A Flexible Approach to Azasugars: Asymmetric Total Syntheses of (+)-Castanospermine, (+)-7-Deoxy-6-epi-castanospermine, and (+)-1-epi-Castanospermine
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The asymmetric total synthesis of natural azasugars (+)-castanospermine, (+)-7-deoxy-6-epi-castanospermine, and synthetic (+)-1-epi-caslanospermine has been accomplished in nine to ten steps from a common chiral building block (S)-8. The method features a powerful chiral relay strategy consisting of a highly diastereoselective vinylogous Mukaiyama-type reaction with either chiral or achiral al-dehydes (>95% de: de = diastereomeric excess) and a diastereodivergent reduction of tetramic acids, which allows formation of three continuous stereogenic centers with high diastereo-selectivities. The method also provides a flexible access to structural arrays of 5-(alpha-hydroxyalkyl)tetramic acids, such as 17/34, and 5-(alpha-hvdroxyalkvI)-4-hydroxy1-2-pyrrolichnones, such as 18 and 25/35a. The method constitutes the first realization of the challenging chiral synthons A and D and thus of the conceptually attractive retrosynthetic analysis shown in Scheme 1 in a highly enantioselective manner.