11 (±)-2,3-ジヒドロキシトリネルビタンの全合成(口頭発表の部)

抄録

Trinervitane (1) is a unique tricyclic diterpene isolated as defence substance of termite soldiers by G. D. Prestwich. Herein we describe the first synthesis of 2,3-dihydroxytrinervitanes (1, 2α and 2β OH). Construction of trinervitane skeleton Bicyclic allyl chloride 9, prepared from 7,16-secotrinervitane (8), was treated with AgClO_4 at -20℃ to provide the dihydroxy trinervitane 10 in 68% yield. On the other hand, kempane diol 13 was formed in 50% yield by the same reagent at +20℃ The cyclization of diacetyl-derivertive 14 to 15 with AgClO_4 proceeded in high yield when carried out in the presence of pyridine. Conformational analysis The conformation of 17 and its plausible cyclization mode was discussed with the help of physical evidence including X-ray crystallographic analysis. The evidence indicates that the bisect conformation of the allyl chloride 17 changes to the bent conformation at allyl cation intermediate generated by the action of AgClO_4. This change is crucial for the facilitation of the five-membered ring formation, leading to the trinervitane skeletons. Total synthesis of (±)-2,3-dihydroxytrinervitanes Regio- and face-selective hydrogenation of triene 20 was achieved by PtO_2 in CH_2Cl_2 to give 21 with the 6: 1 ratio at C-12 position (Scheme 3). Isomerization of 23 to the conjugated enone 24 with DBU in refluxing toluene proceeded in high yield. The ring-opening of the epoxide 25 was carried out by treatment with TMSCl at -10℃ to give allyl alcohol 26, which was transformed to an easily separable 1: 1 mixture of 2,3-dihydroxytrinervitanes, 1a and 1b. Synthetic study of optically active secotrinervitane Aiming at the asymmetric synthesis of trinervitane type diterpenes, a synthetic route to the dihydrosecotrinervitane 37 was explored starting from (R)-citronellol (29) (Scheme 4). Coupling reaction of allylester 30 and Grignard reagent 31 gave dihydrogeranylgeraniol 32, which was converted into epoxy alcohol 36. The cyclization of 36 to 37 is unsuccessful at present.