Diastereo- and enantioselective format synthesis of (+)-conagenin via asymmetric [2,3]-Wittig rearrangement

The formal synthesis of the immunomodulator (+)-conagenin (1) is accomplish ed in a twelve-step sequence with good overall yield (19%), diastereoselect ivity and enantiomeric excess (ds(syn) = 88%, ee = 91%). Key steps of the s ynthesis are the asymmetric [2,3]-Wittig rearrangement of crotyloxyacetalde hyde-SAE;P-hydrazone (S)-5 and the diastereoselective reduction of methylke tone (R,S)-12. The absolute configuration of the (4R)-stereogenic center wa s determined by H-1 NMR NOE-measurements with respect to the known absolute configuration of the (2R,3S)-stereogenic centers of lactone (R,S,R)-14.