giving the same better results (Table , entries 3-7). Notably, Glorius reported that BF3•OEt2 could serve as an effective Lewis acid catalyst to trigger a (3+2) cycloaddition of BCBs with aldehydes, metallic Yb(OTf)3 was chosen as the optimal catalyst, since the removal of undesired metal contamination is often nontrivial and costly in the pharmaceutical industry, to examine other solvents including toluene, tetrahydrofuran (THF) and N,N-dimethylformamide (DMF), while no improvement over CH2Cl2 was seen (Table , entries 10-12). A number of other reaction parameters including temperature, concentration and catalyst loading were then varied, but the key advance was an increased temperature ( With the optimal reaction conditions established, the substrate scope of BCBs was first investigated (Scheme 2). A diverse range of aryl BCB ketones successfully underwent this transformation (4aa-oa).