Endocyclic Oxirane Derivatives Epoxy Sugars

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. 

However, other conformations, such as the B, S, H, and E forms have been evidenced by H NMR studies, X-ray measurements, and calculations,269 271 and their important role in controlling the course of their reactions emphasized. Small energy differences between individual conformations are particularly evident with furanoses, which makes these systems very flexible. Consequently, clear interpretations from their NMR spectra may be difficult. 

Epoxy sugars are frequently used as starting compounds in the synthesis of sugar derivatives (compare Section IV) such as halo, amino, azido, thio, deoxy, and branched-chain derivatives. The oxirane ring is in general more reactive than the oxetane or oxolane ring. It is opened with nucleophiles under base or acid catalysis. On the other hand, the oxirane ring remains unattacked under the conditions of catalytic debenzylation on palladium, 

All cofigurational isomers of 2,3- and 3,4-anhydrohexopyranoses have been described. Generally, they adopt more or less flexible half-chair conformations (H) dependent on the absence or presence of an additional fused ring. Examples of the latter compounds are methyl 2,3-anhydro-4,6-O-benzylidene-a- and /1-hexopyranosides of the alio 70,272,273 manno 71,273 275 gui0 216,211 an(j 276 c0nfigurati0nS. 

The properties of various flexible 2,3- and 3,4-anhydropyranoses, and their deoxy derivatives have been studied. Methyl 2,3-anhydro-4- 

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