Possible Pathway For Prebiotic Uracil Synthesis By Photodehydrogenation

THE synthesis of purines in potentially prebiotic conditions has been demonstrated1,2. Adenine and guanine have been synthesised directly from aqueous cyanide solutions and a mechanism for the concentration of hydrogen cyanide in low melting eutectics has been proposed3. These purines are also obtained from solutions of cyanogen (unpublished). The pyrimidines, however, have provided a more difficult problem. Uracil has been obtained from malic acid and urea, but only by heating in the presence of concentrated sulphuric acid4 or polyphosphoric acid5. Low yields of uracil have been inferred from spectral data for a reaction involving acrylonitrile, urea and ammonium chloride6. Further details concerning the conditions of this reaction, however, have not been offered. Cytosine and, indirectly uracil, have been prepared from cyanoacetylene and cyanate7,8. The rapid hydrolysis of these reactants raises doubts about this route. Cyanoacetaldehyde, a hydrolysis product of cyanoacetylene, condenses with guanidine in aqueous solution to yield 2,4-diaminopyrimidine, which is hydrolysed to cytosine and uracil. The later route is considered to be more plausible9. Thymine has been prepared in 0.1% yield from uracil by a hydroxymethylation-reduction sequence involving formaldehyde and hydrazine hydrate in ammoniacal solution10. In view of uncertainties concerning the abundances on the primitive Earth of a number of the precursors which have been used in these studies, it seemed desirable to consider alternative routes to the synthesis of pyrimidines. In this report we describe the synthesis of uracil by the photodehydrogenation of 5,6-dihydrouracil (DHU), as well as the synthesis of DHU from β-alanine and urea in mild conditions.