Radical-mediated Ring Research Articles

Gas-phase preparation of the dibenzo[e,l]pyrene (C24H14) butterfly molecule via a phenyl radical-mediated ring annulation.

A high temperature phenyl-mediated addition-cyclization-dehydrogenation mechanism to form peri-fused polycyclic aromatic hydrocarbon (PAH) derivatives-illustrated through the formation of dibenzo[e,l]pyrene (C24H14)-is explored through a gas-phase reaction of the phenyl radical (C6H5˙) with triphenylene (C18H12) utilizing photoelectron photoion coincidence spectroscopy (PEPICO) combined with electronic structure calculations. Low-lying vibrational modes of dibenzo[e,l]pyrene exhibit out-of-plane bending and are easily populated in high temperature environments such as combustion flames and circumstellar envelopes of carbon stars, thus stressing dibenzo[e,l]pyrene as a strong target for far-IR astronomical surveys.

Alkoxyl Radical-Mediated Ring Expansion/1,4-Difunctionalization of 1,3-Enynes upon Copper Catalysis.

A redox-neutral copper-catalyzed cascade reaction involving alkoxyl radical-mediated ring expansion/1,4-difunctionalization of 1,3-enynes was developed, offering a straightforward approach to the tetra-substituted allenes with macrolactone, CN, and CF3 functional groups. Remarkably, incorporation of the NH2 group onto the 1,3-enyne moiety enabled further cyclization to give a unique scaffold containing a lactone and an indole moiety.

Iron-catalyzed intermolecular C–C bond vinylation of cycloketoximes promoted by diboron

An iron-catalyzed radical-mediated ring opening of cycloketoxime promoted by diboron compound is reported. This strategy provides alternative catalytic routes for the selective C–C bond vinylation, which provides effective approaches for the preparation of various distal-cyanoalkyl alkenes. The experimental explorations showed that the diboron reagent played important roles during the catalytic cycle. And a reasonable mechanism is proposed.

Front Cover: A Remarkable Ring Opening Observed in the Reaction of Cyclopropyl Ferrocenyl Thioketone with Triiron Dodecacarbonyl Fe3(CO)12 (Eur. J. Inorg. Chem. 36/2022)

The Front Cover shows the radical-mediated ring opening process of a cyclopropyl group resulting in a homoallyl radical, which is illustrated by the blossoming of the flowering plant Cirsium arvense, typical at this time of year in some regions of Europe. Based on a proposed reaction mechanism, this is the crucial and initial step in the reaction of cyclopropyl ferrocenyl thioketone with triiron dodecacarbonyl (Fe3(CO)12) as Fe0 precursor, accompanied by oxidation of a [Fe0] unit to [FeI]. In a follow-up bifurcating cascade, the reaction leads to two different iron carbonyl complexes with ring-opened fragments, a mononuclear η4-1-thia-1,3-diene-type species and a dinuclear η2-acyl-type compound. Cover designed by P. Buday. More information can be found in the Research Article by G. Mlostoń, W. Weigand and co-workers.

Anaerobic Hydroxyproline Degradation Involving C-N Cleavage by a Glycyl Radical Enzyme.

Hydroxyprolines are highly abundant in nature as they are components of many structural proteins and osmolytes. Anaerobic degradation of trans-4-hydroxy-l-proline (t4L-HP) was previously found to involve the glycyl radical enzyme (GRE) t4L-HP dehydratase (HypD). Here, we report a pathway for anaerobic hydroxyproline degradation that involves a new GRE, trans-4-hydroxy-d-proline (t4D-HP) C-N-lyase (HplG). In this pathway, cis-4-hydroxy-l-proline (c4L-HP) is first isomerized to t4D-HP, followed by radical-mediated ring opening by HplG to give 2-amino-4-ketopentanoate (AKP), the first example of a ring opening reaction catalyzed by a GRE 1,2-eliminase. Subsequent cleavage by AKP thiolase (OrtAB) yields acetyl-CoA and d-alanine. We report a crystal structure of HplG in complex with t4D-HP at a resolution of 2.7 Å, providing insights into its catalytic mechanism. Different from HypD commonly identified in proline-reducing Clostridia, HplG is present in other types of fermenting bacteria, including propionate-producing bacteria, underscoring the diversity of enzymatic radical chemistry in the anaerobic microbiome.

