Reexamination of the Evidence for Solvent-Induced Intramolecular Electrophilic Catalysis by acis Vicinal Hydroxyl Group in Ribonucleoside Phosphorylation Reactions

Abstract

The mechanisms of enzymatic and chemical hydrolysis of RNA have been the subject of intense interest for many years. Early demonstration of a dramatically enhanced hydrolytic reactivity of RNA compared to DNA established a crucial mechanistic role for the 2'-hydroxyl group. [1] Hitherto, it has been accepted that the major role of the 2'-hydroxyl group is nucleophilic catalysis and the 2',3'-cyclic phosphate intermediate is well established. The comparatively recent discovery of RNA catalysis (ribozymes) [2] has stimulated renewed interest in the potential role(s) of the 2'-hydroxyl group in an increasingly diverse range of reactions. Thus, the report by Roussev et al. [3] of the electrophilic catalysis of displacement reactions at a neighboring phosphoryl center by the cis vicinal hydroxyl group, if it is correct, offers a potentially novel mechanism that could be of significance to ribozymes. The importance of this claim prompted the present investigation.Roussev and colleagues compared the reactions of 5'protected ribonucleosides and 5'-protected-2'-deoxyribonucleosides with an excess of phosphorus oxychloride and pyridine in CH 2 Cl 2 followed by treatment with an excess of an alcohol. [3] In the case of the ribonucleosides the two-stage sequence led to a mixture of the corresponding 2'-and 3'phosphate triesters (Scheme 1) while in the case of the 2'deoxyribonucleosides the 3'-phosphate triester was produced. Importantly the substrates with a free 2'-hydroxyl group showed much greater reactivity in the second stage, which is the alcoholysis of the intermediate chlorophosphates. This was rationalized in terms of the cis vicinal hydroxyl group hydrogen bonding to the PO group of the ribonucleoside phosphorodichloridate 2 or 3, thereby assisting the subsequent nucleophilic displacements of chlorine by electrophilic catalysis. Crucial to this rationalization was the identification of the intermediate as a mixture of the regioisomeric phosphorodichloridates 2 and 3. This assignment was based on the 31 P NMR spectrum of the reaction mixture. The relevant signals in the case of 5'-O-trityl uridine were reported O TrO HO OH Ura O TrO O OH Ura P O Cl Cl O TrO HO O Ura P O Cl Cl O TrO O OH Ura P O MeO OMe O TrO HO O Ura P O OMe OMe1 + 2 4 3 a) b) + 5 Scheme 1. Reaction scheme proposed by Roussev et al. [3] for the two step synthesis of the 5'-protected ribonucleoside phosphate triesters. Reagents: a) POCl 3 and pyridine in CH 2 Cl 2 ; b) MeOH. as d 22.40 (d, 3 J P,H 12 Hz) and d 20.03 (d, 3 J P,H 10 Hz).The cyclic phosphorochloridate structure 6 a/b was assigned to a minor resonance (6 ± 7 %) at d P 20.01 (t, 3 J P,H 9.2 and 8.4 Hz), and this signal apparently did not increase with time (1 h). Reaction of the 5'-O-trityl-2'-deoxyuridine under the same conditions gave the phosphorodichloridate 7 with a 31 P chemical shift of d 6.7 (d, 3 J P,H 11 Hz). Treatment of the intermediates with an excess of methanol led to the nucleoside dimethylphosphate triesters 4/5 (d P 1.11 and 0.92) and 8 (d P 0.65), respectively. Two aspe...

POCl3 mediated one-pot deoxygenative aromatization and electrophilic chlorination of dihydroxy-2-methyl-4-oxo-indeno[1,2-b]pyrroles

“… 12 Meanwhile, vicinal diols can construct cyclic phosphate ester, which is stable as long as there is no stimulating factor to open the ring. 13 …”