Enol Acetates Research Articles

Electrochemically Induced Synthesis of β‐Ketosulfones from Enol Acetates and Sodium Sulfinates

Abstractβ‐Ketosulfones were synthesized from enol acetates and sodium sulfinates under electrochemical conditions. The starting sulfinates can act both as the supporting electrolyte and as the source of S‐centered radicals, allowing electrolysis to proceed without external salt. Side reactions of enol acetates’ oxidation are not observed, and the developed process is quite selective. The proposed electrochemical sulfonylation is compatible with a wide range of the initial enol acetates and sulfinates. On the basis of literature data, control experiments, and CV studies, the presumable reaction pathway is suggested. The key intermediates are S‐centered sulfonyl radical, C‐centered benzylic radical, and β‐acetoxy‐β‐hydroxysulfone.

Electrochemical Synthesis of α-Thiocyanated/Methoxylated Ketones Using Enol Acetates.

We have developed the synthesis of α-substituted ketone compounds with enol acetates in an electrochemical way. By using cheap NH4SCN and MeOH as the radical sources, a series of valuable α-thiocyanates/methoxy ketones were synthesized under mild electrolysis conditions in acceptable yields with diverse functional group compatibility. Additionally, the scale-up experiment and synthetic transformations reveal potential applications in organic synthesis.

Stereoselective Synthesis of Cyclopropane Derivatives from Mixtures of (E)- and (Z)-Enol Acetates

Stereoselective Synthesis of Cyclopropane Derivatives from Mixtures of (E)- and (Z)-Enol Acetates

Photo-catalyzedα-arylation of enol acetate using recyclable silica-supported heteroleptic and homoleptic copper(I) photosensitizers.

Earth-abundant photosensitizers are highly sought after for light-mediated applications, such as photoredox catalysis, depollution and energy conversion schemes. Homoleptic and heteroleptic copper(I) complexes are promising candidates in this field, as copper is abundant and the corresponding complexes are easily obtained in smooth conditions. However, some heteroleptic copper(I) complexes suffer from low (photo)stability that leads to the gradual formation of the corresponding homoleptic complex. Such degradation pathways are detrimental, especially when recyclability is desired. In here, we report a novel approach for the heterogenization of homoleptic and heteroleptic Cu complexes on silica nanoparticles. In both cases, the photophysical properties upon surface immobilization were only slightly affected. Excited-state quenching with aryl diazonium derivatives occurred efficiently (108-1010M-1s-1) with heterogeneous and homogeneous photosensitizers. Moderate but almost identical yields were obtained for thea-arylation of enol acetate using the homoleptic complex in homogeneous or heterogeneous conditions. Importantly, the silica-supported photocatalysts were recycled with moderate loss in photoactivity over multiple experiments. Transient absorption spectroscopy confirmed that excited-state electron transfer occurred from the homogeneous and heterogeneous homoleptic copper(I) complexes to aryl diazonium derivatives, generating the corresponding copper(II) center that persisted for several hundreds of microseconds, compatible with photoredox catalysis applications.

Electrochemical Radical Reaction Construction of C-C Bonds: Access to 1,4-Dicarbonyl Compounds from Enol Acetates and 1,3-Diketones.

As important substrates for the construction of heterocycles, a simple and efficient approach for synthesis of 1,4-diones is highly desirable. In this work, novel and efficient electrochemical radical reactions of enol acetates and 1,3-diketones have been developed to successfully achieve 1,4-diketones under catalyst-free and oxidant-free conditions. The wide range of substrates, good group tolerance, and simple operation process make the approach have important practical value. Moreover, the obtained 1,4-diketones can be easily further transformed to pyrrole and furan derivatives.

Pre-electrolysis of LiClO4 in Acetonitrile: Electrochemically Induced Protolytic Carbon-Carbon Bond Formation of Benzylic Ethers and Acetals with Allyl Trimethylsilane and Other Carbon Nucleophiles.

