Synthesis of Stereopentad Subunits of Zincophorin and Rifamycin-S through Use of Chiral Allenyltin Reagents

The anti,anti adduct 3, from addition of the allenic stannane (P)-2 to the α-methyl-β-OBn aldehyde (S)-1 promoted by SnCl4, was converted to the stereopentad 6 by a sequence involving reduction to the (E)-allylic alcohol with Red-Al, Sharpless asymmetric epoxidation, and addition of the higher-order methyl cyanocuprate to the derived epoxide 5. Stereopentad 6 was converted to the acetonide acetal 15, an intermediate in Danishefsky's synthesis of zincophorin. By a similar sequence, adduct ent-4 was converted, via diol 19, to stereopentad 22, an intermediate in Kishi's synthesis of rifamycin-S. An alternative route to diol 19 was achieved from the MOM-protected derivative 28 of epoxy diol 18.