Syntheses of Chiral Dispiroacetals from Carbohydrates

The syntheses of trioxadispiroacetals from carbohydrates are described. (5 R,7 S,13 R )-13-Methoxy-1,6,8-trioxadispiro[4.1.5.3]pentadecane ( 23 ) and (5 S,7 S,13 R )-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 (5 R,7 S,13 S )-13-methoxy-1,6,8-trioxadispiro[4.1.5.3]pentadecane ( 37 ) and (5 S,7 S,13 S )-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.