The pre-electrolysis of LiClO4 in acetonitrile in an undivided cell applying only "catalytic" amounts of current (e.g., 0.05 F) led to the formation of a strong acidic medium for the activation of benzylic ethers and acetals. The activated primary and secondary benzylic ethers and acetals could be converted with a range of carbon nucleophiles, such as allyl trimethylsilane, silyl enol ethers, and enol acetates, for the formation of new carbon-carbon bonds. A chemoselective reaction was observed when electron-deficient benzylic acetals were converted with allyl trimethylsilane to the monoallylated products, whereas an electron-rich benzylic acetal led to the double allylated product under activation of both ether groups.

Enantio- and Diastereoenriched Enzymatic Synthesis of 1,2,3-Polysubstituted Cyclopropanes from (Z/E)-Trisubstituted Enol Acetates.

In nature and synthetic chemistry, stereoselective [2 + 1] cyclopropanation is the most prevalent strategy for the synthesis of chiral cyclopropanes, a class of key pharmacophores in pharmaceuticals and bioactive natural products. One of the most extensively studied reactions in the organic chemist's arsenal, stereoselective [2 + 1] cyclopropanation, largely relies on the use of stereodefined olefins, which can require elaborate laboratory synthesis or tedious separation to ensure high stereoselectivity. Here, we report engineered hemoproteins derived from a bacterial cytochrome P450 that catalyze the synthesis of chiral 1,2,3-polysubstituted cyclopropanes, regardless of the stereopurity of the olefin substrates used. Cytochrome P450BM3 variant P411-INC-5185 exclusively converts (Z)-enol acetates to enantio- and diastereoenriched cyclopropanes and in the model reaction delivers a leftover (E)-enol acetate with 98% stereopurity, using whole Escherichia coli cells. P411-INC-5185 was further engineered with a single mutation to enable the biotransformation of (E)-enol acetates to α-branched ketones with high levels of enantioselectivity while simultaneously catalyzing the cyclopropanation of (Z)-enol acetates with excellent activities and selectivities. We conducted docking studies and molecular dynamics simulations to understand how active-site residues distinguish between the substrate isomers and enable the enzyme to perform these distinct transformations with such high selectivities. Computational studies suggest the observed enantio- and diastereoselectivities are achieved through a stepwise pathway. These biotransformations streamline the synthesis of chiral 1,2,3-polysubstituted cyclopropanes from readily available mixtures of (Z/E)-olefins, adding a new dimension to classical cyclopropanation methods.

Electrochemical Radical Reactions of Enol Acetates and Free Alcohols Directly Access to α-Alkoxylated Carbonyl Compounds.

The efficient intermolecular alkoxylation reactions of various enol acetates and different alcohols are developed in the electrochemical process for the first time. Enol acetates derived from either aromatic, alkyl, or alicyclic ketones, and abundant free alcohols directly used in this synthetic strategy, make this transformation very valuable in synthesis and application in the future.

Total Syntheses of (+)-Waixenicin A, (+)-9-Deacetoxy-14,15-deepoxyxeniculin, and (-)-Xeniafaraunol A.

The first asymmetric total synthesis of the Xenia diterpenoid waixenicin A, a potent and highly selective TRPM7 inhibitor, is reported. The characteristic trans-fused oxabicyclo[7.4.0]tridecane ring system was constructed via a diastereoselective conjugate addition/trapping sequence, followed by an intramolecular alkylation to forge the 9-membered ring. While a β-keto sulfone motif enabled efficient ring-closure, the subsequent radical desulfonylation suffered from (E)/(Z)-isomerization of the C7/C8-alkene. Conducting the sequence with a trimethylsilylethyl ester allowed for a fluoride-mediated decarboxylation that proceeded without detectable isomerization. The acid-labile enol acetal of the delicate dihydropyran core was introduced at an early stage and temporarily deactivated by a triflate function. The latter was critical for the introduction of the side chain. Diverting from a common late-stage intermediate provided access to waixenicin A and 9-deacetoxy-14,15-deepoxyxeniculin. A high-yielding base-mediated dihydropyran-cyclohexene rearrangement of 9-deacetoxy-14,15-deepoxyxeniculin led to xeniafaraunol A in one step.

