Asymmetric Synthesis of Highly Substituted β‐Lactones by Nucleophile‐Catalyzed [2+2] Cycloadditions of Disubstituted Ketenes with Aldehydes

α,α-Disubstituted β-lactones can be obtained by the cycloaddition of the corresponding ketenes with aldehydes (see scheme). For the first time, a chiral PPY derivative, 1, serves as an efficient catalyst for the asymmetric synthesis of β-lactones (PPY=4-pyrrolidin-1-ylpyridine). To date, this is the only catalyst that is effective for enantioselective cycloadditions of disubstituted ketenes with aldehydes.