an enantiomerically pure novel GABA analog is also reported.     

Practical method to estimate the standard deviation in absorption coefficients measured with THz time-domain spectroscopy

A model of standard deviation in the intensity spectrum of electric field observed with the terahertz time-domain spectroscopy (THz-TDS) is proposed to estimate the random error in the transmittance and absorption coefficients from a single or a few measurements of a sample. The proposed standard deviation which is derived on the basis of the statistical standard deviation and noise floor of intensity spectrum of reference fits well to the standard deviation of transmittance as well as absorption coefficient computed statistically. This study contributes the simple and computationally efficient method to demonstrate the accuracy in optical constants like imaginary part of refractive index and absorption coefficients measured using the THz-TDS.     

Remarkable potassium selectivity of ion sensors based on supramolecular gel membranes made from low-molecular-weight gelators without any ionophore

Supramolecular gels formed by low-molecular-weight gelators bearing diamide groups were used as ion-sensing membranes for the first time. Even though no special ionophore was added to the membranes, excellent ion selectivities for K(+) over alkali metal ions were realized with the sensor systems.     

3-O-alkylascorbic acids as free radical quenchers. II. Inhibitory effects on some lipid peroxidation models.

We previously found that 3-O-dodecylcarbomethylascorbic acid (3-RASA,3,HX-0112) exhibited a potent inhibitory effect on biochemical lipid peroxidation and that 3-RASA (3) alleviated myocardial lesions induced by ischemia-reperfusion treatment in rats. In this study we examined the mode of action of 3-RASA (3) on the inhibition of lipid peroxidation. There was no reducing activity by 3-RASA (3) (i.e., no oxide was produced) against ferric ions and superoxide anion radicals. The low reducing activity of 3-RASA (3) against a radical as compared to that of alpha-tocopherol was obtained by using a stable radical. However, 3-RASA (3) had a potent inhibitory effect, almost equal to that of alpha-tocopherol, in the model of lipid peroxidation dependent on enzymatic superoxide generation. 3-RASA (3) very strongly inhibited the chain-reaction of the peroxidation induced by Fe(2+)-linoleic acid hydroxyperoxide. On the basis of these findings, it appears that the anti-lipid-peroxidative effects of 3-RASA (3) are due to the inhibition of the radical chain-reaction, as a chain-breaking antioxidant.     

Synthesis of a Structure‐Definite α‐Cyclodextrin‐Based Macromolecular [3]Rotaxane Using a Size‐Complementary Method

The challenging synthesis of an α‐cyclodextrin (CD)‐based macromolecular rotaxane with definite structure was fulfilled using a size‐complementary method. A new peracetylated (PAc) α‐CD‐based size‐complementary [3]rotaxane was prepared and its thermal dissociation kinetics studied. The de‐slippage mechanism was found to be different from that of the native α‐CD‐based system. PAcα‐CD‐based size‐complementary [3]rotaxanes were employed as initiators for a ring‐opening polymerization of ?‐caprolactone to obtain the macromolecular [3]rotaxanes. Detailed investigation of component dissociation showed the highly movable character of the wheel on the polymer main chain. A general method for controlling the movement of wheels in rotaxane frameworks, even in polymer systems, was established. This will enable the development of new supramolecular architectures and molecular machines.     

Double-stranded helical polymers consisting of complementary homopolymers

Two complementary homopolymers of chiral amidines and achiral carboxylic acids with m-terphenyl-based backbones were synthesized by the copolymerization of a p-diiodobenzene derivative with the diethynyl monomers bearing a chiral amidine group and a carboxyl group using the Sonogashira reaction, respectively. Upon mixing in THF, the homopolymer strands assembled into a preferred-handed double helix through interstrand amidinium-carboxylate salt bridges, as evidenced by its absorption, circular dichroism, and IR spectra. In contrast, when mixed in less polar solvents, such as chloroform, the complementary strands kinetically formed an interpolymer complex with an imperfect double helical structure containing a randomly hybridized cross-linked structure, probably because of strong salt bridge formations. This primary complex was rearranged into the fully double helical structure by treatment with a strong acid followed by neutralization with an amine. High-resolution atomic force microscopy revealed the double-stranded helical structure and enabled the determination of the helical sense.     

Aerobic Direct Dioxygenation of Terminal/Internal Alkynes to α-Hydroxyketones by an Fe Porphyrin Catalyst

We herein report a new synthetic method for the preparation of α-hydroxyketones by the dioxygenation of alkynes. The reaction proceeds at room temperature under the action of Fe porphyrin and pinacolborane under air as a green oxidant to produce α-hydroxyketones. The mild reaction conditions allow chemoselective oxidation with functional group tolerance. Terminal alkynes in addition to internal alkynes are applicable, affording unsymmetrical α-hydroxyketones that are difficult to obtain by any reported dioxygenation of unsaturated C−C bonds.     

Enzymatic macrolactonization in the presence of DNA leading to triostin A analogs

Excised thioesterase domains are versatile catalysts for macrocyclization. However, thioesterase-catalyzed cyclization is often precluded due to the occurrence of hydrolysis and product inhibition. To circumvent these obstacles, we devised an unprecedented strategy: coincubation with DNA to capture the cyclic products possessing DNA-binding properties. In experiments involving echinomycin thioesterase-catalyzed macrolactonization leading to the cyclic triostin A analog TANDEM, we found that the addition of DNA drastically improved the yield of TANDEM (19% --> 67%), with a complete reversal of the cyclization:hydrolysis ratio (1:2 --> 18:1). Furthermore, the applicability of this protocol was demonstrated for a variety of substrates. The results described herein provide insight into the mechanism of echinomycin thioesterase-catalyzed conversions and also pave the way for chemoenzymatic synthesis of the quinoxaline antibiotics and their analogs.