Convergent synthesis of the E'FGH ring fragment of ciguatoxin 1B via an acetylene cobalt complex strategy |
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Authors: | Takai Shigeyuki Sawada Naotaka Isobe Minoru |
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Affiliation: | Laboratory of Organic Chemistry, Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa, Nagoya 464-8601, Japan. |
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Abstract: | A convergent synthesis of the E'FGH ring fragment 28 of ciguatoxin 1B, a principal toxin causing widespread seafood poisonings "ciguatera", has been accomplished through (i) coupling between the E' ring-acetylide 9 and the H ring-aldehyde 20, (ii) stereoselective F ring cyclization via an acetylene cobalt complex, (iii) conversion to a carbonyl function under high-pressure hydrogenation, and (iv) reductive hydroxyketone cyclization to construct the G ring. In the (1)H NMR analysis of 28 at room temperature, a considerable broadening phenomenon was observed due to the slow conformational changes of the FG ring, as reported for natural ciguatoxin 1B. When measured in pyridine at -20 degrees C, the spectra of 28 exhibited a 3.5:1 mixture of two conformational isomers (UP and DOWN conformers). |
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