Carboxyboration: The Catalyst-Free Synthesis of Borylated Lactones and Isocoumarins From Esters
Catalyst-free formation of isocoumarins and α-pyrones from the corresponding alkynyl esters is realized. The resulting γ- and δ-lactones are isolated as the pinacolboronic esters, boronic acids or potassium trifluoroborate salts, which provide a variety of methods for downstream functionalization of the newly formed B−C σ bond. Reaction conditions have been optimized and a substrate table has been constructed to show tolerance of various functional groups incompatible with other borylation techniques. A mechanism has been proposed as a result of mechanistic studies and downstream functionalization is in progress to show the versatility of the methodology.