The Reaction Mechanism of Spirocylization and Stereoselectivity Studies for the Calyculin C16–C25 Fragment

Abstract The mechanism of the double intramolecular hetero‐Michael addition, a key reaction in the planned synthesis of the natural product calyculin C, has been studied by NMR. The cyclization follows Baldwin’s rules and proceeds first through a six‐membered ring closure (6‐ endo ‐ dig ), followed by a five‐membered ring cyclization (5‐ exo ‐ trig ). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)