89(P02) 光学活性4-ブロモトリプトファンを利用した麦角アルカロイドChanoclavine-Iの全合成(ポスター発表の部)

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We have already demonstrated the aromatic C-H/olefin coupling reaction with the aid of Pd (II) complex. These include the preparation of 4-bromodehydrotryptophan 2 as a building block for the synthesis of indole alkaloids. Here we would like to report an asymmetric synthesis of chanoclavine-I(3), an ergot alkaloid, which utilizes 4-bromodehydrotryptophan 2 via a Pd (II)-mediated C-H/olefin coupling reaction. Our design is outlined in scheme 2. Treatment of 4-bromoindole 1 with a stoichiometric amount of Pd (II) complex was expected to give intermediate 1A (via route A) without affecting the C-Br bond. Insertion of olefin to 1A followed by syn-β-H elimination was then anticipated to generate 4-bromodehydrotryptophan 2, a compound which was expected to be amenable to further functionalization. As shown in scheme 3, treatment of 4-bromoindole 1 and enamide 4 with Pd(OAc)_2 (1 equiv), NaHCO_3, and chloranil as an additive in 1,2-dichloroethane provided 4-bromodehydrotryptophan 2 in 87% yield. Having completed the synthesis of 4-bromodehydrotryptophan 2, we applied it to the syntesis of (-)-chanoclavine-I (3). Retrosynthetic analysis of (-)-chanoclavine-I (3) suggested that 4-bromodehydrotryptophan 2 might be a good precursor (scheme 5). The key step in the conversion of 4-bromodehydrotryptophan 2 into (-)-chanoclavine-I (3) involves the intramolecular cyclization of 12 (12→11). As shown in Scheme 6, attempts to cyclize 12 to 11 under radical conditions led to disappointing results. On the other hand, Heck cyclization of 12 gave the cyclized product 14. Ultimately, compound 18 was reached (see scheme 7). Not unexpectedly, a variety of attempts to achieve the deprotection of N-Boc group in 18 to complete the synthesis of (-)-chanoclavine-I (3) failed. An alternative route (scheme 8) to (-)-chanoclavine-I (3) required preparation of 22, a key intermediate in Oppolzer's synthesis of (±)-chanoclavine-I. As shown in scheme 8, 14 was converted to the key intermediate 22. This intermediate was further converted to (-)-chanoclavine-I (3) according to Oppolzer's route. As both synthetic sample and natural product have negative optical rotations, the absolute configuration of (-)-chanoclavine-I (3) has been assigned to be the one shown in scheme 8. We wish to add that no racemization occurred in our synthetic route.
天然有機化合物討論会の論文
1996-09-02

89(P02) 光学活性4-ブロモトリプトファンを利用した麦角アルカロイドChanoclavine-Iの全合成(ポスター発表の部)

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