Table 6.
Comparison of Lewis acids in isopropyl addition to 60a.a
| ||||
|---|---|---|---|---|
| entry | Lewis acid | solvent, additive | yield[b] | ee[c] |
| 1 | InCl3 | CH2Cl2 | 33% | 57% |
| 2 | Mg(ClO4)2 | CH2Cl2 | 41% | 66% |
| 3 | Cu(OTf)2 | CH2Cl2 | 41% | 59% |
| 4d,e | Cu(OTf)2 | PhH/CH2Cl2 (2:1), 4Å MS | 66% | 95% |
| 5d,e,f | Cu(OTf)2 | PhH/CH2Cl2 (2:1), 4Å MS | 94% | 86% |
a
Reaction conditions: Lewis acid (1 equiv), tert-butylbisoxazoline ligand (1 equiv), 2-iodopropane (6 equiv), Et3B/O2 (6 equiv).
b
Isolated yield, %.
c
Enantiomeric excess, determined by HPLC.
d
Et3N was added during workup.
e
Preformed aquo complex Cu(tBu-Box)(H2O)2(OTf)2 was used.
f
10 equiv of i-PrI and Et3B were used.