Polar reactions of acyclic conjugated bisallenes

• Reiner Stamm and
• Henning Hopf

Beilstein J. Org. Chem. 2013, 9, 36–48, doi:10.3762/bjoc.9.5

• and epoxides, hydrohalogenation (HCl, HBr addition), halogenation (Br2 and I2 addition), and [2 + 2] cycloaddition with chlorosulfonyl isocyanate. The resulting adducts were fully characterized by spectroscopic and analytical methods; they constitute interesting substrates for further organic

• conjugated dihalodienes and trienes. Finally, the reaction of 2 with chlorosulfonyl isocyanate provides a β-lactam derivative formed in a formal [2 + 2] cycloaddition via a zwitterionic intermediate. Taken together, these studies show that conjugated bisallenes [39], which are readily available by high

• intramolecular chlorine shift from its resonance structure 73 to provide the sulfenimide intermediate 74. Hydrolysis of the latter then provides product 75. In another heterocumulene addition Skattebøl and Boanhave studied the addition of dichloroketene and diphenylketene to 2 [38]. In both cases [2 + 2

Flow photochemistry: Old light through new windows

• Jonathan P. Knowles,
• Luke D. Elliott and
• Kevin I. Booker-Milburn

Beilstein J. Org. Chem. 2012, 8, 2025–2052, doi:10.3762/bjoc.8.229

• santonin. It is generally regarded that the systematic and ground-breaking investigations of Ciamician and Silber [3] paved the way for modern synthetic photochemistry. At the turn of the 20th century they described the first examples of now common reactions such as intramolecular [2 + 2] cycloaddition

• quantities of materials without the need for particularly specialist equipment. By using an inexpensive peristaltic pump, the [2 + 2] cycloaddition of maleimide (76) and n-hexyne was run continuously for 24 hours under optimised conditions for a custom-built Pyrex reactor (Scheme 25). This reaction produced

• product and recycling of the solution. A gas-liquid slug flow was also recently used for the [2 + 2] cycloaddition of a chiral cyclohexenone 89 with ethylene (Scheme 30) [83]. The reactor comprised of FEP tubing (1.0 mm i.d.) wrapped around a quartz immersion well with nine windings. The reaction was

The chemistry of bisallenes

• Henning Hopf and
• Georgios Markopoulos

Beilstein J. Org. Chem. 2012, 8, 1936–1998, doi:10.3762/bjoc.8.225

• product (Scheme 41), the reaction of 24 with either dichloro (170: R = Cl) or diphenylketene (170, R = phenyl) furnished largely the [2 + 2] cycloadducts 171. Only traces of the Diels–Alder adduct 172 are observed in this case. This example seems to be the only one in which the [2 + 4] and [2 + 2

• ] cycloaddition mode compete [123]. Occasionally highly alkylated bisallenes such as 24 also provide ene-addition products. If the double-bond dienophile is replaced by an activated triple-bond dienophile 173, another course of events is observed (Scheme 42) [21][124][125][126]. In this case the Diels–Alder step

Metal–ligand multiple bonds as frustrated Lewis pairs for C–H functionalization

• Matthew T. Whited

Beilstein J. Org. Chem. 2012, 8, 1554–1563, doi:10.3762/bjoc.8.177

•  5) [38]. This reaction is important for the hydroamination of alkynes by Cp2ZrX2 complexes, which proceeds through zirconium imido intermediates [39]. A similar [2 + 2] cycloaddition of symmetrical alkynes across a tungsten nitride is the initial step in Johnson's nitrile-alkyne cross metathesis

• silylenes [54][55], insertion of olefins into hydrosilylenes [56], and bimolecular redistribution of thiolates between ruthenium silyl and silylene complexes [57]. Reactivity that involves metal-ligand cooperation (in the sense described in this article) has been reported in the formal [2 + 2] cycloaddition

• ) imide [42]. Formal [2 + 2] cycloaddition of methyl isocyanate at a ruthenium silylene [58]. Oxygen-atom transfer from phenyl isocyanate to a cationic terminal borylene [60]. Coupling of a phosphorus ylide with an iridium methylene [62]. Reactions of (PNP)Ir═C(H)Ot-Bu with oxygen-containing

