Initial Cycloadduct Research Articles

Room Temperature Diels-Alder Reactions of 4-Vinylimidazoles.

In the course of studying Diels-Alder reactions of 4-vinylimidazoles with N-phenylmaleimide, it was discovered that they engage in cycloaddition at room temperature to give high yields of the initial cycloadduct as a single stereoisomer. In certain cases, the product precipitated out of the reaction mixture and could be isolated by simple filtration, thereby avoiding issues with aromatization observed during chromatographic purification. Given these results, intramolecular variants using doubly activated dienophiles were also investigated at room temperature. Amides underwent cycloaddition at room temperature in modest yields, but the initial adducts were not isolable with Nimid-benzyl-protected systems. Attempts to extend these results to the corresponding esters and hydroxamate were less successful with these substrates only undergoing cycloaddition at elevated temperatures in lower yields. Density functional theory calculations were performed to evaluate the putative transition states for both the inter- and intramolecular variants to rationalize experimental observations.

Enantioselective Synthesis of Tropane Scaffolds by an Organocatalyzed 1,3-Dipolar Cycloaddition of 3-Oxidopyridinium Betaines and Dienamines.

Tropane alkaloids constitute a compound-class which is structurally defined by a central 8-azabicyclo[3.2.1]octane core. A diverse bioactivity profile combined with an unusual aza-bridged bicyclic framework has made tropanes molecules-of-interest within organic chemistry. Enantioselective examples of (5+2) cycloadditions between 3-oxidopyridinium betaines and olefins remain unexplored, despite 3-oxidopyridinium betaines being useful reagents in organic synthesis. The first asymmetric (5+2) cycloaddition of 3-oxidopyridinium betaines is reported, affording tropane derivatives in up to quantitative yield and with excellent control of peri-, regio-, diastereo-, and enantioselectivity. The reactivity is enabled by dienamine-activation of α,β-unsaturated aldehydes combined with in situ formation of the pyridinium reaction-partner. A simple N-deprotection protocol allows for liberation of the tropane alkaloid motif, and synthetic elaborations of the cycloadducts demonstrate their synthetic utility to achieve highly diastereoselective modification around the bicyclic framework. DFT computations suggest a stepwise mechanism where regio- and stereoselectivity are defined during the first bond-forming step in which the pyridinium dipole exerts critical conformational control over its dienamine partner. In the second bond-forming step, a kinetic preference toward an initial (5+4) cycloadduct was identified; however, a lack of catalyst turn-over, reversibility, and thermodynamic bias favoring a (5+2) cycloadduct rendered the reaction fully periselective.

Diazophosphonates: Effective Surrogates for Diazoalkanes in Pyrazole Synthesis.

Diazophosphonates, readily prepared from α-ketophosphonates by oxidation of the corresponding hydrazones in batch or in flow, are useful partners in 1,3-dipolar cycloaddition reactions to alkynes to give N-H pyrazoles, including the first intramolecular examples of such a process. The phosphoryl group imbues a number of desirable properties into the diazo 1,3-dipole. The electron-withdrawing nature of the phosphoryl stabilizes the diazo compound making it easier to handle, whilst the ability of the phosphoryl group to migrate readily in a [1,5]-sigmatropic rearrangement enables its transfer from C to N to aromatize the initial cycloadduct, and hence its facile removal from the final pyrazole product. Overall, the diazophosphonate acts as a surrogate for the much less stable diazoalkane in cycloadditions, with the phosphoryl group playing a vital, but traceless, role. The cycloaddition proceeds more readily with alkynes bearing electron-withdrawing groups, and is regiospecific with asymmetrical alkynes. The potential of diazophosphonates for use in bioorthogonal cycloadditions is demonstrated by their facile addition to strained alkynes.

Intermolecular Crossed [2 + 2] Cycloaddition Promoted by Visible-Light Triplet Photosensitization: Expedient Access to Polysubstituted 2-Oxaspiro[3.3]heptanes.

