Glycosyl Imidates, 52. Synthesis of Globotriaosylceramide (Gb 3 ) and Isoglobotriaosylceramide (isoGb 3 )

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.