Table 1. Optimization of the C-Glycosylation Reaction.
| entrya | D/A | promoter (equiv) | T (°C) | 4 Å MS (wt equiv) | 25/24b | yield of 25c (%) |
|---|---|---|---|---|---|---|
| 1d | 23a/21 | SnCl4 (9.0) | 25 | 0 | 0/100 | 41e |
| 2 | 23a/21 | SnCl4 (3.0) | 25 | 0 | <5 | |
| 3 | 23a/21 | Et2O·BF3 (3.0) | 25 | 4.0 | 0 | |
| 4 | 23a/21 | TMSOTf (3.0) | 25 | 4.0 | <5 | |
| 5 | 23a/21 | Cp2ZrCl2 (3.0), AgClO4 (3.0) | 25 | 4.0 | 0 | |
| 6 | 23a/21 | SnCl4 (3.0), AgClO4 (3.0) | 25 | 4.0 | 60/40 | 14 |
| 7 | 23a/21 | SnCl4 (3.0) | 25 | 4.0 | 72/28 | 25 |
| 8 | 23a/21 | SnCl4 (3.0) | 15 | 4.0 | 52/48 | <15 |
| 9 | 23a/21 | SnCl4 (3.0) | 35 | 4.0 | 55/45 | 10 |
| 10f | 23a/21 | SnCl4 (3.0) | 25 | 4.0 | 0 | |
| 11g | 23a/21 | SnCl4 (3.0) | 25 | 4.0 | 63/37 | 11 |
| 12 | 23a/21 | SnCl4 (3.0) | 25 | 12.0 | 81/19 | 30 |
| 13 | 23a/21 | SnCl4(3.0) | 25 | 20.0 | >95/5 | 50 |
| 14h | 23a/21 | SnCl4 (3.0) | 25 | 4.0 | 50/50 | 11 |
| 15 | 23b/21 | SnCl4 (3.0) | 25 | 20.0 | 90/10 | 26 |
| 16 | 23b/21 | TMSOTf (3.0) | 25 | 20.0 | <5 | |
| 17 | 23b/21 | TMSOTf (0.4) | 25 | 20.0 | <5 | |
| 18 | 23c/21 | SnCl4 (3.0) | 25 | 20.0 | >95/5 | 41 |
| 19 | 23c/21 | SnCl4 (0.5) | 25 | 20.0 | >95/5 | 11i |
| 20 | 23a/55 | SnCl4 (3.0) | 25 | 12.0 | <5j | |
| 21d | 23a/26 | SnCl4 (9.0) | 25 | 0 | 0/100 | 37e |
| 22 | 23a/26 | SnCl4 (3.0) | 25 | 4.0 | 0/100 | <5e |
a
Conditions: 23 (1.0 equiv), aglycon (3.0 equiv), 4 Å MS, promoter (3.0 equiv), solvent (0.017 M), r.t.
b
Ratio determined by 1H NMR of the crude reaction mixture.
c
Combined yield of 25a and 25b.
d
1.0 equiv of aglycon, 4.0 equiv of 23a, and 9.0 equiv of SnCl4 were used.
e
Isolated yield of 24.
f
DCE/THF or MeCN was used as solvent.
g
Concentration = 0.034 M.
h
1.0 equiv of aglycon, 3.0 equiv of 23a, and 3.0 equiv of SnCl4 were used.
i
α-25b was also obtained in 19% yield.
j
Deprotection of 1-hydroxyl group did not happen. D/A = donor/acceptor.