Asymmetric [4+3] cycloadditions between benzofuranyldiazoacetates and dienes: Formal synthesis of (+)-frondosin B

The reaction of benzofuranyldiazoacetates with 1,3-dienes catalyzed by the dirhodium tetracarboxylate Rh-2(R-DOSP)(4), generates formal [4 + 3] cycloadducts with >94% de and 91-98% ee. The reaction proceeds by a tandem cyclopropanation/Cope rearrangement followed by a stereoselective tautomerization. This methodology was extended to a formal synthesis of (+)-frondosin B.