Synthesis of (+)-Bullatacin via the Highly Diastereoselective [3+2] Annulation Reaction of a Racemic Aldehyde and a Nonracemic Allylsilane

[reaction: see text] A total synthesis of (+)-bullatacin has been accomplished via a diastereoselective [3+2] annulation reaction of the highly enantiomerically enriched allylsilane 3 and racemic aldehyde 4, which provides the key bis-tetrahydrofuran fragment 15 with > or = 20:1 ds.