Substituted propargyl acetates are converted into 4boryl-2(5H)-furanones upon thermolysis in the presence of an N-heterocyclic carbene borane (NHC-borane) and di-tertbutyl peroxide.T he acetyl methyl group is lost during the reaction as methane.E vidence suggests that the reaction proceeds by asequence of radical events including:1)addition of an NHC-boryl radical to the triple bond;2 )cyclization of the resultant b-borylalkenyl radical to the ester carbonyl group; 3) b-scission of the so-formed alkoxy radical to providet he 4-boryl-2(5H)-furanone and am ethyl radical;a nd 4) hydrogen abstraction from the NHC-borane to return the initial NHC-boryl radical and methane. Figure 1. Radical cyclizations to carbon-oxygen double bonds. AIBN = 2,2'-azobis(isobutyronitrile),N HC = N-heterocyclic carbene.