Synthesis and structural, electronic, and optical properties of oligo(thienylfuran)s in comparison with oligothiophenes and oligofurans

[structure: see text] Alternate thiophene/furan oligomers having four and six heterocycles, i.e., oligo(thienylfuran) dimer and trimer 2 (n = 4 and 6), were newly synthesized by repetitive Stille coupling reactions. The structural, electronic, and optical properties of these oligomers were investigated by X-ray crystallography (for n = 4), cyclic voltammetry (CV), UV-vis and fluorescence spectroscopy, and DFT calculations, and the results were compared with those of corresponding oligothiophenes (1) and oligofurans (3). The inter-ring torsional energy profiles calculated for bithiophene 1 (n = 2), thienylfuran 2 (n = 2), and bifuran 3 (n = 2) at the B3LYP/6-31G(d) level indicated that the most stable conformers of 2 (n = 2) and 3 (n = 2) are fully coplanar with transoid structure while that of 1 (n = 2) is twisted with a dihedral angle of 158 degrees . In accord with this, X-ray crystallographic analysis of 2 (n = 4) revealed that the pi-conjugated system is nearly planar with the inter-ring C=C-C=C dihedral angles between the thiophene and furan rings of 173.6(7) degrees , -177.0(7) degrees , and 172.6(6) degrees . In the packing structure, these nearly planar molecules are arranged in a herringbone pattern. The CV on a series of oligo(thienylfuran)s 2 showed irreversible oxidation peaks at +0.90, +0.42, and +0.29 V vs Fc/Fc(+) for n = 2, 4, and 6, which were 0.15-0.18 V lower than those for corresponding oligothiophenes 1 and were closer to those for oligofurans 3. On the other hand, the UV-vis spectra of 2 showed the longest wavelength absorption to be almost identical with those of the corresponding 1, and more bathochromically shifted than those of the corresponding 3. The results of CV and UV-vis measurements were supported by DFT calculations (B3LYP/ 6-311+G(2d,p)//B3LYP/6-31G(d)). Thus, oligo(thienylfuran)s 2 have HOMOs which are higher than those of oligothiophenes 1 and close to those of 3, and HOMO-LUMO gaps which are close to those of 1 and smaller than those of 3. In fluorescence spectra, the quantum yield of 2 increased with elongation of the pi-system (n = 2 (3.5%), 4 (19%), 6 (24%)).     

Site-selective photocycloadditions of 2-pyrones with electron-poor olefins and the derivation from the cycloadducts

Photosensitized cycloaddition of 4,6-dimethyl-2-pyrone ( 1 ) with methacrylonitrile ( 3b ) afforded two types of [2 + 2]cycloadducts, 4b and 6b , across the C₅-C₆ and C₃-C₄ double bonds in 1 , respectively. Photosensitized reactions of 1 with dimethyl maleate and dimethyl cyclobutene-1,2-dicarboxylate gave [2 + 2]cycload-ducts 4d, 4e across the C₅-C₆ double bond, in addition to [4 + 2]cycloadduct 9d or bicyclo[4.2.0]octadiene 10e . The photoreactions of methyl 2-pyrone-5-carboxylate ( 2 ) with 3b and 2-chloroacrylonitrile ( 3c ) gave [4 + 2]cycloadducts 5b, 5c in addition to [2 + 2]cycloadducts 11b and 11c across the C₅-C₆ double bond in 2 . The photocycloaddition mechanism was explained from results calculated by means of PM3-CI method. Namely, the site- and/or regio-selective products, 4, 5, 8, 9 and 10 were thought to come from the same site-selective radical intermediates in the case of electron-poor olefins. Pyrolysis and/or hydrolysis of the cycload-ducts 4e, 5b, 5c gave 5,6-dihydro-2-pyrone 12 or benzene derivatives.     

