Radical reaction initiated and stereocontrolled by zinc chloride

The reaction of allyltributyltin with methyl 2-[N-((4S)-4-(1-methylethyl)-2-oxazolidinone-3-carbamoyl)amino]-2-bromoacetate (5 a) was accelerated at -50°C in the presence of AIBN and stopped in the presence of galvinoxyl, indicating that the reaction proceeds through a radical mechanism. The reaction was accelerated dramatically at -78°C in the presence of ZnCl₂·OEt₂, and the ZnCl₂-mediated reaction was stopped in the presence of galvinoxyl. In the presence of 2 equiv ZnCl₂·OEt₂, the reaction afforded methyl (2R)-2-[N-((4SH-(1-methylethyl)-2-oxazolidinone-3-carbamoyl)amino]-4-pentenoate {6a(R)} with high diastereoselectivity (93:7). Taken together, ZnCl₂·OEt₂ works as a radical initiator as well as chelating agent.