Ring opening of benzyl β-D-galactoside cyclic sulfates into galactose monosulfates.: New access to 6-deoxy-galacto-hex-5-enopyranoside and 4-deoxy-3-ketogalactopyranoside.

The behavior of 3,4- and 4,6-cyclic sulfates derived from benzyl 2,6- and 2,3-di-O-benzyl-beta-D-galactopyranosides toward hydrolysis has been studied using aqueous sodium hydroxide under various conditions. Starting from benzyl 2,6-di-0-benzyl-3,4-O-sulfuryl-beta-D-galactopyranoside (5), the reaction with aq NaOH in THF gave both 3- and 4-monosulfates 7 and 8 (83%, in 68:32 ratio), while the reaction in DMF led unexpectedly to the 4-deoxy-3-keto derivative 10 in 77% yield after acidic hydrolysis of the intermediate enolester 9. On the other hand, when benzyl 2,3-di-O-benzyl-4,6-O-sulfuryl-beta-D-galactopyranoside (6) was treated with aq NaOH in THF, a mixture of benzyl 2,3-di-O-benzyl-6-deoxy-4-O-(sodium sulfonato)-alpha-L-arabino-hex-5-enopyranoside (11) and benzyl 2,3-di-O-benzyl-4-deoxy-6-O-(sodium sulfonato)-alpha-L-threo-hex-4-enopyranoside (12) (in 65:35 ratio) was obtained in 93% yield, giving a new and rapid access to 11, a potential precursor of L-sugars derivatives. Alternatively, BzONBu(4), gave a regiospecific opening reaction of 6 and led to the 6-O-benzoate 4-O-sulfate derivative (13) in excellent yield.