Stereoselective photo[4+2]cycloadditions of 2-pyrone-5-carboxylates with maleimides in the solid state and in solution

Solid state photoreactions and thermal reactions of 2-pyrone-5-carboxylates with maleimides gave endo-[4+2]cycloadducts, while the sensitized photoreactions gave exo-[4+2]cycloadducts stereo-selectively.     

Formulation study for lansoprazole fast-disintegrating tablet. III. Design of rapidly disintegrating tablets

Lansoprazole fast-disintegrating tablets (LFDT) are a patient-friendly formulation that rapidly disintegrates in the mouth. LFDT consist of enteric-coated microgranules (mean particle size, approximately 300 microm) and inactive granules. In the design of the inactive granules, mannitol was used as a basic excipient. Microcrystalline cellulose, low-substituted hydroxypropyl cellulose (L-HPC), and crospovidone were used as binders and disintegrants. A new grade of L-HPC (L-HPC-33), with a hydroxypropoxy group content of 5.0-6.9%, was developed and it has no rough texture due to a decrease in water absorption. It was clarified that L-HPC-33 could be useful as a binder and disintegrant in rapidly disintegrating tablets. LFDT contain enteric-coated microgranules in tablet form. The enteric-coated microgranule content in LFDT affect qualities such as tensile strength, disintegration time in the mouth, and dissolution behavior in the acid stage and in the buffer stage of LFDT. The 47.4% content of the enteric-coated microgranules was selected to give sufficient tensile strength (not less than 30 N/cm(2)), rapid disintegration time in the mouth (not more than 30 s), and dissolution behavior in the acid stage and buffer stage similar to current lansoprazole capsules. Compression force affected the tensile strength and the disintegration time in the mouth, but did not affect the dissolution behavior in the acid and buffer stages.     

Regioselective oxidative coupling of 4-hydroxystilbenes: synthesis of resveratrol and epsilon-viniferin (E)-dehydrodimers

Treatment of 5-[2-(4-hydroxyphenyl)vinyl]benzene-1,3-diol (resveratrol) with an equimolar amount of silver(I) acetate in dry MeOH at 50 degrees C for 1 h followed by chromatographic purification with a short silica gel column allowed the isolation of its (E)-dehydrodimer, 5-[5-[2-(3,5-dihydroxyphenyl)vinyl]-2-(4-hydroxyphenyl)-2,3-dihydrobenzofuran-3-yl]benzene-1,3-diol, as a racemic mixture in high yield. The present method was applicable to the oxidative dimerization of 4-hydroxystilbenes such as trans-styrylphenol and 5-[6-hydroxy-2-(4-hydroxyphenyl)-4-[2-(4-hydroxyphenyl)vinyl]-2,3-dihydrobenzofuran-3-yl]benzene-1,3-diol (epsilon-viniferin) leading to the corresponding 2-(4-hydroxyphenyl)-2,3-dihydrobenzofurans possessing various types of biological activities.     

Cis-Trans Isomerization and Oxidation of Radical Cations of Stilbene Derivatives

Isomerization from cis stilbene derivatives (c-S (S = RCH=CHC(6)H(5): 1, R = C(6)H(5); 2, R = 4-CH(3)C(6)H(4); 3, R = 4-CH(3)OC(6)H(4) (= An); 4, R = 2,4-(CH(3)O)(2)C(6)H(3); 5, R = 3,4-(CH(3)O)(2)C(6)H(3); 6, R = 3,5-(CH(3)O)(2)C(6)H(3); 7, AnCH=C(CH(3))C(6)H(5); 8, AnCH=CHAn)) to trans isomers (t-S) and oxidation of S with O(2) were studied in gamma-ray radiolyses of c-S in Ar-saturated 1,2-dichloroethane (DCE) and of S in O(2)-saturated DCE, respectively. On the basis of product analyses, it is suggested that a smaller barrier to c-t unimolecular isomerization for c-3(*+)-5(*+) and 8(*+) than for c-1(*+), 2(*+), and 6(*+) due to the single bond character of the central C=C double bond for c-3(*+)-5(*+) and 8(*+) with a p-methoxyl group but not for c-1(*+), 2(*+), and 6(*+) without a p-methoxyl group because of the contribution of a quinoid-type structure induced by charge-spin separation. The isomerization proceeds via chain reaction mechanisms involving c-t unimolecular isomerization and endergonic hole transfer or dimerization and decomposition. The isomerization of c-3(*+) to t-3(*+) is catalyzed by addition of 1,4-dimethoxybenzene but terminated by triethylamine. The regioselective formation of 3d in oxidation of 3(*+) with O(2) is explained by spin localization on the beta-olefinic carbon in 3(*+). The results of product analyses are compared with the rate constants of the unimolecular isomerization and the oxidation for S(*+) measured with pulse radiolyses.     

