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A highly diastereoselective and enantioselective Cu(II)/SaBOX-catalyzed 2 + 2 cycloaddition of methylidenemalonate and multisubstituted alkenes was developed to furnish optically active cyclobutanes in high yields with >99/1 dr and up to >99% ee. By application of the newly developed method, the total synthesis of (+)-piperarborenine B was completed in eight steps from methylidenemalonate and olefin in 17% overall yield with >99/1 dr and 99% ee.
Hu et al. (Thu,) studied this question.