Synthesis of carbon‐bridged N‐acetyl‐c‐lactosamine and derivatives

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Abstract
4‐C‐Formyl‐2‐azidoglucopyranoside 12a, required for N‐acetyl‐C‐lactosamine synthesis as electrophile, was obtained from thexyldimethylsilyl 2‐azido‐2‐deoxy‐glucopyranoside 3 via readily available 4‐O‐unprotected 6a and then 4‐C‐methylene derivative 8a in overall seven steps. Alternatively, regioselective silylation of 3 with tert‐butyldimethylsilyl chloride gave 4‐O‐unprotected 6b which was transformed by a similar reaction sequence into 12a. In order to circumvent a Wittig reaction, 6a was transformed into triflate 13 the reaction of which with 4‐C‐cyano derivative 14 followed by reduction with DIBAH and base‐catalyzed isomerization also afforded 12a. Reaction of 12a with 1‐C‐lithiated 2‐phenylsul‐finyl‐D‐galactal 15 as nucleophile furnished C‐disaccharide intermediates 16a and 16b as diastereoisomers. Ensuing removal of the phenylsulfinyl group with Raney nickel and diastereospecific 2b‐hydrogen and 3b‐hydroxy transfer afforded β(1→4)‐connected N‐acetyl‐C‐lactosamines 19a and 19b; their structures were deduced from derivatives 20a, b and 21a, b on the basis of 1H‐NMR data. Hydrogenolytic O‐debenzylation of 19b afforded hydroxymethylene‐bridged N‐acetyl‐C‐lactosamine 2b′.

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