Convergent approach to pumiliotoxin alkaloids. Asymmetric total synthesis of (+)-pumiliotoxins A, B, and 225F

A versatile convergent approach for preparing the pumiliotoxin alkaloids has been developed employing Pd(0)-catalyzed cross-coupling reactions between homoallylic organozincs and vinyl iodides. The (Z)-iodoalkylidene indolizidine 34, which served as a common key intermediate, was synthesized through highly stereoselective addition of the chiral silylallene 19 to (S)-acetylpyrrolidine followed by a palladium-catalyzed intramolecular carbonylation-cyclization sequence. This synthetic process allowed the first total synthesis of (+)-pumiliotoxin 225F. The intermediate (Z)iodoalkylidene indolizidine 34 obtained was converted to a homoallylzinc chloride derivative and subjected to homoallyl-vinyl cross-coupling with the (E)-vinyl iodide 42 using Pd(PPh3)(4) catalyst to give the cross-coupled product 47 with a 1,5-diene side chain. Subsequent deprotection provided (+)-pumiliotoxin A. On the other hand, the (Z)-iodoalkylidene indolizidine 34 was transformed into the homoallyl-tert-butyl zinc derivative, which underwent palladium-catalyzed cross-coupling with the (E)-vinyl iodide 50 and subsequent deprotection to afford (+)-pumiliotoxin B.