Table 1. Lewis Acid-Promoted (4 + 3) Cycloaddition between Alkenylindole 2a and Dimethyl Acetal 3a.
| entry | Lewis acid (equiv) | solvent | temp. | yield (%)b |
|---|---|---|---|---|
| 1 | SnCl4 (1.0) | CH2Cl2 | –78 °C | -c |
| 2 | ZnCl2 (1.1) | CH2Cl2 | 0 °C | 50 |
| 3 | TMSOTf (1.0) | CH2Cl2 | –78 °C | 67 |
| 4 | TMSOTf (1.0) | PhMe | –78 °C | 62 |
| 5 | TMSOTf (1.0) | tBuOMe | –78 °C | 54 |
| 6 | TMSOTf (1.0) | Et2O | –78 °C | 85 |
| 7 | TMSOTf (1.0) | THF | –78 °C | 97 (91)d |
| 8 | TMSOTf (1.0) | EtNO2 | –78 °C | 95 |
a
Reactions were performed with 13–15 mg of alkenylindole 2a (0.04M) and 1.0–1.5 equiv of 3a.
b
Yields were determined by NMR with 1,3,5-trimethoxybenzene as an internal standard.
c
Decomposition of the starting materials.
d
Isolated yield.