1,2-anhydrohexopyranoses, preparation

E. Wu and Q. Wu, Preparation of hexopyranose 2-tosylates and their facile conversion into 1,2-anhydrohexopyranoses by internal displacement, Carbohydr. Res., 250 (1993) 327-333. 

Anhydro sugars have proven to be versatile glycosyl donors for the preparation of many glycoconjugates. The 1,2-anhydro bridge is an epoxide that is also part of an acetal and is responsible for the unique reactivity of 1,2-anhydrohexopyranoses. The first well-defined member of this class was prepared by treatment of the partially functionalized pyra-nose (17) with ammonia in benzene, and is commonly referred to as BrigTs anhydride (O Scheme 15) [46]. 

The selectivity of alkylation of 1,6-anhydrohexopyranoses is lower than drat of acylations. The relative rate-constants for the three hydroxyl groups in 1,6-anhydro-jS-D-glucopyranose (6) during methyla-tion with dimethyl sulfate in 19% sodium hydroxide solution689 are k2 ks k4 = 2.5 1 1.8, so that the formation of pure monomethyl or dimethyl ethers of 6 by this method is of little preparative value (compare Refs. 224 and 671). l,6-Anhydro-/3-D-galactopyranose (12) affords mostly the 2,4-di-O-methyl derivative,690 and 2-acetamido-l,6-anhy-dro-2-deoxy-/3-D-galactopyranose is mainly methylated at 0-4 by the... 

In the 1,6-anhydrohexopyranose series, derivatives having a ketone group have been prepared either by partial oxidation of the unsubsti-... 

The second example of a 2,6-anhydrohexopyranose was prepared by an intramolecular displacement of a methylsulfonyloxy group on the ring methyl 2,6-anhydro-3,4-di-0-methyl-a-D-mannopyranoside (112) was obtained in 47 % yield by heating398 a dilute solution (in methyl sulfoxide) of methyl 3,4-di-0-methyl-2-0-(methylsulfonyl)-6-0-sodio-a-D-glucopyranoside (111) at 85°. 

Other Anhydrides. - Stereoselective chemical and chemoenzymatic approaches to the preparation of 1,6-anhydrohexopyranoses have been reviewed, and base treatment of pentabromophenyl glycosides has been investigated as an improved procedure for the synthesis of 1,6-anhydro sugars. Treatment of O-benzylated-3-O-acetyl-glycosyl chlorides with base (KO Bu, THF) has allowed syntheses of l,3-anhydro-2,4,6-tri-0-benzyl-P-D-talopyranose, l,3-anhydro-2,4-di-C -benzyl-a-L-arabinopyranose and 1,3-anhydro-2,4-di-0-benzyl-6-deoxy-P-L-talopyranose. The theoretical solution conformation of methyl 3,6-anhydro-a-D-galactopyranoside has been studied using ab initio calculations. The computed conformation was similar to that adopted in the crystal (X-ray analysis). ... 

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