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Glycosyl Imidates, 55. Synthesis of the Pentasaccharide Moiety of an Asterosaponin

✍ Scribed by Bing, Han Xiao ;Schmidt, Richard R.

Publisher
John Wiley and Sons
Year
1992
Tongue
English
Weight
715 KB
Volume
1992
Category
Article
ISSN
0947-3440

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✦ Synopsis

Abstract

Reaction of 2‐O‐acetyl‐protected O‐galactosyl trichloroacetimidate 3 as glycosyl donor and 2,4‐di‐O‐unprotected xylopyranoside 2 as glycosyl acceptor furnished in the presence of Et~2~O · BF~3~ as catalyst regioselectively β‐(1→4)‐connected disaccharide 4 which gave upon subsequent reaction with O‐quinovosyl trichloroacetimidate 5 as donor β‐(1→2)‐connection, thus affording trisaccharide 6. Removal of the 2‐O‐acetyl group from the galactosyl moiety yielded acceptor 7; its glycosylation with donor 5 furnished β‐connected tetrasaccharide 8. This compound was transformed via 1‐O‐desilylation and then treatment with trichloroacetonitrile in the presence of a base into O‐tetraosyl trichloroacetimidate 10 as glycosyl donor. Reaction of 10 with 3‐O‐unprotected 2,4‐di‐O‐acetyl‐ and 2,4‐di‐O‐benzyl‐protected quinovosides 13 and 16 furnished the desired fully O‐protected pentasaccharides 17 and 18, respectively. Hydrogenolytic O‐debenzylation of 18 furnished the O‐unprotected target molecule 19 which was characterized as its O‐acetyl product 20.

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