Table 1.
Selection of optimization experiments.
| # | Lewis acid (equiv) | Conditions | Yield (s. m.)a [%] |
| 1 | Hf(OTf)4 (2.0) | MeCN, rt, 48 h | 72 (15) |
| 2 | Hf(OTf)4 (0.2) | MeCN, rt, 65 h | 25 (69) |
| 3 | Hf(OTf)4 (0.2) | MeCN/H2O 10:1, rt, 65 h | 3 (81) |
| 4 | AlBr3 (2.0) | MeCN, rt, 65 h | 50 (39) |
| 5 | AlBr3 (2.0) | MeCN, 50 °C, 90 h | 65 (20) |
| 6 | AlBr3 (2.0), Hf(OTf)4 (0.1) | MeCN, rt, 65 h | 64 (24) |
| 7 | TMSOTf (2.0) | MeCN, rt, 65 h | 83 (9) |
| 8 | TMSCl (2.0) | MeCN, rt, 65 h | 33 (59) |
| 9 | TMSOTf (2.0) | MeCN, Et3Nb, rt, 65 h | 80 (10) |
aYields of recovered starting material (s. m.) given in parentheses. bEt3N (1 equiv) was added.