Practical enantioselective synthesis of β-lactones catalyzed by aluminum bissulfonamide complexes

The development of an efficient and practical aluminum-bissulfonamide complex catalyzed enantioselective formation of beta-lactones by [2+2] cycloaddition of ketene (generated in situ from acetyl bromide by dehydrobromination) with various alpha-unbrancbed and -branched aliphatic aldehydes is presented. The methodology offers the advantage of operational simplicity not only as the ligand synthesis requires just a single sulfonylation step from commercially available enantiomerically pure diamines. The products are formed in high to excellent yields with ee values typically ranging from 78 to 90% using 10 mol% of the bissulfonamide ligand. The key finding of this work was a remarkable rate acceleration by using an aluminum/ ligand ratio of 1.5:1.