Cascade radical mediated macrocyclisation-transannulation reactions leading to ring-fused bicycles

Abstract

The scope for tandem radical mediated macrocyclisation–transannulation processes in the elaboration of ring-fused carbocycles has been examined. Thus, a range of E-iodo dienones viz. 21, 30b, 40, 42 and 44 were first prepared using synthetic sequences based on sound literature precedent. Treatment of the iodo dienone 21 with Bu3SnH–AIBN led to a 3 : 2 mixture of trans-and cis-isomers of 1-decalone, 35 and 36, respectively, in a combined yield of 72%, whereas the positional isomer 30b of 21 underwent 10-endo macrocyclisation and transannulation to a 1 : 1 mixture of trans-1-decalone 35 and cis-octahydroazulen-1-one 39 (combined yield 68%), resulting from competitive 6-exo/5-exo transannular cyclisation from the intermediate cyclodecenone radical 38.In further investigations of the scope for sequential radical macrocyclisation–transannulations in the synthesis of bicyclic systems, the iodo dienone 40 was found to undergo regioselective cyclisation to the cis-tetralone 41(50%), whereas the iodo dienone 42 produced only (Z)-cyclooct-3-enone 54 and none of the expected bicyclo[3.3.0] octanone 43 on treatment with Bu3SnH–AIBN. Only the 4-cyclo-pentylcyclohexanone 61, and none of the hoped for 7,6-bicyclic ketone 45, was produced from radical cyclisation of the iodo dienone 44.The differing reaction pathways followed by the iodo dienones 21, 30b, 40 and 42 have been rationalised in terms of the conformational preferences of the macrocyclic α-keto radical intermediates, e.g. 32, 38 and 52 involved in the various cyclisations supported by some preliminary MM2 studies and calculations.