Andrey A. Tabolin

Bio: Andrey A. Tabolin is an academic researcher from Russian Academy of Sciences. The author has contributed to research in topics: Cycloaddition & Nitroso. The author has an hindex of 13, co-authored 51 publications receiving 607 citations. Previous affiliations of Andrey A. Tabolin include D. Mendeleev University of Chemical Technology of Russia.

Rearrangement of N-Oxyenamines and Related Reactions

Abstract: ion from either the RCH2 or the R CH2 group. However, some selective cases have been reported. The products 294 and 296 can be converted to imidazoles and oxazoles, respectively. Reaction of the oximes 297 with dimethyl carbonate in the presence of potassium carbonate under heating in an autoclave afforded N-methyloxazolones 299 in moderate yields (Scheme 93). The primary O-methoxycarbonylation of the initial oximes 297 has been established as leading to the target products. The intermediates 298 are further subjected to Nmethylation and thus give rise to the oxyenamines A. Subsequently, these undergo [3,3]-rearrangement with ultimate ring closure. Reaction is facile only for α-CH2-ketone-derived oximes. Tautomerization leading to A proceeds exclusively through abstraction of the more mobile proton. Recently, a similar transformation was reported for the O-perfluorobenzoyl oxime 300 (Scheme 94). In this case the reaction proceeds under milder conditions than those for dimethyl carbonate. An older multistep approach to the mentioned transformation consists of oxime O-acylation, followed by Nmethylation with a Meerwein salt and subsequent proton abstraction (Scheme 95). This method is the predecessor for the direct α-oxygenation methodology discussed in the next section. Acid-catalyzed conversion of conjugated cyclohexenone oximes into aniline derivatives has been known since the end of the 19th century. This reaction is often referred as “Semmler−Wolff aromatization”. Typical conditions consist of treating the oximes with acetic anhydride in the presence of a strong acid such as hydrogen chloride. The reaction was used in the syntheses of substituted anilines, quinolines, and other heteroaromatics as well as in the total syntheses of miltirone, sanguinarine, and chelerythrine. It also proved successful for more complex targets such as pseudopteroxazole, penitrem D, and HKI 0231B. Ketene and 1-ethoxyvinyl acetate were found to be useful reagents for the transformation (Scheme 96, eq 1). Other mild reaction conditions involve treatment of 303 with acetyl chloride in toluene at 80 °C. One of the mechanisms proposed includes a N,O-bis(acetylation) of the starting oximes, leading to the oxy-enamines A and/or B. Although alternative ways involving dienimine 307 or azirine 308 are also possible, acetic acid elimination from A or B affords the anilides 304. Another way for generation of the initial enoxime is the in situ enolization of the monooximes 305, which allows synthesis of the acetylated m-aminophenols 306 (eq 2). A substituent shift can be observed if the initial cyclohexenone oxime possesses quaternary carbon atoms. In certain cases a Beckmann rearrangement is a side process in the rearrangement. However, judicious choice of reaction conditions may allow selective transformations. Due to the acidic medium there is a high probability for formation of cationic species that can be trapped onto aromatics. In contrast, sometimes basic media may be preferable for aromatization, for example, if the cyclohexenone oxime ring possesses electron-withdrawing substituents. Scheme 94 Scheme 95 Chemical Reviews Review dx.doi.org/10.1021/cr400196x | Chem. Rev. 2014, 114, 5426−5476 5456 4.6. Direct α-Oxygenation of Carbonyl Compounds Reaction of carbonyl compounds with secondary amines is a classical method for enamine synthesis. The enamines formed can be involved in [3,3]-rearrangements with subsequent hydrolysis, affording functionalized carbonyl compound. Thus, it seems somewhat amazing that only recently has the reaction of aldehydes and ketones 309 with N-alkyl-O-acylhydroxylamines been applied for reliable introduction of the α-hydroxyl moiety (Scheme 97, Table 1). The bulky reagent 310 (X = C, R = t-Bu, R = Ph) reacts selectively with aldehydes but is, however, unactive toward ketones. The less sterically demanding derivative 310 (X = C, R = Me, R = Ph) reacts with cyclic ketones at room temperature and with acyclic and aromatic ones under mild heating (50 °C). This synthesis methodology tolerates different functionalities, such as esters, acetals, or phenols. Unsymmetrical ketones give selective rise to CH2-group oxygenation in the presence of a CH3 group. Note that methyl ketones (acetone, acetophenone) fail to react. A similar reaction is applicable for synthesis of carbonates (X = C, R = Me, R = OR′) and carbamate derivatives (X = C, R = Me, R = NR′2) 311. Use of N-methyl-O-tosylhydroxylamine in the presence of methanesulfonic acid converts aldehydes to the corresponding 2-tosyloxy derivatives 311 (X = SO, R = Me, R = n-Tol). In such a manner the functionalization of methyl ketones 309 (R = H) is possible but with only moderate regioselectivity (functionalization of the secondary site/primary site ≈ 2.6− 4.2:1). More electron-accepting substituents at the sulfur, such as p-nitrophenyl, lead to Beckmann-like rearrangements. The applicability of asymmetric reagents such as 310 was also studied. Substituents on both the nitrogen and the oxygen atoms, reaction temperature, solvent, and counteranion were all found to have a dramatic effect on both the yield and the asymmetric induction. After thorough screening the best Scheme 96

Copper-mediated oxidative [3 + 2]-annulation of nitroalkenes and pyridinium ylides: general access to functionalized indolizines and efficient synthesis of 1-fluoroindolizines.

