Carbon–carbon bond formation occurs under mild conditions when 2-methylthiazolines react with acid chlorides to form N-acyl-β-keto cyclic ketene-N, S-acetals. N-Methyl cyclic ketene-N, S-acetals, generated from 2-methyl-thiazolines, can react further with acid chlorides to form N-methyl-β-keto cyclic ketene-N, S-acetals.