Palladium-mediated synthesis of C-5 pyrimidine nucleoside thioethers from disulfides and mercurinucleosides

Thioether-linked side chains can be created at C-5 of pyrimidine nucleosides via a palladium-mediated reaction of mercurated nucleosides with organic disulfides. 5-(Chloromercuri)-2′-deoxyuridine reacts with butyl disulfide, phenyl disulfide, dimethyl 3,3′-dithiodipropionate, and N,N′-bis(trifluoroacetyl)cystamine to yield respectively 5-(1-thiapentyl)-2′-deoxyuridine, 5-(phenylthio)-2′-deoxyuridine, 5-[3-(methoxycarbonyl)-1-thiapropyl]-2′-deoxyuridine, and 5-[3-(trifluoroacetamido)-1-thiapropyl]-2′-deoxyuridine in yields ranging from 46 to 73%. Other mercurated nucleosides, including 5-(chloromercuri)-2′-deoxycytidine, 5-(chloromercuri)cytidine, and 5-(chloromercuri)tubercidin react with N,N′-bis(trifluoroacetyl)cystamine and lithium-palladium chloride in methanol to yield the corresponding coupled products, but the yields are much lower (5-10%). The nucleoside coupling reaction is complicated by competing side reactions between disulfides and Pd²⁺, which remain to be elucidated.