Synthesis of linear beta-(1 --> 4)-galacto-hexa- and heptasaccharides and studies directed towards cyclogalactans

Synthesis of an exclusively beta-(1 --> 4)-linked galactohexa- and heptasaccharide is described by coupling a 2-O-pivaloyl-3,6-O-allyl-protected thiogalactobioside donor with an equally protected, yet terminally 4-OH-free galactopentaoside. The same approach though failed to elaborate cyclic oligomers, as neither cyclodimerization of the correspondingly protected thiogalactotriosides with a 4"-OH could be effected, nor intramolecular glycosidation of the respective hexa- and heptagalactosides with an unprotected 4-OH at one, and phenylthio or sulfoxido groups at the reducing end. The causative factors underlying this are attributed to an inadequate predisposition of the linear beta-(1 --> 4)-galactan chains to adopt the tightly coiled molecular geometry necessary for cyclization--at least at the hexa- and heptasaccharide stage.