The First Ring-Enlargement of a 1-Azabicyclo[1.1.0]butane to a 1-Azabicyclo[2.1.1]hexane

The reactions of 3‐phenyl‐1‐azabicyclo[1.1.0]butane (4) with dimethyl dicyanofumarate ((E)‐8) and dimethyl dicyanomaleate ((Z)‐8) lead to the same mixture of cis‐ and trans‐4‐phenyl‐1‐azabicyclo[2.1.1]hexane 2,3‐dicarboxylates (cis‐11 and trans‐11, resp.; Scheme 3). This result of a formal cycloaddition to the central CN bond of 4 is interpreted by a stepwise reaction mechanism via a relatively stable zwitterionic intermediate 10, which could be intercepted by morpholine to give a 1 : 1 : 1 adduct 12, which undergoes a spontaneous elimination of HCN to yield the fumarate 13 (Scheme 4).