At the beginning of our study, we selected two known compounds, 2-naphthyl BCB ketone 1a and N-PMP substituted imidazolidine 2a, as model substrates because both are relatively stable, easy to handle and store (Table ). We were pleased to find that by using Cu(OTf)2 as catalyst in CH2Cl2 at room temperature for 2 h, the desired 2,5-diazabicyclo[5.1.1]nonane product 4aa was obtained in 59% isolated yield (Table , entry 1). A control experiment in the absence of catalyst led to no product formation (Table , entry 2). Then, various Lewis acids were evaluated. To our delight, Ga(OTf)3, 44 Yb(OTf)3, AgOTf, TMSOTf and B(C6F5)3 all successfully promoted this (5+3) cycloaddition, with Yb(OTf)3 and B(C6F5)3