Recent developments in enantioselective photocatalysis

• Callum Prentice,
• James Morrisson,
• Andrew D. Smith and
• Eli Zysman-Colman

Beilstein J. Org. Chem. 2020, 16, 2363–2441, doi:10.3762/bjoc.16.197

• the [2 + 2] photocycloaddition of 2-pyridones 186 and acetylenedicarboxylates 187 catalysed by ent-183 to give cyclobutenes 188 (Scheme 28b) [82]. Another intermolecular reaction was later developed, this time using catalyst 185 for the [2 + 2] photocycloaddition of quinolones 189 and electron

Reactions of 3-aryl-1-(trifluoromethyl)prop-2-yn-1-iminium salts with 1,3-dienes and styrenes

• Thomas Schneider,
• Michael Keim,
• Bianca Seitz and
• Gerhard Maas

Beilstein J. Org. Chem. 2020, 16, 2064–2072, doi:10.3762/bjoc.16.173

• concerted conrotatory process [47][48] would create a strained cis,trans-dihydrobenzo[8]annulene ring system. Cyclobutenes resulting from a [2 + 2] cycloaddition of electrophilic alkynes and alkenes under moderate thermal conditions have been isolated also from the reaction of CF3-free propyniminium salts

Aldehydes as powerful initiators for photochemical transformations

• Maria A. Theodoropoulou,
• Nikolaos F. Nikitas and
• Christoforos G. Kokotos

Beilstein J. Org. Chem. 2020, 16, 833–857, doi:10.3762/bjoc.16.76

• photocatalytically active species can be initiated by the reaction mixture, containing traces of different benzaldehyde photolysis products together with benzaldehyde. In 2020, Landais and co-workers developed a photosensitized sulfonylcyanation of chiral cyclobutenes [61]. Although cyclobutanes are substrates of

• significant pharmaceutical use, the unsaturated analogs, cyclobutenes, are characterized by a poor reactivity of the π-bond, restricting the access to chiral cyclobutanes. This research group worked on a photocatalyzed addition of an electrophilic sulfonyl radical and a cyanide group across the π-bond of a

• chiral cyclobutene 180, providing highly functionalized cyclobutanes 182 and an access to the enantioenriched cyanosulfones 183, resulting from cyclobutane ring opening, or the new tetrasubstituted cyclobutanes 184 (Scheme 42). The cyclobutenes 180 were subjected to a photocatalyzed radical addition, and

Recent advances in Cu-catalyzed C(sp³)–Si and C(sp³)–B bond formation

• Balaram S. Takale,
• Ruchita R. Thakore,
• Elham Etemadi-Davan and
• Bruce H. Lipshutz

Beilstein J. Org. Chem. 2020, 16, 691–737, doi:10.3762/bjoc.16.67

• the details of the mechanism for the hydrofunctionalization of internal alkenes and vinyl arenes (Scheme 55) [100]. Unlike acyclic alkenes, Tortosa et al. utilized cyclobutenes as well as bicyclic cyclobutenes as educts using (R)-DM-Segphos for Cu-catalyzed enantioselective borylation, affording

Selective ring-opening metathesis polymerization (ROMP) of cyclobutenes. Unsymmetrical ladderphane containing polycyclobutene and polynorbornene strands

• Yuan-Zhen Ke,
• Shou-Ling Huang,
• Guoqiao Lai and
• Tien-Yau Luh

Beilstein J. Org. Chem. 2019, 15, 44–51, doi:10.3762/bjoc.15.4

• in polymer synthesis [24]. Since the first living ROMP methods for cyclobutenes were reported in 1992 [25], cyclobutene-containing block copolymers are well documented [26][27][28][29][30][31][32][33][34]. Alternating cyclobutene–cyclohexene copolymers have been synthesized by ROMP of the

• , therefore, the possibility for similar intramolecular metathesis cyclopolymerization might take place to form intermediate 16 for further transformations. However, no such reaction was observed in this study. Presumably, the 6-catalyzed metathesis reactivity of cyclobutenes would be much higher than that of

A tutorial review of stereoretentive olefin metathesis based on ruthenium dithiolate catalysts

