Synthesis of Briarane Diterpenoids: Biomimetic Transannular Oxa-6π electrocyclization Induced by a UVA/UVC Photoswitch

A biomimetic synthesis of briareolate ester B (3) from briareolate ester L (1) via the intermediate briareolate ester G (2) has been achieved through a unique transannular oxa-6 pi electrocyclization induced by UVA light. UVC irradiation of 3 triggered a rapid retro-6 pi electrocyclization to establish an unprecedented photochromic switch. In the ground state, reaction of 1 led to the formation of a polycyclic gamma-spiroketal gamma-lactone 5, architecturally related to the ether-bridged cembranoids of the dadiellin class.