Agarans and carrageenans are abundant natural polysaccharides which are obtained in large scale by water extraction from a variety of red seaweeds. These galactans in addition of being valuable products for pharmaceutical and food industries are low cost starting material 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 (β-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 in meaningfully 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.
Production of Carbohydrate Building Blocks from Red Seaweed Polysaccharides. Efficient Conversion of Galactans into C-Glycosyl Aldehydes
MASSI, Alessandro;DONDONI, Alessandro;
2009
Abstract
Agarans and carrageenans are abundant natural polysaccharides which are obtained in large scale by water extraction from a variety of red seaweeds. These galactans in addition of being valuable products for pharmaceutical and food industries are low cost starting material 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 (β-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 in meaningfully 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.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
