A Flexible Approach to Azasugars: Asymmetric Total Syntheses of (+)‐Castanospermine, (+)‐7‐Deoxy‐6‐ epi ‐castanospermine, and (+)‐1‐ epi ‐Castanospermine

The asymmetric total synthesis of natural azasugars (+)‐castanospermine, (+)‐7‐deoxy‐6‐ epi ‐castanospermine, and synthetic (+)‐1‐ epi ‐castanospermine 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 aldehydes (≥95 %  de ; de =diastereomeric excess) and a diastereodivergent reduction of tetramic acids, which allows formation of three continuous stereogenic centers with high diastereoselectivities. The method also provides a flexible access to structural arrays of 5‐(α‐hydroxyalkyl)tetramic acids, such as 17/34 , and 5‐(α‐hydroxyalkyl)‐4‐hydroxyl‐2‐pyrrolidinones, such as 18 and 25/35 a . 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.