Imidazolidine substrates were next examined. Symmetrical Nalkyl imidazolidines were compatible with this transformation under rhodium catalysis, affording the desired products 33-35 in up to 95% yield, while HFIP instead of CH2Cl2 as solvent was necessary to achieve good results under copper catalysis. In sharp contrast, the symmetrical N-aryl imidazolidines underwent the cycloaddition sluggishly under both conditions, furnishing the corresponding products 36 and 37 in only moderate yields, which may be attributed to the weak nucleophilicity as well as low solubility of N-aryl imidazolidines. For 2-methylimidazolidine, unfortunately, only trace product 38 was formed as a mixture of two inseparable diastereoisomers under either rhodium or copper catalysis. When 4,4-dimethylimidazolidine was employed, only one regioisomer 39a was detected and isolated in good yield under both conditions, the structure of which was confirmed by Xray diffraction analysis (CCDC 2285030). In addition, three types of unsymmetric N-substituted imidazolidines (N-Bn/N-Me, N-Bn/N-Ar, N-Ar 1 /N-Ar 2 ) were synthetically available, and all were gratifyingly suitable substrates for this cycloaddition, leading to the target products 40-42 in good yields. Notably, much better regioselectivity was achieved for N-Bn/N-Me and N-Bn/N-Ar imidazolidines under rhodium than copper catalysis (40 and 41), while similar results were obtained for N-Ar 1 /N-Ar 2 imidazolidine under both conditions (42). The structure of 40b was determined by the N-selective deprotection/methylation process (see Scheme 4C), and both structures of 41a and 42a were confirmed by X-ray diffraction analysis (CCDC 2285031 for 41a; CCDC 2296400 for 42a). Scheme 3. Substrate scope. Isolated yields under Condition B are in parentheses. The r.r. and d.r. values were determined by 1 H NMR analysis of the reaction.