Electron transfer initiated cyclizations: cyclic acetal synthesis through carbon-carbon sigma-bond activation

Route to Chiral Tetrahydrofuran Acetals via Pd-Catalyzed Asymmetric Allylic Cycloaddition of Vinyl Epoxides with β-Keto Enol Ethers

An efficient method for the synthesis of functionalized chiral tetrahydrofuran (THF) acetals via Pd-catalyzed asymmetric allylic cycloaddition has been developed. With a palladium catalyst coordinated by a chiral phosphine ligand, the protocol is enabled to combine readily available vinyl epoxides and β-keto enol ethers to produce THF acetals bearing three stereocenters in a broad substrate scope with uniformly high levels of enantio- and diastereoselectivity.

Synthesis and Olfactory Properties of Seco-Analogues of Lilac Aldehydes

Lilac aldehydes are considered as principal olfactory molecules of lilac flowers. We have designed, prepared, and evaluated a set of racemic seco-analogues of such natural products. The synthesis employs commercially available α-chloroketones as substrates that are transformed in four steps to target compounds. Their qualitative olfactory analysis revealed that the opening of the tetrahydrofuran ring leads to a vanishing of original flowery scent with the emergence of spicy aroma accompanied by green notes, and/or fruity aspects of novel seco-analogues. These results suggest the important osmophoric role of THF moiety for the generation of the typical flowery aroma associated with lilac aldehydes.

Cleavage of carbon–carbon bonds by radical reactions

This review provides insights into the in situ generated radicals triggered carbon–carbon bond cleavage reactions.

Chiral Diaryliodonium Phosphate Enables Light Driven Diastereoselective α-C(sp3)-H Acetalization

C(sp³)-H bond functionalization has emerged as a robust tool enabling rapid construction of molecular complexity from simple building blocks, and the development of asymmetric versions of this reaction creates a powerful methodology to access enantiopure sp³-rich materials. Herein, we report the stereoselective functionalization of C(sp³)-H bonds of cyclic ethers employing a photochemically active diaryliodonium salt in combination with an anionic phase-transfer catalyst. The synthetic strategy outlined herein allows for regio- and stereochemical control in the α-C-H acetalization of furans and pyrans using alcohol nucleophiles, thus providing the ability to control the configuration at the stereogenic exocyclic acetal carbon.

Enantioselective Synthesis of Pyrroloindolines via Noncovalent Stabilization of Indole Radical Cations and Applications to the Synthesis of Alkaloid Natural Products

While interest in the synthetic chemistry of radical cations continues to grow, controlling enantioselectivity in the reactions of these intermediates remains a challenge. Based on recent insights into the oxidation of tryptophan in enzymatic systems, we report a photocatalytic method for the generation of indole radical cations as hydrogen-bonded adducts with chiral phosphate anions. These noncovalent open-shell complexes can be intercepted by the stable nitroxyl radical TEMPO· to form alkoxyamine-substituted pyrroloindolines with high levels of enantioselectivity. Further elaboration of these optically enriched adducts can be achieved via a catalytic single-electron oxidation/mesolytic cleavage sequence to furnish transient carbocation intermediates that may be intercepted by a wide range of nucleophiles. Taken together, this two-step sequence provides a simple catalytic method to access a wide range of substituted pyrroloindolines in enantioenriched form via a standard experimental protocol from a common synthetic intermediate. The design, development, mechanistic study, and scope of this process are presented, as are applications of this method to the synthesis of several dimeric pyrroloindoline natural products.

Reversible Bond/Cation-Coupled Electron Transfer on Phenylenediamine-Based Rhodamine B and Its Application on Electrochromism

A biomimetic system on reversible bond-coupled electron transfer (BCET) has been proposed and investigated in a switchable Rh-N molecule with redox active subunits. We discover that energy barrier of C-N bond breaking is reduced dramatically to less than 1/7 (from 40.4 to 5.5 kcal/mol), and 1/3 of the oxidation potential is simultaneously lowered (from 0.67 to 0.43 V) with the oxidation of Rh-N. The concept, cation-coupled electron transfer (CCET), is highly recommended by analyzing existing proton coupled electron transfer (PCET) and metal coupled electron transfer (MCET) along with aforementioned BCET, which have same characteristic of transferring positive charges, such as proton, metal ion, and organic cation. Molecular switch can be controlled directly by electricity through BCET process. Solid electrochromic device was fabricated with extremely high coloration efficiency (720 cm²/C), great reversibility (no degradation for 600 cycles), and quick respond time (30 ms).

