Intramolecular general base induced

Although, in general, the -excessive nature of the heterocyclic rings under discussion reduces their reactivity to nucleophilic substitution, there are a number of reactions in which the leaving group is a nitro group. An example of intramolecular nucleophilic displacement of a pyrrole nitro group is provided by the base-induced cyclization of 2-acetyl-l-(2-hydroxyethyl)-5-nitropyrrole (413 414). 

A general and convenient route to 2-substituted benzofurans or indoles is based on intramolecular annulation of 2-alkynylphenols or 2-alkynylanilines (Scheme 23), which are in turn readily available via alkynylaryl cross-coupling reactions. In a representative example, base-induced cyclization of the precursors 37 with concomitant elimination of the TMS-moiety affords the indoles 38 in good yields (Scheme 24) <1997JOC6507>. 

Oxindoles substituted in unusual positions have been obtained by base-induced intramolecular nucleophilic substitution of hydrogen in OT-nitroacylanilides. For example, treatment of the substrate 228 with potassium /-butoxide in DMSO gave the product 229 (Equation 71). However, the yields in such annulations are generally low, and some substrates give mixtures of isomeric oxindoles <2002S2203>. 

Pascal, R. Induced intramolecularity in the reference reaction can be responsible for the low effective molarity of intramolecular general acid-base catalysis. J. Phys. Org. Chem. 2002, 15, 566-569. 

A certain class of RNA molecules called ribozymes catalyze aqueous degradation of their own as well as other RNAs through an efficient induced intramolecular general acid (GBH+) and general base (GB)-catalyzed intramolecular... 

Disruption of Necessary RNA Structure. RNA adopts a variety of three-dimensional structures induced by intramolecular hybridization, the most common of which is the stem loop. These structures play crucial roles in a variety of functions. They are used to provide additional stability for RNA and as recognition motifs for a number of proteins, nucleic acids, and ribonucleoproteins that participate in the intermediary metabolism and activities of RNA species. Thus, given the potential general activity of the mechanism, it is surprising that occupancy-based disruption RNA has not been more extensively exploited. 

Intramolecular cyclization reactions with ureas and related nitrogen sources usually require a base at least at one stage of the reaction. As a result, aerobic reaction conditions for terminal reoxidation cannot be realized in this type of intramolecular vicinal difunctionalization. In order to demonstrate the general feasibility of such a process, Muniz employed a platinum catalyst in the cycHzation of six-membered annelation products 132 (Scheme 16.33). Aminometallation under platinum catalysis is known to proceed under neutral conditions, and hence without the requirement of a base addition [92]. The resulting intermediate AA is oxidized by copper(II) to a platinum(IV) intermediate AB, which induces reductive elimination to 133. 

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