Highly stereoselective directed reactions and an efficient synthesis of azafuranoses from a chiral aziridine

Polyhydroxylated pyrrolidines, such as biologically important azafuranoses represented by the natural product (+)-2,5-imino-2,5,6-trideoxy-gulo-heptitol and its C(3)-epimer, were elaborated from a commercially available enantiomerically pure (2R)-hydroxymethylaziridine by highly stereoselective directed reactions in more than 61% overall yield. At first, the nucleophile 2-trimethylsilyloxyfuran was directed to (2R)-aziridine-2-carboxaldehyde by ZnBr₂ to yield the unusual anti-addition product as a single isomer via the chelation-controlled transition. The ring opening of aziridine was followed by conjugate addition to give a cis-fused bicycle, which was converted to the target molecule after the required reductive operations.