Synergistic effect of oxygen vacancies and edging/corner-connected MnO6 structural motifs in multi-dimensional manganese oxides to enhance OVOCs catalytic oxidation

The structure-activity relationships for vinyl acetate catalytic oxidation are challenging to explore at the atomic scale due to the ambiguity of the structural defect types and sites of manganese oxides. Our work elaborates, at the atomic level, through in-situ experimental and theoretical methods, the synergistic effects of two types of structural defect sites of VO-e (edge-sharing oxygen) and VO-c (corner-sharing oxygen) and MnO6 structural motifs of manganese oxides. Multi-dimensional manganese oxides, namely those with corner-connected MnO6 structural motifs and VO-c structural oxygen defect sites, significantly improved the activation of vinyl acetate. Enhancement of enol structure formation, acetate and formate intermediate species, and tautomerism between enol structure and acetaldehyde were detected when oxygen vacancies of manganese oxides were present in combination with corner/edge-connected MnO6. Moreover, the activation of chemical bonds and deep catalytic oxidation of vinyl acetate depend on the presence of a redox couple, surface oxygen species, and weakened MnO bonds. It provides a valuable notion for investigating and designing catalytic systems and reaction processes for the purpose of emission reduction and the management of environmental contaminants.

Visible-Light Promoted Radical Fluoroalkylation of O- and N-Substituted Alkenes.

Interaction of enol ethers enol acetates, enamides and enamines with fluorinated reagents may be considered as a reliable method for the synthesis of organofluorine compounds. While classic nucleophile/electrophile substitution or addition mechanisms cannot be realized for coupling of these components, their intrinsic reactivities are revealed with the aid of photoredox catalysis. A combination of these electron donating and accepting components provides a perfect balance needed for individual redox steps, which in some cases may proceed even without a photocatalyst. The same electronic factors also support the key C,C-bond forming event involving addition of fluorinated radical at the electron rich double bond.

Total Synthesis of Puberuline C.

Puberuline C (1) is an architecturally complex C19-diterpenoid alkaloid with a unique ring fusion pattern. The 6/7/5/6/6/6-membered rings (ABCDEF-rings) contain one tertiary amine and six oxygen functionalities, and possess 12 contiguously aligned stereocenters, three of which are quaternary. These structural features of 1 make its chemical construction exceptionally challenging. Here, we disclose the first total synthesis of 1. The synthesis was accomplished from 2-cyclohexenone (9) by integrating radical cascade and Mukaiyama aldol reactions as the key transformations. A double Mannich reaction fused the A- and E-rings, and Sonogashira coupling attached the C-ring, efficiently leading to ACE-rings with the requisite 19 carbons of 1. The chemically stable tertiary chloride of the ACE-ring structure was then transformed to the corresponding bridgehead radical, which participated in the simultaneous cyclization of the B- and F-rings via a highly organized radical cascade process. This unusual step installed five contiguous stereocenters, including two quaternary carbons, without damaging the preexisting multiple polar functionalities. Subsequently, the intramolecular Mukaiyama aldol reaction between silyl enol ether and acetal was realized by applying a combination of SnCl4 and ZnCl2, forging the last remaining D-ring of the hexacycle. Finally, 3 was elaborated into 1 through regio- and stereoselective functionalizations of the BCD-rings. Our novel radical-based strategy achieved the total synthesis of 1 in 32 total steps from simple 9, demonstrating the power of the radical cascade reaction to streamline the assembly of highly complex molecules.

Activation of O-Electrophiles via Structural and Solvent Effects: SN2@O Reaction of Cyclic Diacyl Peroxides with Enol Acetates.

The reactions of O-electrophiles, such as organic peroxides, with carbon nucleophiles are an umpolung alternative to the common approaches to C-O bond formation. Nucleophilic substitution at the oxygen atom of cyclic diacyl peroxides by enol acetates with the following deacylation leads to α-acyloxyketones with an appended carboxylic acid in 28-87% yields. The effect of fluorinated alcohols on the oxidative functionalization of enol acetates by cyclic diacyl peroxides was studied experimentally and computationally. Computational analysis reveals that the key step proceeds as a direct substitution nucleophilic bimolecular (SN2) reaction at oxygen (SN2@O). CF3CH2OH has a dual role in assisting in both steps of the reaction cascade: it lowers the energy of the SN2@O activation step by hydrogen bonding to a remote carbonyl and promotes the deacylation of the cationic intermediate.