Synthesis and in silico screening of a library of β-carboline-containing compounds

• Kay M. Brummond,
• John R. Goodell,
• Matthew G. LaPorte,
• Lirong Wang and
• Xiang-Qun Xie

Beilstein J. Org. Chem. 2012, 8, 1048–1058, doi:10.3762/bjoc.8.117

• Tantillo [6] regarding the thermal [2 + 2] cycloaddition reaction suggest that the energy barrier for this reaction should not be effected by the presence of two radical stabilizing groups over one, it was postulated that this cycloaddition process was facilitated by exposure to incident light [7]. Thus

• , the mixtures of compounds 2 and 9 were reconstituted in CH2Cl2 and placed in front of two 6 W UV lamps for 16 h at rt to afford the desired cyclobutenes (Method B, Table 2, entries 5–8). During optimization of the conditions for the [2 + 2] cycloaddition reaction (Method A), it was found that reducing

• not be identified (see spectral data in Supporting Information File 1). Aryl-substituted alkynones 9{1,5}–9{1,8} were not available for the molybdenum-mediated cyclocarbonylation process due to competing [2 + 2] cycloaddition reactions (Table 2). The majority of these β-carboline-containing products

Facile isomerization of silyl enol ethers catalyzed by triflic imide and its application to one-pot isomerization–(2 + 2) cycloaddition

• Kazato Inanaga,
• Yu Ogawa,
• Yuuki Nagamoto,
• Akihiro Daigaku,
• Hidetoshi Tokuyama,
• Yoshiji Takemoto and
• Kiyosei Takasu

Beilstein J. Org. Chem. 2012, 8, 658–661, doi:10.3762/bjoc.8.73

• , Japan 10.3762/bjoc.8.73 Abstract A triflic imide (Tf2NH) catalyzed isomerization of kinetically favourable silyl enol ethers into thermodynamically stable ones was developed. We also demonstrated a one-pot catalytic reaction consisting of (2 + 2) cycloaddition and isomerization. In the reaction

• procedure would be required. In this communication, we describe isomerization of silyl enol ethers by an organocatalyst under mild conditions and its application to a one-pot catalytic reaction involving isomerization of silyl enol ethers and (2 + 2) cycloaddition. Results and Discussion During our research

• ) cycloaddition of silyl enol ethers with acrylates generating substituted cyclobutanes [10]. We are intrigued that the isomerization of silyl enol ethers and successive (2 + 2) cycloaddition could be promoted by Tf2NH in a one-pot reaction. When 1a was treated with Tf2NH (1 mol %) under the isomerization

Alkoxide-induced ring opening of bicyclic 2-vinylcyclobutanones: A convenient synthesis of 2-vinyl-substituted 3-cycloalkene-1-carboxylic acid esters

• Xiufang Ji,
• Zhiming Li,
• Quanrui Wang and
• Andreas Goeke

Beilstein J. Org. Chem. 2012, 8, 650–657, doi:10.3762/bjoc.8.72

• derivatives. Frequently employed ones include the thermal [2 + 2] cycloaddition of ketenes to alkenes and the polar addition of cyclopropyl ylides to carbonyls [1][2]. These methods generally allow regioselective as well as stereoselective syntheses of extensively substituted four-membered ring carbocycles

• ring opening of 7-methyl-7-vinylbicyclo[3.2.0]hept-2-en-6-one to a linear polyene ketone. Synthesis of vinyl or phenyl substituted cyclobutanones 4a–i. Ring opening of cyclobutanones 4 to afford products 5 or 6. Reaction of 4a with LDA. Plausible mechanism for ring opening of 4a. Formal [2 + 2

• ] cycloaddition of vinylketenes 2 with olefins.a Cleavage reaction of 4a with metal alkoxide.a Reaction of 4 with MeONa or t-BuOK.a Supporting Information Supporting Information File 61: Detailed experimental procedures. Supporting Information File 62: NMR spectral data for unknown compounds.

