Pyrrolizidine alkaloid analogues. Synthesis of ten-membered macrocyclic diesters of (+)-retronecine

Abstract
The first synthesis of ten-membered macrocyclic pyrrolizidine diesters [(3)–(6)] has been achieved by treatment of (+)-retronecine (2) with succinic anhydride, (±)-trans-cyclohexane-1,2-dicarboxylic anhydride, and phthalic anhydride respectively, followed by lactonisation via the pyridine-2-thiol esters.