Production of carbohydrate building blocks from red seaweed polysaccharides. Efficient conversion of galactans into C-glycosyl aldehydes

Agarans and carrageenans are abundant natural polysaccharides which are obtained on a large scale by water extraction from a variety of red seaweeds. These galactans, in addition to being valuable products for the pharmaceutical and food industries, are low cost starting materials for the preparation of useful and rare carbohydrate-based building blocks whose access by total synthesis is difficult and expensive. The semisynthesis of two sets of C-glycosyl aldehydes with L- and D-configuration from agarose and kappa-carrageenan respectively is described. Succinctly, the partial acid-catalyzed mercaptolysis of the two galactans under mild conditions afforded agarobiose and carrabiose (beta-D-Galp-(1 -> 4)-3,6-anhydro-aldehydo-L- and D-galactose, respectively) derivatives. Complete depolymerization of agarose and kappa-carrageenan under harsher conditions produced 3,6-anhydro L- and D-galactose aldehyde derivatives. Chain shortening of these products via alditol formation and oxidative carbon-carbon bond cleavage furnished C-formyl alpha-L- and alpha-D-threofuranosides. The above C-glycosyl aldehydes were all prepared on a meaningful preparative scale starting from gram quantities of galactans. Finally, a new procedure for the preparation of the 2,3-O-benzyl L-threofuranose was established by Baeyer-Villiger oxidation of the benzylated C-formyl alpha-L-threofuranoside here prepared from agarose.