α,β-Unsaturated and cyclopropyl acyl radicals, and their ketenealkyl radical equivalents. Ring synthesis and tandem cyclisation reactions

Abstract

Treatment of the α,β-unsaturated selenyl esters 12 and 14 with Bu₃SnH–AIBN produces the corresponding 2-cyclohexenones 13 and 15 respectively via presumed α-ketene alkyl radical intermediates, viz.10. By contrast, the 2,7-diene esters 34 and 39 undergo tandem radical cyclisations producing diquinanes, e.g.38 (76%), and the corresponding allene-substituted α,β-unsaturated selenyl ester 48 gives the cyclooctadienone 56 on treatment with Bu₃SnH–AIBN in refluxing benzene. The selenyl ester 19 derived from chrysanthemic acid produces a mixture of the γ,δ-unsaturated aldehyde 22 and the corresponding dimer 25a on treatment with Bu₃SnH–AIBN. Furthermore, in the presence of methanol the only product from this reaction was the bis(methyl ester) dimer 25b, thereby lending further credence to the involvement of ketene alkyl radical intermediates in these reactions, and in the aforementioned reactions involving 2,6- and 2,7-diene selenyl esters. Treatment of the cyclopropane selenyl esters 59 and 61, containing keto- and oxy-group functionality in their side-chains, with Bu₃SnH–AIBN led to excellent syntheses of the enol lactone 66 (76%) and the trans-fused bicyclo[6.1.0]nonane 67 (80–95%) respectively.