Development of a regioselective N-methylation of (benz)imidazoles providing the more sterically hindered isomer

Borinic Acid Catalyzed Regioselective N-Alkylation of Azoles

A method for regioselective N-alkylation of ambident, azole-type heterocycles with alkene or epoxide electrophiles is described. In the presence of diphenylborinic acid (Ph₂BOH) and an amine cocatalyst, heterocyclic nucleophiles such as 1,2,3- and 1,2,4-triazoles, substituted tetrazoles, and purine are activated toward selective N-functionalization. The scope of electrophilic partners includes enones, 2-vinylpyridine, phenyl vinyl sulfone, a dehydroalanine derivative, and epoxides. Mechanistic studies, including in situ ¹¹B NMR spectroscopy and kinetic analysis, are discussed.

Diarylborinic Acid-Catalyzed Regioselective Ring Openings of Epoxy Alcohols with Pyrazoles, Imidazoles, Triazoles, and Other Nitrogen Heterocycles

A method for regioselective ring openings of 3,4- and 2,3-epoxy alcohols with ambident nitrogen heterocycles is described. Using a diarylborinic acid catalyst, a single regioisomer is favored in couplings of nucleophile and electrophile partners that display low regioselectivity under conventional conditions. The method provides access to aromatic heterocycles bearing stereochemically defined, functionalized alkyl substituents, a product class similar in structure to medicinally relevant compounds such as the acyclic nucleoside analogues.

Regioselective Radical Arene Amination for the Concise Synthesis of ortho-Phenylenediamines

The formation of arene C–N bonds directly from C–H bonds is of great importance and there has been rapid recent development of methods for achieving this through radical mechanisms, often involving reactive N-centered radicals. A major challenge associated with these advances is that of regiocontrol, with mixtures of regioisomeric products obtained in most protocols, limiting broader utility. We have designed a system that utilizes attractive noncovalent interactions between an anionic substrate and an incoming radical cation in order to guide the latter to the arene ortho position. The anionic substrate takes the form of a sulfamate-protected aniline and telescoped cleavage of the sulfamate group after amination leads directly to ortho-phenylenediamines, key building blocks for a range of medicinally relevant diazoles. Our method can deliver both free amines and monoalkyl amines allowing access to unsymmetrical, selectively monoalkylated benzimidazoles and benzotriazoles. As well as providing concise access to valuable ortho-phenylenediamines, this work demonstrates the potential for utilizing noncovalent interactions to control positional selectivity in radical reactions.

Synthesis of a Nonhydrolyzable Nucleotide Phosphoroimidazolide Analogue That Catalyzes Nonenzymatic RNA Primer Extension

We report the synthesis of guanosine 5′-(4-methylimidazolyl)phosphonate (ICG), the third member of a series of nonhydrolyzable nucleoside 5′-phosphoro-2-methylimidazolide (2-MeImpN) analogues designed for mechanistic studies of nonenzymatic RNA primer extension. The addition of a 2-MeImpN monomer to a primer is catalyzed by the presence of a downstream activated monomer, yet the three nonhydrolyzable analogues do not show catalytic effects under standard mildly basic primer extension conditions. Surprisingly, ICG, which has a pK_a similar to that of 2-MeImpG, is a modest catalyst of nonenzymatic primer extension at acidic pH. Here we show that ICG reacts with 2-MeImpC to form a stable 5′–5′-imidazole-bridged guanosine-cytosine dinucleotide, with both a labile nitrogen–phosphorus and a stable carbon–phosphorus linkage flanking the central imidazole bridge. Cognate RNA primer–template complexes react with this GC-dinucleotide by attack of the primer 3′-hydroxyl on the activated N–P side of the 5′-5′-imidazole bridge. These observations support the hypothesis that 5′–5′-imidazole-bridged dinucleotides can bind to cognate RNA primer–template duplexes and adopt appropriate conformations for subsequent phosphodiester bond formation, consistent with our recent mechanistic proposal that the formation of activated 5′–5′-imidazolium-bridged dinucleotides is responsible for 2-MeImpN-driven primer extension.

Total synthesis of kealiiquinone: the regio-controlled strategy for accessing its 1-methyl-4-arylbenzimidazolone core

A practical, concise and straightforward total synthesis of kealiiquinone 1, a naphtho[2,3-d]imidazole alkaloid obtained from the Micronesian marine sponge Leucetta sp. was accomplished. The squaric acid chemistry to construct the 1,4-quinoid ring and the regioselective N-methylation through a benzo[c][1,2,5]selenadiazolium heterocycle are the key features in this report. The full details of the representative approaches involving the different attempted synthetic strategies are also presented. Finally a successful total synthesis of this complex secondary metabolite is described.

A practical, concise and straightforward total synthesis of kealiiquinone 1, a naphtho[2,3-d]imidazole alkaloid obtained from the Micronesian marine sponge Leucetta sp. was accomplished.

Regioselective C–H alkenylation of imidazoles and its application to the synthesis of unsymmetrically substituted benzimidazoles

A palladium-catalyzed C5-selective alkenylation of imidazoles has been developed and applied to the synthesis of alkenyl imidazoles and multi-substituted benzimidazoles.