98(P18) アゼピノインドール型麦角アルカロイド、クラビシピチン酸の合成(ポスター発表の部)
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概要
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The formal synthesis of optically active cis- and trans-clavicipitic acids (1a, b) has been achieved. The key steps are i) the introduction of 1-hydroxy-3,3-dimethylally group at 4-position of the indole ring via directed lithiation of 1-(triisopropylsilyl)gramine (2) and ii) the stereoselective coupling of Shollkopf's optically active dihydropyrazine 4 with the 3, 4-disubstituted lindole 10. The efficient synthesis of 10 using directed lithiation of 2 as a key reaction is shown in Scheme 2. The coupling of 4 with 10 under the standard Shollkopf's conditions (BuLi, THF, -78〜50℃) went in poor stereoselectivity (11a: 11b=2.5: 1). However, the selectivity was dramatically improved by using a solvent or an additive of good coordinating ability. When TMEDA was used as an additive, the highest stereoselectivity was achieved (11a: 11b=30: 1). The poor stereoselectivity under the standard conditions may be rationalized by steric shielding of the less hindered side of the metalated 4 with 10 by coordination with Boc group. After hydrolysis of 11a to 12, its enantiomeric excess was estimated to be 89% by HPLC analysis. The enatiomeric excess was improved to 98% by using less than 1 equiv of BuLi against the dihydropyrazine 4 in the coupling reaction. The amino-alcohol 12 (a sample of 89% ee) was reacted with PPTS in reluxing CH_2Cl_2, for 7 days to give azepinoindoles 13a and 13b in 45% and 32% yields, respectively. Although the reaction time was quite long, no racemization was observed at this stage. Deprotection of Boc group from indole nitrogen was effected by adsorbing the substrate on silica gel, followed by gentle heating under reduced pressure. By using this mild method, clavicipitic acid methyl esters 14a (88% ee) and 14b (87% ee) were obtained from 13a and 13b in 33% and 52% yields, respectively. Since 14a, b have been shown to undergo hydrolysis to clavicipitic acids (1a, b), this therefore represents a formal synthesis of the optically active alkaloids. During the final deprotection reactions, we observed an interesting isomerization between 14a and 14b. Since the configuration at C-5 was essentially retained (from HPLC analysis), this isomerization must occurr via the acid-catalyzed ring opening-recyclization process at C-10 as shown in Scheme 5.
- 天然有機化合物討論会の論文
- 1998-08-31
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