A New Enantioselective Total Synthesis of Natural Vincamine via an Intramolecular Mannich Reaction of an Silyl Enol Ether |
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Authors: | Wolfgang Oppolzer Hartmut Hauth Paul Pfffli Roland Wenger |
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Institution: | Wolfgang Oppolzer,Hartmut Hauth,Paul Pfäffli,Roland Wenger |
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Abstract: | Natural Vincamine ( 1 ) has been synthesized in an enantioselective manner starting from the ethylpentenal 7 . In the key step a mixture of the diastereoisomeric racemates, 14 and 15 , was directly obtained from the silyl enol ether 11 and the dihydro-β-carboline 12 by the way of an intramolecular Mannich reaction of the intermediate 13 (Scheme 4). The undesired stereoisomers, 14 and 15b , were recycled to 15a using the related reversible Mannich reaction 18 ? 14 + 15 , followed by crystallization of the salt from 15a and (+)malic acid. 15a was converted to natural vincamine ( 1 ) in several steps including the known transformation 20→1 . |
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