A Surprising Ring Opening Mechanism in the Formation of α-D-Arabinofuranosyl Nucleosides from 5-Substituted Uracils

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Abstract
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 to give 8. Compound 7 was also reacted with Lawesson's Reagent to generate the corresponding 4-thio-α-D-arabinofuranoside nucleoside 14 which was deprotected by treatment with methanolic ammonia to give the nucleosides 15. Deprotected acyclo nucleosides 10 were likewise obtained from compounds 9. The mechanism for formation of the nucleosides 7 is discussed and the acyclo nucleosides 9 are believed to be intermediates.

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