102(P-38) シガトキシンCTX3CのABCDE環部の収束合成(ポスター発表の部)

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CTX3C (1) was isolated as congener of ciguatoxin (CTX) from cultured dinoflagellate Gambierdiscus toxicus by the Yasumoto group. Its unique ladder-shaped trans-fused polycyclic structure possessing 13 ether rings is a synthetic challenge and has attracted the attention of synthetic chemists. During the course of our studies directed toward CTXs, we achieved the syntheses of their individual medium ring parts and developed a convergent method for the construction of their X/6/7/X ring systems. Here, we disclose alternative successful convergent construction of the ABCDE ring system 2 of 1 from the AB- and E-ring segments (5 and 6, respectively). Previously, effective convergent method applying coupling reaction of an acyl anion equivalent with an aldehyde followed by reductive cyclization reactions was developed for the synthesis of trans-fused tetracyclic ethers in this laboratory. Therefore, dimethyldithioacetal mono-S-oxide 5 and aldehyde 6 were selected as AB-and E-ring segments, respectively, in our synthetic plan for 1. The segment 5 was prepared in 16 steps from 7 in 12% total yield. The aldehyde 6 was synthesized in 20 steps from 16 in 38% total yield. Coupling reaction of deprotonated 5 with 6 followed by simultaneous removal of TBS and dithioacetal groups afforded ketodiol 4, which was subjected to reductive cyclization and the subsequent oxidation and deprotection to give 3. Hydroxy ketone 3 was efficiently cyclized into pentacyclic ether 35 under the similar reductive conditions as the cyclization of 4. Treatment of 35 with an excess amount of LDBB induced not only benzyl group detachment but also conversion of the PBBO group to a BnO group to produce 2 successfully. Thus, the ABCDE-ring part 2 was efficiently constructed in 10 steps from 6 in 11% total yield.
天然有機化合物討論会の論文
2004-10-01

102(P-38) シガトキシンCTX3CのABCDE環部の収束合成(ポスター発表の部)

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