Yoshihiko Nakashita

Regiospecific (biogenetic-type) synthesis of 2-methyl-5H-pyrano[3,2-c][1]benzopyran-4-one, the basic skeleton in citromycetin

Regiopspecific cyclization of the acetal (4g), derived in 5 steps from 2′-benzyloxyacetophenone (3a), gave the pyrone (6a), which was easily converted into 2-methyl-5H-pyrano[3,2-c][1]benzopyran-4-one (1b), the basic skeleton in citromycetin (1a).

Studies on the synthesis of heterocyclic compounds containing benzopyrone. Part 4. Synthesis of 4,10-dihydro-3-hydroxy-3-methyl-1H,3H-pyrano[4,3-b][1] benzopyran-10-one, the basic skeleton in fulvic acid

The synthesis of 4,5-dihydro-3-hydroxy-3-methyl-lH,3H-pyrano[4,3-b][1]benzopyran-10-one (1b), the basic skeleton in fulvic acid, is described. The acetal (4), chosen as a common intermediate for syntheses of the basic skeletons in fungal metabolites such as fulvic acid and citromycetin, was cyclized into the dihydropyrone (7) with 5% HCl-tetrahydrofuran (1 : 2) regioselectively. Debenzylation followed by cyclization of the boron complex of (7) gave the tricyclic pyrone [2b], which was converted into the benzopyranone (1b) with 5% HCl–(1 : 1)

Studies of the syntheses of heterocyclic compounds containing benzopyrone. Part 3. Synthesis of 2-methyl-4H,5H-pyrano[3,2-c][1]benzopyran-4-one, the basic skeleton in citromycetin

The synthesis of 2-methyl-4H,5H-pyrano[3,2-c][1]benzopyrano-4-one (1b), the basic skeleton in citromycetin, is described. The alcohol (6a), chosen as the starting material, was oxidized to the dione (6c) which, after methylenation, was treated with concentrated hydrochloric acid–methanol (1 :100) at ambient temperature to afford the pyrone (9a) regioselectively. Hydrogenation and bromine substitution of the pyrone (9a) gave the bromide (9e), which was converted into the benzopyranone (1b) with aqueous sodium hydrogen carbonate.

Regioselective synthesis of 4,10-dihydro-3-hydroxy-3-methyl-1H,3H-pyrano[4,3-b][1]benzopyran-10-one, the basic skeleton in fulvic acid

The first synthesis of 4,10-dihydro-3-hydroxy-3-methyl-1H,3H-pyrano[4,3-b][1]benzopyran-10-one, the basic skeleton in fulvic acid, was achieved by sequential cyclization, which involved hydrogenation of the boron complex of the dione (5) obtained by regioselective cyclization of the acetal (4) with HCl in tetrahydrofuran.

Studies of the syntheses of heterocyclic compounds containing benzopyrone. Part 2. Syntheses of khellin analogues

6-Acetoxy-4,7-dimethoxybenzofuran-5-carbonyl chloride (4), prepared from khellin (1) by three steps, was condensed with enamines or lithium enolates. Further treatment with acid afforded the khellin analogues (10). Better yields were obtained with lithium enolates than with enamines.

New synthetic route to chromones

Condensation of o-acetoxybenzoyl chloride with lithium enolates, obtained by treatment of trimethylsilyl ethers with phenyl-lithium, gives chromones.