3.6- anhydrosugar

O-stannylene acetals,84 glycals and 1,2-anhydrosugars,85 and to selective de-O-benzylation of position-2 with TIBAL, DIBAL-H57 or Lewis acid catalysts,86 or to the one-pot access to 3-O-benzyl-4,6-0-benzylidene glucosides by tandem catalysis recently reported.87 The 1,2-lactones recently reported by Linker and co-workers35 are also synthons which provide... 

SOLID SUPPORT GLYCAL ASSEMBLY VIA 1,2-ANHYDROSUGAR DONORS... 

Polymer-supported glucal 37 was converted to the protected thioethyl glucosyl donor 39 as outlined in Scheme 2.11. Compound 37 was first epoxidized by the action of DMDO. The resulting 1,2-anhydrosugar was opened by a mixture of ethanethiol and dichloromethane (1 1) in the presence of a trace of trifluoroacetic acid. Polymer-bound 38 was thus obtained in 91% yield. This was a substantial improvement over the 78% yield obtained by the same protocol in solution. Protection by reaction with pivaloyl chloride occurred in quantitative yield to furnish 39a. 

M. Cavicchioli, A. Mele, V. Montanari, and G. Resnati, A stereoselective and preparative entry to 1,2-anhydrosugars through oxidation of glycals with perfluoro-c -2,3-dialkyl-oxaziridines, J. Chem. Soc., Chem. Common., (1995) 901-902. 

In the cationic polymerization of 1,2-anhydrosugars, however, racemization is clearly detected indicating unimolecular opening of 3-mem-bered ring [154] ... 

Scheme 25 C-Glycoside synthesis via the reductive opening of 1,2-anhydrosugars...

Perfluorinated oxaziridines epoxidize glycols to give the corresponding 1,2-anhydrosugars in high yield and moderate to complete diastereoselection <1995CC901>. For example, treatment of 226 with 80 in chloroform afforded 227 in >90% yield and >98% de. 

SCHEME 5.37 Solid-phase synthesis using glycal-derived 1,2-anhydrosugars. 

Anhydrosugars are an important class of compounds in which the three-membered oxirane ring is connected to the anomeric and the C-2 atoms of a sugar. The first compound of this class, the so-called Brigl s anhydride (26), was prepared by treatment of 8-chloro-3,4,6-tri-0-acetyl-D-glucose (25) with ammonia ( Scheme 11) [3]. 

The 1,2-anhydrosugars are convenient synthons for the preparation of a variety of important derivatives. Generally, opening of the oxirane ring with various nucleophiles (alcoholates, amines, sulfides, carboanions, etc.) proceeds with the inversion of the configuration at the anomeric center [28,29,30]. 

In contrast to the reaction of 1,2-anhydrosugar, C-glycosylation of 1,6-anhydropyranose requires a strong Lewis acid in order to open the anhydro bridge, generating the corresponding oxocarbenium cation (O Eq. 6). Addition of nucleophiles to the cation could be controlled by the stereoelectronic effect, chelation between the nucleophile and the Lewis acid coordinated to the hydroxyl group, or intramolecular delivery of nucleophiles as shown below. 

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