70 光学活性ベーテノン類の合成研究

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Betaenones are phytotoxins isolated from culture filtrate of Phoma betae Fr., causal fungus of leaf spot disease of sugar beet. Among betaenones A(1), B(2), and C(3), betaenone C(3) exhibited the highest phytotoxic activity to cause wilting of the host plant. Since in the course of structural studies, betaenones B and C were converted to betaenone A, total synthesis of chiral betaenone C (or B) was undertaken using highly stereocontrolled strategy, in which the intramolecular Diels-Alder reaction of chiral trienone 5a, containing a (E,E)-diene, an (E)-enone and three asymmetric-carbons involves. From the molecular models, it is expected that among four possible diastereomers, only prefered isomer 4 would be produced in the Diels-Alder reaction, since the other three diastereomers involve appreciable steric hindrance in the transition state. The trienone 5a will be constructed by condensation of three segments A, B and C. The segments A and B have one and two asymmetric carbons respectively, and would be derived from an intermadiate 6 which is easily accessible from D-glucose according to known procedure. Thus, the intermediate 6 was converted to a tetraacetate 8, which will be transformed to aldehyde 11. The racemic 11 was converted to a sulfone 14 through 12 and 13 in 21% overall yield. Segment B(24) was synthesized from the same intermediate 6 as follows. Methylation of 17 with CH_3MgI gave alkylated products 18 and 19 in ratio of 67:33. The prefered isomer 18 was converted to segment B(24) in low yield. An alternative route for 24 is now under investigation.
天然有機化合物討論会の論文
1983-09-15

70 光学活性ベーテノン類の合成研究

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