22 発芽誘因物質ソルゴラクトンの合成研究(口頭発表の部)

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Seed germination of parasitic weeds of the genera Striga and Orobanche, which cause considerable damages to important gramineous crops such as corn, sorghum, and rice, is known to be stimulated by chemicals released from their host plants. The attractive idea to control such harmful weeds by using effective germination stimulants, inducing suicidal germination in the absence of their host plants, prompted various syntheses of strigol (2), the first discovered naturally occurring stimulant originally isolated from cotton (the non-host plant) root exudates, and its analogues. Sorgolactone was isolated from Sorghum bicolor, a genuine host plant, as a highly potent germination stimulant for Striga and Orobanche. Although spectroscopic details were not sufficient due to the very small amount of the stimulant obtained, the structure 1 was proposed based on ^1H-NMR, MS, and CD comparison with those of (+)-strigol. To ascertain the proposed structure of sorgolactone and also to elucidate structure-activity relationship, we studied syntheses of racemic and optically active sorgolactone and evaluated their biological activities. Synthesis of racemic sorgolactone The synthesis of the racemates of the structure proposed and its three stereoisomers was achieved by confirming the stereostructure of the intermediate (±)-12, obtained from known diketo acid 8 in three steps followed by MPLC separation from the diastereomer mixture, and the final product (±)-1 by X-ray analysis. Bioassay employing Orobanche minor seeds revealed that all of stereoisomers stimulate the germination and the order of activity was (±)-strigol=(±)-15≧(±)-18>(±)-1>(±)-17. Synthesis of optically active sorgolactone Radical cyclization of the phenylselenyl derivative of alkynyl ester 23, obtained from (S)-citronellal (19) in eight steps, afforded exo-cyclic alkene 25 in good yield. The alkene was readily transformed into a diastereomeric mixture of tricyclic lactones, which was separated into oily 11 and crystalline 12. The procedure employed for the synthesis of the racemates led to 1 from 11. Recrystallization of the product improved enantiomeric purity to 100%e.e. (determined by chiral HPLC). Bioassay of 1 and its stereoisomers is now in progress.
天然有機化合物討論会の論文
1997-07-20

22 発芽誘因物質ソルゴラクトンの合成研究(口頭発表の部)

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