A cascade radical macrocyclisation–transannulation approach towards the construction of ring-fused tricycles and polycycles

Abstract

Treatment of the iodo trienone 6 with Bu₃SnH–AIBN results in the formation of the angular 5,7,5-ring-fused tricyclic ketone 20 by way of a novel sequential 13-endo-trig macrocyclisation followed by two successive 5-exo-trig transannulation processes, viz 7→8→/18/19→20. The cis-anti-trans stereochemistry of 20 was established from an X-ray crystal structure determination of the corresponding 2,4-dinitrophenylhydrazone. By contrast, treatment of the iodo trienone 21 with Bu₃SnH–AIBN, under the same conditions, led to the substituted cyclopropane 33 (instead of the hoped-for tricyclic ketone 22), and only the product 38 of macrocyclisation (without further transannulation to the triquinane 24) was produced when the iodo trienone 23 was treated similarly.