Self-organizing biochemical cycles -- Orgel 97 (23): 12503 -- Proceedings of the National Academy of Sciences
: "However, nucleotides are intrinsically more complicated than amino acids, and it is by no means obvious that they can be obtained in a few simple steps under prebiotic conditions. A remarkable synthesis of adenine (3) and more or less plausible syntheses of the pyrimidine nucleoside bases (4) have been reported, but the synthesis of ribose and the regiospecific combination of the bases, ribose, and phosphate to give -nucleotides remain problematical. The recent work of Eschenmoser and his co-workers (5) has led to a specific synthesis of ribose 2,4-bisphosphate, but prebiotically plausible sequences of steps to the precursors of this ribose derivative and from it to the standard nucleotides are not obvious."
Orgel LE. Self-organizing biochemical cycles -- Orgel 97 (23): 12503 -- Proceedings of the National Academy of Sciences Proc Natl Acad Sci U S A. 2000 Nov 7;97(23):12503-7. Free Full Text article