N−H Insertion Reactions of Boc-Amino Acid Amides: Solution- and Solid-Phase Synthesis of Pyrazinones and Pyrazines

2(1H)-Pyrazinones from acyclic building blocks: methods of synthesis and further derivatizations

Pyrazinones (2(1H)-pyrazinones) are found as components of a range of natural substances and are involved in the preparation of a great number of bioactive molecules. Synthesis of such compounds, and analogues, requires knowledge of the heterocyclic properties of pyrazinones and, in particular, methods for their ring construction. This review deals with the strategies that have been developed for the synthesis of pyrazinones from acyclic precursors, especially α-amino acid-derived units, from the first examples in 1905 up to the most recent in 2021.

Four-Component Synthesis of Polysubstituted Pyrazin-2(1H)-ones through a Ugi/Staudinger/Aza-Wittig/Isomerization Sequence

A new efficient synthesis of polysubstituted pyrazin-2(1H)-ones via the sequential Ugi/Staudinger/aza-Wittig/isomerization reaction has been developed. The four-component Ugi reactions of arylglyoxals 1, primary amines 2, α-azidovinyl acids 3, and isocyanides 4 produced the azides 5, which were treated with triphenylphosphine to give pyrazin-2(1H)-ones 6 in good yields through domino Staudinger/aza-Wittig/isomerization reactions.

Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis?

Visible-light photocatalysis has evolved over the last decade into a widely used method in organic synthesis. Photocatalytic variants have been reported for many important transformations, such as cross-coupling reactions, α-amino functionalizations, cycloadditions, ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temperature, and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible-light photocatalysis make a difference in organic synthesis? The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible-light-absorbing photocatalyst holds the promise to improve current procedures in radical chemistry and to open up new avenues by accessing reactive species hitherto unknown, especially by merging photocatalysis with organo- or metal catalysis.

Synthesis of substituted pyrazines from N-allyl malonamides

Synthesis of highly substituted pyrazines from geminal diazides via thermal or copper-mediated cyclization and modification of the obtained pyrazines.

Catalyst design based on agostic interactions: synthesis, characterization, and catalytic activity of bis(pyrazolyl)borate copper complexes

The first example of designed catalysts based on the agostic-like weak interaction acts as a “switch” which will be turned “on” and “off” during the catalytic process.

Myoglobin-catalyzed intermolecular carbene N-H insertion with arylamine substrates

Engineered variants of the heme-containing protein myoglobin can efficiently catalyze the insertion of α-diazo esters into the N-H bond of arylamines, featuring a combination of high chemoselectivity, elevated turnover numbers, and broad substrate scope.

Cytochrome P450-Catalyzed Insertion of Carbenoids into N-H Bonds

Expanding nature's catalytic repertoire to include reactions important in synthetic chemistry will open new opportunities for 'green' chemistry and biosynthesis. We demonstrate enzyme-catalyzed insertion of carbenoids into N-H bonds. This type of bond disconnection, which has no counterpart in nature, can be mediated by variants of the cytochrome P450 from Bacillus megaterium. The N-H insertion reaction takes place in water, provides the desired products in 26-83% yield, forms the single addition product exclusively, and does not require slow addition of the diazo component.

Synthesis of N-aryl substituted, five- and six-membered azacycles using aluminum-amide complexes

N-Aryl substituted, five- and six-membered azacycles, isoindoles and tetrahydroisoquinolines, were synthesized from the reaction of corresponding cyclic ethers (n = 1, 2) with dimethylaluminum-amide reagents.

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.

A water-soluble ruthenium glycosylated porphyrin catalyst for carbenoid transfer reactions in aqueous media with applications in bioconjugation reactions

