Syntheses of chiral dispiroacetals from carbohydrates

The syntheses of trioxadispiroacetals from carbohydrates are described. (5R,7S,13R)-13-Methoxy-1,6,8-trioxadispiro[4.1.5.3]pentadecane (23) and (5S,7S,13R)-13-methoxy-1,6,8-trioxadispiro[4.1.5.3]pentadecane (24) were prepared starting from D-galactose. The construction of the lateral tetrahydrofuran and tetrahydropyran rings was realized by homologation at C-1 and C-6 by appropriate tethers possessing a terminal primary alcohol. The cyclization of these alcohols at C-1 and C-5, respectively, was performed with (diacetoxyiodo)benzene and iodine in order to generate the alkoxy radicals which undergo an intramolecular hydrogen abstraction reaction. The diastereoisomers (5R,7S,13S)-13-methoxy-1,6,8-trioxadispiro[4.1.5.3]pentadecane (37) and (5S,7S,13S)-13-methoxy-1,6,8-trioxadispiro[4.1.5.3]pentadecane (38) were prepared starting from tri-O-acetyl-D-glucal using a suitable methodology in which homologation and intramolecular hydrogen abstraction were again the key steps. We believe that this protocol could be easily extended to other tricyclic dispiroacetal ring systems.