Two approaches to the synthesis of the title compounds are described.
In the first route, a reactive 5-oxo-6-methylene pyrimidine intermedia
te that is generated by treating the bis-acetylated or bis-benzoylated
nucleosides 10 and 11 with sodium hydroxide undergoes intramolecular
attack by the 5'-thiol group to afford the 5-hydroxy cyclonucleoside 1
2. In the second and higher yielding approach, the S-5',6-methano link
age is established by an internal allylic displacement reaction that o
ccurs when the 5-bromo-6-mmethyl nucleoside 24 is treated with base. T
he conformational properties of S-5',6-methano-5'-thiouridine (3) and
certain long-range spin-spin couplings observed in the NMR spectra of
the intermediate nucleosides are discussed.