Fragmentation of carbohydrate anomeric alkoxy radicals.: A new general method for the synthesis of alduronic acid lactones

Alduronic acid 4,1-, 5,1-, and 5,2-lactones can be specifically obtained when hexuronic and penturonic acids belonging to the erythrose and threose carbohydrate series undergo a tandem beta-fragmentation-intramolecular cyclization reaction. In this way, gamma-lactones such as 3-O-formyl-1,2-O-isopropylidene-D-threurono-4,1-lactone (38), 3-O-formyl-1,2-di-O-methyl-D-threurono-4,1-lactones (39), or 3-O-formyl-1,2-O-isopropylidene-D-erythrurono-4,1-lactone (41), and delta-lactones such as 1-O-(tert-butyldimethylsilyl)-4-O-formyl-2,3-O-isopropylidene-D-lyxurono-5,1-lactones (40), or 4-O-formyl-1,2,3-tri-O-methyl-D-arabinurono-5,1-lactones (42), or 3-O-benzyl-4-O-formyl-1,2-O-isopropylidene-D-arabinurono-5,1-lactone (43), were obtained. Alternatively, an intermolecular reaction took place when the carboxyl group was lactonized. Thus, 1,4-di-O-acetyl-3-formyl-1-iodo-D-arabininurono-5,2-lactone (45) was prepared from 2,5-di-O-acetyl-D-glucurono-6,3-lactone (37). The reaction is promoted by two different systems: (diacetoxyiodo)benzene (DIB)-iodine, under mild conditions, or diphenylhydroxyselenium acetate (DHSA)-iodine under visible light irradiation. With this new strategy, nor-aldopyranosuronic and aldofuranosuronic acid lactones are formed via 1,5 and 1,6 intramolecular cyclization.