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 β-fragmentation−intramolecular cyclization reaction. In this way, γ-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 δ-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 -arabinurono-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.