Table 1. Screening of Chiral Phosphoric Acidsa.
| entry | catalyst | conversion | global yield (%)b | 3a:4a ratioc | dr 3a (cis:trans)c | ee (%) (cis-3a/trans-3a/4a)d |
|---|---|---|---|---|---|---|
| 1 | 2c | 100 | 81 | 1.2:1 | 6:1 | 50/4/50 |
| 2 | 2d | 90 | 80 | 2:1 | 5:1 | 20/19/11 |
| 3 | 2e | 90 | 77 | 2:1 | 5:1 | 13/17/24 |
| 4 | 2f | 80 | 69 | 1.2:1 | 5:1 | 14/14/5 |
| 5 | 2g | 79 | 51 | 2:1 | 5:1 | 3/17/13 |
| 6 | 2h | 100 | 83 | 5:1 | 4:1 | 38/61/18 |
| 7 | 2i | 68 | 53 | 4:1 | 5:1 | 37/10/29 |
| 8 | 2a | 28 | 25 | 6:1 | 7:1 | 16/18/15 |
| 9 | 2j | 78 | 79 | 2.5:1 | 4:1 | 3/13/9 |
a
Reactions were carried out with 0.10 mmol of cyclobutanol 1a and 0.11 mmol of NBS in toluene (0.2 M) at room temperature.
b
Yields refer to the mixture of isolated products by column chromatography.
c
Ratio calculated in the reaction crude by 1H NMR.
d
Calculated by HPLC on a chiral stationary phase.