14 5,,6,11-トリデオキシテトロドトキシンの全合成(口頭発表の部)

抄録

Tetrodotoxin (TTX) is a toxic principle of puffer fish poisoning. TTX is a useful biological tool because of its specificity for blocking voltage-dependent sodium channels. 5,6,11-Trideoxy TTX (1), one of the congeners of TTX, has also been isolated from ovaries of puffers, Fugu poecilonotus, by Yasumoto et al. in 1995 and received significant interests in view of biosynthetic relationship with TTX. Synthetic studies of TTX and its congeners have been extensive due to their biological activities as well as their molecular complexity consisting of a quaternary carbon center attached to an amino group, concomitant polyol system, a cyclic guanidine containing an aminal, and an ortho acid for TTX or a δ-lactone for some of the congeners. In this report, we wish to describe the first total synthesis of 5,6,11-trideoxy TTX (1) and its 4-epimer 2. The synthesis was commenced with the known triol 7, prepared from (-)-quinic acid in 4 steps, which was converted to Strecker precursor 6 in 6 steps. 6 was subjected to an asymmetric transferring Strecker synthesis for the diastereoselective construction of a quaternary carbon center involving an amino group at C8a. Installation of a hydroxy group to C7 was accomplished by the Mislow-Evans rearrangement to give 20. Initial hydrogenation of 20 for the introduction of β-methyl group at C6 and subsequent pyrrolidine ring opening via lactam 24 gave alcohol 25. The hydroxyethyl group of 25 was transformed to a vinyl group in 3 steps to give 26. Inversion of the hydroxy group at C7 was performed by oxidation to a diketone followed by diastereoselective reduction to give cis-diol 4, This was converted to aldehyde 28, which, upon treatment with TMSCN in the presence of Et_3N, gave a cyanohydrin as an inseparable mixture of diastereomers. The mixture was guanidylated to give 29. Ozonolysis of the desired (9S)-29 followed by exposure to 20% TFA afforded a mixture of 5,6,11-trideoxyTTX (1), and its 4-epimer 2 and anhydro derivative 31. Thus, the total syntheses of 1 and 2 were accomplished.