Water-soluble [Ru(II)(4-Glc-TPP)(CO)] (1, 4-Glc-TPP = meso-tetrakis(4-(beta-D-glucosyl)phenyl)porphyrinato dianion) is an active catalyst for the following carbenoid transfer reactions in aqueous media with good selectivities and up to 100% conversions: intermolecular cyclopropanation of styrenes (up to 76% yield), intramolecular cyclopropanation of an allylic diazoacetate (68% yield), intramolecular ammonium/sulfonium ylide formation/[2,3]-sigmatroptic rearrangement reactions (up to 91% yield), and intermolecular carbenoid insertion into N-H bonds of primary arylamines (up to 83% yield). This ruthenium glycosylated porphyrin complex can selectively catalyze alkylation of the N-terminus of peptides (8 examples) and mediate N-terminal modification of proteins (four examples) using a fluorescent-tethered diazo compound (15). A fluorescent group was conjugated to ubiquitin via 1-catalyzed alkene cyclopropanation with 15 in aqueous solution in two steps: (1) incorporation of an alkenic group by the reaction of N-hydroxysuccinimide ester 19 with ubiquitin and (2) cyclopropanation of the alkene-tethered Lys(6) ubiquitin (23) with the fluorescent-labeled diazoacetate 15 in the presence of a catalytic amount of 1. The corresponding cyclopropanation product (24) was obtained with approximately 55% conversion based on MALDI-TOF mass spectrometry. The products 23, 24, and the N-terminal modified peptides and proteins were characterized by LC-MS/MS and/or SDS-PAGE analyses.

Multiplicity of diverse heterocycles from polymer-supported alpha-acylamino ketones

Polymer-supported alpha-acylamino ketones were transformed to seven types of structurally unrelated heterocyclic compounds. Syntheses involved variety of chemical routes and comprised diverse chemistries (C-C, C=C, C-N, C=N, and C-O bond formations). Different sizes of heterocycles (4-, 5-, 6-, and 7-membered rings) were prepared, including dihydro-pyrrol-2-ones, pyrazin-2-ones, dihydro-triazepin-6-ones, morpholin-3-ones, imidazoles, beta-lactams, and isoquinolin-1-ones. Further elaboration to fused ring systems was also documented.

Rh(II)-Catalyzed Intramolecular C−H Insertion of Diazo Substrates in Water: Scope and Limitations

Preferential Rh(II) carbenoid intramolecular C-H versus O-H insertion derived from alpha-diazo-acetamides can be achieved in water by using an appropriate combination of the catalyst and amide groups, which creates a larger hydrophobic environment around the reactive carbenoid center.

Synthesis of cyclic endiamino peptides

Several building blocks for endiamino peptides, as well as several cyclic endiamino peptides themselves, and pyrazin-6-one, which embodies the endiamino group, were prepared. The variety of synthesized compounds shows the potential of this synthesis in the preparation of many different groups of compounds.

N-H insertion reactions of primary ureas: the synthesis of highly substituted imidazolones and imidazoles from diazocarbonyls

Primary ureas have been used as substrates in rhodium-catalyzed N-H insertion reactions with an array of diazocarbonyls. The insertion reaction is efficient and gives excellent selectivity and yields. The products from the insertion reaction with diazoketones cyclize readily in the presence of acid to yield the corresponding imidazolones that can be further derivatized by N-alkylation with alkyl, allyl, and benzyl halides. Alternatively, the imidazolones were treated with phosphorus oxybromide to form the corresponding 2-bromoimidazoles that were further functionalized using a Suzuki coupling reaction.

Cyclic and acyclic sulfonimides in reactions with Rh(ii)-ketocarbenoids: a new access to chemoselective O-functionalization of the imidic carbonyl groups

Catalytic decomposition of diazoacetylacetone, diazoacetoacetic, diazomalonic, and diazoacetic esters using dirhodium tetraacetate in the presence of isothiazol-3(2H)-one 1,1-dioxides and a number of N-(arenesulfonyl)carboxamides in solutions of methylene chloride or dichloroethane gives rise to O-alkylation of the imidic carbonyl groups by Rh(II)-carbenoids and the formation of O-alkylimidates as the final products. The reaction proceeds with high chemoselectivity via carbonyl ylides and offers a powerful method for the synthesis in good yields of the imidates with polyfunctional O-alkyl groups. On the basis of X-ray analysis and 1H- and 13C-NMR studies it was shown that the resulting acyclic O-alkylimidates have the E-configuration in the solid state and in solution. Unlike acyclic analogues, the cyclic carbonyl ylide derived from substituted diazosaccharin by intramolecular cyclization of the appropriate diketocarbenoid is capable of reacting with DMAD in a 1,3-cycloaddition process.