Hairpin oligodeoxynucleotides incorporating a (2R,3S)-4-(methoxyamino)butane-1,2,3-triol residue in the middle of the double-helical stem and opposite to either one of the canonical nucleobases or an abasic 2-(hydroxymethyl)tetrahydrofuran-3-ol spacer were synthesized. Under mildly acidic conditions, aromatic aldehydes reacted reversibly with these oligonucleotides, converting the (2R,3S)-4-(methoxyamino)butane-1,2,3-triol unit into a 2-aryl-N-methoxy-1,3-oxazinane nucleoside analogue. The equilibrium of this reaction was found to be dependent on both the aldehyde and the nucleobase opposite to the modified residue. 9-Formyl-9-deazaadenine, combining a large stacking surface with an array of hydrogen bond donors and acceptors, showed the highest affinity as well as selectivity consistent with the rules of Watson-Crick base pairing. 5-Formyluracil or indole-3-carbaldehyde, lacking in either stacking or hydrogen bonding ability, were incorporated with a much lower affinity and selectivity.