We have synthesized four conformationally restricted bicyclic 2′-spiro nucleosides via 2′-C-allyl nucleosides as key intermediates. The ribo-configured 2′-spironucleosides 9b and 14b were obtained by a convergent strategy starting from 2-ketofuranose 1 whereas the arabino-configured 2′-spironucleosides 21 and 27 were obtained by a linear strategy with a 2′-ketouridine derivative as starting material. The furanose ring of 9b/14b adopts N-type conformations whereas the furanose ring of 21/27 exists as an N⇄S equilibrium. These compounds showed no anti-HIV-1 activity or cytotoxicity. Incorporation of the four 2′-spironucleosides (as monomers X4 and X5) into oligodeoxynucleotides was accomplished using the phosphoramidite approach on an automated DNA synthesizer. Irrespective of monomeric configuration, hybridization studies revealed that these 2′-spironucleotide monomers (X4 and X5) induce decreased duplex thermostabilities compared with the corresponding DNA:DNA and DNA:RNA duplexes. Molecular modelling indicated that steric constraints are a possible reason for the lowered binding affinities of the modified oligodeoxynucleotides towards complementary single-stranded DNA and single-stranded RNA complements.