Synthesis of the [6-6-7-5-5] Pentacyclic Core of Calyciphylline N.

A new approach for the concise 11-step synthesis of the [6-6-7-5-5] BCDEF pentacyclic core of calyciphylline N is described. A type II [5 + 2] cycloaddition was employed to construct the strained BCD skeleton, which encompasses the challenging bicyclo[2.2.2] and bicyclo[4.3.1] ring systems. With a regio- and diastereoselective Lu's [3 + 2] cycloaddition, followed by intramolecular aldol cyclization and elimination, the desired [5-5]-fused EF ring system has been successfully installed, resulting in the complete carbocyclic skeleton of calyciphylline N.