Tin-free radical chemistry: intramolecular addition of alkyl radicals to aldehydes and ketones

Ni-Catalyzed Enantioselective Dialkyl Carbinol Synthesis via Decarboxylative Cross-Coupling: Development, Scope, and Applications

The first enantioselective decarboxylative Negishi-type alkylations of α-oxy carboxylic acids are reported via the intermediacy of redox-active esters (RAEs). This transformation enables a radical-based retrosynthesis of seemingly trivial enantiopure dialkyl carbinols. This article includes a discussion of the history of such couplings, the retrosynthetic ramifications of such a coupling, the development of general conditions, and an extensive series of applications that vividly demonstrate how it can simplify synthesis.

Applications of Halogen-Atom Transfer (XAT) for the Generation of Carbon Radicals in Synthetic Photochemistry and Photocatalysis

The halogen-atom transfer (XAT) is one of the most important and applied processes for the generation of carbon radicals in synthetic chemistry. In this review, we summarize and highlight the most important aspects associated with XAT and the impact it has had on photochemistry and photocatalysis. The organization of the material starts with the analysis of the most important mechanistic aspects and then follows a subdivision based on the nature of the reagents used in the halogen abstraction. This review aims to provide a general overview of the fundamental concepts and main agents involved in XAT processes with the objective of offering a tool to understand and facilitate the development of new synthetic radical strategies.

Et3 B/Et2 AlCl/O2 -Mediated Radical Coupling Reaction between α-Alkoxyacyl Tellurides and 2-Hydroxybenzaldehyde Derivatives

A newly devised radical-based strategy enabled coupling between multiply oxygenated α-alkoxyacyl tellurides and 2-hydroxybenzaldehyde derivatives. A reagent combination of Et₃ B, Et₂ AlCl, and O₂ promoted the formation of the α-alkoxy carbon radical from the α-alkoxyacyl telluride and the addition of the radical to the carbonyl group of 2-hydroxybenzaldehyde. The reaction chemo- and stereoselectively forged the hindered C-C bond between two oxygen-functionalized carbons at ambient temperature. The method was applied to the preparation of 12 coupling adducts with three to six contiguous stereocenters and to the concise synthesis of an antitumor compound, LLY-283.

Iron-Catalyzed Radical Intermolecular Addition of Unbiased Alkenes to Aldehydes

The intermolecular reductive radical coupling of aldehydes with nonactivated alkenes, employing metal hydride atom transfer (MHAT) catalysis with a combination of Fe^II and Fe^III salts, is described. This constitutes the first use of aldehydes as viable acceptor groups in MHAT reactions. The insights gained in this study led to the reexamination of the previously reported intramolecular version of the reaction, and the addition of Fe^II salts allowed the development of a more efficient second-generation approach.

Total Synthesis of Diospyrodin and Its Three Diastereomers

Antibacterial diospyrodin (1) was synthesized in 13 steps. Et₃B and O₂ promoted the formation of an α-alkoxy carbon radical from l-ribose-derived α-alkoxyacyl telluride 5, which reacted with d-glucose-derived aldehyde 4. The radical addition realized the convergent assembly of the contiguously hydroxylated carbon-chain of 3-α and greatly contributed to streamlining the synthetic route. Compound 3-α was transformed not only to 1 but also to its three diastereomers by functional group manipulations.

Convergent Total Synthesis of Hikizimycin Enabled by Intermolecular Radical Addition to Aldehyde

