C-disaccharides of ketoses

Reaction of gluconolactone 2 with allylmagnesium bromide at low temperatures afforded ketopyranose 3, which could easily be converted into open-chain ketoses (R)-6 and (S)-6. Their reaction with lithioacetylide 9 afforded propargylic alcohol derivatives (R)-10 and (S)-10, which could not be cyclized directly to the desired C-ketosides. They were converted by standard procedures into (R)-14 and (S)-14 and then into dicobalthexacarbonyl complexes (R)-16 and (S)-16. A facile acid-catalyzed ring closure gave the desired C-ketosides (R)-18 alpha/beta and (S)-18 alpha/beta, respectively, in different ratios. In order to demonstrate that removal of the protective groups and hydrogenation of the CC triple bond proceed smoothly, (R)-18% was transformed into the deprotected target molecule (R)-1x. For the assignment of the new chiral centers at C-2/2' and at C-8, (S)-18 alpha was transformed into azido derivative (S)-22 alpha, which underwent intramolecular cycloaddition to afford the spiro derivative (S)-25 alpha. Because of the conformational constraints in this molecule, unequivocal configurational assignment was possible with the help of NMR data.