Table 1. Optimization of the Reaction Conditionsa.
| entry | catalyst (mol %) | solvent | time (h) | 2a (%)b | 3a (%)b |
|---|---|---|---|---|---|
| 1 | BSA (20) | THF | 48 | 42 | 4 |
| 2 | p-TsOH (20) | THF | 30 | 55 | 8 |
| 3 | p-TsOH (20) | 1,4-dioxane | 16 | 60 | 5 |
| 4 | MSA (20) | THF | 12 | 65 | trace |
| 5 | MSA (20) | 1,4-dioxane | 8 | 68 | trace |
| 6 | MSA (20) | H2O | 5 | 80 | ND |
| 7 | MSA (10) | H2O | 6 | 75 | ND |
| 8c | MSA (20) | H2O | 5 | 82 | ND |
| 9 | H2SO4 (20) | H2O | 6 | 70 | ND |
| 10 | TfOH (20) | H2O | 8 | 65 | ND |
a
Reactions were carried out with 1a (0.5 mmol) in refluxing solvent (10 mL).
b
Isolated yield.
c
Reaction was performed under air.