Syntesis of Chiral Dispiroacetals from Carbohydrates

Abstract

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 C1 and C6 by appropriate tethers possessing a terminal primary alcohol. The cyclization of these alcohols at C1 and C5, 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.