Possible pathway for prebiotic uracil synthesis by photodehydrogenation

Abstract

THE synthesis of purines in potentially prebiotic conditions has been demonstrated^1,2. 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 These purines are also obtained from solutions of cyanogen (unpublished).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 cyanate^7,8. 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.