Peri-selective photocycloadditions of methyl 2-pyrone-5-carboxylate with unsaturated cyclic ethers

Photosensitized cycloaddition reaction of methyl 2-pyrone-5-carboxylate ( 1 ) with 2,3-dihydrofuran gave cis- exo- and cis-endo-[2 + 2] cycloadducts across the C₃-C₄ double bond in 1 , and a [4 + 2] cycloadduct which was different in addition-orientation from the Diels-Alder adducts. Each [2 + 2] cycloadduct was obtained by the use of sensitizers having different triplet energies. Photosensitized reactions of 1 with 3,4-dihydro-2H-pyrans afforded cis-endo-[2 + 2] cycloadducts, respectively. The photocycloaddition mechanism was also explained from the excited state of 1 calculated by means of MNDO-Cl method.     

Occurrence of Stilbene Glucosides in Upuna borneensis

Four new stilbene glucosides, upunosides A ( 1 ), B ( 5 ), C ( 6 ), and D ( 7 ), were isolated from the stem of Upuna borneensis (Dipterocarpaceae) together with the three known glucosides 3, 4 , and 8 . Upunoside A ( 1 ) is the first natural instance of a glucoside of a resveratrol pentamer, and its aglycone has a dibenzo‐fused bicyclo[5.3.0]octadiene and two dihydrobenzofuran moieties. The relative structure of the aglycone was determined by spectral analysis including 1D and 2D NMR experiments.     

Reversible intramolecular triplet-triplet energy transfer in benzophenone-N-methylphthalimide dyad

In the present paper, we synthesized a series of benzophenone (BP)-N-methylphthalimide (MePI) dyads (Cn, n = 3, 6, and 9, where n denotes the number of methylene in the linker) and investigated the photochemical properties and intramolecular triplet-triplet energy transfer from BP(T1) to MePI. Formation of two different intramolecular complexes was found, that is, a ground-state complex and a singlet exciplex. The formation of the triplet-equilibrium between MeBP and MePI was observed. The triplet-equilibrium constant (1.0 and 1.1 for C6 and C9, respectively) and forward ((3.8 +/- 1.3) x 107 and (3.9 +/- 1.2) x 107 s-1 for C6 and C9, respectively) and back ((3.8 +/- 1.3) x 107 and (3.6 +/- 1.2) x 107 s-1 for C6 and C9, respectively) energy transfer rates were estimated from the result of transient absorption measurements. From the van't Hoff plots, enthalpy and entropy change for the equilibrium formation were estimated.     

Charge separation in acridine- and phenothiazine-modified DNA

The formation of the long-lived, charge-separated state in DNA upon visible light irradiation is of particular interest in molecular-scale optoelectronics, sensor design, and other areas of nanotechnology. However, the efficient generation of the charge-separated state is hampered by fast charge recombination within a contact ion pair, which limits the application of DNA for photoelectrochemical sensors and devices. In this study, a series of protonated 9-alkylamino-6-chloro-2-methoxyacridine (Acr+)- and phenothiazine (Ptz)-modified DNAs were synthesized for the further understanding of the mechanism of charge separation in DNA to generate a long-lived, charge-separated state with a high quantum yield (Phi). The Acr+ serves as a photosensitizer to produce a hole on guanine (G), and the G-C base pairs were used as a hole-transporting pathway to separate a hole from Acr* (the one-electron-reduced form of Acr+) to be trapped at Ptz. Since Acr+ oxides only G upon photoexcitation, the A-T base pair can be used as a spacer between Acr+ and the G-C base pair to avoid the formation of a contact ion pair. The charge injection dynamics was investigated by steady-state fluorescence spectra and fluorescence lifetime measurements, and the Phi and the lifetime of the charge-separated state produced upon photoirradiation were assessed by nanosecond laser flash photolysis of the Acr+- and Ptz-modified DNA. A long-lived, charge-separated state was successfully formed upon visible-light irradiation, and the Phi was the highest for the DNA having a single intervening A-T base pair between Acr+ and the G-C base pair. These results clearly demonstrated that the charge separation process in DNA can be refined by putting a redox-inactive intervening base pair as a spacer between a photosensitizer and the nucleobase to be oxidized to slow down the charge recombination rate.     

Synthesis and characterization of tetraanthroporphyrins

Linearly pi-expanded novel tetraanthroporphyrins with and without aromatic rings at the meso-positions were prepared quantitatively for the first time from the corresponding precursors by a retro-Diels-Alder reaction.     

Single-molecule Detection of Reactive Oxygen Species: Application to Photocatalytic Reactions

In this review, we have focused on the oxidation reactions of single dye molecules by reactive oxygen species (ROS). The methodologies for the single-molecule detection of ROS, such as hydroxyl radical (HO^•), singlet oxygen (O₂(a¹Δ_g)), and hydrogen peroxide (H₂O₂), have been introduced together with examples. In particular, a successful application using the single-molecule fluorescence technique for the investigation of the TiO₂ photocatalytic oxidation reactions is demonstrated in detail.