This paper describes an intermolecular cross-selective [2 + 2] photocycloaddition reaction of exocyclic arylidene oxetanes, azetidines, and cyclobutanes with simple electron-deficient alkenes. The reaction takes place under mild conditions using a commercially available Ir(III) photosensitizer upon blue light irradiation. This transformation provides access to a range of polysubstituted 2-oxaspiro[3.3]heptane, 2-azaspiro[3.3]heptane, and spiro[3.3]heptane motifs, which are of prime interest in medicinal chemistry as gem-dimethyl and carbonyl bioisosteres. A variety of further transformations of the initial cycloadducts are demonstrated to highlight the versatility of the products and enable selective access to either of a syn- or an anti-diastereoisomer through kinetic or thermodynamic epimerization, respectively. Mechanistic experiments and DFT calculations suggest that this reaction proceeds through a sensitized energy transfer pathway.

First thia-Diels-Alder reactions of thiochalcones with 1,4-quinones.

Aryl and hetaryl thiochalcones react smoothly with 1,4-quinones in THF solution at 60 °C yielding the corresponding fused 4H-thiopyrans after spontaneous dehydrogenation of the initially formed [4 + 2] cycloadducts. In general, the yields of the isolated products were high. With 5-chloro-10-hydroxy-1,4-anthraquinone, the thia-Diels–Alder reaction occurred with complete regioselectivity. In the case of the reaction of vitamin K3 (menadione) with diphenylthiochalcone, the initial cycloadduct was isolated in 37% yield.

1,3-Dipolar cycloaddition of chalcones and arylidene-1,3-dicarbonyls with diazosulfone for the regioselective synthesis of functionalized pyrazoles and pyrazolines

A convenient method for the synthesis of 3-acylpyrazoles and pyrazole-3-carboxylates using diazosulfone as a reactive 1,3-dipole and a diazomethane equivalent is reported here. Chalcones, arylidenemalonates and other arylidene-1,3-dicarbonyls performed well as the dipolarophiles in the reaction with the diazosulfone which took place under simple base mediated conditions (Cs2CO3 or NaOEt in EtOH). In few cases, the initial cycloadducts, the intermediate pyrazoline derivatives, could also be isolated and characterized. The pyrazoline derivatives undergo an alkoxide mediated 1,4-elimination, viz. decarboxylation-detosylation to afford the pyrazole derivatives.

アラインを用いた複素環化合物の合成

Synthesis of nitrogen, oxygen, sulfur, and selenium containing heterocycles from aryne intermediates was reviewed. Reaction of (2-trimethylsilylphenyl)phenyliodonium triflate with thiobenzophenones in the presence of CsF gave the corresponding [4+2] cycloadducts, whereas reaction with thiopivalophenones afforded the [2+2] cycloadducts. Reaction of selones with benzyne produced from (2-trimethylsilyl)phenyl triflate also produced benzoselenetes. In the case of selenofenchone, initial cycloadduct further reacted with another molar of benzyne to give the corresponding selenoxanthene. Benzyne also reacted with ortho-substituted hydroxybenzoyl derivatives to afford the xanthene derivatives. Three component reaction involving benzyne, THF, and chloroform afforded phenyl ω-trichloroalkyl ethers via 1,3-oxonium zwitterion intermediates. Carbon-carbon single bond insertion of benzyne into β-diketones followed by intramolecular aldol reaction gave 4-substituted 2-naphthols regioselectively, which further oxidized by vanadium oxide to afford 4,4’,2,2’-binaphthols in good yields. This methodology would be applicable for the synthesis of biologically or pharmaceutically active products such as isocoumarins and benzodiazepines in one-pot operation.

Structure and properties of 9,10,11,12,13,14-hexahydro-9,10[1′,4′]-benzenoanthracene and 9,10,11,12,13,14,15,16-octahydro-9,10[1′,4′]-benzenoanthracene

The title compounds were prepared by 4πs+4πs photochemical cycloaddition between anthracene and 1,3-cyclohexadiene, followed by catalytic hydrogenation. The results confirm the structure of the initial cycloadduct, 9,10,11,12,13,14-hexahydro-9,10[1′,4′]-benzenoanthracene (1), which in the crystal exhibits positional disorder about the cyclohexadiene-derived fragment that is only partially resolved. The origin of the disorder is considered in light of the crystallographic packing interactions and compared with intermolecular contacts in the hydrogenated derivative, 9,10,11,12,13,14,15,16-octahydro-9,10[1′,4′]-benzenoanthracene (2), which is not disordered. The asymmetric unit of 2 contains one-half of a molecule situated about a two-fold symmetry axis. Both structures contain relatively long interannular bonds between the bridgehead carbons of the anthracene- and cyclohexadiene-derived fragments, in agreement with ab initio calculations and considered in terms of bond strain in the carbon framework of these compounds.

