Chemo-enzymatic total synthesis of 3-epiaustraline, australine, and 7-epialexine

Sequential enzymatic aldol reaction and bis-reductive amination leads to th e total syntheses of tetrahydroxylated pyrrolizidine alkaloids, 3-epiaustal ine (14), australine (1), and 7-epialexine (11). This approach allows for t heir rapid construction without the need for protecting group manipulation of the hydroxyl functionality. In addition, an improved procedure for the a symmetric epoxidation of divinyl carbinol (3) was described, and the produc t was used in a concise synthesis of the required triol 7 and ent-7.