A surprising ring opening mechanism in the formation of α-D-arabinofuranosyl nucleosides from 5-substituted uracils

Reaction of silylated 5-substituted uracil derivatives 6 with methyl 2,3,5-tri-O-benzoyl-α-D-arabinofuranoside (3) in the presence of trimethylsilyl trifluoromethanesulfonate afforded a mixture of the corresponding 5-substituted 1-(2,3,5-tri-O-benzoyl-α-D-arabinofuranosyl)uracils 7 and the acyclo 2,3,5-tri-O-benzoyl-1-O-methyl-1-(uracil-1-yl)-D-arabinitols 9 with the methoxy group intact at C-1. Compound 7 was deprotected with methanolic ammonia. Compound 7 was also reacted With Lawesson's reagent to generate the corresponding 4-thio-α-D-arabinofuranoside nucleoside which was deprotected by treatment with methanolic ammonia