Highly stereoselective C-α- d-ribofuranosylation. Reactions of d-ribofuranosyl fluoride derivatives with enol trimethylsilyl ethers and with allyltrimethylsilane

Abstract

2,3,5-Tri- O-methyl- d-ribofuranosyl flouride ( 6), 2,3-di- O-benzyl-5- O-methyl- d-ribofuranosyl fluoride ( 7), and 5- O-benzyl-2,3-di- O-methyl- d-ribofuranosyl fluoride ( 8) were obtained in 57 ( 6α, 15; and 6β, 42), 87 ( 7α, 22; and 7β, 65), and 85.5 ( 8α, 35.5; and 8β, 50%) yields, respectively, from the corresponding OH-1 derivatives by the reaction with N,N-diethyl-1,1,2,3,3,3-hexafluoropropylamine, adduct of hexafluoropropene with diethylamine. These fluorides and 2,3,5-tri- O-benzyl- d-ribofuranosyl fluoride ( 5) reacted with isopropenyl trimethylsilyl ether, (Z)-1-ethyl-1-propenyl trimethylsilyl ether, and allyltrimethylsilane, in the presence of boron trifluoride·diethyl etherate to give the corresponding 1- d-ribofuranosyl-2-propanones, 2- d-ribofuranosyl-3-pentanones, and 3- d-ribofuranosyl-1-propenes in good yields. C-Acetonylation was confirmed to afford the α- d anomer as the initial product, and the α- d anomer was isomerized into the corresponding β- d anomer to give a mixture. The C-allylation reaction gave only the α- d anomer. C-Pentanonylation, however, gave a mixture of diastereoisomers that could not be isolated. All reactions afforded almost the same results starting with either α- or β- d-ribofuranosyl fluoride. No reaction of the β anomer of 5 with 1-isopropyl-2-methyl-1-propenyl trimethylsilyl ether took place.