A Novel and Versatile Entry to Asymmetrically Substituted Pyrazines

Synthesis and base-pairing properties of pyrazine nucleic acids

The diversity of backbone modifications in the study of primitive informational polymers is partly limited by the plausible formation of their prebiotic starting components. In this paper, we synthesize pyrazine nucleic acid, an acyclic polymer, with the nucleoside derivable from a prebiotic one-pot synthesis containing alanine amide and D-ribose. Pyrazine nucleic acid (PzNA) which has a backbone structurally similar to glycerol nucleic acid (GNA), contain pyrazine derived nucleosides as informational elements that may exhibit base pairing properties similar to the pyrimidines present in RNA.^[1] We found that insertion of pyrazinone nucleotides into DNA oligonucleotide sequences is not well-tolerated, and that homogenous sequences of PzNA are unable to form duplexes with RNA or DNA. Reasons for our results may be attributed to the pyrazine-2-one moiety, which is purposed to be a thymine analog, but has a lower pK_a (pK_a ∼ 8.5) than thymine and uracil. Additionally, we discovered an "apparent" regioselective protection of pyrazine-2-one derivatives in the presence of a secondary hydroxyl group that proved crucial in the preparation of the pyrazine-2-one phosphoramidite. The regioselectivity observed is proposed to be of general interest in the context of heterocyclic chemistry. In the larger context of origins of life studies, it points to the importance of keto-enol preferences of the canonical nucleobases versus pyrazine heterocycles in functioning as recognition elements.

A rhodium-catalysed Sonogashira-type coupling exploiting C-S functionalisation: orthogonality with palladium-catalysed variants

This report concerns the development of an efficient Sonogashira-type coupling of arylmethylsulfides and terminal alkynes to generate aryl alkyne motifs. Orthogonal reactivity between traditional Pd catalysts, and the Rh catalysts employed, results in the ability to selectively activate either the C-S bond or C-X bond through catalyst choice. The Rh-bisphosphine catalyst has further been shown to be able to effect a hydroacylation-Sonogashira tandem sequence, and in combination with further onward reactions has been used in the synthesis of heterocycles and polycyclic systems.

Synthesis of quinoxalines or quinolin-8-amines from N-propargyl aniline derivatives employing tin and indium chlorides

The utility of stannous chloride dihydrate as well as indium/InCl₃ in combination with “ortho-nitrogen”-N-propargylanilines for the construction of either quinoxaline- or quinolin-8-amine-scaffolds is demonstrated. Depending on the nature of the alkyne, different bicyclic nitrogen heterocycles are formed under aerobic conditions.

Transition metal-catalyzed functionalization of pyrazines

Transition metal-catalyzed reactions are generally used for carbon-carbon bond formation on pyrazines and include, but are not limited to, classical palladium-catalyzed reactions like Sonogashira, Heck, Suzuki, and Stille reactions. Also a few examples of carbon-heteroatom bond formation in pyrazines are known. This perspective reviews recent progress in the field of transition metal-catalyzed cross-coupling reactions on pyrazine systems. It deals with the most important C-C- and C-X-bond formation methodologies.

The Liebeskind-Srogl C-C cross-coupling reaction

Although a plethora of highly selective and reliable methods for the construction of C-C bonds are known to organic chemists, there is growing interest in the development of new protocols that offer different or orthogonal reactivity to that of existing methods. In 2000, Liebeskind and Srogl described a mechanistically unprecedented transition-metal-catalyzed cross-coupling of thioesters with boronic acids to produce ketones under neutral conditions. This desulfitative cross-coupling process is catalytic in palladium(0), stoichiometric in copper(I), and applicable to a range of organosulfur derivatives and nucleophilic organometallic reagents. In this Minireview, we highlight recent applications of this intriguing cross-coupling reaction in modern organic synthesis, with an emphasis on cases in which traditional methods for C-C bond formation have failed.

Diversity-Oriented Synthesis of Substituted Furo[2,3-b]pyrazines

A highly efficient method for the synthesis of diversely substituted furo[2,3-b]pyrazines has been elaborated. The Ag+- or iodine-mediated electrophilic cyclization of readily generated 5-chloro-3-substituted ethynyl-1-(4-methoxybenzyl)-pyrazin-2(1H)-ones affords substituted furo[2,3-b]pyrazines, which undergo various palladium catalyzed reactions to generate a library of difficult to attain diversely substituted furo[2,3-b]pyrazines.