49(P08) パン酵母還元を活用する天然物の不斉合成研究(ポスター発表の部)

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Enantioselective total syntheses of octalactin A(1), quassiol A and 1(15), 8(19)-trinervitadien-9β-ol (4) have been investigated by the extensive use of baker's yeast reduction as the chirality induction method. Formal total synthesis of octalactin A Octalactin A(1) is a novel natural product having potent cytotoxic activity. We have achieved an efficient synthesis of the protected tetraol 6 which has been converted into 1 by Buszek et al. from (-)-citronellol (5). The synthesis involves baker's yeast reduction of α-hydroxyketone 7 giving (R)-diol 8 (〜100%ee), fragmentation of the benzyl ether of 9 with n-BuLi to yield the olefin 10, asymmetric dihydroxylation of 10 using AD-mix-α(selectivity; 9: 1), and stereoselective hydroboration of the ester 15(selectivity; 9: 1) as the key steps. Synthesis of quassiol A and its absolute stereochemistry Quassiol A is a triterpene ether having five chiral centers isolated from Quassia multiflora together with its 18-acetate. We attempted to determine the absolute stereostructure by synthesis, since only the relative configuration around tetrahydrofuran part has been elucidated. Three stereoisomers 2a, 2b, and 2c were synthesized enantioselectively by the coupling of 20 with 25, 28, and 31. The ^1H and ^<13>C NMR spectra of 2a and 2c were found to be identical to those of natural product. On the basis of the fact that the optical rotation of 2a is similar to that of quassiol A with opposite sign, the structure 2, an enantiomer of 2a, was proposed as the absolute stereostructure of quassiol A. Synthetic studies of trinervitane diterpenes Trinervitane diterpenes were isolated as the component of frontal gland secretions of termites. Since none of the natural products belonging to this series has been synthesized though their novel tricyclic carbon framework, the synthesis of 4, the simplest member of the diterpenes, was attempted. The starting diketone (-)-3 was obtained in good yield in 98%ee by the kinetic resolution of (±)-3 with baker's yeast. (-)-3 was converted into the enol trifrate 34, which was subjected to Suzuki coupling with the borane derivative 36 derived from (+)-citronellol. Malonic ester unit was introduced to the acetate 39 again by the Pd-catalyzed cross-coupling reaction. After conversion of 40 into the chlorosufide 43 through an allylic alcohol 42, 11-membered ring formation was examined in various conditions resulting in the formation of 44 in low yield. The conversion of 44 into natural 4 is currently in progress.
天然有機化合物討論会の論文
1997-07-20

49(P08) パン酵母還元を活用する天然物の不斉合成研究(ポスター発表の部)

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