Competing Knorr and Fischer–Fink pathways to pyrroles in neutral aqueous solution

A Method for Rigorously Selective Capture and Simultaneous Fluorescent Labeling of N-Terminal Glycine Peptides

Although chemical methods for the selective derivatization of amino acid (AA) side chains in peptides and proteins are available, selective N-terminal labeling is challenging, especially for glycine, which has no side chain at the α-carbon position. We report here a double activation at glycine's α-methylene group that allows this AA to be differentiated from the other 19 AAs. A condensation reaction of dibenzoylmethane with glycine results in the formation of an imine, and subsequent tautomerization is followed by intramolecular cyclization, leading to the formation of a fluorescent pyrrole ring. Additionally, the approach exhibits compatibility with AAs possessing reactive side chains. Further, the method allows for selective pull-down assays of N-terminal glycine peptides from mixtures without prior knowledge of the N-terminal peptide distribution.

Palladium-catalyzed oxidative cross-coupling for the synthesis of α-amino ketones

A novel oxidative cross-coupling reaction for the synthesis of α-aryl α-amino ketones in the presence of palladium catalysts using T⁺BF₄⁻ as an oxidant has been developed. This transformation was achieved by direct C–H oxidation of α-aminocarbonyl compounds and arylation. The mild reaction has a broad reaction scope and gives desired α-aryl α-amino ketones in moderate to excellent yields.

A novel oxidative cross-coupling reaction for the synthesis of α-aryl α-amino ketones in the presence of palladium catalysts using T⁺BF₄⁻ as an oxidant has been developed.

The Porphobilinogen Conundrum in Prebiotic Routes to Tetrapyrrole Macrocycles

Attempts to develop a credible prebiotic route to tetrapyrroles have relied on enzyme-free recapitulation of the extant biosynthesis, but this process has foundered from the inability to form the pyrrole porphobilinogen (PBG) in good yield by self-condensation of the precursor δ-aminolevulinic acid (ALA). PBG undergoes robust oligomerization in aqueous solution to give uroporphyrinogen (4 isomers) in good yield. ALA, PBG, and uroporphyrinogen III are universal precursors to all known tetrapyrrole macrocycles. The enzymic formation of PBG entails carbon-carbon bond formation between the less stable enolate/enamine of one ALA molecule (3-position) and the carbonyl/imine (4-position) of the second ALA molecule; without enzymes, the first ALA reacts at the more stable enolate/enamine (5-position) and gives the pyrrole pseudo-PBG. pseudo-PBG cannot self-condense, yet has one open α-pyrrole position and is proposed to be a terminator of oligopyrromethane chain-growth from PBG. Here, 23 analogues of ALA have been subjected to density functional theoretical (DFT) calculations, but no motif has been identified that directs reaction at the 3-position. Deuteriation experiments suggested 5-(phosphonooxy)levulinic acid would react preferentially at the 3- versus 5-position, but a hybrid condensation with ALA gave no observable uroporphyrin. The results suggest efforts toward a biomimetic, enzyme-free route to tetrapyrroles from ALA should turn away from structure-directed reactions and focus on catalysts that orient the two aminoketones to form PBG in a kinetically controlled process, thereby avoiding formation of pseudo-PBG.

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.

Base-promoted intramolecular cyclization of N-alkyl, N-propargylic β-enaminones for the synthesis of polysubstituted pyrroles

An efficient, mild, metal-free, base-mediated intramolecular cyclization of N-alkyl, N-propargylic β-enaminones has been realized for the generation of polysubstituted pyrrole derivatives.

Complexity in structure-directed prebiotic chemistry. Unexpected compositional richness from competing reactants in tetrapyrrole formation

Acyclic reactants afford “partially defective” pyrroles that interfere with chain growth of “normal” pyrroles on the path to tetrapyrrole macrocycles.

Scope and limitations of two model prebiotic routes to tetrapyrrole macrocycles

Aqueous reaction (35 °C, 72 h) of two acyclic compounds, an α-aminoketone + β-ketoester or β-diketone (not shown), affords a pyrrole that self-condenses to give the porphyrinogen.

Synthesis of diverse acyclic precursors to pyrroles for studies of prebiotic routes to tetrapyrrole macrocycles

Some 50 ketones, β-diketones, β-ketoesters and α-aminoketones have been prepared for studies of the formation of trisubstituted pyrroles equipped for self-condensation leading to tetrapyrrole macrocycles.

Cobalt(iii)-catalyzed synthesis of pyrroles from enamides and alkynes

An efficient and regioselective cobalt-catalyzed synthesis of multi-substituted pyrroles is reported by the use of readily available enamides and alkynes.

Complexity in structure-directed prebiotic chemistry. Reaction bifurcation from a β-diketone in tetrapyrrole formation

An unsymmetrical β-diketone with δ-aminolevulinic acid affords both a “defective” and a “normal” pyrrole; upon combinatorial reaction the former terminates chain-growth of the latter on the path to tetrapyrrole macrocycles.

Complexity in structure-directed prebiotic chemistry. Effect of a defective competing reactant in tetrapyrrole formation

A reactive but defective pyrrole, derived from the simple β-diketone acetylacetone, terminates chain-growth in a quantitative combinatorial manner in tetrapyrrole formation.