Cascade Radical Cyclization-Fragmentation-Transannular-Ring Expansion Reactions Involving Oximes. A New Approach to Synthesis of Angular Triquinanes

Abstract

Treatment of the acetylene-substituted cyclobutanone oxime ether (13) with (Me₃Si)₃SiH leads, in one pot, to the 5,6-bicyclic enone oxime (14) in 70% yield. It is suggested that the formation of (14) from (13) proceeds by way of a novel double ring expansion-transannulation process involving aminyl radical intermediates (see Scheme 3, below). By contrast, treatment of the cyclobutanone (3a) with Bu₃SnH instead leads only to the product (11) of hydrostannylation of the acetylene unit in (3a). Neither (3a) nor (13) produced any of the expected bicyclo[3.3.0]octane products, viz. (5a) and (15), following treatment with silanes or stannanes. When a solution of the vinyl bromide-substituted cyclobutanone oxime (42) was irradiated with a sunlamp in the presence of (Me₃Si)₃SiH, the main product isolated was the triquinane (45) (38%) resulting from a cascade radical 6-exo-trig cyclization, aminyl radical fragmentation, 5-exo-trig radical transannulation, 5-exo-trig cyclization sequence.