Initially, γ−2-indolyl phenol
1a and α, β‐alkynal
2a were chosen as model substrates for screening the reaction conditions (
Table 1). The catalytic transformation of
1a with
2a was investigated under NHC catalysis using 3, 3′, 5, 5′-tetra‑
tert-butyldiphenoquinone (DQ) as the oxidant (
Table 1). We were encouraged to find that the reaction using the triazolium salt
C as an NHC precursor and DBU as a base in the presence of Sc(OTf)
3 provided the desired pentacyclic ten-membered lactam
3a in 65% yield without observing its regioisomer
3a', revealing that the reaction demonstrates complete regioselectivity (entry 1). Based on these preliminary results, various reaction parameters, such as the NHC source, base, solvent and additive, were then carefully investigated. Exchanging triazolium salt
C with imidazolium salt
C or thiazolium salt
C returned inferior yields of
3a (entries 2 and 3). After screening several bases commonly used in NHC catalysis, 4-dimethylaminopyridine (DMAP) was found to be the best choice for this transformation (80%, entry 5), as reactions involving other organic (Et
3N) and inorganic (Cs
2CO
3 and K
2CO
3) bases were tested but resulted in traces or no desired products (entries 4, 6 and 7). The application Mg(OTf)
2 and Zn(OTf)
2 as Lewis acid catalysts could make this reaction to proceed readily; however, the yields of
3a obtained with both catalysts remain inferior to those achieved with Sc(OTf)₃ (entries 8 and 9
vs. entry 5). Next, the effect of the solvents was evaluated. The yields of product
3a associated with Pre-NHC
C in the presence of Sc(OTf)
3 and DMAP in several solvents are summarized as follows (entries 10–15): MeCN (60%), ethyl acetate (EA, 67%), toluene (58%), 1, 2-dichloroethane (DCE, trace), and 1, 4-dioxane (54%), indicating that none of them can improve the efficiency of this transformation as compared with THF. Decreasing the amount of Sc(OTf)
3 (10 mol%) or DMAP (1.0 equiv.) resulted in a markedly reduced yield of
3a (entries 15 and 16). Without Sc(OTf)
3, the yield of
3a decreased remarkably to 57%, showing that Sc(OTf)
3 plays an important role in increasing the yield of
3a (entry 17). The reaction did not proceed with the use of MnO
2 or 2, 3-dichloro-5, 6-dicyano-1, 4-benzoquinone (DDQ) as the oxidant (entry 18). Control experiments carried out without the NHC precursor or DQ did not yield
3a, indicating the indispensable role of the carbene catalyst as well as the oxidant in this reaction (entries 19 and 20).