Carboxybenzyl Group as an O‑Nucleophile in the C–H Allylic Oxidation: Total Synthesis of (−)-Castanospermine

The first palladium-mediated C–H allylic oxidation with a Cbz group acting as an O-nucleophile is reported. It was found that this transformation is promoted by rare-earth metal triflates: Yb­(OTf)₃ or Sc­(OTf)₃. A possible catalytic cycle is proposed. This reaction was applied in the synthesis of a d-xylose derived oxazolidinon, a versatile intermediate used further in the stereoselective synthesis of unnatural (−)-castanospermine. Cyclization of the key intermediate with PhSeBr afforded the desired bicyclic scaffold. In an alternative route, hydroboration/oxidation followed by DPPA-mediated cyclization was used.