Synthesis of trisaccharides and tetrasaccharides by means of intramolecular glycosylation supported by rigid spacers

Treatment of alpha,alpha'-dibromo-m-xylylene with 6-O-unprotected thiomalto side 4 as glycosyl donor (--> 5), followed by 4-O-unprotected galactoside d erivative 6 as acceptor, afforded beta -linked macrocyclic trisaccharide SP in high yield after removal of the 3-O-MPM protective group and subsequent intramolecular glycoside bond formation. Similarly, by the same sequence o f steps, the corresponding tetrasaccharide 14 beta was obtained from 5 and 4b-O-unprotected lactoside 11. For reiterative glycoside bond formation, tr eatment of alpha,alpha 'dibromo-m-xylylene with 3-O-unprotected thioglycosi de 15 as donor (--> 16), followed by 4,6-O-unprotected glucoside, and subse quent glycosylation afforded macrocyclic maltotrioside 22, which was transf ormed into known maltotrioside 23. A slight modification of the protecting- group pattern in maltotrioside synthesis resulted in generally higher yield s in the ligation of the building blocks to the m-xylylene spacer, particul arly in the second glycosylation step, thus providing macrocyclic maltotrio side 40 alpha, which was transformed into known maltotriosides 41 alpha and 41 beta.