(Bio)isosteres of ortho- and meta-substituted benzenes

• H. Erik Diepers and
• Johannes C. L. Walker

Beilstein J. Org. Chem. 2024, 20, 859–890, doi:10.3762/bjoc.20.78

• was also employed for the synthesis of 1,2,4-BCHs [35]. Heterocycle-substituted BCHs were accessed by [2π + 2σ] photocycloaddition by Glorius and co-workers [79] and Houk, Glorius and co-workers [80]. This reactivity was enabled by triplet energy transfer catalysis activating the heterocyclic

Strategies to access the [5-8] bicyclic core encountered in the sesquiterpene, diterpene and sesterterpene series

• Cécile Alleman,
• Charlène Gadais,
• Laurent Legentil and
• François-Hugues Porée

Beilstein J. Org. Chem. 2023, 19, 245–281, doi:10.3762/bjoc.19.23

A comprehensive review of flow chemistry techniques tailored to the flavours and fragrances industries

• Guido Gambacorta,
• James S. Sharley and
• Ian R. Baxendale

Beilstein J. Org. Chem. 2021, 17, 1181–1312, doi:10.3762/bjoc.17.90

Insight into functionalized-macrocycles-guided supramolecular photocatalysis

• Minzan Zuo,
• Krishnasamy Velmurugan,
• Kaiya Wang,
• Xueqi Tian and
• Xiao-Yu Hu

Beilstein J. Org. Chem. 2021, 17, 139–155, doi:10.3762/bjoc.17.15

• of functional photocatalytic systems [16][17]. The diversity of crown ether-based photocatalytic reactions is relatively low, involving mainly photocycloaddition reactions. Saltiel and co-workers reported the regiospecific intermolecular [2 + 2]-photocycloaddition based on a supramolecular assembly

• issues in industrial engineering. Chemical structures of representative macrocycles. Ba2+-induced intermolecular [2 + 2]-photocycloaddition of crown ether-functionalized substrates 1 and 2 to form cycloadduct 3. Republished with permission of The Royal Society of Chemistry from [18] (“Supramolecular

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

• energy charge transfer state that can be photoexcited to generate singlet intermediate 89*. Subsequent enantioselective photocycloaddition with 88 via diradical 90 gives iminium ion intermediate 91, which after hydrolysis affords the desired cyclobutane products 92 in excellent yields and good

• that is trapped intermolecularly by N-hydroxycarbamates 152 (Scheme 21b) [71]. Bach et al. has recently reported a bifunctional catalyst 154, which contains both a photoactive thioxanthone unit and a CPA (Scheme 22) [72]. They have applied this photocatalyst to the [2 + 2] photocycloaddition of

• ] photocycloaddition of quinolone 170. The proposed mechanism proceeds via hydrogen-bonded complex 171, which is sensitised by the pendant benzophenone to its triplet excited state 171*. The following cycloaddition completes the cycle and generates the desired cyclobutane product 173 in excellent conversion but poor

Chan–Evans–Lam N1-(het)arylation and N1-alkеnylation of 4-fluoroalkylpyrimidin-2(1H)-ones

• Viktor M. Tkachuk,
• Oleh O. Lukianov,
• Mykhailo V. Vovk,
• Isabelle Gillaizeau and
• Volodymyr A. Sukach

Beilstein J. Org. Chem. 2020, 16, 2304–2313, doi:10.3762/bjoc.16.191

• analogous enamides [57], the facile [2 + 2] photocycloaddition process likely occurred due to favorable orientation of the interacting molecules and close contacts between alkene carbon atoms in the crystal structure. Consequently, the synthesis of product 5c was performed in darkness. N1-Styryl-substituted

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

• presumably renders an ionic [2 + 2] cycloaddition pathway competitive with the Diels–Alder reaction. The few reported examples of cyclobutene formation from alkynes and unactivated 1,3-dienes include the sensitized photocycloaddition of phenylacetylene and DMBD [30] and the gold(I)-catalyzed reaction of

Recent synthesis of thietanes

• Jiaxi Xu

Beilstein J. Org. Chem. 2020, 16, 1357–1410, doi:10.3762/bjoc.16.116

• -donating substituents present in the allenes enhanced the overall reaction rate. There was little effect of the solvent polarity on the reaction rate. The formation of thietanes involved the excited triplet thiones and the π-bond of allenes [66]. In 1984, Bos and co-workers realized the photocycloaddition

