Eunsang Kwon

A novel metal-free panchromatic TiO2 sensitizer based on a phenylenevinylene-conjugated unit and an indoline derivative for highly efficient dye-sensitized solar cells

A novel donor-π-spacer-acceptor type organic dye (AK01) bearing a phenylenevinylene-conjugated system and a derivative of indoline donor was synthesized for dye-sensitized solar cells. AK01 showed panchromatic TiO(2) sensitization with high overall conversion efficiency of 6.2% under AM 1.5 illumination (100 mW cm(-2)).

Highly Efficient Cu(OAc)2‐Catalyzed Dimerization of Monofunctionalized Hydrofullerenes Leading to Single‐Bonded [60]Fullerene Dimers

The single-bonded fullerene dimer RC60 C60R, which has a direct covalent bond between two C60 cages, is an interesting and unusual structure that is expected to display interesting optical and electronic properties through the interaction of two adjacent fullerene cages. Since the pioneering work by Krusic and co-workers, studies on ESR and X-ray crystalstructure analysis revealed that singly bonded fullerene dimers consist of racemic and meso isomers, which are in equilibrium with the monomer radical (RC60C) in solution. [3]

Total Syntheses of Complanadines A and B

Naturally occurring dimeric alkaloids often show distinct and/ or improved biological activities compared with those of their monomers. To investigate the details of their biogenesis and bioactivity, it is attractive to synthesize dimeric natural products from their monomers, in the same way as nature may do. This approach is even more challenging when the dimer is unsymmetrical, because it requires position-controlled union of the two monomers. Complanadines, isolated from the club moss Lycopodium complanatum by Kobayashi et al. , are dimeric Lycopodium alkaloids (Scheme 1). These compounds are unsymmetrical as a result of a C2 C3’ bipyridyl linkage and/or different oxidation levels of the lycodine units. Complanadines A (1), B (2), D, and E have been reported to induce secretion of neurotrophic factors (NTFs) from 1321N1 human astrocytoma cells, thus they are lead compounds for the development of drugs for treating Alzheimer s disease. Because of their challenging unsymmetrical structures and intriguing biological activities, complanadines have attracted much attention from organic chemists. Two groups reported a total synthesis of complanadine A in 2010. In these pioneering studies, the 2,3’-bipyridyl framework was elegantly constructed, but there are still no reports of efficient and versatile ways of assembling unsymmetrical structures like that of complanadine B (2). Herein, we report the total syntheses of 1 and 2 by straightforward coupling of the monomeric units, which were prepared from a single N-protected lycodine, and the subsequent correction of the reported specific rotation ([a]D) of natural complanadine A (1) to levorotatory. Retrosynthetically, pyridine mono-N-oxide 4 could be a common intermediate for 1 and 2 (Scheme 2). Reduction of 4 would afford 1. Regioselective benzylic oxidation using the

Oxidative Amidation of Nitroalkanes with Amine Nucleophiles using Molecular Oxygen and Iodine

The formation of amides and peptides often necessitates powerful yet mild reagent systems. The reagents used, however, are often expensive and highly elaborate. New atom-economical and practical methods that achieve such goals are highly desirable. Ideally, the methods should start with substrates that are readily available in both chiral and non-chiral forms and utilize cheap reagents that are compatible with a wide variety of functional groups, steric encumberance, and epimerizable stereocenters. A direct oxidative method was developed to form amide and peptide bonds between amines and primary nitroalkanes simply by using I2 and K2 CO3 under O2 . Contrary to expectations, a 1:1 halogen-bonded complex forms between the iodonium source and the amine, which reacts with nitronates to form α-iodo nitroalkanes as precursors to the amides.

Donor–acceptor dyes incorporating a stable dibenzosilole π-conjugated spacer for dye-sensitized solar cells

Four novel organic dyes including three based on dibenzosilole (YS01–03) and one based on fluorene (YS04) were synthesized, and their photophysical properties and dye-sensitized solar cell (DSC) performances were characterized. The silicon-containing dibenzosilole-based dyes (YS01–03) were superior to the carbon analogue fluorene-based dye YS04 in incident-photon-to-current conversion efficiency (IPCE), and total solar-to-electric conversion efficiency (η), with YS03, which has the bulkiest and most branched electron donor group, achieving the highest η of 5.07% compared to 2.88% of YS04. To better understand how silicon influences the excited state oxidation potentials (S+/*) and absorption maxima (λmax), the equilibrium molecular geometries of dyes YS01–04 were calculated using density functional theory (DFT) utilizing B3LYP energy functional and DGDZVP basis set. It was shown that the torsion angles (θ1 and θ2) across the biphenyl linkages of dyes containing silicon (YS01–03) were less twisted than that of the silicon-free dye (YS04), which enhanced the π–π* overlap, and that translated into photocurrent enhancements in the silicon-containing dyes YS01–03. Moreover, the vertical electronic excitations and S+/* of dyes YS01–04 were studied using different long-range corrected time-dependent DFT methods, including CAM-B3LYP, LC-BLYP, WB97XD, and LC-wPBE at the basis set level DGDZVP. Excellent agreement between the calculated, using CAM-B3LYP/DGDZVP, and experimental results was found.

