80(P69) 下痢性貝毒Yessotoxinの相対、絶対配置と類縁体の構造(ポスター発表の部)
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Yessotoxin (1, YTX) was isolated as one of the causative toxins of diarrhetic shellfish poisoning (DSP). Its planar structure was elucidated by using NMR spectroscopies and by negative FAB MS/MS. Structurally, YTX is closely related to brevetoxin B and ciguatoxin 4B in having a ladder-shape polycyclic ether skeleton and an unsaturated side chain. In view of the increasing interest in the molecular mechanism of the action of polycyclic ether toxins on sodium channels or cell membranes, we elucidated the stereostructure of YTX to compare with that of brevetoxin B and searched for its analogs. YTX and its analogs were isolated from the digestive glands of DSP-infested scallops, Patinopecten yessoensis, collected in 1993 in Mutsu Bay, Japan, following the previously reported procedures, modifying only the final step of purification; a Develosil ODS-7 column (Nomura Chemicals) with MeOH:MeCN:H_2O (4:2:7) for 1, and MeOH: MeCN:H2O (4:2:9) for 2 and 3 by Yasumoto and co-workers. The relative stereochemistry of 1, 2, and 3 was determined by NOE experiments. We have been developing new chiral anisotropic reagents, such as 2NMA, 9ATMA, 2ATMA, and PGME, which make it possible to determine the absolute configurations of organic compounds by NMR. Among these reagents, 2NMA [methoxy-(2-naphthyl)acetic acid] is the most convenient for secondary alcohols in that (i) the anisotropy of its naphthalene ring is much greater than that of the benzene ring of MTPA (conventional modified Mosher's method), giving greater Δδ(δR- δS) values, and (ii) the enantiomerically pure reagent is easily prepared in substantial amounts. We reported here determination of the absolute configuration of YTX by means of 2NMA. Attempted esterification at 32-OH of YTX with 2NMA was unsuccessful because of its axial orientation. Therefore, we focused on introducing 2NMA at 4-position, which is blocked by a sulfate group. Solvolysis of YTX was facilitated by heating a solution of YTX in pyridine-dioxane (1:1, 120℃, 3h) to give bisdesulfated YTX (5: DS-YTX). When DS-YTX was treated with an excess of acetic anhydride in pyridine at 0℃ for 45 min, a mixture of recovered DS-YTX, monoacetate (6), and diacetate, separable by HPLC, was obtained. Monoacetate 6 (600μg) having free OH at C-4 was divided in two portions, and each portion was treated with (R) and (S)-2NMA, EDC, DMAP, and NEt_3 in chloroform for 43.5 h at room temperature. Each product was purified by HPLC to give pure (R) (7) and (S)-2NMA esters (8), respectively. By analyzing the COSY and HOHAHA spectra of 7 and 8, much higher Δδvalues [δR(7)-δS(8)] of the protons with much greater magnitude than those anticipated for MTPA were obtained (Fig. 4). From these results, the absolute configuration of the hydroxy group at C-4 was determined to be S, and, since the relative stereochemistry of the other asymmetric centers has been established, the present finding led to the absolute configuration of YTX as shown in 1.
- 1996-09-02
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