• photocycloaddition of ring-substituted cyclic dithiosuccinimides 223 with 2,3-dimethyl-2-butene (215a), affording a series of spirothietanes 245 [72] (Scheme 47). In 1986, Coyle and Rapley reported that the photochemical cycloaddition reactions of N-methylthiophthalimide (237a) and N-methyldithiophthalimide (225

• were obtained via a ring cleavage of the thietanes, that had formed in the [2 + 2] photocycloaddition of the thiocarbonyl moiety and the olefin. However, the reaction of 1,3,3-trimethylindoline-2-thione (300) and isobutene (215c) afforded the corresponding spiroindoline-thietane derivative 301 [80

Synthesis of esters of diaminotruxillic bis-amino acids by Pd-mediated photocycloaddition of analogs of the Kaede protein chromophore

• Esteban P. Urriolabeitia,
• Pablo Sánchez,
• Alexandra Pop,
• Cristian Silvestru,
• Eduardo Laga,
• Ana I. Jiménez and
• Carlos Cativiela

Beilstein J. Org. Chem. 2020, 16, 1111–1123, doi:10.3762/bjoc.16.98

• -face arrangement, which is optimal for their [2 + 2] photocycloaddition. Irradiation of dimers 3 in CH2Cl2 solution with blue light (465 nm) promoted the chemo- and stereoselective [2 + 2] photocycloaddition of the exocyclic C=C bonds and the formation of cyclobutane-containing ortho-palladated

• ; photocycloaddition; Introduction Truxillic acid derivatives (Figure 1a) are a special family of cyclobutanes that have been known since 1888 and show properties of high interest [1]. In this respect, their pharmacological activity has prompted extensive research that has focused on both their synthesis and the

• date [16][17][18]. The properties outlined above highlight the importance of the wide scope of applications of this set of cyclobutanes. Despite their importance, access to most of these compounds mainly relies in the well-known [2 + 2] photocycloaddition of olefins [19]. Unfortunately, the synthesis

Reversible photoswitching of the DNA-binding properties of styrylquinolizinium derivatives through photochromic [2 + 2] cycloaddition and cycloreversion

• Sarah Kölsch,
• Heiko Ihmels,
• Jochen Mattay,
• Norbert Sewald and
• Brian O. Patrick

Beilstein J. Org. Chem. 2020, 16, 111–124, doi:10.3762/bjoc.16.13

• methoxy-substituted styrylquinolizinium derivatives form the corresponding syn head-to-tail cyclobutanes in a selective [2 + 2] photocycloaddition, as revealed by X-ray diffraction analysis of the reaction products. These photodimers bind to DNA only weakly by outside-edge association, but they release

• deactivation of a stilbene tyrosine kinase inhibitor by a [2 + 2] photocycloaddition [59]. As the quinolizinium ion has been established as a versatile platform for the development of DNA intercalators [60], we identified styryl-substituted quinolizinium derivatives as a promising basis for the search for

• photoswitchable DNA binders based on the photocycloaddition–photocycloreversion equilibrium. In fact, some selected styrylquinolizinium derivatives have already been shown to bind to DNA [61][62][63][64][65][66][67], however, their photocycloaddition reaction and the propensity of the corresponding photodimers to

Spiro annulation of cage polycycles via Grignard reaction and ring-closing metathesis as key steps

• Sambasivarao Kotha,
• Mohammad Saifuddin,
• Rashid Ali and
• Gaddamedi Sreevani

Beilstein J. Org. Chem. 2015, 11, 1367–1372, doi:10.3762/bjoc.11.147

• Claisen rearrangement and RCM as key steps [21][30]. Here, we have prepared the cage dione 10 by the known route involving two atom-economic protocols such as Diels–Alder reaction and [2 + 2] photocycloaddition [42][43][44][45] (Scheme 1). Later, the hexacyclic cage dione 10 was subjected to a Grignard

Selected synthetic strategies to cyclophanes

• Sambasivarao Kotha,
• Mukesh E. Shirbhate and
• Gopalkrushna T. Waghule

Beilstein J. Org. Chem. 2015, 11, 1274–1331, doi:10.3762/bjoc.11.142

• treated with LAH to generate diol 266 (72–92%). The dehydration of diol 266 with pyridinium p-toluenesulfonate in benzene gave diolefin 267. [2 + 2] Photocycloaddition of diolefin 267 was carried out by irradiation with a 400 W high-pressure Hg lamp (Pyrex filter) in benzene for 26–92 h. After evaporation

Synthesis of carbohydrate-scaffolded thymine glycoconjugates to organize multivalency

