Spiroketal Synthesis. — A Case of Intramolecular Glycoside Bond Formation

From 4‐ O ‐unsubstituted glucose derivatives 1a , b the 4‐hydroxymethyl‐substituted glucose derivatives 9a , b were obtained via oxidation to the ketone, Wittig reaction with methylenetriphenylphosphorane, borane addition, and subsequent oxidation to yield the diastereomeric galactose‐derived compounds 5a , b ; the required inversion of configuration at C‐4 was accomplished through oxidation to the formyl derivatives, base‐catalyzed isomerisation, and then reduction. Transformation of hydroxymethyl derivatives 9a , b into the iodo derivatives 11a , b and reaction with n ‐butyllithium generated the C ‐lithiated species 11aA , bA which furnished with O ‐benzyl‐protected gluconolactone 12 the ketose derivatives 13a, b . Treatment with Et 2 O–BF 3 afforded under concomitant 6‐ O ‐debenzylation the spiroketals 15a , b . Hydrogenolytic O ‐debenzylation and subsequent O ‐acetylation led to compounds 16a and 16b , respectively. The anomeric ketoside configuration was derived from NOE experiments.