Metal- and Acid-Free C-H Formylation of Nitrogen Heterocycles: Using Trioxane as an Aldehyde Equivalent Enabled by an Organic-Soluble Oxidant

First total synthesis of caerulomycin K: a case study on selective, multiple C-H functionalizations of pyridines

Caerulomycins, natural alkaloids with antimicrobial properties, have been previously synthesized starting with highly pre-functionalized building blocks or requiring many functional group manipulations. In this work, we report the first total synthesis of caerulomycin K, a diversely trifunctionalized pyridine readily assembled in three steps exploiting the recent advancements in the C-H activation of N-heterocycles.

Synthesis of 4-sulfonyl-1,7-diesters via K2CO3-mediated alkylative debenzoylation of α-sulfonyl o-hydroxyacetophenones with acrylates

The synthesis of 4-sulfonyl-1,7-diesters was well developed, under open-vessel conditions, by K2CO3-mediated double alkylation of α-sulfonyl o-hydroxyacetophenones with acrylates and tandem debenzoylation of the resulting α,α-disubstituted o-hydroxyacetophenones. A plausible mechanism is proposed and discussed here. This high-yielding protocol provides a highly effective intermolecular alkylation and intramolecular debenzoylation via the formation of two carbon-carbon (C-C) single bonds and the cleavage of a carbon-carbon (C-C) single bond.

A Fenton Approach to Aromatic Radical Cations and Diarylmethane Synthesis

Manipulating carbon-centered radicals to add to electron-deficient systems is a well-precedented process. By coupling the Fe(II)-mediated Fenton reaction with the Fe(III)-mediated single-electron oxidation of anisolic compounds, we demonstrate how electron-rich carbon-centered radicals can react with electron-rich arenes through a radical-polar cascade pathway. This bioinspired approach produces diarylmethane derivatives from simple unfunctionalized precursors.

Recent Advances On Direct Formylation Reactions

Aldehydes serve as the key functional group in organic synthesis and are valuable intermediates. The various advanced methods of direct formylation reactions have been reviewed in this article. Overcoming the drawbacks of the traditional methods of formylation, newer methods involving homo and heterogenous catalysts, one pot reactions, solvent free techniques are elaborated, which can be performed under mild conditions and using inexpensive resources.

Photoredox Catalyzed Thioformylation of Terminal Alkynes Using Nitromethane as a Formyl Source

A photoredox thioformylation of terminal alkynes using nitromethane as a formyl anion equivalent, thereby leading to the synthesis of (E)-1,2-difunctionalized acrylaldehyde, has been described. The current strategy introduces an adaptable aldehyde function across an alkyne and offers a new route to synthesizing α-alkyl/aryl aldehydes.

(NH4)2S2O8-Mediated Metal-Free Decarboxylative Formylation/Acylation of α-Oxo/Ketoacids and Its Application to the Synthesis of Indole Alkaloids

A metal-free method for the formylation/acylation of indoles and β-carbolines with (NH₄)₂S₂O₈ via direct decarboxylative cross-coupling of α-oxo/ketoacids in moderate to good yields is described. The reaction occurs between ambient temperature and 40 °C under mild reaction conditions with commercially available starting materials. This methodology can be expanded to some biologically active indole alkaloids like pityriacitrins, eudistomins Y₁ and Y₃, and marinacarbolines A-D.

A Metal-Free Visible-Light Photoredox Construction and Direct C-H Functionalization of Pyridines: Green Synthesis of Polysubstituted Picolinaldehydes

The development of a novel environmental benign and sustainable synthetic method for highly efficient construction and direct C-H functionalization of N-heterocycles remains a pivotal central research topic for organic and medicinal chemistry. Herein, a novel visible-light-enabled biomimetic aza-6π electrocyclization for efficient assembly of diverse pyridines and further tandem Minisci-type reaction were developed. A broad spectrum of polysubstituted picolinaldehydes were readily constructed with high efficacy and good functional group tolerance under metal- and oxidant-free conditions.

Methanol as a formylating agent in nitrogen heterocycles

A radical mediated C-H direct formylation of N-heteroarenes with methanol is reported. The reaction features a novel iron-catalyzed Minisci oxidative coupling process using commercially available methanol as a formylating reagent. It effectively solved the long-standing problems associated with using methanol as a formylating reagent in these types of reactions. Compared to the traditional Minisci C-H formylation methods, this protocol is highly atom-economical, simple to operate, and environmentally friendly and shows good functional group tolerance. This Minisci formylation strategy is a straightforward approach for the late-stage functionalization of N-heteroarenes.

Iron-catalyzed tandem oxidative coupling and acetal hydrolysis reaction to prepare formylated benzothiazoles and isoquinolines

The aldehyde group is one of the most versatile intermediates in synthetic chemistry, and the introduction of an aldehyde group into heteroarenes is important for the transformation of molecular structure. Herein, we achieved the direct formylation of benzothiazo/les and isoquinolines. The reaction features a novel iron-catalyzed Minisci-type oxidative coupling process using commercially available 1,3-dioxolane as a formylated reagent followed by acetal hydrolysis without a separation process. The reaction can be performed under exceedingly mild reaction conditions and exhibits broad functional group tolerance.

Synthesis of Polysubstituted Isoindolinones via Radical Cyclization of 1,3-Dicarbonyl Ugi-4CR Adducts Using Tetrabutylammonium Persulfate and TEMPO

The development of an efficient method for the synthesis of polysubstituted isoindolinones from 1,3-dicarbonyl Ugi-4CR adducts, employing an aromatic radical cyclization process promoted by tetrabutylammonium persulfate and 2,2,6,6-tetramethyl-1-piperidine 1-oxyl (TEMPO), is described. The protocol allowed the construction of a library of isoindolinones bearing a congested carbon in good to excellent yields under mild conditions and in short reaction times.

Catalytic Approach toward Chiral P,N-Chelate Complexes Utilizing the Asymmetric Hydrophosphination Protocol

A synthetic procedure for obtaining a chiral P,N ligand was developed by exploiting the versatility of the asymmetric hydrophosphination protocol catalyzed by a phosphapalladacycle complex. The addition of the synthesized ligand to various metal sources led to the generation of chiral and enantioenriched chelate complexes, which can be useful prototypes for catalyst design in the future. The resulting coordination compounds were comprehensively characterized by solid-state (X-ray crystallography) and solution-based (one- and two-dimensional NMR spectroscopy) techniques and natural bond orbital (density functional theory) analysis to determine their structural and key electronic features.

Highly regioselective α-formylation and α-acylation of BODIPY dyes via tandem cross-dehydrogenative coupling with in situ deprotection

A metal-free C-H formylation and acylation of BODIPY dyes using a variety of dioxolane derivatives as aldehyde equivalents is reported, providing a postfunctionalization method for controllable synthesis of BODIPYs with carbonyl groups at 3,5-positions via a radical process. The photophysical properties of resultant dyes from this efficient one-pot, chemo- and site-selective transformation have been studied.