Lewis acid promoted Diels–Alder reactions of cyclopentadiene and dienophiles 8a–f.
| Entry | Dienophile | Lewis acid (equiv.) | Temp (°C) (time, h) | Yielda (%) | Endo b (dr) |
|---|---|---|---|---|---|
| 1 | 8a | Et2AlCl (1.4) | −78 (2) | 36 | 99 : 1 |
| 2 | 8a | Et2AlCl (2) | −78 (24) | 50 | 99 : 1 |
| 3 | 8a | Et2AlCl (3) | −78 (24) | 76 | 99 : 1 |
| 4 | 8a | Et2AlCl (2) | −78 to 23 (24) | 71 | 97 : 3 |
| 5 | 8a | TiCl4 (1.4) | −78 (24) | Traces | 90 : 10 |
| 6 | 8a | TiCl4 (2) | −78 (24) | Traces | 90 : 10 |
| 7 | 8a | Yb(OTf)3 (2) | 23 (24) | SM | — |
| 8 | 8b | Et2AlCl (1.5) | −78 (8) | 58 | 99 : 1 |
| 9 | 8c | Et2AlCl (1.5) | −78 (8) | 70 | 99 : 1 |
| 10 | 8d | Et2AlCl (1.5) | −78 (8) | 73 | 99 : 1 |
| 11 | 8d | SnCl4 (1) | 0 (48) | 23 | 99 : 1 |
| 12 | 8e | Et2AlCl (1.5) | −78 (8) | 70 | 99 : 1 |
| 13 | 8f | Et2AlCl (1.6) | −78 (8) | 98 | 99 : 1 |
| 14 | 8f | Et2AlCl (1) | −78 to 0 (48) | 15 | 90 : 10 |
| 15 | 8f | EtAlCl2 (1) | −78 (48) | SM | — |
| 16 | 8f | SnCl4 (1) | 0 (24) | 27 | 99 : 1 |
a
Reactions were carried out in 0.1 to 1 mmol scale. Isolated yield after silica gel chromatography.
b
Endo diastereomeric ratios were determined by 1H NMR; SM: starting material recovered.