A characteristic CH band in VCD of methyl glycosidic carbohydrates

Novel vibrational circular dichroism (VCD) studies in the CH region of a series of methyl glycosidic carbohydrates were examined. The specific CH stretching VCD band predicts absolute stereochemistries of their anomeric positions. The C-1 chiral information was extracted to the methoxy substituent as a probe. The concept of the vibrational chirality probe from a single chiral center in the presence of numerous such centers might be useful in determining the absolute configuration, when a multiplicity of chiral centers is present in a molecule such as a carbohydrate.     

Method to determine quadruple points of a two-component system containing a simple hydrate phase and behavior of the system near these points

A method for determining quadruple points of a two-component system containing a simple hydrate phase is proposed. This method utilizes the quasi-static change of the system along three-phase equilibrium lines and was proved to be able to determine the quadruple points as accurately as the conventional method. By using this method, even though some preparation is necessary, a quadruple point can be determined in just a single experimental run. The behavior of the system near the quadruple points was also examined experimentally, for both the quasi-static and the irreversible change cases. At the quadruple points, the temperature and pressure of the system were kept constant for a while, as at the triple point of water. In both cases, the representative point of the state of the system passed through the quadruple point on a p–T diagram.     

The reactions of benzofuroxan with carbonyl compounds on the surface of solid catalysts

The cyclocondensations of benzofuroxan 1a with carbonyl compounds were smoothly and efficiently carried out by the adsorption of the components on the surface of silica gel or a molecular sieve to form a 2,3-disubstituted quinoxaline 1,4-dioxide. When the reactions using a molecular sieve 3A (powder) were carried out at 90°, the actual reaction times were reduced to 0.5-2 hours. Although Duerckheimer has reported the isolation of only the 7-substituted quinoxaline 1,4-dioxide when 5-methoxybenzofuroxan 1e was allowed to react with ethyl acetoacetate 2j , it produced only the 6-methoxy isomer as the reaction product by our method. 5-Carboxybenzofuroxan 1f did not react with the carbonyl compound.     

Computer-aided structure elucidation methods

Computer-aided structure elucidation methodology is discussed. Major processes involved in computer-aided structure elucidation systems are partial-structure elucidation, structure generation, and structure examination. For the three representative systems CONGEN, CASE, and CHEMICS, these processes are examined. There are four necessary conditions for automated chemical structure elucidation systems: reliability, width of application, ease of modification and portability.     

Solubilization of lanthanide ions by cyclodextrins in basic aqueous solutions

Cyclodextrins form complexes with lanthanide ions in basic aqueous solutions. This complex formation in basic solution dramatically enhances the solubility of lanthanide ions, which are otherwise insoluble due to the formation of hydroxide gels. Solutions of the -cyclodextrin-Ce³⁺ complex effectively hydrolyze 2-deoxyadenosine-5-monophosphate to 2-deoxyadenosine.     

Sweet pregnane glycosides from Telosma procumbens

An intensely sweet polyoxypregnane glycoside, telosmoside A15 (15), was isolated from an Asian Asclepiadaceae plant, Telosma procumbens, collected in Vietnam. This is the first time a sweet pregnane glycoside has been found, and its sweetness intensity is 1000 times greater than that of sucrose. From the same plant, 17 other new glycosides were isolated, having the same aglycone; they are named telosmosides A1-A14 (1-14) and A16-A18 (16-18). Some of these glycosides are also sweet, but others are tasteless or bitter. Chemical structures of the 18 glycosides were determined, and the structure-taste relationship was discussed.     

A new approach to the synthesis of pyridazino[4,5-c]pyridazinones

The reaction of 5-hydrazinopyridazin-3(2H)-ones 1 with α-keto diester 2 in acetic acid afforded the corresponding 4,6-dihydropyridazino[4,5-c]pyridazin-5(1H)-ones 3 and pyrrolo[2,3-d)pyridazin-4(5H)-ones 4 . Compounds 3 were also obtained from 4-bromo-5-hydrazinopyridazin-3(2H)-ones 8 and 2 under milder conditions. 5-Bromo-4-hydrazinopyridazin-3(2H)-one 9 , the regioisomer of 8b , also reacted readily with 2a to give 4,7-dihydropyridazino[4,5-c]pyridazin-8(1H)-one 10b , the regioisomer of 3b .