Toward the Synthesis of Antascomicin B. Synthesis of a Model of the C22-C34 Fragment via Ireland-Claisen and Allylic Diazene Rearrangements.

The C22-C34 fragment of antascomicin B lacking the C31 and C32 hydroxyl groups has been prepared in 11 steps from commercially available 2-OH-cyclohexanone. An Ireland-Claisen rearrangement was employed to install the C26 and C27 stereocenters. Our recently reported diastereoselective acyclic 1,3-reductive transposition was used to establish the remote C23 stereocenter. Directed hydrogenation was employed to set the C29 stereocenter. The model compound contains 5 of the stereocenters and all of the carbons of the corresponding fragment of antascomicin B.