A DFT study on the mechanisms for the cycloaddition reactions between 1−aza-2-azoniaallene cations and acetylenes

The mechanisms of cycloaddition reactions between 1-aza-2-azoniaallene cations 1 and acetylenes 2 have been investigated using the global electrophilicity and nucleophilicity of the corresponding reactants as global reactivity indexes defined within the conceptual density functional theory. The reactivity and regioselectivity of these reactions were predicted by analysis of the energies, geometries, and electronic nature of the transition state structures. The theoretical results revealed that the reaction features a tandem process: an ionic 1,3-dipolar cycloaddition to produce the cycloadducts 3 H-pyrazolium salts 3 followed by a [1,2]-shift affording the thermodynamically more stable adducts 4 or 5. The mechanism of the cycloaddition reactions can be described as an asynchronous concerted pathway with reverse electron demand. The model reaction has also been investigated at the QCISD/6-31++G(d,p) and CCSD(T)/6-31++G(d,p)//B3LYP/6-31++G(d,p) levels as well as by the DFT. The polarizable continuum model, at the B3LYP/6-31++G(d,p) level of theory, was used to study solvent effects on all the studied reactions. In solvent dichloromethane, all the initial cycloadducts 3 were obtained via direct ionic process as the result of the solvent effect. The consecutive [1,2]-shift reaction, in which intermediates 3 are rearranged to the five-membered heterocycles 4/5, is proved to be a kinetically controlled reaction, and the regioselectivity can be modulated by varying the migrant. The LOL function and RDG function based on localized electron analysis were used to analysis the covalent bond and noncovalent interactions in order to unravel the mechanism of the title reactions.

ChemInform Abstract: Unprecedented Intramolecular [3 + 2] Cycloadditions of Azido‐Ketenimines and Azido‐Carbodiimides. Synthesis of Indolo[1,2‐a]quinazolines and Tetrazolo[5,1‐b]quinazolines.

AbstractA series of azido‐ketenimines (V) and azido‐carbodiimines (XII) is prepared.

Unprecedented intramolecular [3 + 2] cycloadditions of azido-ketenimines and azido-carbodiimides. Synthesis of indolo[1,2-a]quinazolines and tetrazolo[5,1-b]quinazolines

N-(2-azidomethyl)phenyl ketenimines and N-(2-azidomethyl)phenyl-N'-alkyl(aryl) carbodiimides undergo, under mild thermal conditions, intramolecular [3 + 2] cycloaddition reactions between the azido group and either the C=C or the distal C=N double bonds of the ketenimine and carbodiimide functions respectively. The reaction products are indolo[1,2-a]quinazolines and/or indolo[2,1-b]quinazolines in the case of azido-ketenimines, and tetrazolo[5,1-b]quinazolines in the case of azido-carbodiimides. The formation of the two classes of indoloquinazolines implies the ulterior dinitrogen extrusion from the non-isolated, putative [3 + 2] cycloadducts between the azide and ketenimine functions, whereas in the case of azido-carbodiimides the initial cycloadducts, tetrazoloquinazolines, were cleanly isolated and further converted into 2-aminoquinazolines by thermally induced dinitrogen extrusion.