Electro-reductive C-H cyanoalkylation of quinoxalin-2(1H)-ones

Herein, we report a practical electro-reductive protocol for the direct C–H cyanoalkylation of quinoxalin-2(1H)-ones via iminyl radical-mediated ring opening. These mild reactions proceed under metal-, reductant-, and reagent-free conditions to provide synthetically useful cyanoalkylated quinoxalin-2(1H)-ones.

Photoinduced Fragmentation Borylation of Cyclic Alcohols and Hemiacetals.

A visible-light photoinduced fragmentation borylation of O-phthalimido cycloalkanols with bis(catecholato)diboron is described. Structurally diverse keto and formyloxy alkyl boronic esters are shown to be conveniently prepared by radical-mediated ring opening of cyclic alcohols and hemiacetals, respectively. The reactions proceed under mild conditions in the absence of additives or photocatalysts, display excellent functional group tolerance, and are shown to allow cleavage of 4-, 5-, 6-, and 7-membered ring substrates. The mechanism proceeds via sequential homolytic N-O and C-C bond cleavages, the latter of which involves β-scission of an alkoxy radical, generating a carbonyl and an alkyl radical that is trapped by the diboron reagent. Spectroscopic studies suggest direct photoexcitation of either the phthalimide or diboron substrates with blue light can initiate a radical chain mechanism.

Visible-Light-Driven Strain-Increase Ring Contraction Allows the Synthesis of Cyclobutyl Boronic Esters.

There are a limited number of ring-contraction methodologies which convert readily available five-membered rings into strained four-membered rings. Here we report a photo-induced radical-mediated ring contraction of five-membered-ring alkenyl boronate complexes into cyclobutanes. The process involves the addition of an electrophilic radical to the electron-rich alkenyl boronate complex, leading to an α-boryl radical. Upon one-electron oxidation, ring-contractive 1,2-metalate rearrangement occurs to give a cyclobutyl boronic ester. A range of radical precursors and vinyl boronates can be employed, and chiral cyclobutanes can be accessed with high levels of stereocontrol. The process was extended to the preparation of benzofused cyclobutenes and the versatility of the boronic ester was demonstrated by conversion to other functional groups.

Nickel-catalyzed, ring-forming aromatic C–H alkylations with unactivated alkyl halides

The development of a nickel-catalyzed C–H alkylation of aromatic substrates with unactivated alkyl halides is described. This carbocyclization facilitates the synthesis of diverse fused ring systems from simple aromatic substrates and is an attractive alternative to traditional polar or radical-mediated ring formations. The present system uses unactivated primary and secondary alkyl bromides and chlorides, while avoiding the use of precious palladium catalysts and more reactive alkyl halides commonly used in related C–H alkylations.

A Radical Intermediate in Bacillus subtilis QueE during Turnover with the Substrate Analogue 6-Carboxypterin.

7-Carboxy-7-deazaguanine (CDG) synthase (QueE), a member of the radical S-deoxyadenosyl-l-methionine (SAM) superfamily of enzymes, catalyzes a radical-mediated ring rearrangement required to convert 6-carboxy-5,6,7,8-tetrahydropterin (CPH4) into CDG, forming the 7-dezapurine precursor to all pyrrolopyrimidine metabolites. Members of the radical SAM superfamily bind SAM to a [4Fe-4S] cluster, leveraging the reductive cleavage of SAM by the cluster to produce a highly reactive 5'-deoxyadenosyl radical which initiates chemistry by H atom abstraction from the substrate. QueE has recently been shown to use 6-carboxypterin (6-CP) as an alternative substrate, forming 6-deoxyadenosylpterin as the product. This reaction has been proposed to occur by radical addition between 5'-dAdo· and 6-CP, which upon oxidative decarboxylation yields the modified pterin. Here, we present spectroscopic evidence for a 6-CP-dAdo radical. The structure of this intermediate is determined by characterizing its electronic structure by continuous wave and pulse electron paramagnetic resonance spectroscopy.

QueE: A Radical SAM Enzyme Involved in the Biosynthesis of 7-Deazapurine Containing Natural Products.

7-Carboxy-7-deazaguanine (CDG) is a common intermediate in the biosynthesis of 7-deazapurine-containing natural products. The biosynthesis of CDG from GTP requires three enzymes: GTP cyclohydrolase I, 6-carboxy-5,6,7,8-tetrahydropterin (CPH4) synthase, and CDG synthase (QueE). QueE is a member of the radical S-adenosyl-l-methionine (SAM) superfamily and catalyzes the SAM-dependent radical-mediated ring contraction of CPH4 to generate CDG. This chapter focuses on methods to reconstitute the activity of QueE in vitro.

