Table 1.
Stereoselectivity of aldol additions using chiral amino alkoxides and antipodes of Evans enolates (Chart 5).a
| Entry | Lithium alkoxide | Aggregate | syn/syn (2/38) |
syn/anti ([2+38]/39) |
Yield (%) |
|---|---|---|---|---|---|
| 1 | — | 3a/4a | 12:1 | 16:1 | 91 |
| 2 | — | 5a | 1:1 | 5:1 | <20 |
| 3 | 11 | (S)-15a | 6:1 | 7:1 | 54 |
| 4 | 11 | (R)-15a | 6:1 | 7:1 | 56 |
| 5 | 11 | (S)-16a | 18:1 | 50:1 | 90 |
| 6 | 11 | (R)-16a | 2:1 | 5:1 | 88 |
| 7 | — | 3b/4b | 5:1 | 4:1 | 90 |
| 8 | — | 5b | 1:5 | 3:1 | <20 |
| 9 | 11 | (S)-15b | 50:1 | 15:1 | 48 |
| 10 | 11 | (R)-15b | 50:1 | 14:1 | 45 |
| 11 | 11 | (S)-16b | 30:1 | >100:1 | 92 |
| 12 | 11 | (R)-16b | 1:1 | 6:1 | 88 |
| 13 | 8 | (S)-36a | 10:1 | 16:1 | 55 |
| 14 | 8 | (R)-36a | 10:1 | 14:1 | 50 |
| 15 | 8 | (S)-37a | 10:1 | 25:1 | 85 |
| 16 | 8 | (R)-37a | 10:1 | 8:1 | 78 |
| 17 | 8 | (S)-36b | 9:1 | 13:1 | 52 |
| 18 | 8 | (R)-36b | 9:1 | 13:1 | 55 |
| 19 | 8 | (S)-37b | 4:1 | 3:1 | 80 |
| 20 | 8 | (R)-37b | 9:1 | 4:1 | 79 |
a
Reaction in neat tetrahydrofuran/–78 °C for 30 min after aging the enolate at 25 °C for 10 min; 2 equiv of electrophile.