Application of ruthenium-induced macrocyclization for the construction of macrocyclic depsipeptides

Functionalized Spiro- and Fused-Ring Heterocycles via Oxidative Demetalation of Cyclohexadienyl Ruthenium Complexes

Electron-rich alkoxy- and chloro-substituted azaspirocyclic cyclohexadienyl Ru(II) complexes have been converted to either azaspiro[4.5]decane derivatives or functionalized tetrahydroisoquinolines by treatment with suitable oxidizing agents. Copper(II) chloride was found to provide demetalated products in high yield relative to the other oxidants examined (FeCl(3), DDQ, CAN, [Cp(2)Fe][PF(6)], phenyliodine diacetate, IBX). In certain instances, the efficiency of oxidative demetalation was enhanced by the inclusion of chloride ion additives in the reaction. Pyridinium dichromate (PDC) was also found to effect the demetalation of a wide range of cyclohexadienyl Ru complexes; however, isolated yields of metal-free products were exceedingly low. The cyclohexadienyl ruthenium complexes used in this study were prepared from (arene)Ru(II) precursors; thus, the isolation of alicyclic cyclohexadienone derivatives upon demetalation constitutes completion of a Ru-mediated dearomatization process.