Table 3. Optimal Temperatures for Stereoselective Epoxide-Opening Spirocyclization of Glycal Epoxide Substrates.a.
entry | substratea | T (°C) | CD3OD (M) | inversion (18, 8-10) |
retention (19, 11-13) |
glycoside (20, 14-16) |
---|---|---|---|---|---|---|
1 | 17 | −63 | 17.8b | nr | ||
2 | 17 | −35 | 17.8 | 93 | 0 | 7 |
| ||||||
3 | 5c (R = H) | −35 | 17.8 | >98 | <2 | 0 |
4 | 5c (R = H) | −35 | 11.9c | >98 | <2 | 0 |
5 | 5c (R = H) | −35 | 0 | nr | ||
| ||||||
6 | 6c (R = H) | −20 | 17.8 | 70 | 30 | 0 |
7 | 7c (R = H) | 0 | 17.8 | 0 | 41 | 59 |
Glycal epoxides generated in situ from glycal precursors with DMDO (−78 °C, 10 min), then warmed to the indicated temperature in the NMR probe.
6:1:1.3 CD3OD/CDCl3/acetone.
4:3:1.3 CD3OD/CDCl3/acetone.
R = CH2CH2OTBDPS; nr = no reaction.