Amines as key building blocks in Pd-assisted multicomponent processes

• Didier Bouyssi,
• Nuno Monteiro and
• Geneviève Balme

Beilstein J. Org. Chem. 2011, 7, 1387–1406, doi:10.3762/bjoc.7.163

• elimination. This münchnone is in equilibrium with its ketene isomeric form 6, and a formal [2 + 2] cycloaddition with a second equivalent of imine generates the lactam (Scheme 3). The authors pointed out that the trapping of HCl by a sterically hindered base (NEtiPr2) is the key point in this methodology to

Recent developments in gold-catalyzed cycloaddition reactions

• Fernando López and
• José L. Mascareñas

Beilstein J. Org. Chem. 2011, 7, 1075–1094, doi:10.3762/bjoc.7.124

• allenic esters 42; compounds which evolve in situ in the presence of the same metal catalyst to give adducts resulting from formal (3 + 2) and/or (2 + 2) annulation processes (Scheme 23). In the presence of Ph3PAuCl/AgSbF6, the (2 + 2) cycloaddition is favored furnishing 43 (upper arrow) [85]. However

• , formally resulting from an internal (2 + 2) cycloaddition [87]. However, in 2009, Toste demonstrated that, when a gold catalyst such as Au1/AgSbF6, with a more readily donating phosphine ligand is employed, the alternative (3 + 2) cycloaddition leading to bicyclo[4.3.0]nonanes 58 is favored (Scheme 30) [99

• )-catalyzed intermolecular (2 + 2) cycloaddition of alkynes with alkenes [62]. Metal-catalyzed cycloaddition of alkynes tethered to cycloheptatriene [65]. Gold-catalyzed cycloaddition of functionalized ketoenynes: Synthesis of (+)-orientalol F [68]. Gold-catalyzed intermolecular cyclopropanation of enynes

Cationic gold(I) axially chiral biaryl bisphosphine complex-catalyzed atropselective synthesis of heterobiaryls

• Tetsuro Shibuya,
• Kyosuke Nakamura and
• Ken Tanaka

Beilstein J. Org. Chem. 2011, 7, 944–950, doi:10.3762/bjoc.7.105

• synthesis [1][2][3][4] has attracted significant interest due to its great utility in asymmetric catalysis and natural product synthesis. In 2004, three research groups, including ours, independently reported transition-metal catalyzed asymmetric [2 + 2 + 2] cycloaddition reactions to produce axially chiral

• -aryl-2-pyridones has already been achieved by rhodium catalyzed [2 + 2 + 2] cycloaddition [18], while the atropselective synthesis of 4-aryl-2-pyridones has not yet been realized. The application of this intramolecular hydroalkenylation reaction to the atropselective synthesis of 4-aryl-2-pyridones

Intraannular photoreactions in pseudo-geminally substituted [2.2]paracyclophanes

• Henning Hopf,
• Vitaly Raev and
• Peter G. Jones

Beilstein J. Org. Chem. 2011, 7, 658–667, doi:10.3762/bjoc.7.78

• of Z,Z-22 with a 1 kW halogen lamp in a Pyrex flask over 12 h (Scheme 8) gave only two [2 + 2] cycloaddition products: The hydrocarbons 23 and 24 in 3:5-ratio with a total yield of 70%. The isomers were separated by column chromatography and their structures established by NMR spectroscopy and single

Complete transfer of chirality in an intramolecular, thermal [2 + 2] cycloaddition of allene-ynes to form non-racemic spirooxindoles

• Kay M. Brummond and
• Joshua M. Osbourn

Beilstein J. Org. Chem. 2011, 7, 601–605, doi:10.3762/bjoc.7.70

• Kay M. Brummond Joshua M. Osbourn University of Pittsburgh, Department of Chemistry, Chevron Science Center, 219 Parkman Avenue, Pittsburgh, PA 15260, USA 10.3762/bjoc.7.70 Abstract A thermal [2 + 2] cycloaddition reaction of allene-ynes has been used to transform chiral non-racemic allenyl

• cyclobutene; allene; allenyloxindole; chiral lanthanide shift reagent; chiral transfer; Introduction The [2 + 2] cycloaddition reaction of allenes and alkynes provides rapid entry into synthetically challenging alkylidene cyclobutene ring systems. We, along with others, have demonstrated the intramolecular

