Spectacular advances in structural and molecular biology have added support to the 'RNA world' hypothesis, and provide a mandate for chemistry to explain how RNA might have been generated prebiotically on the early earth. Difficulties in achieving a prebiotically plausible synthesis of RNA, however, have led many to ponder the question posed in the title of this paper. Herein, we review recent experimental work on the assembly of potential RNA precursors, focusing on methods for stereoselective C-C bond construction by aldolisation and related processes. This chemistry is presented in the context of a broader picture of the potential constitutional self-assembly of RNA. Finally, the relative accessibility of RNA and alternative nucleic acids is considered.

For the RNA world hypothesis to be accepted, the constitutional self-assembly of RNA will have to be demonstrated. Conceptually, the simplest route to RNA involves nucleotide polymerisation. Activated pyrimidine nucleotides can be derived from arabinose-3-phosphate under potentially prebiotic conditions, but the prebiotic synthesis of this sugar phosphate has not hitherto been investigated. The results of synthetic approaches involving phosphorylation, phosphate migration and 2,3-C--C bond construction are described herein.

Monoalkyl phosphates can take the place of carboxylic acids in the Passerini reaction, but excesses of aldehyde and isonitriles are required, and transfer of the phosphate to the newly formed hydroxyl group does not take place. By rendering the phosphate addition intramolecular, the efficiency of the reaction is substantially increased. Phosphate transfer can take place when cyanovinyl phosphate is used rather than a monoalkyl phosphate.As part of a long-standing interest in multicomponent reactions involving isonitriles, 1 we recently started to investigate the possibility that such reactions played a role in prebiotic chemistry. 2 In the Passerini and Ugi reactions, isonitriles add reversibly to aldehydes and iminium ions, respectively, to give nitrilium ions 1 3 (Scheme 1).Attack of these nitrilium ions by carboxylate anions gives intermediates 2 that undergo intramolecular acyl transfer giving the products 3. We speculated that alkyl phosphate dianions 4 might be able to substitute for carboxylate anions in this chemistry, and considered it possible that the resultant imidoyl phosphates 5 would undergo reactions other than the intramolecular phosphoryl transfer that might be expected on the basis of the Passerini and Ugi reactions. Specifically, we reasoned that nucleophilic attack on phosphorus by water, or an internal hydroxyl group from the alkyl phosphate, if one were available, and suitably disposed, would displace an a-hydroxy-or a-amino acid amide 6, and either regenerate the original phosphate dianion, or give a cyclic phosphate diester 7. These expectations were realised using nucleotides as the phosphate dianions, and we found that treatment of a 2¢-or 3¢-nucleotide with an excess of an isonitrile, and an aldehyde (+ ammonia) gave the corresponding nucleoside-2¢,3¢-cyclic phosphate in extremely high yield. 2 In the absence of ammonia, a-hydroxy acid amides were formed as co-products, and, in its presence, a-amino acid amides were also formed. The amounts of the co-products 6 that were formed were greater than the amounts of the cyclic nucleotide product suggesting that the nitrilium ions 1 also underwent direct hydration to 6. When 5¢-nucleotides were subjected to the same reaction conditions, the co-products 6 were formed more rapidly than they were in no nucleotide controls, but no altered nucleotide products were formed. We explained these observations by invoking hydrolysis of the intermediate 5 in the case of the 5¢-nucleotide due to the low effective concentration of the 3¢-hydroxyl group. In the case of the 2¢/3¢-nucleotides, the 3¢/ 2¢-hydroxyl group in the intermediates 5 has a much higher effective concentration, and out-competes water in the attack of the activated phosphate. From a prebiotic chemistry standpoint, the chemistry of the 2¢/3¢-nucleotides in the presence of ammonia is the more interesting as simultaneous formation of stably activated nucleotides, and aamino acid derivatives takes place. From the standpoint of conventional organic synthesis, however, the chemistry of the 5¢-n...

Biochemistry Z 0250 RNA: Prebiotic Product, or Biotic Invention?-[40 refs.] -(ANASTASI, C.; BUCHET, F. F.; CROWE, M. A.; PARKES, A. L.; POWNER, M. W.; SMITH, J. M.; SUTHERLAND*, J. D.; Chem. Biodiversity 4 (2007) 4, 721-739; Sch. Chem., Univ. Manchester, Manchester M13 9PL, UK; Eng.) -Nuesgen 26-271