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-hydroxy-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 five of the stereocenters and all of the carbons of the corresponding fragment of antascomicin B.