• Daniel S. Müller,
• Olivier Baslé and
• Marc Mauduit

Beilstein J. Org. Chem. 2018, 14, 2999–3010, doi:10.3762/bjoc.14.279

• catalyst loading Scheme 2b) [2]. Allylic alcohol (7) reacted cleanly with norbornene (1), albeit with lower stereoretention (8; 88:12 Z/E) [17]. Cyclobutenes (e.g., 9) and cyclopropenes also delivered the corresponding products with good yields and excellent selectivity (Scheme 2b) [17]. It should also be

Synthesis of nanodiamond derivatives carrying amino functions and quantification by a modified Kaiser test

• Gerald Jarre,
• Steffen Heyer,
• Elisabeth Memmel,
• Thomas Meinhardt and
• Anke Krueger

Beilstein J. Org. Chem. 2014, 10, 2729–2737, doi:10.3762/bjoc.10.288

• depicted in Scheme 1. Besides the novel functionalization methods using pyrazines and cyclobutenes (see below) several nanodiamond conjugates were prepared for comparison similar to already reported procedures. It is known that pyrazine derivatives react with fullerenes in a cycloaddition reaction [13

• Diels–Alder reaction were based on the 1,4-elimination from suitable precursors. This step requires reagents such as potassium iodide and 18-crown-6 and hence the removal of the resulting side products [11]. Another approach to ortho-quinodimethanes makes use of the thermal ring opening of cyclobutenes

• obvious that the thermal ring opening of suitable cyclobutenes is another valuable addition to the portfolio of cycloaddition related surface reactions of nanodiamond. It allows the efficient immobilization of quite complex organic moieties without the use of auxiliaries or further reagents. No

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

• -functionalised poly-L-lysine with alkyne tethered glycol-dendrons to form cyclobutenes [84]. The reactions were performed on a sub-millimolar scale, but the precise control over reaction conditions with the flow apparatus allowed for high yields of the complex dendronic products. Nettekoven et al. [85] trialled

The chemistry of bisallenes

• Henning Hopf and
• Georgios Markopoulos

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

• various 1,2-disubstituted cyclobutenes do not add to 2 [122]. To investigate the stereochemistry of addition both 1,2,4,5-heptatetraene (161) [122] and the erythro-bisallene 163 [116] were subjected to Diels–Alder additions (Scheme 39) with maleic anhydride (151) and N-phenylmaleimide (154), respectively

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

• , 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

Gold-catalyzed propargylic substitutions: Scope and synthetic developments

• Olivier Debleds,
• Eric Gayon,
• Emmanuel Vrancken and
• Jean-Marc Campagne

Beilstein J. Org. Chem. 2011, 7, 866–877, doi:10.3762/bjoc.7.99

• been described by Fairlamb [82]. We were also able, from compound 2, to develop a 1,5-enyne metathesis that leads to functionalized cyclobutenes 38 (Scheme 20) [83], which was subsequently nicely illustrated by Goess in a total synthesis of grandisol [84]. From isoxazolines 23, we were also able to

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

• % (isolated yield) of mixed photocycloadducts 2. Careful acidic hydrolysis of the acetal function of 2 gives the title compounds 3, the overall sequence representing a first approach to a (formal) [2 + 2] photocycloadduct of a 1,2-naphthoquinone to an alkyne. Keywords: cyclobutenes; photocycloaddition

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

• 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

• resulting alkylidene cyclobutenes, the regioselectivity for reaction of the distal double bond of the allene, and ready cyclization of substrates possessing heteroatom tethers to give heterocyclic structures (Scheme 1) [19][20][21][22][23][24][25][26]. Inspired by the unique skeleton of welwitindolinone A

Allylsilanes in the synthesis of three to seven membered rings: the silylcuprate strategy

• Asunción Barbero,
• Francisco J. Pulido and
• M. Carmen Sañudo

Beilstein J. Org. Chem. 2007, 3, No. 16, doi:10.1186/1860-5397-3-16

• substituted cyclobutenes after cyclization of the vinylsilane or vinylstannane intermediate. [23] Cyclization of the corresponding vinylsilanes gave poor results of no synthetic utility, however the vinylstannane strategy results in formation of 1- and 3-substituted cyclobutenes 42 and 43 in good yield