Stereoselective Synthesis of C-Glycosylated Pyrrolizidines through Nitrone Cycloadditions

Polyhydroxylated alkaloids, among which iminosugars represent a prominent subclass, are a structurally diverse family of carbohydrate mimics with significant biological activity. Herein, we report a concise and stereoselective synthesis of five novel C-glycosylated pyrrolizidines (<b>15</b>-<b>19</b>) related to casuarine and hyacinthacines, achieved through 1,3-dipolar cycloadditions of nitrones with <i>E</i>-alkenes. The reactions of nitrone <b>21</b> with fructose-derived α,β-unsaturated ester <b>20a</b> and ketone <b>20b</b> proceeded with predictable anti-selectivity, affording isoxazolidine intermediates that were transformed into pyrrolizidines via reductive N-O cleavage and subsequent ring closure. Careful stereochemical assignments were established by NOE analyses.