The isopropylidenation of d-ribose diethyl dithioacetal and ribitol. A new synthesis of α- and β-d-ribofuranosylethyne via 2,3: 4,5-di-O-isopropylidene-aldehydo-d-ribose

The reaction of d-ribose diethyl dithioacetal with acetone and sulphuric acid in the presence of anhydrous copper sulphate gives the 2,3:4,5-di-O-isopropylidene derivative 14 (40%) and the isomeric 2,5:3,4-di-O-isopropylidene acetal 17 (40%), contrary to the conclusions of some previous investigators. Earlier work on the structures of the mono-O-isopropylidene derivatives formed when copper sulphate alone is the catalyst has been confirmed and extended. The diacetonides 14 and 17 were converted into the aldehydo-d-ribose derivatives 11 and 30, respectively, and thence into the di-O-isopropylideneribitol derivatives 24 and 25. When ribitol was treated with acetone and sulphuric acid, 1,2:4,5-di-O-isopropylideneribitol (66%) was the major product, together with dl-24 (22%) and dl-25 (11%). The aldehydo-d-ribose 11 reacted with ethynylmagnesium bromide in tetrahydrofuran to give the d-altro and d-allo alcohols 39 and 40 in the ratio 2:1. Toluene-p-sulphonylation of the mixture of 39 and 40, followed by solvolysis in buffered, boiling aqueous ethanol, afforded the 2,3-O-isopropylidene-d-ribofuranosylethynes 45 and 46 with loss of the terminal O-isopropylidene group. ¹³C-N.m.r. spectroscopy was used extensively to determine the ring sizes of the isopropylidene derivatives. © 1985.