Electrochemical synthesis for α-arylation of ketones using enol acetates and aryl diazonium salts

A new electrochemical method has been developed to achieve the α-arylation of ketones by reacting aryl diazonium salts with enol acetates.

Electrochemical Synthesis of Fluorinated Ketones from Enol Acetates and Sodium Perfluoroalkyl Sulfinates.

The electrochemical synthesis of fluorinated ketones from enol acetates and RfSO2Na in an undivided cell under constant current conditions was developed. The electrosynthesis proceeded via perfluoroalkyl radical generation from sodium perfluoroalkyl sulfinate followed by addition to the enol acetate and transformation of the resulting C radical to a fluorinated ketone. The method is applicable to a wide range of enol acetates and results in the desired products in yields of 20 to 85%.

Difluoromethylation of Enol Acetates

Key words iridium catalysis - photocatalysis - difluoromethylation - enol acetates

Cover Feature: Mechanistic Insights into the Formation of δ‐Lactones by Cerium‐Catalyzed Aerobic Coupling of β‐Oxoesters with Enol Acetates (Eur. J. Org. Chem. 6/2021)

Cover Feature: Mechanistic Insights into the Formation of δ‐Lactones by Cerium‐Catalyzed Aerobic Coupling of β‐Oxoesters with Enol Acetates (Eur. J. Org. Chem. 6/2021)

Mechanistic Insights into the Formation of δ‐Lactones by Cerium‐Catalyzed Aerobic Coupling of β‐Oxoesters with Enol Acetates

Abstractδ‐Valerolactone derivatives are formed by the cerium‐catalyzed, aerobic coupling of β‐oxoesters with enol acetates and dioxygen. The products possess a 1,4‐diketone moiety, thus, the conversion can be regarded as an Umpolung since the β‐oxoesters are oxidized to electrophilic α‐radicals. The transformation has similarities to the Baeyer‐Villiger oxidation (BVO) where the higher substituted residue migrates. An endoperoxidic oxycarbenium ion comparable to the Criegee intermediate in the BVO is proposed as a reaction intermediate in this case of the oxidative C−C coupling reaction, but in contrast to the BVO, the less substituted alkyl residue migrates. It was demonstrated by the conversion of β‐oxoesters with two stereocenters that this 1,2‐alkyl shift proceeds with retention of configuration. A radical chain mechanism of the coupling reaction was furthermore evidenced by the conversion of enol acetates and β‐oxoesters with cyclopropyl substituents. Isolation and characterization of products with opened cyclopropane rings established the constitution of radical intermediates.

Theoretical Study of the Reactivity of Phenyl Radicals Toward Enol Acetates

Arylation reactions are an important class of reactions and allow the synthesis of natural and synthetic products. Despite the efficient, but high cost and toxic methodologies involving transition metals, radical arylations have gained importance after the advent of photoredox catalysis. Arylation of enol acetates is an important tool for obtaining aryl ketones but the scope of the reaction is limited to the pattern of substitution at phenyl radical and α-carbon of the enol. Theoretical calculations ((U)BHandHLYP/6-311G**) show that the polar effect is the key factor in this reaction. A good correlation of calculated rate constants with field effect explained why phenyl radicals with electron-withdrawing groups react faster toward enol acetate. The presence of alkyl groups at α carbon at the enol showed some influence of enthalpic effect but strong influence of steric effect, evidenced by great correlations with Taft and Charton parameters. Finally, substitution at β carbon showed no significant effect at reaction rates.

Visible-Light-Promoted Synthesis of α-CF2H-Substituted Ketones by Radical Difluoromethylation of Enol Acetates

An efficient and novel visible-light-promoted radical difluoromethylation of enol acetates for the synthesis of α-CF2H-substituted ketones has been described. Upon irradiation under blue LED with catalytic amounts of fac-Ir(ppy)3, this photocatalytic procedure employs difluoromethyltriphenylphosphonium bromide as a radical precursor. Various α-CF2H-substituted ketones are successfully created via designed systems based on the SET process. The methodology has also provided an operationally simple process with broad functional group compatibility.