Stereoselective Total Syntheses of (+)-Castanospermine and Neu5Ac Methyl Ester.

Concise and stereocontrolled total syntheses of (+)-castanospermine and N-acetylneuraminic acid methyl ester were achieved from diastereomerically enriched anti, syn, syn-1,3-oxazine and anti, syn, anti-1,3-oxazine, respectively. The key step in this strategy was the stereoselective BF3·OEt2-mediated allylation.