Generation of the ylide of methyl 5-deoxy-2,3-O-isopropylidene-5-(triphenylphosphonio)-β-d-ribofuranoside iodide (2a) is described. Treatment of the ylide with aldehydes affords good yields of olefinic products of the α-l-lyxo configuration, resulting from epimerization of the ylide prior to reaction. Ketones do not react cleanly with the ylide. Addition of a proton source to the ylide under appropriate conditions allows the formation of good yields of a self-condensation product 14. © 1977, American Chemical Society. All rights reserved.