Synthesis of 5-[2-(guanin-9-yl)- and 5-[2-(2-aminopurin-9-yl)ethyl]- 2-d-ribo-(1′,2′,3′,4′-tetrahydroxybutyl)-1,3-dioxane

Stereoselective synthesis of 5-[2-(guanin-9-yl)- and 5-[2-(2-aminopurin-9-yl)ethyl]-2-D-ribo(1′,2′,3′,4′- tetrahydroxybutyl)-1,3-dioxane, 2-5, as potential prodrugs of penciclovir, has been accomplished in six steps from readily available 2,3,4,5-tetra-O-acetyl-aldehydo-D-ribose (6) and the 1,3-diol 7. It has been demonstrated that the use of boron trifluoride diethyl etherate (BF₃·Et₂O) in dichloromethane along with excess anhydrous copper(II) sulfate was crucial for the efficient formation of cyclic acetal 8. In addition, the chromatographic separation of cis and trans isomers of the cyclic acetal at the bromide stage 10 was feasible, which was requisite for the successful stereoselective synthesis of the ribosyl derivatives 2-5.