76 光学活性なシクリトール誘導体を不斉源として用いた不斉合成反応の開発(ポスター発表の部)
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L-Quebrachitol (1L-(-)-2-O-methyl-chiro-inositol) is a naturally abundant chiral cyclitol obtained from the serum of the rubber tree. Although total syntheses of natural products starting from L-quebrachitol have been studied extensively, asymmetric reaction employing L-quebrachitol as chiral source had not been reported. Thus diastereoselective asymmetric reaction employing chiral cyclitol (2 and 3) as chiral auxiliaries were investigated. Reduction of α-keto ester (4) derived from 1L-3-O-t-butyldimethylsilyl-1,2:5,6-di-O-cyclohexylidene-chiro-inositol, which is readily available from L-quebrachitol, with K-Selectride in Et_2O underwent highly diastereoselectively and α-hydroxy ester with 2'S isomer was obtained in 96% de. On the other hand, addition of 18-Crown-6 completely reversed the diastereoselectivity and 2'R-isomer was obtained in 92%de. While addition of Grignard reagents to the α-keto ester (4) proceeded via re-face attack of the ketone to afford α-hydroxy ester with 2'S isomer in 96% de, those of alkyl lithium took place via si-face attack to afford 2'R isomer preferentially. SnCl4 promoted addition of allyl silane or sillyl enol ether to 4 also took place to afford single diastereomer exclusively. 1,3-Dipolar cycloaddition of an acryloyl ester (5) bearing 1L-3-O-t-butyldiphenylsilyl-1,2:5,6-di-O-cyclohexylidene-chiro-inositol with nitrile oxide furnished Δ^2-isoxazolines of high diastereomeric excess. It was found chiral cyclitols derived from L-quebrachitol are efficient chiral auxiliaries.
- 天然有機化合物討論会の論文
- 1993-09-10
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