Abstract: A general method for the synthesis of substituted indolizines by copper(II) acetate-promoted oxidative [3 + 2]-annulation of α-fluoronitroalkenes with in situ generated pyridinium ylides was developed. Application of the copper(II) acetate–2,6-lutidine system provides efficient access to various 1-fluoroindolizines in up to 81% yield. Both electron-rich and electron-deficient nitroalkenes as well as different pyridinium and isoquinolinium salts can be involved in the reaction. Moreover, it was found that copper-mediated annulation is applicable for other α-substituted (alkyl, chloro, and ester) nitroalkenes giving rise to the corresponding indolizines. First synthesis of monofluorinated [3,2,2]cyclazines was demonstrated via oxidative annulation of 3-unsubstituted fluoroindolizines with diethyl acetylene dicarboxylate.

Radical Nitration-Debromination of α-Bromo-α-fluoroalkenes as a Stereoselective Route to Aromatic α-Fluoronitroalkenes—Functionalized Fluorinated Building Blocks for Organic Synthesis

Abstract: A new highly efficient method for the synthesis of 2-fluoro-2-nitrostyrenes was described. Radical nitration of readily available 2-bromo-2-fluorostyrenes with Fe(NO3)3·9H2O resulted in the formation of the corresponding α-fluoro-nitroalkenes in isolated yields up to 92%. The reaction proceeded as a nitration-debromination sequence to highly stereoselectively give α-fluoro-nitroalkenes as Z-isomers only. The broad scope of this method was demonstrated. Prepared monofluorinated alkenes were shown to be versatile building blocks for the synthesis of various fluorinated products.

A new golden age for donor-acceptor cyclopropanes.

Abstract: The effective use of ring strain has been applied to considerable advantage for the construction of complex systems. The focus here is directed towards cyclopropanes as building blocks for organic synthesis. Although thermodynamics should take the side of synthetic chemists, only a specific substitution pattern at the cyclopropane ring allows for particularly mild, efficient, and selective transformations. The required decrease in the activation barrier is achieved by the combined effects of vicinal electron-donating and electron-accepting moieties. This Review highlights the appropriate tools for successfully employing donor–acceptor cyclopropanes in ring-opening reactions, cycloadditions, and rearrangements.

Transition-Metal-Catalyzed Cleavage of C-N Single Bonds.

Abstract: Kunbing Ouyang,†,‡ Wei Hao,† Wen-Xiong Zhang,*,†,§ and Zhenfeng Xi† †Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of the Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China ‡Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China

Transition metal-catalyzed C-H functionalization of N-oxyenamine internal oxidants.

Abstract: The transition metal-catalyzed C–H functionalization with hydroxylamine derivatives serving as both reactants and internal oxidants has attracted a lot of interest. These reactions obviate the need for external oxidants and therefore result in high reactivity and selectivity, as well as excellent functional group tolerance under mild reaction conditions, and moreover, water, methanol or carboxylic acid is generally released as the by-product, thus leading to reduced waste. This review focuses on the transition metal-catalyzed oxidative C–H functionalization of N-oxyenamine internal oxidants, with an emphasis on the scope and limitations, as well as the mechanisms of these reactions.

The [3 + 3]-cycloaddition alternative for heterocycle syntheses: catalytically generated metalloenolcarbenes as dipolar adducts.

Abstract: ConspectusThe combination of two or more unsaturated structural units to form cyclic organic compounds is commonly referred to as cycloaddition, and the combination of two unsaturated structural units that forms a six-membered ring is formally either a [5 + 1]-, [4 + 2]-, [2 + 2 + 2]-, or [3 + 3]-cycloaddition. Occurring as concerted or stepwise processes, cycloaddition reactions are among the most useful synthetic constructions in organic chemistry. Of these transformations, the concerted [4 + 2]-cycloaddition, the Diels–Alder reaction, is by far the best known and most widely applied. However, although symmetry disallowed as a concerted process and lacking certifiable examples until recently, stepwise [3 + 3]-cycloadditions offer advantages for the synthesis of a substantial variety of heterocyclic compounds, and they are receiving considerable attention.In this Account, we present the development of stepwise [3 + 3]-cycloaddition reactions from virtual invisibility in the 1990s to a rapidly growing syn...

Ein neues goldenes Zeitalter in der Chemie Donor‐Akzeptor‐substituierter Cyclopropane

Abstract: Ringspannung gewinnbringend zu nutzen, ist seit jeher fur den Aufbau komplexer Systeme von grosem Vorteil. Schnell richtet sich somit der Blick auf Cyclopropane als Bausteine fur die organische Synthese. Auch wenn die Thermodynamik dabei auf der Seite des Synthesechemikers steht, ermoglicht erst ein spezielles Substitutionsmuster am Cyclopropan besonders milde, effiziente und selektive Umsetzungen. Die erforderliche Absenkung der Aktivierungsbarriere wird durch die Kombination von zueinander vicinal stehendem Elektronendonor und -akzeptor erreicht. Dieser Aufsatz beleuchtet die richtigen Hilfsmittel, um mit Donor-Akzeptor-substituierten Cyclopropanen Ringoffnungen, Cycloadditionen und Umlagerungen durchzufuhren.