Glycosyl Imidates, 52. Synthesis of Globotriaosylceramide (Gb3) and Isoglobotriaosylceramide (isoGb3)

The synthesis of globotriaosylceramide (1) was based on O‐galactosyl trichloroacetimidate 5α as donor and 4b‐O‐unprotected lactose 7 as acceptor; 7 was readily accessible from lactose. Glycosylation by an “inverted procedure” afforded preferentially the α‐trisaccharide 8α. Its transformation into the O‐acetyl‐protected trichloroacetimidate 11α led to an efficient triaosyl donor for the β‐selective glycosylation of 3‐O‐benzoyl‐azidosphingosine 12. The obtained lysoglycosphingolipid derivative 13 was directly converted into the N‐palmitoyl derivative 14 which gave upon O‐deacylation the target molecule 1. For the synthesis of isoglobotriaosylceramide (2) essentially the same procedure was applied. Thus, by starting from 5α and 3b, 4b‐O‐unprotected lactose acceptor 15 the use of the inverted procedure for glycoside‐bond formation gave preferentially trisaccharide 16α, which was transformed into triaosyl donor 24α. Application of the azidosphingosine glycosylation procedure afforded lysosphingolipid 25 and subsequently glycosphingolipid 26; after deprotection the target molecule 2 was obtained.