12 ビシクロ[2.2.2]オクタ-5-エン-2-オン系の橋頭位置換法の開発とテルペン合成への応用(口頭発表の部)  [in Japanese] 12 A NOVEL METHOD FOR BRIDGEHEAD SUBSTITUTION OF THE BICYCLO-[2.2.2]OCT-5-EN-2-ONE AND APPLICATION TO TERPENOID SYNTHESES  [in Japanese]

A novel method to replace the bridgehead ether group of 1-methoxybicyclo[2.2.2]oct-5-en-2-ones (12) by an alkyl or aryl group or hydrogen has been developed. By treatment with BF_3-2MeOH, ketones 12 were transformed into 1-methoxybicyclo[3.2.1]oct-3-en-2-ones (13). Reactions of 13 with organolithium reagents (RM) gave the endo alcohols (15). By treatment with TsOH in benzene, pinacol-type rearrangement of 15 proceeded smoothly to give the C-1 substituted bicyclo[2.2.2]oct-5-en-2-ones (17). The C-1 unsubstituted bicyclo[2.2.2]octenones (17, R=H) were derived similarly using DIBAH instead of RM. The overall transformation proceeded in a range of 20-68% yields. This bridgehead substitution has been applied to the first total synthesis of (+/-)-nakafuran-8 (7), antifeedant against common reef fishes isolated from the marine sponges Dysidea fragilis and etheria and some nudibranchs, and to the formal total synthesis of (+/-)-gymnomitrol (25), the unique tricyclic sesquiterpene.