• spirooxindoles for application to natural product synthesis [5][6][7]. Herein, we disclose preliminary results demonstrating a complete transfer of chiral information from a chiral non-racemic allene-yne to form an enantiomerically enriched spirooxindole in a [2 + 2] cycloaddition reaction. Findings This study

Synthesis of 5-(2-methoxy-1-naphthyl)- and 5-[2-(methoxymethyl)-1-naphthyl]-11H-benzo[b]fluorene as 2,2'-disubstituted 1,1'-binaphthyls via benzannulated enyne–allenes

• Yu-Hsuan Wang,
• Joshua F. Bailey,
• Jeffrey L. Petersen and
• Kung K. Wang

Beilstein J. Org. Chem. 2011, 7, 496–502, doi:10.3762/bjoc.7.58

• aromaticity to furnish 13a as proposed previously [5]. An intramolecular [2 + 2] cycloaddition reaction of 10a or a direct radical–radical coupling of 11a could account for the formation of 14a [5]. From 9b, 5-[2-(methoxymethyl)phenyl]-11H-benzo[b]fluorene 13b and the [2 + 2] cycloaddition adduct 14b were

Effects of anion complexation on the photoreactivity of bisureido- and bisthioureido-substituted dibenzobarrelene derivatives

• Heiko Ihmels and
• Jia Luo

Beilstein J. Org. Chem. 2011, 7, 278–289, doi:10.3762/bjoc.7.37

• resulting in regio- or stereoselective product formation. Indeed, this approach has been employed to carry out stereoselective photoreactions, for example [2 + 2] cycloaddition [4], [4 + 4] photocycloaddition [5], Norrish–Yang cyclization [6], and [6π] photocyclization [7]. Asymmetric photoreactions have

• is multiplicity-dependent: The direct irradiation of 1a leads to the dibenzocyclooctatetraene 3a in a singlet reaction that occurs via an initial [2 + 2] cycloaddition followed by a [4 + 2] retro-Diels–Alder reaction [27][28][29][30]. In the presence of a triplet sensitizer, e.g., acetone or

Formation of macrocyclic lactones in the Paternò–Büchi dimerization reaction

• Junya Arimura,
• Tsutomu Mizuta,
• Yoshikazu Hiraga and
• Manabu Abe

Beilstein J. Org. Chem. 2011, 7, 265–269, doi:10.3762/bjoc.7.35

• : furans; macrocyclic lactone; oxetane; Paternò–Büchi reaction; photochemical reaction; Findings Photochemical [2 + 2] cycloaddition reaction of alkenes with carbonyls, so-called Paternò–Büchi reaction [1][2][3][4][5][6][7][8][9][10][11][12][13], is one of the most efficient methods for preparing

Reciprocal polyhedra and the Euler relationship: cage hydrocarbons, C_nH_n and closo-boranes [B_xH_x]²⁻

• Michael J. McGlinchey and
• Henning Hopf

Beilstein J. Org. Chem. 2011, 7, 222–233, doi:10.3762/bjoc.7.30

• expected it to be readily available by photolytic [2 + 2] cycloaddition of hypostrophene, 34, or by extrusion of nitrogen from either 35 or 36, as in Scheme 6 [17][18][19][20]; surprisingly, all these routes were found to be ineffective. Success was finally achieved via ring contraction of a

• manipulation led to the iodo-tosylate 39 which, in the presence of base, generated the homohypostrophene, 40; [2 + 2] cycloaddition then furnished the homopentaprismanone 41. Introduction of a bridge head bromine (with the intent of carrying out a Favorskii ring contraction) proved to be impossible. Instead it

Heavy atom effects in the Paternò–Büchi reaction of pyrimidine derivatives with 4,4’-disubstituted benzophenones

• Feng-Feng Kong,
• Jian-Bo Wang and
• Qin-Hua Song

Beilstein J. Org. Chem. 2011, 7, 113–118, doi:10.3762/bjoc.7.16

• ; Introduction The regio- and stereoselectivity in the Paternò–Büchi reaction, which is a photochemical [2 + 2] cycloaddition of a carbonyl compound with an olefin, has been extensively studied [1][2][3][4]. The ene–carbonyl photocycloaddition generally proceeds through attack of the excited carbonyl state

