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THE introduction of the exocyclic C(4')-C(5') double bond in the sugar unit of adenosine is a key step in the synthesis of antibiotics related to angustmycinl and nucleocidin.2 These syntheses use routes which involve blocking and deblocking procedures to achieve the introduction of the double bond ; generally E, elimination of H(4') and a good leaving group a t C(5') (p-MeC6H,S0,, MeSO,, Br, or I), are used. The yields in this elimination step are limited by the instability of the starting compound and its ability to undergo nucleophilic displacement leading to N(3)-C(5') cyclonucleosides.3 This side reaction can be hampered by benzoylation of the 6-NH2 group, which decreases the nucleophilicity of N(3).We now report a new method for generating the exocyclic double bond based on the following considerations : (i) adenosine (1) can be specifically and quantitatively converted into 5'-chloro-5'-deoxy-adenosine (2) ,4 a compound which is stable with respect to intramolecular cyclisation ; (ii) selenoxides readily undergo syn elimination under very mild conditions to give the corresponding ethylenic c o m p o ~n d . ~Treatment of 5'-chloro-5'-deoxy-adenosine (2) (90% yield from adenosine) with sodium benzene selenolate in tetrahydrofuran-hexamethylphosphoric triamide (THF-HMPT) 6 gave 5'-phenylseleno-5'-deoxyadenosine which was oxidised in situ by H,O, to the stable crystalline selenoxide (3a)t (m.p. 169-171 "C; 54% yield). The n.m.r. spectrum: showed one set of signals indicating the presence of a single diastereoisomer. This compound is stable in