ChemInform Abstract: Synthesis of Fluorinated Homoallylic Compounds by Fluoroalkyl Radical Mediated Ring Opening of Methylenecyclopropanes.

ChemInform Abstract: Synthesis of Fluorinated Homoallylic Compounds by Fluoroalkyl Radical Mediated Ring Opening of Methylenecyclopropanes.

Investigation of the Radical SAM Enzyme CDG Synthase

7‐Deazapurine containing secondary metabolites and hypermodified tRNA bases are produced in a variety of marine and terrestrial organisms. Although these 7‐deazapurines vary in both their structure and biological roles, they share a common precursor, 7‐cyano‐7‐deazaguanine (preQ0), which is derived from GTP through the action of four enzymes. CDG synthase (QueE) catalyzes the penultimate step, which generates the 7‐deazapurine moiety. QueE is a member of the radical S‐adenosyl‐L‐methionine (SAM) superfamily and catalyzes a complex radical‐mediated ring contraction to convert 6‐carboxy‐5,6,7,8‐tetrahydropterin (CPH4) into 7‐carboxydeazaguanine (CDG). This paper will discuss the structure function relationships that are emerging from studies of this fascinating transformation. This work was supported by NIH grant GM72623 to VB.

Radical SAM enzyme QueE defines a new minimal core fold and metal-dependent mechanism.

7-Carboxy-7-deazaguanine synthase (QueE) catalyzes a key S-adenosyl-L-methionine (AdoMet)- and Mg2+-dependent radical-mediated ring contraction step, which is common to the biosynthetic pathways of all deazapurine-containing compounds. QueE is a member of the AdoMet radical superfamily, which employs the 5′-deoxyadenosyl radical from reductive cleavage of AdoMet to initiate chemistry. To provide a mechanistic rationale for this elaborate transformation, we present the first crystal structure of a QueE, along with structures of pre- and post-turnover states. We find that substrate binds perpendicular to the [4Fe-4S]-bound AdoMet, exposing its C6 hydrogen atom for abstraction and generating the binding site for Mg2+, which directly coordinates to the substrate. The Burkholderia multivorans structure reported here varies from all other previously characterized members of the AdoMet radical superfamily in that it contains a hypermodified (β6/α3) protein core and an expanded cluster-binding motif CX14CX2C.

Studies toward the total synthesis of clavulactone.

Synthetic studies directed toward a total synthesis of clavulactone are reported. In light of the analysis made in our previous work, cyclopentane 4a (a key intermediate in the present work) was synthesized through a radical-mediated ring closure of a rationally designed substrate 25. Using HWE reactions, the lower and upper side-chains of 4a were converted into an allyl chloride and an allyl cyanohydrin, respectively. Subsequent treatment of the allyl chloride/cyanohydrin in a highly diluted THF solution with sodium bis(trimethylsiliyl)amide led to intramolecular alkylation and thus completed a major endeavor in synthesizing the dolabellane framework, construction of the eleven-membered ring. SmI(2)-mediated lactonization as a model reaction for the formation of the alpha,beta-unsaturated delta-lactone segment of clavulactone is also described.

Cyclopropylcarbinyl radical-mediated ring expansion to seven-membered carbocycles

Abstract Radical-mediated ring expansion methodology is presented wherein 7-membered carbocycles can be prepared from the corresponding xanthate derivatives of bicyclo[4.1.0]heptan-1-methanol. In certain systems, an intermediate cycloheptyl radical appears to be kinetically favored over the cyclohexyl radical, but the direction of cyclopropylcarbinyl radical fragmentation is subject to substitution about the bicyclo[4.1.0]heptan-1-methyl ring.

Radical-mediated ring enlargement of cyclobutenones: new synthetic potential of squaric acid.

Radical-mediated ring enlargement of cyclobutenones: new synthetic potential of squaric acid.

Free radical-mediated ring expansion and related annulations. [Erratum to document cited in CA119(17):180410r

Free radical-mediated ring expansion and related annulations. [Erratum to document cited in CA119(17):180410r

Free radical-mediated ring expansion and related annulations

Free radical-mediated ring expansion and related annulations

Total synthesis of the monoterpenes (−)-mintlactone and (+)-isomintlactone

The first total stereodirected synthesis of the monoterpenes (−)-mintlactone and (+)-isomintlactone from the same chiral starting product is described. A stereoselective, radical-mediated ring closure was the key step in both syntheses.