Keywords: Squaric acid / Acidity / Nucleotide analogues / Cyclic nucleotide analogues New analogues of 2Ј-deoxynucleotides and ribonucleotides incorporating a unique squaramide structure were synthesized. Because of the strong acidity of this moiety (pK a = 2.3), these nucleotide analogues exist in a monoanionic form, which can be regarded as an electronic isoster of 5Ј-nucleotides under physiological conditions. The synthesis of the nucleotide analogues was achieved through the condensation of 5Ј-or 3Ј-aminonucleosides with dimethyl squarate, whilst the selective removal of the methyl group was effectively accomplished by treatment with sodium bromide. In addition,