Yasuhiko Kawamura

13C NMR spectral assignment of the A-ring of polyoxygenated flavones

Abstract The 13 C NMR spectra of polyhydroxyflavones were systematically examined by using about 70 examples which have a 4-methoxyphenyl B-ring, such as 5,6,7-, 5,7,8 and 5,6,8-trioxygenated and 5,6,7,8-tetraoxygenated flavones and flavonols, and the following results were obtained. In the 13 C NMR spectra of flavones, the signals at the 2-, 3- and 4-positions in the C-ring were hardly affected by the substituents on the A-ring except for those at the 5-position and shifted regularly by introduction of a methoxy or hydroxy group at the 3-position. The signals at the 5- to 10-positions in the A-ring were greatly affected by the substituents and oxygenated patterns on the A-ring and exhibited the respective characteristic pattern reflecting the A-ring substituents without the effects of the substituents on the B- and C-rings. The substituent effects on the A-ring were estimated from the spectral data. Consequently, it is found that the structures of polyhydroxyflavones can be correctly defined by their 13 C NMR spectral assignment. Additionally, we examined the structures of some natural flavones previously reported.

Synthesis of 5,8-dihydroxy-6,7-dimethoxyflavones and revised structures for some natural flavones

Abstract Six 5,8-dihydroxy-6,7-dimethoxyflavones and three 8-hydroxy-5,6,7-trimethoxyflavones were synthesized from 2′,5′-dihydroxy-3′,4′,6′-trimethoxyacetophenone by adapting the selective O -alkylation and dealkylation, and the differentiation between the flavones and their isomeric 6-hydroxyflavones was clarified by 1 H NMR and UV spectra. Four natural flavones proposed as 5,8-dihydroxy-6,7-dimethoxyflavones, must have other structures and three are shown to be the isomeric 5,7-dihydroxy-6,8-dimethoxyflavones. A flavone, isolated from Ageratum conyzoides , is correctly identified as 8-hydroxy-5,6,7,3′,4′,5′-hexamethoxyflavone, but the structure of a flavone, isolated from Helichrysum , is revised to the isomeric 7-hydroxy-5,6,8-trimethoxyflavone.

Photochemistry of 3-aryl-2-isoxazoline1

Abstract 3-Phenyl-2-isoxazoline (1) was irradiated to give 4-phenyl-2-oxazoline (3), β-aminoaldehyde (14) and benzonitrile from its π-π* singlet excited state. Several related derivatives afforded similar photoproducts on irradiation. The quantum yields of the photoreactions were dependent on the magnitudes of the singlet energies of the 2-isoxazolines. p-Cyanophenyl-2-isoxazoline (1c) formed a one-to-one photoadduct (22) with benzene.

DEHYDRATION INDUCED BY INTRAMOLECULAR REDOX CHARACTER OF A STABLE ALLYLIDENETRIBUTYLPHOSPHORANE

Abstract An air and moisture stable P-ylide, dimethyl fluorenylidenetributylphosphoranylidene-succinate acts as a new type of dehydrating agent for synthesizing acid anhydride, ester, and amide. The ylide is most suitable for inducing these reactions in analogous P-ylides. The reaction is considered to be caused by internal reductive and oxidative nature of the P-ylide.

Curious oxygen effect on photosensitized electron-transfer reactions of benzophenone oxime O-methyl ethers: one-way photoisomerization of an iminic double bond

Abstract Under an oxygen atmosphere, the E and Z isomer ratio of N -methoxy-4-methoxyphenyl-4′-methylphenylmethanimine (initially, [ E ]/[ Z ]=1/1) reached 4/96 upon irradiation (>360 nm) of 9,10-dicyanoanthracene (DCA) as a photosensitizer in acetonitrile.

Revised structure of a natural flavone from Ageratum conyzoides

Abstract The structure of a natural flavone from Ageratum conyzoides , assumed to be 5,6,8,3′,4′,5′-hexamethoxyflavone, is revised to 5,6,7,3′,4′,5

3,5-Dihydroxy-7,8-dimethoxyflavones and revised structures for some natural flavones

Abstract Seven 3,5-dihydroxy-7,8-dimethoxyflavones were synthesized from the corresponding 3,5,7,8-tetramethoxyflavones using selective demethylation and protection of 3- or 5- methoxy group, and their properties were clarified by the UV and 1 H NMR spectral data. The structures of the four natural flavones were revised as follows: the three flavones, isolated from Heteromma simplicifolium , are 5,7-dihydroxy-3,8,3′,4′-tetramethoxyflavone, 5,7-dihydroxy-3,8,3′,4′,5′-pentamethoxyflavone, and 5,7,4′-trihydroxy-3,8,3′-trimethoxyflavone, respectively; the flavone glycoside, isolated from Rudbeckia bicolor , is proposed to be 3,5,4′-trihydroxy-6,7-dimethoxyflavone 3-rhamnoside.

CHARGE-TRANSFER REACTION OF BUTATRIENE: FORMATION OF DIHYDRONAPHTHALENE DERIVATIVE

The charge-transfer reaction of tetraarylbutatriene 1 with tetracyanoethene (TCNE) in dichloromethane at room temperature was studied and we found a novel addition reaction. A red crystalline material 2 was isolated as an intermediate product which is converted slowly into dihydronaphthalene derivative 3 in dichloromethane but rapidly in protic solvent. The structure of the compounds was determined by X-ray crystallography. The detailed structure and the plausible reaction mechanism have also been discussed.

Photochemical behaviour of cyclic imino ethers: The NO bond fission, syn—anti isomerization and cycloaddition reactions in the CNO chromophore

Abstract Photochemical reactions of 3-aryl-2 isoxazolines and six-, seven- and eight- membered analogues were investigated. 3-Phenyl-dihydro-1,2-oxazine was irradiated to give several products which were derived from the initial NO bond fission, whereas the seven- and eight-membered analogues were found to be stable to light. Only the syn — anti isomerization was observed. In addition, a novel [2 + 2] photocycloaddition reaction of 2-isoxazolines having cyano or methoxy-carbonyl groups on the 3-aryl moiety with some aromatic compounds such as benzene, furan, indene and thiophene was discovered. The UV absorption and emission spectra of 3-phenyl-2-isoxazoline and its related heterocyclics are also discussed.

Regioselective Synthesis of 6‐Prenylpolyhydroxyisoflavone (Wighteone) and Wighteone Hydrate with Hypervalent Iodine

Abstract The oxidative rearrangement of 3′‐iodotetraalkoxychalcone with [hydroxyl(tosyloxy)iodo]benzene, followed by cyclization of the resultant acetal gave 6‐iodotrialkoxyisoflavone. The coupling reaction of the isoflavone with 2‐methyl‐3‐butyn‐2‐ol gave 6‐alkynylisoflavone, whose hydrogenation gave wighteone hydrate. Wighteone was synthesized by dehydration of wighteone hydrate.