Gas-phase Diels–Alder cycloaddition of benzyne to an aromatic hydrocarbon bay region: Groundwork for the selective solvent-free growth of armchair carbon nanotubes

Benzyne, generated in the gas phase by pyrolysis of phthalic anhydride, has been shown to undergo Diels–Alder cycloaddition to the bay region of perylene, a typical polycyclic aromatic hydrocarbon, under solvent-free conditions in a high temperature flow system. The initial cycloadduct spontaneously loses two hydrogen atoms, thereby rearomatizing to give naphtho[1,2,3,4- ghi]perylene. Analogous Diels–Alder cycloadditions of benzyne to bay regions on the rims of suitable cylindrical hydrocarbon templates, when followed by rearomatizations and thermal cyclodehydrogenations to join adjacent benzo groups, are proposed as key steps for a directed chemical synthesis of uniform diameter armchair carbon nanotubes.

ChemInform Abstract: Synthesis of Mesomeric Betaines Containing a Pyrrolo- or Imidazotriaziniumolate System and Their Cycloaddition with Acetylenic Dipolarophiles Leading to Triazocinone Derivatives.

Abstract A series of 2-substituted mesomeric betaines containing a cyclic azomethine imine unit were synthesized and their cycloaddition with acetylenic dipolarophiles were examined. Unexpectedly the cycloaddition of the betaines with electron-deficient dipolarophiles gave ring-expanding adducts having a triazocinone structure. With electron-rich dipolarophiles such as ynamines, the reactions proceeded more readily leading regioselectively to the same type of triazocinones in almost quantitative yields. In particular in the case of an imidazobetaine and an ynamine the cycloaddition took place at room temperature to afford the initial cycloadducts exclusively and in excellent yields. The isolated tricyclic precursors were found to rearrange quantitatively to the final products on heating to 60°C.

A novel and facile stereocontrolled synthetic method for polyhydro-quinolines and pyridopyridazines via a diene-transmissive Diels–Alder reaction involving inverse electron-demand hetero Diels–Alder cycloaddition of cross-conjugated azatrienes

Cross-conjugated azatrienes bearing an electron-withdrawing sulfonyl or benzoyl group on the nitrogen atom underwent, on heating or in the presence of a Lewis acid (TMSOTf), an initial inverse electron-demand hetero Diels–Alder reaction with electron-rich dienophiles (vinyl ether, vinyl thioether, and allenyl ether) to produce 1:1 cycloadducts with high endo selectivity. The initial cycloadducts thus obtained underwent a second Diels–Alder reaction stereoselectively on the newly formed diene unit with electron-deficient dienophiles to give the crossed bis-cycloadducts, octahydroquinolines, with high diastereo-π-facial selectivity. The N-sulfonylazatrienes tethering an ortho-cinnamyloxyphenyl dienophile at the triene terminal underwent an initial intramolecular hetero Diels–Alder reaction of the inverse electron-demand type. The subsequent second Diels–Alder reaction of the formed mono-cycloadducts completed the diene-transmissive hetero Diels–Alder protocol to give benzopyrano[3,4- c]quinolines in a highly stereoselective manner.

4 + 2] cycloadditions of N-alkenyl iminium ions: structurally complex heterocycles from a three-component Diels-Alder reaction sequence.

N-Alkenyl iminium ions serve as conduits to three-component [4 + 2] cycloaddition reactions accessing structurally and stereochemically diverse piperidine derivatives. These cationic 2-azadienes participate in endo- or exo-selective [4 + 2] cycloadditions with electron-rich and neutral alkene dienophiles to generate a tetrahydropyridinium ion as the initial cycloadduct. In situ nucleophilic addition to the cycloaddition-derived iminium ion completes the three-component coupling sequence and affords a versatile synthesis of structurally complex piperidines.

Preparation and Diels−Alder Chemistry of 4-Vinylimidazoles

Various 4-vinylimidazole derivatives have been prepared from the corresponding 4-iodoimidazoles or from urocanic acid. Several methods for the elaboration of these vinylimidazoles and their Diels-Alder reactions are reported. All of the vinylimidazoles prepared in the course of this study react with N-phenylmaleimide quite readily with mild thermal activation providing a single cycloadduct, in most cases the initial, nonaromatic adduct. With more electron rich substrates, there is a tendency for these initial cycloadducts to undergo aromatization, ene reaction, and oxidation although this can be circumvented to a large extent by the choice of reaction conditions. Limited reactions were observed with other dienophiles, providing the expected cycloadducts in most cases, although an abnormal adduct was obtained in one case with dimethyl acetylene dicarboxylate. These substrates also participate in regioselective Diels-Alder reactions with monoactivated dienophiles, but require fairly forcing conditions, thus only providing the aromatized cycloadducts in modest yields. An investigation of substituent effects at the 2-position of the imidazole moiety was undertaken, in which electron-donating and weakly electron-withdrawing substituents are tolerated. In addition, several substrates with terminally substituted vinyl moieties have been investigated.

