With these considerations in mind, we began our investigations with screening studies of Lewis acids on their abilities to promote the reaction of triisopropylsilyl phenyl enol ether (1a) from acetophenone and naphthyl BCB (2a). In the presence of late transition metal derived triflate salts, only trace amount of desired cycloadduct 3a was observed by NMR analysis of the crude reaction mixture (Table , entry 1-4). The major product was identified to be cyclobutyl silyl enol ether 4, presumably derived from nucleophilic addition followed by silyl migration. Additionally, B(C6F5)3 would only give byproduct 4 (Table , entry 6). On the other hand, we found that with lanthanide triflates such as Eu(OTf)3, Gd(OTf)3, Tm(OTf)3, Lu(OTf)3 and Yb(OTf)3 the reaction afforded the desired product 3a in high yield (Table , entry 7-11). Among them, Yb(OTf)3 was the optimal catalyst, allowing the reaction to proceed in 97% yield ( With the optimal conditions in hand, the generality of substrates in this cycloaddition was investigated as depicted in Table . The reactions of BCB 2a and various silyl enol ethers 1a-d derived from acetophenones were examined, which proceeded to completion, affording the desired products 3a-d in 45%-93% yields. Within these substrates, those bearing more stable silyl groups resulted in higher yields.