摘要：

Uracil or thymine nucleosides were converted into a series of analogs in which the oxygen atom at position 4 has been replaced by other functional groups. This procedure involves thiation of suitably-protected nucleosides and has led to a facile synthesis of the hitherto-rare, naturally-occurring nucleoside, 5-methyl-2′-deoxycytidine, from thymidine. Similarly, 5-methylcytidine and cytidine were prepared from 1-β-D-ribofuranosylthymine and uridine, respectively. The 4-thio intermediates of blocked nucleosides served as excellent chemical precursors for the synthesis of a host of 4-substituted derivatives of 1-β-D-ribofuranosyl-, 2′-deoxyribofuranosyl-, -xylofuranosyl-, and -glucopyranosyl-pyrimidine nucleosides such as the 4-alkylamino, 4-hydrazino, 4-hydroxylamino, 4-thio, 4-azido (or tetrazolo) and other analogs. A comparison of the spectrally-determined pKa values of nucleosides with and without a 5-methyl substituent is given and the ultraviolet absorption spectra of the more important nucleosides at different pH values are described. 1-Methyl-4-thiouracil was synthesized from 1-methyluracil and converted to 1-methylcytosine. 4-Hydrazino-2(1H)-pyrimidinone was prepared from 4-ethoxy-2(1H)-pyrimidinone and 1,5-dimethylcytosine was synthesized from 1,5-dimethyl-4-ethoxy-2(1H)-pyrimidinone.

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