Recent advances in photoinduced glycosylation: oligosaccharides, glycoconjugates and their synthetic applications

Carbohydrates have been demonstrated to perform imperative act in biological processes. This review highlights recent uses of photoinduced glycosylation in carbohydrate chemistry for the synthesis of oligosaccharides, thiosugars, glycoconjugates and glycoprotein.

Catalytic Carbocation Generation Enabled by the Mesolytic Cleavage of Alkoxyamine Radical Cations

A new catalytic method is described to access carbocation intermediates via the mesolytic cleavage of alkoxyamine radical cations. In this process, electron transfer between an excited state oxidant and a TEMPO-derived alkoxyamine substrate gives rise to a radical cation with a remarkably weak C-O bond. Spontaneous scission results in the formation of the stable nitroxyl radical TEMPO(.) as well as a reactive carbocation intermediate that can be intercepted by a wide range of nucleophiles. Notably, this process occurs under neutral conditions and at comparatively mild potentials, enabling catalytic cation generation in the presence of both acid sensitive and easily oxidized nucleophilic partners.

Total synthesis of theopederin D

The total synthesis of the potent cytotoxin theopederin D has been achieved through the use of an oxidative carbon–carbon bond cleavage reaction to form an acyliminium ion in the presence of acid labile acetal groups Other key transformations include an acid mediated functionalization of a tetrahydrofuranyl alcohol in the presence of a tetrahydropyranyl alcohol, a syn -selective glycal epoxide opening, and a catalytic asymmetric aldehyde-acid chloride condensation.

An experimental and computational approach to defining structure/reactivity relationships for intramolecular addition reactions to bicyclic epoxonium ions

In this manuscript we report that oxidative cleavage reactions can be used to form oxocarbenium ions that react with pendent epoxides to form bicyclic epoxonium ions as an entry to the formation of cyclic oligoether compounds. Bicyclic epoxonium ion structure was shown to have a dramatic impact on the ratio of exo- to endo-cyclization reactions, with bicyclo[4.1.0] intermediates showing a strong preference for endo-closures and bicyclo[3.1.0] intermediates showing a preference for exo-closures. Computational studies on the structures and energetics of the transition states using the B3LYP/6-31G(d) method provide substantial insight into the origins of this selectivity.

Intramolecular Electron Transfer Initiated Cation and Radical Formation through Carbon−Carbon Bond Activation

Radical cations can be formed in a spatially and temporally controlled manner by appending a sacrificial photooxidant to an easily oxidized substrate, leading to intramolecular electron transfer upon irradiation. The anthraquinone carboxyl group is an effective photooxidant that can promote single electron oxidation from an appended arene. The resulting intermediates undergo a cleavage reaction through carbon-carbon bond activation to provide either cations or radicals that react to form a range of products.

Reactivity and acid-base behavior of ring-methoxylated arylalkanoic acid radical cations and radical zwitterions in aqueous solution. Influence of structural effects and pH on the benzylic C-H deprotonation pathway

A product and time-resolved kinetic study of the one-electron oxidation of ring-methoxylated phenylpropanoic and phenylbutanoic acids (Ar(CH2)nCO2H, n = 2, 3) has been carried out at different pH values. Oxidation leads to the formation of aromatic radical cations (Ar.+(CH2)nCO2H) or radical zwitterions (Ar.+(CH2)nCO2-) depending on pH, and pKa values for the corresponding acid-base equilibria have been measured. In the radical cation, the acidity of the carboxylic proton decreases by increasing the number of methoxy ring substituents and by increasing the distance between the carboxylic group and the aromatic ring. At pH 1.7 or 6.7, the radical cations or radical zwitterions undergo benzylic C-H deprotonation as the exclusive side-chain fragmentation pathway, as clearly shown by product analysis results. At pH 1.7, the first-order deprotonation rate constants measured for the ring-methoxylated arylalkanoic acid radical cations are similar to those measured previously in acidic aqueous solution for the alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations. In basic solution, the second-order rate constants for reaction of the radical zwitterions with (-)OH (k-OH)) have been obtained. These values are similar to those obtained previously for the (-)OH-induced alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations, indicating that under these conditions the radical zwitterions undergo benzylic C-H deprotonation. Very interestingly, with 3,4-dimethoxyphenylethanoic acid radical zwitterion, that was previously observed to undergo exclusive decarboxylation up to pH 10, competition between decarboxylation and benzylic C-H deprotonation is observed above pH 11.

Intramolecular Participation in Alkoxycarbenium Ion Pools

[reaction: see text] Alkoxycarbenium ions having no substituents on the cationic carbon have been accumulated as "cation pools" by the introduction of an ether group in an appropriate position. Intramolecular participation of the ether oxygen is suggested to be responsible for stabilization of the alkoxycarbenium ions.

