Preparation of indole alkaloids via 1,4-dihydropyridine stage or cyano-masked iminium intermediates
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2004-01-23
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en
Pages
32, [51]
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Report / Helsinki University of Technology, Department of Chemistry, Laboratory of Organic Chemistry, 4/2003
Abstract
Methods to synthesise substituted pyridine derivatives utilising 1,4-dihydropyridines were studied. Addition of nucleophiles to N-alkyl pyridinium salts and application of the products to alkaloid synthesis were investigated. Application of the Kröhnke procedure to the synthesis of indole alkaloids is described. The feasibility of applying the Kröhnke procedure to pyridine derivatives without an electron withdrawing β-substituent at the pyridinium ring was demonstrated by adding dimethyl malonate to the γ-position of Boc protected 1-[2-(3-indolyl)ethyl]pyridinium salts. The method permits access to the indoloquinolizidine skeleton present in several indole alkaloids. The total synthesis of (±)-tangutorine, a novel indole alkaloid, was achieved. The dithionite reduction leading to a 1,4-dihydropyridine derivative provided easy access to the tangutorine skeleton with good yields. In a second part of the work, dihydropyridines were stabilised through the introduction of cyanide ion to iminium intermediates. The Polonovski–Potier reaction and the cyano-trapping method were used in the preparation of dimethyl malonyl substituted indolo[2,3-α]quinolizidine derivative, a potential synthon of antirhine. A novel synthetic approach to the preparation of 2,6-dicyanopiperidine derivatives via 1,4-dihydropyridine intermediates was examined. The formation of 2,6-dicyanopiperidines in the Fry reaction was verified. The stereochemistry of 2,6-dicyanopiperidine derivatives is discussed.Description
Keywords
indole alkaloids, 1,4-dihydropyridines, cyano-trapping
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