Enantioselective Diels–Alder Reaction of 1,2-Dihydropyridines with Aldehydes Using β-Amino Alcohol Organocatalyst

The enantioselective Diels-Alder reaction of 1,2-dihydropyridines with aldehydes using an easily prepared optically active β-amino alcohol catalyst was found to provide optically active isoquinuclidines, an efficient synthetic intermediate of pharmaceutically important compounds such as oseltamivir phosphate, with a satisfactory chemical yield and enantioselectivity (up to 96%, up to 98% ee). In addition, the obtained highly optically pure isoquinuclidine was easily converted to an optically active piperidine having four successive carbon centers.

Equatorenes: synthesis and properties of chiral naphthalene, phenanthrene, chrysene, and pyrene possessing bis(1-adamantyl) groups at the peri-position.

Chiral polycyclic aromatic hydrocarbons containing bis(1-adamantyl) groups at the peri-positions, named equatorenes, were synthesized in optically pure form starting from optically pure 4,5-bis(1-adamantyl)-8-methoxy-1-naphthol. A sequential Diels-Alder reaction of furan and arynes generated from 1,2-bromotriflates provided tricyclic and tetracyclic epoxides, and acid-catalyzed aromatization gave phenanthrol and chrysenol. Deoxygenation reactions involving the hydrogenolysis of triflates gave 1,8-bis(1-adamantyl)naphthalene, 1,10-bis(1-adamantyl)phenanthrene, and 1,12-bis(1-adamantyl)chrysene. 3,4-Bis(1-adamantyl)pyrene was synthesized from phenanthrol by Sonogashira coupling and Pt-catalyzed cyclization. Essentially no racemization occurred during the synthesis. X-ray analysis indicated the distorted naphthalene moiety possessing the peri-diadamantyl groups and the flat structure of the other benzene rings. UV-vis analysis of the equatorenes showed considerable redshifts compared with that of the corresponding achiral arenes. Electrochemical analysis of the naphthalene and pyrene indicated that the distortion decreased the highest occupied molecular orbital stability with no marked effect on the lowest unoccupied molecular orbital energy level, and the origin was discussed on the basis of calculation results.

Isopimarane diterpene glycosides, isolated from endophytic fungus Paraconiothyrium sp. MY-42.

Six isopimarane diterpenes, compounds 1-6, were isolated from the endophytic fungus Paraconiothyrium sp. MY-42. Compound 1 possesses a 19-glucopyranosyloxy group. Its structure was first elucidated by spectroscopic data analysis and finally confirmed by X-ray crystallography, whereas structures 2-6 were mainly elucidated based on the analysis of spectroscopic evidence. Compounds 2 and 3 showed moderate cytotoxicities against the human promyelocytic leukemia cell line HL60 (IC₅₀ 6.7 μM value for 2 and 9.8 μM for 3).

Highly enantioselective organocatalytic oxidative kinetic resolution of secondary alcohols using chiral alkoxyamines as precatalysts: catalyst structure, active species, and substrate scope.

The development and characterization of enantioselective organocatalytic oxidative kinetic resolution (OKR) of racemic secondary alcohols using chiral alkoxyamines as precatalysts are described. A number of chiral alkoxyamines have been synthesized, and their structure-enantioselectivity correlation study in OKR has led us to identify a promising precatalyst, namely, 7-benzyl-3-n-butyl-4-oxa-5-azahomoadamantane, which affords various chiral aliphatic secondary alcohols (ee up to >99%, k(rel) up to 296). In a mechanistic study, chlorine-containing oxoammonium species were identified as the active species generated in situ from the alkoxyamine precatalyst, and it was revealed that the chlorine atom is crucial for high reactivity and enantioselectivity. The present OKR is the first successful example applicable to various unactivated aliphatic secondary alcohols, including heterocyclic alcohols with high enantioselectivity, the synthetic application of which is demonstrated by the synthesis of a bioactive compound.

A concise and unified strategy for synthesis of the C1-C18 macrolactone fragments of FD-891, FD-892 and their analogues: formal total synthesis of FD-891.

A concise and unified strategy for the synthesis of C1-C18 macrolactone fragments of FD-891 and FD-892 as well as their analogues is reported. The strategy includes a stereoselective vinylogous Mukaiyama aldol reaction (VMAR) using chiral silyl ketene N,O-acetal to construct C6-C7 stereocenters, an E-selective ring closing metathesis to construct a C12-C13 olefin, and stereodivergent construction of a C8-C9 epoxide.