Remarkable stereocontrol in the palladium-catalysed cyclopropanation of vinyl- and dienylboronates by substituted diazoalkanes

Substituted diazoalkanes react smoothly, in the presence of catalytic amounts of Pd(OAc)(2), with a range of vinyl- and dienylboronates, affording in good to excellent yields, the corresponding trisubstituted cyclopropanes. The reaction is remarkably regio-, chemo-, and diastereoselective. The synthetic utility of this novel protocol is illustrated by the efficient assembly of the middle fragment of ambruticin