Stereocontrolled [4+2]-Annulation Accessing Dihydropyrans: Synthesis of the C1a-C10 Fragment of Kendomycin

[reaction: see text] Development of new organosilane reagents bearing C-centered chirality where the stereocenter is fully substituted, and their use in the stereocontrolled synthesis of cis- and trans-dihydropyrans containing a trisubstituted olefin is described. The reagents participate in Lewis acid promoted [4+2]-annulations providing useful levels of selectivity with both aliphatic and aromatic aldehydes. A stereoselective synthesis of the C1a-C10 fragment of kendomycin (1) is also described.

Oxidative Cyclorelease from Soluble Polymeric Supports

[reaction: see text] Single electron oxidation is shown to be a viable method for effecting concomitant cyclization and cleavage (cyclorelease) of a series of polymer bound homobenzylic ethers. Soluble oligonorbornene polymers are stable toward redox chemistry and are isolable through precipitation with methanol, making them excellent supports for this process. These oxidative conditions are also shown to cleave secondary and tertiary alcohols and ethers in a new traceless approach to polymer-supported aldehyde and ketone synthesis.

Structure-reactivity relationships in oxidative carbon-carbon bond forming reactions: a mild and efficient approach to stereoselective syntheses of 2,6-disubstituted tetrahydropyrones

Homobenzylic ethers with pendent enol acetate nucleophiles undergo highly efficient cleavage reactions followed by 6-endo cyclizations to form 2,6-disubstituted tetrahydropyrones with excellent stereocontrol at room temperature in the presence of the mild oxidant ceric ammonium nitrate. Cyclizations proceed through either stabilized or nonstabilized oxocarbenium ions. Structure-reactivity relationships are presented to provide a predictive guide for the design of radical cation cleavage processes. Unique sequences for preparing cyclization substrates based on stereoselective Lewis acid mediated acetal openings have been developed for the synthesis of complex substrates that are suitable for applications to the synthesis of biologically active natural products.

An oxidative entry into the amido trioxadecalin ring system

The amido trioxadecalin ring system is a key structural component of the architecturally interesting anticancer and immunosuppressive agents of the mycalamide, theopederin, and onnamide families of natural products. We report a new entry into this structure in which a mixed acetal serves as a surrogate for a formaldehyde hemiacetal in an addition to an oxidatively generated acyliminium ion. The stereochemical outcome of this process can be explained by the conformational preferences of the product ring system. [reaction: see text]

Tuning reactivity and chemoselectivity in electron transfer initiated cyclization reactions: applications to carbon-carbon bond formation

In this communication, we demonstrate that the scope of our electron transfer initiated cyclization reaction can be significantly broadened by exploiting the relationship between the oxidation potentials of homobenzylic ethers and the mesolytic benzylic carbon-carbon bond dissociation energies of their radical cations. By lowering the oxidation potential of the electrophore and the benzylic carbon-carbon bond dissociation energy, we can initiate reactions under mild, nonphotochemical conditions. The selectivity of the arene oxidation and the mild reaction conditions allow a variety of electron-rich olefins to serve as nucleophilic groups to form carbon-carbon bonds with excellent efficiency.

Oxidative synthesis of cyclic acyl aminals through carbon-carbon sigma-bond activation

Acyliminium ions can be prepared through photoinitiated single-electron oxidation reactions of homobenzylic amides and carbamates. Cyclic acyl aminals are formed when these acyliminium ions are appended to nucleophiles such as hydroxyl, ether, and sulfonamide groups. The scope of these reactions is discussed along with mechanistic issues relating to the energetics, chemoselectivity, and stereoelectronic effects of bond activation. [reaction: see text]

Electron transfer initiated heterogenerative cascade cyclizations: polyether synthesis under nonacidic conditions

[reaction: see text] Single-electron oxidation has been employed to initiate heterogenerative cascade cyclization reactions that form polyether compounds under essentially neutral conditions. The reactions proceed through mesolytic benzylic carbon-carbon bond cleavages of homobenzylic ether-derived radical cations followed by intramolecular epoxonium ion formation, leading to further cyclizations. Both oligotetrahydrofuran and tetrahydropyran structures can be prepared by altering substrate topography.

Aerobic organocatalytic photoinitiated arene oxidations: application to electron transfer initiated cyclization reactions

[reaction: see text] Electron transfer initiated oxidative cyclization reactions can be effected by catalytic amounts of N-methylquinolinium hexafluorophosphate through photoirradiation in the presence of dioxygen. Solid sodium thiosulfate serves as an effective reducing agent to remove the reactive oxygen species formed from catalyst regeneration and radical coupling reactions with O(2), allowing these cyclizations to proceed efficiently on preparatively useful scales.