The first ring-enlargement of a 1-Azabicyclo[1.1.0]butane to a 1-Azabicyclo[2.1.1]hexane

Abstract

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).