72(P54) 分枝シクリトール類の合成と天然物合成への応用 : PA-48153Cならびにレストリクチシンの合成(ポスター発表の部)

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Cyclitols (polyhydroxycycloalkanes), which are classified as a type of carbohydrate, occur widely in nature. Among them, myo-inositol (1) and L-quebrachitol (2) are obtained in large quantities from plants. Given that stereoselective introduction of methyl group into cyclitol ring and regioselective ring cleavage of the resulting branched-chain cyclitols are possible, cyclitols are expected to be versatile chiral building blocks for the synthesis of acyclic and heterocyclic natural products. We report here preparation of methyl branched-chain cyclitols from 2 and synthesis of restricticin (3) and PA-48153C (4) utilizing branched-chain cyclitol derivatives as the starting material. Stereoselective Introduction of Methyl Group into L-Quebrachitol. L-Quebrachitol was transformed to epoxides (7-10), which were treated with several organometallic reagents. It was found that reaction of cyclitol epoxides with Me_3Al and methylcopper reagents generally afforded the methylated product in high stereoselectivity and in moderate-to-good yield. In some cases, reactions with methylcopper reagents showed different stereoselectivity from those with Me_3Al. Formal Synthesis of Restricticin. Baeyer-Villiger reaction of the ketone (16) derived for branched-chain cyclitol (14) afforded 7-membered acetal-lactone (17) as the sole product, which was transformed into tetrahydropyran (21). Exchange of the protecting groups and introduction of methyl group afforded the known synthetic intermediate (26) of antifungal agent, restricticin (3). Total Synthesis of PA48153C. To the methyl branched cyclitol (30) derived from 2 in five steps, second methyl group was introduced stereoselectively via epoxide (32). Ring cleavage reaction of 35, followed by stereoselective construction of E-alkene by CH_3CHI_2-CrCl_2 provided epoxide (44). Coupling reaction of 44 with dithian derivative (49) derived from L-malic acid gave 50, whose thioketal function was effectively converted to β-OH group to provide triol (53). MnO_2 oxidation of 53 generated 2-pyranone skeleton and afforded novel immunosuppresive agent, PA-48153C (4).
天然有機化合物討論会の論文
1997-07-20

72(P54) 分枝シクリトール類の合成と天然物合成への応用 : PA-48153Cならびにレストリクチシンの合成(ポスター発表の部)

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