Phosphine-catalyzed formal Buchner [6+1] annulation: construction of cycloheptatrienes.

An unprecedented phosphine-catalyzed formal Buchner [6+1] annulation of a newly designed allenoate has been developed, providing a series of cycloheptatriene derivatives in moderate to good yields (up to 99%). This reaction demonstrates that the introduction of an electrophilic allylic group to allenoates effectively extends the reaction scope of phosphine-catalyzed annulation, providing a concise route to cycloheptatrienes. Mechanistic study indicated that this reaction involves a [4+2] Diels-Alder reaction and ring expansion of the bicyclo[4.1.0] moiety, which is similar to the Buchner reaction. Notably, an enantioselective variant of this [6+1] annulation can also be performed using a chiral iminophosphine catalyst.