Abstract
A general reaction protocol for catalytic enantioselective 1,3-dipolar cycloaddition reaction of nitrones, activated by chiral Lewis acids, with electron-rich alkenes has been developed. The nitrones are activated by various chiral 2,2‘-dihydroxy-1,1‘-binaphthyl (BINOL)-AlMe complexes, and it has been found that 3,3‘-diaryl-BINOL-AlMe complexes catalyze a highly regio-, diastereo-, and enantioselective 1,3-dipolar cycloaddition reaction of aromatic nitrones with vinyl ethers, giving the exo-diastereomer of the isoxazolidines with de's up to >90% and up to 97% ee. The reaction has been investigated under various conditions with different nitrones and vinyl ethers (and alkenes), and a general synthetic procedure is presented. The mechanism for the reaction is discussed on the basis of a linear stereochemical effect of the catalyst, the diastereoselectivity, and absolute stereochemistry of the isoxazolidines formed, and theoretical calculations of the 3,3‘-diphenyl-BINOL-AlMe−nitrone intermediate.
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