A novel route to functionalized linearly benzannulated medium ring carbocyclics through regioselective aryl radical cyclization

Abstract

The scope for n-Bu₃SnH-AIBN-mediated regioselective endo-trig aryl radical cyclization processes in elaboration of linearly benzannulated medium ring carbocycles is described. A range of vinylcyclohexanols 3a,b, 4a,b, 28a,b,c, |allyl-cyclohexanols 11a,b, 12a, 18a,b, 32, 36a,b and butenylcyclohexanols 43a,b and 44a, prepared using synthetic sequence based on sound literature precedent, were first examined. Radical cyclization of 3a,b, 4a,b and 28a-c led to the corresponding seven- and eight-membered ring-annulated tricyclic alcohols 5a,b, 6a,b and 13a,b, 14a,b and 30a-c in good yields. Radical cyclization of 11a,b, 12a, 18a,b and 32a,b, 36a,b, 43 a,b and 44a produced the corresponding 8- endo- and 9-endo-trig products 13a,b, 14a, 19a,b and 33, 39a,b, 45a,b and 47a in moderate to good yields, in addition to the respective uncyclized debrominated alcohols. The X-ray crystal structure of the g-lactones 22a and 42b, derived from the respective hydroxy esters 19a and 39b, have been determined. The ring size of the starting cycloalkanols affecting the propensity of the endo-cyclizations have been examined with the allyl- and butenyl cyclopentanols 23, 49 and the cycloheptanols 26, 52 leading to corresponding eight- and nine-membered ring products 24, 50 and 27, 53. The vinylcyclopentanol 8 produced exclusively the 6-exo-trig products 9, which on oxidative dehydration led to the hydrocarbon 10.