This paper deals with the synthesis of a stable biotin-phosmidosine conjugate molecule 3 that is required for isolation of biomolecules that bind to phosmidosine (1). It was found that introduction of a biotin residue into the 6-N position of phosmidosine could be carried out by reaction of an N-7-Boc-7,8-dihydro-8-oxoadenosine derivative 13 with phenyl chloroformate followed by displacement with a diamine derivative 6 along with the simultaneous removal of the Boc group and one of the two phenoxycarbonyl groups and the successive condensation with an N-tritylated biotin derivative 5. The condensation of an N-prolylphosphorodiamidite derivative 4 with an appropriately protected 7,8-dihydro-8-oxoadenosine derivative 17 having the biotin residue gave the coupling product 18, which was deprotected to give the biotin-phosmidosine (O-ethyl ester) conjugate 3. (C) 2004 Elsevier Ltd. All rights reserved.