Hikizimycin (1), which exhibits powerful anthelmintic activity, has the most densely functionalized structure among nucleoside antibiotics. A central 4-amino-4-deoxyundecose of 1 possesses 10 contiguous stereocenters on a C1-C11 linear chain and is decorated with a cytosine base at C1 and a 3-amino-3-deoxyglucose at C6-OH. These distinctive structural features of 1 make it an extremely challenging target for de novo construction. Herein, we report a convergent total synthesis of 1 from four known components: 3-azide-3-deoxyglucose derivative 4, bis-TMS-cytosine 5, d-mannose 9, and d-galactose derivative 10. We first designed and devised a novel radical coupling reaction between multiply hydroxylated aldehydes and α-alkoxyacyl tellurides. The generality and efficiency of this process was demonstrated by the coupling of 7c and 8, which were readily accessible from two hexoses, 9 and 10, respectively. Et₃B and O₂ rapidly induced decarbonylative radical formation from α-alkoxyacyl telluride 8, and intermolecular addition of the generated α-alkoxy radical to aldehyde 7c yielded 4-amino-4-deoxyundecose 6-α with installation of the desired C5,6-stereocenters. Subsequent attachments of the cytosine with 5 and of the 3-azide-3-deoxyglucose with 4 were realized through selective activation of the C1-acetal and selective deprotection of the C6-hydroxy group. Finally, the 3 amino and 10 hydroxy groups were liberated in a single step to deliver the target 1. Thus, the combination of the newly developed radical-coupling and protective-group strategies minimized the functional group manipulations and thereby enabled the synthesis of 1 from 10 in only 17 steps. The present total synthesis demonstrates the versatility of intermolecular radical addition to aldehyde for the first time and offers a new strategic design for multistep target-oriented syntheses of various nucleoside antibiotics and other bioactive natural products.

Chain propagation determines the chemo- and regioselectivity of alkyl radical additions to C-O vs. C-C double bonds

Investigations into the selectivity of intermolecular alkyl radical additions to C-O- vs. C-C-double bonds in α,β-unsaturated carbonyl compounds are described. Therefore, a photoredox-initiated radical chain reaction is explored, where the activation of the carbonyl-group through an in situ generated Lewis acid - originating from the substrate - enables the formation of either C-O or the C-C-addition products. α,β-Unsaturated aldehydes form selectively 1,2-, while esters and ketones form the corresponding 1,4-addition products exclusively. Computational studies lead to reason that this chemo- and regioselectivity is determined by the consecutive step, i.e. an electron transfer, after reversible radical addition, which eventually propagates the radical chain.

Iron and cobalt catalysis: new perspectives in synthetic radical chemistry

Iron and cobalt complexes are at the origin of high valuable synthetic pathways involving radical intemediates.

Construction of a meroterpenoid-like compound collection by precursor-assisted biosynthesis

Natural products (NPs) and their derivatives play a pivotal role in drug discovery due to their complexity and diversity. The strategies to rapidly generate NP-like compounds offer unique opportunities to access bioactive compounds. Here we present a new approach, precursor-assisted biosynthesis (PAB), for the creation of NP-like compounds by combination of artificial supplementation of common precursors and divergent post-modifications of precursor-deficient fungi. This method was applied to construct a meroterpenoid-like compound collection containing 43 compounds with diverse molecular scaffolds. Extensive bioactive screening of the collection revealed novel STING (stimulator of interferon genes) inhibitors, cytotoxic and antifungal compounds. This result indicates that PAB is an effective methodology for producing compound collections for the purpose of drug discovery.

Design and synthesis of new antitumor agents with the 1,7-epoxycyclononane framework. Study of their anticancer action mechanism by a model compound

This article describes the design, synthesis and biological evaluation of a new family of antitumor agents having the 1,7-epoxycyclononane framework. We have developed a versatile synthetic methodology that allows the preparation of a chemical library with structural diversity and in good yield. The synthetic methodology has been scaled up to the multigram level and can be developed in an enantioselective fashion. The study in vitro of a model compound, in front of the cancer cell lines HL-60 and MCF-7, showed a growth inhibitory effect better than that of cisplatin. The observation of cancer cells by fluorescence microscopy showed the presence of apoptotic bodies and a degradation of microtubules. The study of cell cycle and mechanism of death of cancer cells by flow cytometry indicates that the cell cycle arrested at the G₀/G₁ phase and that the cells died by apoptosis preferably over necrosis. A high percentage of apoptotic cells at the subG₀/G₁ level was observed. This indicates that our model compound does not behave as an antimitotic agent like nocodazole, used as a reference, which arrests the cell cycle at G₂/M phase. The interaction of anticancer agents with DNA molecules was evaluated by atomic force microscopy, circular dichroism and electrophoresis on agarose gel. The results indicate that the model compound has not DNA as a target molecule. The in silico study of the model compound showed a potential good oral bioavailability.

