Abstract
Reaction of 2,3- O-isopropylidene- d-ribofuranose ( 13) with the stabilized Wittig reagents Ph 3PC(Me)CO 2Me ( 14) and Ph 3PC(Me)CN ( 15) gave olefinic products which, upon treatment with dilute base, afforded the corresponding anhydro sugars having the β- d-anomer configuration exclusively. Treatment of these kinetic products with a strong base did not affect the ester obtained from 14, but the nitrile from 15 gave a β-to-α-anomer ratio at equilibrium of 4:1. Reaction of 13 with Ph 3PCHCOMe led directly to a 7:3 mixture of β- and α- d anomers, and this ratio was changed to 1:4 upon prolonged exposure to base. Treatment of 4,6- O-ethylidene- d-glucopyranose with the Wittig reagent Ph 3PCHCOCH 2CO 2Et led directly to a 1:1 mixture of the anomers of the anhydro sugar in which a β-keto ester residue is attached at the (original) anomeric center. This ratio of anomeric forms could not be changed by treatment with base, but the ethylidene-protecting group could be removed, and the resulting tetrol tritylated at the primary position.
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