101(P-18) ラジカル反応を用いたアミノ酸類の新規不斉合成法の開発(ポスター発表の部)
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In recent years, much attention has been paid to the development of concise and flexible synthetic approaches to α- and β-amino acids, allowing facile incorporation of functional groups and structural variety. We now report a novel asymmetric synthesis of α- and β-amino acids based on newly-found carbon radical addition to oxime ethers. We first investigated the diastereoselective carbon radical addition to the Oppolzer's camphorsultam derivative of glyoxylic oxime ether. Following the initial study on the stannyl radical-mediated reaction which proceeded smoothly with high diastereoselectivities, we continued our investigation on the radical addition to oxime ether in the absence of Bu_3SnH from economical and ecological points of view. As expected, the radical reaction proceeded effectively via a route involving the iodine atom-transfer process which provided an efficient method for the preparation of α-amino acids based on the addition of more nucleophilic and stable alkyl radicals. The radical reaction was also successfully extended to the one-pot synthesis of α-amino acids. Considering recent interest in the synthesis and characterization of β-amino acids, so-called β-peptides, we next investigated the asymmetric synthesis of β-amino acids based on the radical addition to the oxime ethers bearing camphorsultam. The requisite α-alkyl oxime ether was stereoselectively prepared by the alkylation under the conditions using a phase-transfer catalyst. In the presence of BF_3・OEt2, the carbon radical addition to the α-alkyl oxime ethers under the iodine atom-transfer reaction conditions proceeded smoothly to give the α,β-dialkyl-β-amino acid derivatives with excellent diastereoselectivities. Finally, we investigated a new tandem radical reaction of the substrates having two different radical acceptors such as acrylate and aldoxime ether moieties, which was successfully applied to the asymmetric synthesis of γ-butyrolactones and β-amino acids. A remarkable feature of this addition-cyclization reaction is the construction of two C-C bonds and two chiral centers via Mannich-type radical process.
- 2000-10-01
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