Synthesis of the 10-oxabicyclo[5.2.1]decane framework present in bioactive natural products

The present work deals with the synthesis of the 10-oxabicyclo[5.2.1]decane framework present in bioactive natural products like physalins, with potential as antitumor agents. This synthetic methodology involves several key reactions: (a) synthesis of polyfunctionalized cycloheptenones by [4 + 3] cycloaddition reactions of furan precursors with oxyallyl cations; (b) Nicholas reaction with propargyl cations stabilized as dicobalt hexacarbonyl complexes; (c) demetallation and hydration of the resulting acetylenes; (d) stereoconvergent aldol cyclization to generate a key oxatricyclic intermediate and (e) a β-fragmentation process that affords, under hypoiodite photolysis, the desired product with moderate to good yield. The final compounds are the result of a radicalary β-fragmentation at the level of C2-C6 with respect to the tertiary hydroxyl group on C6, with an unexpected contraction from a ten- to a nine-membered ring system, via a radical addition to the carbonyl group on C4. The synthetic methodology has been scaled up to multigram level with good overall yield. Further biological, biochemical and biophysical studies are being carried out in our laboratory on these 1,7-epoxycyclononane derivatives to determine the potential of this kind of oxabicyclic compound as future hits and/or leads for the development of new anticancer drugs. The preliminary evaluation of the anticancer activity of the representative synthesized compounds, against the leukaemia cancer cell lines K-562 and SR, shows a promising activity with a GI₅₀ = 0.01 μM and a LC₅₀ = 7.4 μM for a conveniently functionalized 10-oxabicyclo[5.2.1]decane.

Copper-catalyzed cascade annulation of unsaturated α-bromocarbonyls with enynals: a facile access to ketones from aldehydes

A Cu-catalyzed cascade annulation of enynals with alkenyl or alkynyl α-bromocarbonyls for the synthesis of various cyclohexenone-fused polycyclic compounds is described. Up to six new C-C bonds and four new carbocycles can be established in a single reaction, highlighting the high efficiency and step-economics of this protocol. This reaction offers a novel and straightforward entry to the synthesis of ketones featuring the addition of carbon radicals to aldehydes.

Intramolecular functional group differentiation as a strategy for the synthesis of bridged bicyclic β-amino acids

Differentiation of identical electrophilic functional groups (carboxylates) by a strategically placed internal nucleophile (an amino group) in cyclic precursors was used as a key general approach to functionalized azabicyclic scaffolds.

Endeavors to access molecular complexity: strategic use of free radicals in natural product synthesis

Free radicals, which in the past were considered unruly chemical species, have become manageable and indispensable for synthetic organic chemistry. The unique nature of free radicals has allowed practitioners in organic synthesis to design flexible approaches to produce various materials ranging from small molecules to polymers. The present Personal Account describes the author's endeavors to create molecular complexity by the strategic use of free radicals, with an emphasis on the synthesis of bioactive natural products.

Practical radical cyclizations with arylboronic acids and trifluoroborates

Practical radical cyclizations using organoboronic acids and trifluoroborates take place in water, open to air, and in a scalable fashion employing catalytic silver nitrate and stoichiometric potassium persulfate. Both Pschorr-type cyclizations and tandem radical cyclization/trap cascades are described, illustrating the utility of these mild conditions for the generation of polycyclic scaffolds.

Dimethylzinc-initiated radical reactions

Developments in modern organic synthesis owe much to the field of radical chemistry. Mild reaction conditions, high selectivity, good functional group tolerance and high product yield are features that have made reactions involving radical species indispensable tools for synthetic chemists. In part, the discovery of new radical initiators has led to the efficiency that now characterizes most radical reactions. This Account describes our investigations of radical reactions initiated by dimethylzinc. In 2001, we unexpectedly observed this reaction while investigating the amidophosphane-copper-catalyzed asymmetric addition of dimethylzinc to N-sulfonyl imines with tetrahydrofuran (THF) as reaction solvent. However, instead of adding the desired methyl group to the N-sulfonyl imine, we produced the THF adduct in excellent yield. This result laid the foundation for our discovery of novel modes of reactivity. Further investigations of the unexpected addition reaction revealed that a trace amount of air was needed for reaction progress, indicating that radical intermediates were involved. Indeed, controlled injection of air into the reaction flask by a syringe pump through a sodium hydroxide tube afforded the products in good to excellent yield. In addition, the reaction proved to be chemoselective for a C=N bond over a C=O bond, as well as for 1,4-addition over 1,2-addition. We developed asymmetric variants of the radical addition reaction of ethers to imines using chiral N-sulfinyl imines to produce the adducts in reasonably high stereoselectivity (up to 11:1). A 93:7 diastereomeric ratio of the adduct was obtained when bis(8-phenylmenthyl) benzylidenemalonate was used in the radical addition of ethers to C=C bonds. Interestingly, in the presence of dimethylzinc and air, arylamines, alkoxyamines, and dialkylhydrazines react with THF to give amino alcohols, oximes, and hydrazones, respectively, in moderate to high yields. We performed a tin-free intermolecular addition of functionalized primary alkyl groups, generated from their corresponding iodides, to N-sulfonyl imines using dimethylzinc, air, boron trifluoride diethyl etherate, and a catalytic amount of copper(II) triflate. Direct C-H bond cleavage from cycloalkanes was also feasible in the presence of dimethylzinc, air, and boron trifluoride diethyl etherate to give the corresponding cycloalkyl radicals, which were suitable nucleophiles for N-sulfonyl imines. In all of the above reactions, dimethylzinc was a superior radical initiator than other conventional initiators such as dibenzoyl peroxide, diethylzinc, and triethylborane. We hope the coming decades will witness the report of other novel radical initiators that would complement the reactivity modes of existing ones.

