Table 1.
Selective 5-O-benzoylation of methyl α/β-xylofuranosides (9) with PhCOCl
| Entry | Regents and conditions | Base (equiv.) | BzCl (equiv.) | Time (h) | Ratios of 11b/11aa | Yields (%)b |
|---|---|---|---|---|---|---|
| 1 | BzCl/CH2Cl2/Py 0 °C→rt | Et3N (1.0) | 1.2 | 18 | 1.24:1 | 29 |
| 2 | BzCl/CH2Cl2 0 °C→rt | Et3N/iPr2NEt(1.7/0.7) | 1.3 | 18 | 1.29:1 | 39 |
| 3 | BzCl/CH2Cl2 0 °C→rt | Et3N/iPr2NEt(1.7/0.8) | 1.3 | 18 | 1.32:1 | 46 (39) |
| 4 | Bu2SnO/MeOH reflux; BzCl/toluene, rt | iPr2NEt (1.9) | 2.0 | 5 | 1:0.72 | 46 (35) |
| 5 | Bu2SnO/MeOH reflux; BzCl/1,2-DME, rt | iPr2NEt (2.3) | 3.0 | 5 | 1:0.80 | 97 (82) |
a
Ratio of 5-O-benzoyl α- and β-xylofuranosides determined from 1H NMR spectrum of the anomeric mixture after chromatography.
b
Combined isolated yields of 5-O-benzoates 11a,b after chromatography on silica gel. Figures in parentheses refer to isolated yield of methyl 5-O-benzoyl α-xylofuranoside 11a (calculated from α:β ratio of 1:1.25/1.14 in the starting methyl xyloside 9).