SNTHESIS AND "ANOMERIZATION" OF C-GLYCOSYL COMPUNDS RELATED TO SOME HETEROCYCLIC NATURAL PRODUCTS

Article abstract of DOI:10.1016/S0008-6215(00)90877-2

Reaction of 2,3-O-isopropylidene-D-ribofuranose (13) with the stabilized Wittig reagents Ph3P=C(Me)CO2Me (14) and Ph3P=C(Me)CN (15) gave olefinic products which, upon treatment with dilute base, afforded the corresponding anhydro sugars having the β-D-anomer configuration excusively.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 Ph3P=CHCOMe 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 Ph3P=CHCOCH2CO2Et 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 coud not be changed by treatment with base, but the ethylidene-protecting group could be removed, and the resulting tetrol tritylated at the primary position.