99(P-46) Yessotoxinの全合成研究(ポスター発表の部)
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概要
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Yessotoxin (1) was isolated as one of the causative toxins of diarrhetic shellfish poisoning (DSP) from the digestive glands of DSP-infested scallops, Patinopecten yessoensis, collected at Mutsu Bay, Japan, in 1986. The absolute structure was elucidated in 1996 by using NMR spectroscopic methods. Yessotoxin is structurally related to brevetoxins and ciguatoxins in having a ladder-shape polycyclic ether skeleton and an unsaturated side chain. We report here a convergent synthesis of the ABCDEF fragment 2 of yessotoxin based on acetylene coupling and oxiranyl anion strategies. The retrosynthetic analysis revealed that 2 was disconnected to the A-ring unit 3, acetylene unit, and the DEF-ring unit 4. The A-ring unit would be prepared from the chiral building block of erythritol derivative (+)-5 and epoxy sulfone 6. The DEF ring could be obtained (-)-5 and three epoxy sulfones 7, 8. and 9. The chiral erythritol derivatives (+)-5 was prepared from 4,6-O-benzylidene-D-glucose and the enantiomeric derivative (-)-5 was prepared from tri-O-acetyl-D-glucal. Both enantiomers are interesting and useful as a four-carbon chiral building block because erythritol itself is meso compound. The coupling reaction of (+)-21 with the oxiranyl anion generated from the trisubstituted epoxy sulfone 6 gave 22, which was converted into the A-ring fragment 3 via the stereospecific 6-endo cyclization. The triflate 3 was then transformed into the A-ring acetylene fragment 25 by reaction with lithium TMS-acetylide followed by detrimethylsilylation. The synthesis of DEF ring fragment was started from the coupling of triflate (-)-21 and the racemic epoxy sulfone 7, and the coupling product was converted into triflate 27 via cyclic ketone 26. The second coupling of 27 with the chiral epoxy sulfone 8 followed by 6-endo cyclization to 28 and the subsequent ring expansion with trimethylsilyldiazomethane gave the 6-7 ring system 30, which was transformed into 32. The third coupling reaction of 32 with the oxiranyl anion generated from 9 followed by 6-endo cyclization gave the DEF ketone 34. Coupling reaction of the triflate 4 prepared from 34 with the lithium acetylide of 25 gave 37, ruthenium oxidation of which afforded diketone 38. Heating of 38 in HCH(OMe)_3-MeOH in the presence of TsOH gave the hexacyclic diacetal 39 with the desired ring system. Finally, reduction of the two methyl acetal groups with Et_3SiH-TMSOTf afforded the ABCDEF ring system of yessotoxin.
- 天然有機化合物討論会の論文
- 2002-09-01
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