Challenge To Detect 1,4-Zwitterions Spectroscopically in a Ketene−Alkene Reaction

The first example of direct spectroscopic detection of transient species, 1,4-zwitterions, generated in a ketene-alkene reaction is reported. Also, a striking result of the intervention of an unprecedented "1,4-zwitterion neutral dimer" is presented in a new mechanistic pathway; the ketene-alkene reaction gives the product cyclobutanone from the initial cycloadduct alpha-methyleneoxetane.

Photochemistry of 5-Phenyl-1-pentene in the Gas Phase

The photochemistry of the title compound has been studied in the gas phase using 254-nm irradiation. In addition to meta cycloadducts analogous to those observed in solution, population of S1(vib) in the gas phase gives several products, the relative amounts of which depend on quencher gas pressure but not on excitation wavelength. For example, in the absence of butane, the major photoproduct is compound 5. This product is formed by a [1,5] hydrogen shift in the primary photoproduct, compound 4. Compound 4 is an intramolecular meta cycloadduct that is generated in the gas phase with sufficient excess vibrational energy to undergo rearrangement unless quencher gas is present. Likewise, there is evidence that two other meta cycloadducts (2 and 3) are also formed with appreciable vibrational energy in the absence of a quencher gas. A unique intramolecular ortho cycloadduct is also formed from 1 but only within a narrow range of quencher gas pressures. This is a two-photon product, with the initial cycloadduct (11) ring opening to a cyclooctatriene (12) that photochemically closes to 6. The pressure dependence of this ortho cycloaddition may be due to a requirement for vibrational deactivation of 11 (Scheme 5) or a precursor species (Scheme 6). The overall chemistry is outlined in Scheme 7.

The Azomethine Ylide Strategy for β‐Lactam Synthesis. Azapenams and 1‐Azacephams.

AbstractFor Abstract see ChemInform Abstract in Full Text.

1,3-Dipolar Cycloaddition to the Fe−OC Fragment. 19. Synthesis and Properties of Fe(CO)2(PR3)(R1NC(R2)−C(R3)O) and Fe(CO)(Ph2PCH2CH2PPh2)(R1NC(R2)−C(R3)O) and Their Reactivity toward Dipolarophiles. First X-ray Crystal Structure of the Initial Bicyclo[2.2.1] Adduct

Complexes 7ak−an, 7bk,bm [(R1NC(R2)−C(R3)O)Fe(CO)2PR3 a: R1 = tBu, R2 = H, R3 = Ph; b: R1 = Me, R2 = Ph, R3 = Ph; k: R = OMe; l: R = Ph; m: R = Et; n: R = nPr], and 7ao,bo [(R1NC(R2)−C(R3)O)Fe(CO)Ph2PCH2CH2PPh2] have been prepared by substitution of CO in the respective tricarbonyl complexes 1a,b at room temperature. Various dynamic processes have been investigated by 13C and 31P NMR. On the IR time scale, complex 7bk exists in two distinct isomeric forms. Complexes 7 have been characterized by IR, UV−vis, and 1H, 13C, and 31P NMR spectroscopy. Surprisingly, in the 1,3-dipolar cycloaddition reaction of complexes 7 with the dipolarophiles dimethyl acetylenedicarboxylate (DMAD, v), methyl propynoate (w), phenyl acetylene (x), and p-methoxyphenyl-isothiocyanate (y) the bicyclic ferra[2.2.1] complexes 8, i.e., the initial cycloadducts, are stable and isolable. With dimethyl maleate (z) it is formed reversibly. Complexes 8 containing one phosphorus ligand are formed as two non-interconverting isomers. Th...