Synthesis of stereopentad analogues of the C14-C22 segment of callystatin A through additions of chiral allenylzinc reagents to stereotriads

The addition of (P)- and (M)-allenylzinc reagents, prepared in situ through Pd-catalyzed metalation of (R)- and (S)-8-butyn-2-ol mesylates, to diaster eomeric stereotriad aldehydes 8, 13, 18, and 23 of syn,syn, syn,anti, anti, anti, and anti,syn stereochemistry was examined. Additions to the former tw o aldehydes afforded the four anti adducts with high diastereoselectivity a nd negligible mismatching. Significant mismatching was observed with the la tter two aldehydes and the (All)-allenylzinc reagent. An evaluation of poss ible transition states is presented in consideration of steric and dipolar control elements.