Synergistic Acid-Promoted Synthesis of Highly Substituted Butenolides via the Annulation of Keto Acids and Tertiary Alcohols.

An efficient and practical approach to highly substituted butenolides via the annulation of keto acids and tertiary alcohols is described. The synergistic effect provided by the combination of Lewis and Brønsted acids is crucial to the reaction outcome. A variety of highly substituted butenolides are readily produced in synthetically useful yields. Water is the only waste from the reaction, therefore making it an environmentally benign and atom-economical process for butenolide synthesis.