The reaction of sugar lactols with Wittig-type reagents and cyclization to C-glyco-sides is a traditional method for preparing C-glycosides. This section addresses the reactions of sugar lactones, the Wittig reaction of sugar lactols with unstabilized ylides and their subsequent cyclizations, and reactions of lactols with stabilized ylides and their conversion into C-glycosides.
Chapleur and coworkers have published a full account on their work dealing with the dichloroolefination of sugar lactones . They found that sugar-based lactones give good yields of dichloroolefins when treated with a carbon tetrachloride-tri-phenylphosphine reagent combination. Some of the products along with their yields are boxed in Scheme 26. The same group has also found that reaction of certain chloroolefins such as 121 with mCPBA in dichloromethane gives compounds of the general structure 122 . The reaction is thought to proceed via formation from dichloromethane of chlorine radicals that do not undergo addition to chloroolefins, since they are poor radical acceptors, but instead combine to give molecular chlorine that then undergo ionic addition to the double bond to give the observed products.
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