To further improve the enantioselectivity, the protecting group effect of both substrates were examined under Cu(OTf)2/L5 catalytic system in HFIP at -4 o C (Scheme 5, top). Diazooxindoles bearing either acetyl or benzyl group led to inferior enantioselectivities (4 and 7). Besides tosyl group, several other sulfonyl groups were also tested (58-60), and 1-naphthyl sulfonyl group yielded a bit better result (60). The absolute configuration of the major enantiomer of product 59 was determined by X-ray crystallography to be (R) (CCDC 2306191). For Nalkyl N-aryl imidazolidines, the reaction proceeded very sluggishly with poor stereocontrol (34 and 36). Several other benzyl groups were evaluated as well (61-63), and 4methoxybenzyl (PMB) group gave 90.5:9.5 e.r. (61). When diazooxindole with 1-naphthyl sulfonyl group and imidazolidine with PMB group were subjected to the asymmetric conditions, the target product 64 was obtained in 77% yield and 93:7 e.r. The generality of this catalytic asymmetric (5+1) cycloaddition was then investigated on a selection of typical substrates (Scheme 5, bottom). Good levels of enantioselectivities were achieved for C5/C6-substituted diazooxindoles (66-71), whereas C4-substituted diazooxindole led to only moderate stereocontrol (65). In addition, C-substituted imidazolidines were compatible with this reaction, giving rise to products 72 and 73 with acceptable yields and enantioselectivities. Unfortunately, the analogous (6+1) cycloaddition of diazooxindole with hexahydropyrimidine could hardly happen under the asymmetric conditions (74). Scheme 5. Protecting group effect and substrate scope.