Initially, coupling reaction between
ortho-alkynylbenzoate donor
4-2 and acceptor
5a was conducted using the typical Au catalyst as promotor. Gratifyingly,
6a with exclusive stereoselectivities was afforded when the reaction was catalyzed by Ph
3PAuCl (0.1 equiv.) and AgNTf
2 (or AgSbF
6) (0.1 equiv.) (
Table 2, entries 2 and 4). However, the acceptor
5a was not yet completely consumed, leading to the formation of the glycosyl product in 60% yield for AgNTf
2 and in 25% yield for AgSbF
6 respectively. Furthermore, when Ph
3PAuCl (0.1 equiv.) and AgOTf (0.1 equiv.) (
Table 2, entry 5) was used to calalyze the reaction, the yield for the target disaccharide was only increased to 68%. According to the aforementioned results, Hg(Ⅱ) (
Table 1, entries 7–9) exhibited better activity than Au(Ⅰ) or Au(Ⅲ) (
Table 1, entries 1–6) for the activation of the alkynoate. Therefore, Hg(NTf
2)
2 and Hg(OTf)
2 were applied to catalyze the reaction (
Table 2, entries 6 and 7). Again, Hg(OTf)
2 displayed better catalytic activity in comparison with Hg(NTf
2)
2, and 77% yield was afforded when the reaction was conducted in CH
2Cl
2 at concentration of 0.1 mol/L (
Table 2, entry 7). Thus, Hg(OTf)
2 was considered as the optimal catalyst for the further studies of glycosyl condition. Then, the amount of catalyst, reaction concentration and solvents were screened for the optimal glycosylation condition. It was shown that 89% yield was obtained for using 5% catalyst (
Table 2, entry 8) at a reaction concentration of 0.1 mol/L, and 91% yield for using 10% catalyst (
Table 2, entry 10) at a reaction concentration of 0.05 mol/L. To reduce the usage of Hg(Ⅱ), the amount of catalyst was determined as 5% for the following studies. At last, different solvents (including toluene, ether, and acetonitrile,
Table 2, entries 14–16) were also examined, and the yield was further increased to 95% when using Et
2O as the solvent (
Table 2, entry 15). Generally, Et
2O was considered to be helpful for construction of
α-configuration glycosides. However, the selectivity was not influenced in this work, demonstrating the strong tendency to
β-configuration of 2,6-lactone-bridged mannosyl donor.