Regioselective and Enantioselective Intermolecular Buchner Ring Expansions in Flow.

The first example of a regioselective and enantioselective intermolecular Buchner ring expansion is reported using continuous flow. The practicality and scope of the reaction are greatly improved under flow conditions. Reactions of ethyl diazoacetate with symmetric and nonsymmetric arenes afford cycloheptatrienes in good yield and excellent regioselectivity. The first example of an asymmetric intermolecular Buchner reaction is demonstrated with disubstituted diazo esters in good to excellent enantioselectivity. The asymmetric reactions proceed with absolute regioselectivity to afford cycloheptatrienes with an all-carbon quaternary center.