Effect of core-shell micelle formation on the redox properties of phenothiazine-labeled poly(ethyl glycidy ether)-block-poly(ethylene oxide)

Redox properties of phenothiazine-labeled poly(ethyl glycidy ether)-block-poly(ethylene oxide) (PT-EGE_n-b-EO_m) are reversibly changed by core-shell micelle formation. In the temperature range higher than the critical micellization temperature (cmt), the anodic potential of PT group positively shifts and concomitantly its anodic current decrease, or levels off compared to those of the reference polymer PT-EO_m without the thermo-responsive EGE_n segment. The former alteration is caused by incorporation of hydrophobic PT groups into a core of the micelle and the latter by the decrease in the diffusion coefficient of PT groups due to formation of the core-shell micelles. The cmt value and the temperature-dependent alteration in the redox properties strongly depend on the polymer structure, especially the length of thermo-responsive EGE_n segment. The electrochemically determined hydrodynamic radii of the polymer aggregates seem to be overestimated, compared to the values reported for the aggregates of other thermo-responsive polymers with similar molecular weights, implying the presence of electrochemically inactive PT groups in the copolymers having longer thermo-responsive segments.     

Mechanism for conversion of spirosolane derivative into pregnane

Previously, we reported an interesting reaction by which esculeogenin A [(5alpha,22S,23S,25S)-3beta,23,27-trihydroxyspirosolane], a sapogenol of tomato-saponin, esculeoside A, was easily converted into a pregnane derivative, 5alpha-pregn-16-en-3beta-ol-20-one, merely by refluxing with pyridine and water. Its chemical mechanism including air oxidation is here described.     

Activation parameters for the recombination reaction of intramolecular radical pairs generated from the radical diffusion-inhibited HABI derivative

Activation parameters were determined for the recombination of radical pairs arising from newly designed, photochromic, radical diffusion-restricted hexaarylbiimidazole (HABI) derivative. We have developed a new type of radical diffusion-inhibited HABI derivative, which contains two equivalent HABI units and yields a tetraradical with four equivalent 2,4,5-triphenylimidazolyl radical (TPIR) units by photoirradiation. This radical dimerization proceeds by a successive first-order reaction from the tetraradical to the parent molecule via a diradical. The rate constants of each reaction were determined from the decay profile of EPR signal intensities. The entropies of activation (DeltaS(double dagger)) for the first and the successive dimerization steps were estimated to be -178.5 and -205.5 J K(-1) mol(-1), respectively. Within the experimental temperature range, the radical dimerizations are entropy-controlled (-TDeltaS(double dagger) > DeltaH(double dagger)). The large negative DeltaS(double dagger) values imply a highly ordered transition state, indicating that the radical dimerizations occur when the TPIR units interact at a specific orientation. The present study demonstrates the availability of radical diffusion-inhibited HABI for the kinetic study of radical-radical reaction.     

A synthesis of RGD model cyclic peptide by palladium-catalyzed carbonylative macrolactamization

Cyclic peptidic RGD models were efficiently synthesized by Pd(P(t-Bu)3)2-catalyzed carbonylative macrolactamization in the presence of 4 A molecular sieves under 10 atm of carbon monoxide.     

Photochromism of a radical diffusion-inhibited hexaarylbiimidazole derivative with intense coloration and fast decoloration performance

We report the synthesis and the photochromic behavior of a newly designed, photochromic, radical diffusion-inhibited hexaarylbiimidazole (HABI) derivative with markedly improved photochromic performance in coloration and decoloration rates as well as greater optical density in the colored state. The thermal bleaching rate (tau1/2 = 260 ms at 295 K) is the fastest among the reported ones for HABI derivatives.     

Combinatorial synthesis of RGD model cyclic peptides utilizing a palladium-catalyzed carbonylative macrolactamization on a polymer support

A combinatorial synthesis of 24-member RGD models was accomplished on polymer-support. Ortho-, meta-, and para-iodobenzylamines loaded on an aldehyde linker by reductive amination were coupled with RGD sequences and various omega-amino acids by a split-and-pool method. Palladium-catalyzed carbonylative macrolactamization of the polymer-supported cyclization precursors, followed by acid cleavage, provided conformationally restricted RGD model cyclic peptides.     

Stereospecific Synthesis of a Dihydroxyethylene Isostere of Cyclohexylalanine Amide, (2S,3R,4S)-4-Amino-5-Cyclohexyl-1-Morpholino-2,3-Pentanediol(ACMP) from a Protected Sugar

ABSTRACT

A transition-state analogue of a renin inhibitor at the scissile site, a dihydroxyethylene isostere of cyclohexylalanine amide, (2S,3R,4S)-4-amino-5-cyclohexyl-1-morpholino-2,3-pentanediol(ACMP), was synthesized from 1,2:5,6-di-O-isopropylidene-α-D-allofuranose stereospecifically.