The Scope and Limitations of 1,3-Stannyl Shift-Promoted Intramolecular Cyclizations of α-Stannyl Radicals with a Formyl Group

Alpha-tributylstannyl radicals can be generated from the corresponding bromides or xanthates. These radicals undergo efficient intramolecular 1,5-cyclizations with a formyl group. The resulting beta-stannyl alkoxy radicals proceed through a 1,3-stannyl shift from carbon to oxygen to afford beta-stannyloxy radicals. This novel rearrangement is most likely irreversible and serves as a driving force to promote the cyclizations. Although the cyclization rates can be accelerated when the formyl group carries alpha-dimethyl substituents, unfortunately beta-scission of the alkoxy radicals becomes competitive with the 1,3-stannyl shift. The beta-stannyloxy radicals can be employed in further cyclizations to obtain tandem cyclization products.

Photoinduced Atom-Transfer Cyclization of α-Iodocycloalkanones Bearing an Allenyl Side Chain

[reaction: see text] Irradiation of alpha-iodocycloalkanones bearing an allenyl side chain with a sunlamp effected atom-transfer cyclization to give cyclized products in good yield. A mechanism, involving radical atom-transfer cyclization accompanied by 1,5- and 1,4-hydrogen transfers, is proposed.

Radical Cyclization Cascade Involving Ynamides: An Original Access to Nitrogen-Containing Heterocycles

A radical cascade involving a 5-exo-dig cyclization followed by a 6-endo-trig radical trapping transforms ynamides into heterogeneous polycyclic compounds in good yields. This leads interestingly to the formation of isoindols, isoindolinones, and pyridoisoindolones. [reaction: see text]

Hydroxyalkylation of alpha-C-H bonds of tetrahydrofuran with aldehydes in the presence of triethylborane and tert-butyl hydroperoxide

The alpha-hydroxyalkylation of tetrahydrofuran with aldehydes via radical C-H abstraction was conducted using triethylborane in the presence of tert-butyl hydroperoxide. This study presents a rare instance of direct intermolecular radical addition of unactivated cyclic ether to aldehydes.

The application of intramolecular radical cyclizations of acylsilanes in the regiospecific formation of cyclic silyl enol ethers

Acylsilanes with terminal alpha-stannyl bromide or xanthate functionalities are prepared. Alpha-stannyl radicals generated from these acylsilanes undergo intramolecular cyclizations to give cyclic silyl enol ethers regiospecifically. The radical processes involve radical cyclization, Brook rearrangement, and beta-fragmentation in sequence. A tributylstannyl group serves as the radical leaving group. The newly formed sigma-bond and pi-bond are located between the same two carbon atoms. This approach is limited to the formation of five-membered rings. In another route, omega-bromo-alpha-phenylsulfonylacylsilanes are synthesized. The radical cyclizations of these alpha-sulfonylacylsilanes also give cyclic silyl enol ethers. The phenylsulfonyl moiety is the radical leaving group in this system. Furthermore, the newly formed sigma-bond and pi-bond are located at adjacent positions sharing a single carbon atom. The latter approach is effective for both five- and six-membered ring formation.