Recent advances in the development of alkyne metathesis catalysts

• Xian Wu and
• Matthias Tamm

Beilstein J. Org. Chem. 2011, 7, 82–93, doi:10.3762/bjoc.7.12

• the reaction of 5 with an excess of 3-hexyne confirmed that the [2 + 2]-cycloaddition (Katz) mechanism is operative [73][74]. The prototype 5 of our new catalyst system was used for the ACM of 1-phenylpropyne (7) and was shown to be significantly more active than the classic Schrock alkylidyne complex

• formation of deprotonated, inactive metallacyclobutadienes [96]. Hence, future efforts should also re-address this issue, e. g., by adjusting the properties of the metallacyclobutadiene key intermediates [97] in order to prevent their degeneration and therefore ineffectiveness in undergoing the Katz [2 + 2

• ]cycloaddition/cycloreversion mechanism (Scheme 1). Alkylidyne complexes 9 and 10. Alkyne metathesis based on the Katz mechanism. Reaction patterns of alkyne metathesis. Typical examples from traditional catalyst systems. Ligand synthesis and catalyst design. Catalysts synthesis using high- and low-oxidation

Stereoselectivity of supported alkene metathesis catalysts: a goal and a tool to characterize active sites

• Christophe Copéret

Beilstein J. Org. Chem. 2011, 7, 13–21, doi:10.3762/bjoc.7.3

• steps: alkene coordination, [2 + 2]-cycloaddition generating metallacyclobutanes and the corresponding reverse steps, i.e., cycloreversion and alkene dissociation (Scheme 1b). The approach of an alkene of a given configuration towards a metal–alkylidene intermediate in a given configuration will

Synthesis of 2a,8b-Dihydrocyclobuta[a]naphthalene-3,4-diones

• Kerstin Schmidt and
• Paul Margaretha

Beilstein J. Org. Chem. 2010, 6, No. 76, doi:10.3762/bjoc.6.76

• ; quinone monoacetals; Introduction The behaviour of excited 1,2- and 1,4-quinones towards ground-state molecules differs greatly. Whereas the former typically react via H-abstraction by an excited carbonyl group [1], the latter smoothly undergo [2 + 2] cycloaddition to alkenes to afford cyclobutane-type

Acid catalyzed cyclodimerization of 2,2-bis(trifluoromethyl)-4-alkoxy-oxetanes and -thietanes. Synthesis of 2,2,6,6-tetrakis(trifluoromethyl)-4,8-dialkoxy-1,5-dioxocanes and 3,3,7,7-tetrakis(trifluoromethyl)-9-oxa-2,6-dithia-bicyclo[3.3.1]nonane

• Viacheslav A. Petrov and
• Will Marshall

Beilstein J. Org. Chem. 2010, 6, No. 46, doi:10.3762/bjoc.6.46

• are readily available materials, prepared by [2 + 2] cycloaddition of vinyl ethers with hexafluoroacetone [1][2][3] or hexafluorothioacetone [4][5], respectively. Although both groups of compounds have been known for over 40 years, reports on their chemical transformations are limited. Among the

A thermally-induced, tandem [3,3]-sigmatropic rearrangement/[2 + 2] cycloaddition approach to carbocyclic spirooxindoles

• Kay M. Brummond and
• Joshua M. Osbourn

Beilstein J. Org. Chem. 2010, 6, No. 33, doi:10.3762/bjoc.6.33

• Kay M. Brummond Joshua M. Osbourn Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, U.S.A 10.3762/bjoc.6.33 Abstract The synthesis of C3-carbocyclic spirooxindoles was realized by way of an intramolecular [2 + 2] cycloaddition reaction between a vinylidene indolin

• observed, but are likely intermediates of an infrequently encountered thermal [3,3]-sigmatropic rearrangement of a propargylic acetate. Keywords: allene; propargylic acetate; spirooxindole; thermal [2 + 2] cycloaddition; thermal [3,3]-sigmatropic rearrangement; vinylidene indolin-2-one; Introduction

• discovery of new biological probes and pharmaceuticals [8]. We recently disclosed a thermal [2 + 2] cycloaddition reaction of allene-ynes to provide a variety of alkylidene cyclobutenes in good yields [9][10][11][12][13][14][15][16][17][18]. Notable features of this reaction were the stability of the