Other Naturally Occurring Pyrimidine Nucleosides

Carbon-13 magnetic resonance spectra of the naturally occurring cytidines have been described in several papers.79-82 The electronic structure of the compounds is reflected in the carbon-13 shifts. For instance, the observed chemical shifts for these and other pyrimidine and purine nucleosides were correlated, at least qualitatively, with the calculated charge densities (see Section VIII) and with the known reactivity of these molecules. It is difficult to draw conclusions from the carbon-13 spectra about the tautomerism of cytosine. 

The pentose component of naturally occurring nucleotides is ribose or 2-deoxyribose (i.e., ribose with a hydrogen instead of a C-2 —OH). In nucleotides the purine or pyrimidine is attached to C-1 of the pentose in the /3 configuration. This means that the base is cis relative to C-5 —OH and trans relative to the C-3 —OH. The major function of deoxyribonucleotides (those that have 2-deoxyribose as the pentose) is to serve as building blocks for DNA. Although ribonucleotides similarly serve as the units for RNA synthesis, they also have a multitude of other functions in cell metabolism. In some synthetic nucleosides with therapeutic properties, other pentose components such as arabinose are present. (See fig. 12.2 for the structure of arabinose.)... 

There are thirty-three pyrimidine derivatives identified in tobacco and tobacco smoke. The vast majority of these compounds (67%) have been identified in tobacco. Only fourteen are known to exist in tobacco smoke. As previously mentioned, the naturally occurring derivatives of pyrimidine are components of nucleic acids cytosine, thymidine, and uracil. Free pyrimidine and functionalized pyrimidine compounds in tobacco are believed to be formed from the catabolism of various nucleosides (17B21). Several of the pyrimidine-containing compounds in tobacco are agronomic chemical residues while other compounds identified in tobacco smoke are formed from those agronomic residues. 

Notably, the Dimroth rearrangement has been shown to occur in nature with the purine and pyrimidine bases of nucleosides and nucleotides upon exposure to certain chemical entities. For example, 3,4-epoxybutene, styrene oxide and other aromatic hydrocarbon based epoxides, butadiene and butadiene monoxide, chloroethylene oxirane, chlorambucil, and acrolein, " among others, have been shown to facilitate Dimroth rearrangement, and in some cases subsequent cross-linking of DNA. While interesting from a mechanistic and biological perspective, these reactions will not be reviewed here. 

---