Sugars with fused rings, conformation

The racemic form of methyl 3,4-anhydro-/3-mycaroside (82) has been synthesized 35 it is an epoxide of a branched-chain sugar having the oxirane ring fused to the pyranose ring. Hydrolysis of the oxirane ring with hydrochloric acid in aqueous p-dioxane occurs with substitution at C-4, the (more accessible) secondary position, as would be expected from its favored °H, conformation (82) anticipated, because it is an analog of the 3,4-anhydro-D-allopyranoside derivatives considered previously (see p. 148). 

Anhydro sugars of this type, having an oxirane ring fused with a pyranose or furanose, possess 3,7-dioxabicyclo[4.1.0]heptane (67) and 3,6-dioxa-bicyclo[3.1.0]hexane (68) basic skeletons, respectively. 2,3-Anhydro-pyranoses usually adopt 5HQ or °HS and 3,4-anhydropyranoses adopt 1H0 and °H conformations in the ground state. 

Formation of cyclic acetals by sugar hydroxyls generally retards nucleophilic displacements at the anomeric centre of the same sugar residue. In the case of 4,6-benzylidene derivatives, the mechanism of deactivation appears to be that the dipole of the C6-06 bond is constrained with its positive end directed towards the anomeric centre.In the case of the 1,2-diketals, the deactivation arises from the increased difficulty of forming half-chair or boat conformations in a six-membered ring, which is part of a traw -fused decalin structure. 

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