Abstract Anomeric pairs of per-
O-acetylated-D-xylopyranosyl halides were individually treated with a wide variety of nucleophiles under mild PTC conditions. Thus, 2,3,4-tri-
O-acetyl-α-D-xylopyranosyl bromide
1 provided exclusively the β-D-xylopyranosyl anomers
2-11 in good to excellent yields (65-95%). Alternatively, under the same PTC conditions, 2,3,4-tri-
O-acetyl-β-D-xylopyranosyl chloride
13 afforded solely the inverted α-D-anomers
15 (82%) and
16 (67%) upon treatment with thiophenol and sodium azide, respectively. Similarly, 2,3,4,6-tetra-
O-acetyl-β-D-glucopyranosyl chloride
19 provided the analogous products
20 (63%) and
21 (31%) upon treatment with thiophenol and sodium azide. In the presence of tetrabutylammonium chloride as PTC catalyst, β-xylopyranosyl chloride
13 was shown to slowly equilibrate to the α-chloride
14. Therefore, care must be taken to avoid PTC catalyst for which counter anions can cause anomerization of the starting glycosyl halides.
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