• Anna K. Ciuk and
• Thisbe K. Lindhorst

Beilstein J. Org. Chem. 2015, 11, 668–674, doi:10.3762/bjoc.11.75

• dimerized by [2 + 2] photocycloaddition. Thus, thymine functions as a control element that allows to restrict the conformational flexibility of the scaffolded sugar ligands and thus to “organize” multivalency. With this work we add a parameter to multivalency studies additional to valency. Keywords: [2 + 2

• ] photocycloaddition; carbohydrate scaffolds; multivalency; thymine glycoconjugates; Introduction Multivalency of molecular interactions is a fundamental principle in carbohydrate recognition. It influences the avidity and specificity of carbohydrate–protein interactions as well as it enables supramolecular changes

• appropriate carbohydrate conjugation the same molecular architecture could be applied for organization of a divalent glycoconjugate (Figure 1B). We selected thymine as a photocontrollable element as it can undergo [2 + 2] photocycloaddition upon irradiation with light of hν ≥ 290 nm [5][9][10]. Furthermore

Articulated rods – a novel class of molecular rods based on oligospiroketals (OSK)

• Pablo Wessig,
• Roswitha Merkel and
• Peter Müller

Beilstein J. Org. Chem. 2015, 11, 74–84, doi:10.3762/bjoc.11.11

• distance dependent change in the fluorescence spectrum. Furthermore, we were interested in functional groups in the terminal positions of the ARs enabling reversible switching between the two states. This was supposed to be accomplished both by [2 + 2] photocycloaddition of derivatives of cinnamic acid and

• [4 + 4] photocycloaddition of anthracene derivatives. At first we pursued a sequential approach by introducing the pyrene-1-ylacetyl and the cinnamoyl moieties in a two-step sequence in 4-hydroxypiperidine with subsequent oxidation to give piperidin-4-ones 27a,b. An appropriate anthracene derivative

Design and synthesis of novel bis-annulated caged polycycles via ring-closing metathesis: pushpakenediol

• Sambasivarao Kotha and
• Mirtunjay Kumar Dipak

Beilstein J. Org. Chem. 2014, 10, 2664–2670, doi:10.3762/bjoc.10.280

• ] cycloaddition strategies, which involve the DA reaction of 2,5-dialkyl-1,4-benzoquinone 10 and 1,3-cyclopentadiene (9) followed by the formation of the cyclobutane ring through a [2 + 2] photocycloaddition reaction (Figure 2). Later on, one can introduce two allyl groups by using traditional carbonyl chemistry

Photochemical approach to functionalized benzobicyclo[3.2.1]octene structures via fused oxazoline derivatives from 4- and 5-(o-vinylstyryl)oxazoles

• Ivana Šagud,
• Simona Božić,
• Željko Marinić and
• Marija Šindler-Kulyk

Beilstein J. Org. Chem. 2014, 10, 2222–2229, doi:10.3762/bjoc.10.230

• )oxazole small quantities of electrocyclization product, 4-(1,2-dihydronaphthalen-2-yl)oxazole, are isolated and spectroscopically characterized. Keywords: bicyclo[3.2.1]octane; intramolecular photocycloaddition; oxazole; styryl; vinyl; Introduction The bicyclo[3.2.1]octane skeleton is the basic

• ][23][24][25]. There are examples of oxazole photochemical intermolecular cycloadditions [26][27][28][29][30][31][32], but to the best of our knowledge, there are no examples of intramolecular photocycloaddition. We describe herein, the synthesis of new 4- and 5-(2-vinylstyryl)oxazoles (1, 2) and their

• intramolecular photocycloaddition to diverse fused tetracyclic oxazoline compounds which further spontaneously or during the work-up procedure hydrolyze to benzobicyclo[3.2.1]octenone derivatives. This is a new method for the synthesis of functionalized benzobicyclo[3.2.1]octenes. Results and Discussion cis

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

• competitive [29]. In a similar [2 + 2] photocycloaddition (Scheme 5), the authors demonstrated that by extending the chain length by one carbon, the 4,6-fused ring system 14 could be formed, albeit with some formation of regioisomer 15. In this instance the batch reaction gave slightly better selectivity than

• productivity of the Vycor reactor illustrates the importance of glassware choice for UV transmission. The N-pentenyl substituted dimethyl maleimide 78 underwent a [5 + 2] photocycloaddition to the corresponding azepine 79 with a productivity of 39 mmol/h. The same reactor was also used to optimise the [5 + 2

Fine-tuning alkyne cycloadditions: Insights into photochemistry responsible for the double-strand DNA cleavage via structural perturbations in diaryl alkyne conjugates

• Wang-Yong Yang,
• Samantha A. Marrone,
• Nalisha Minors,
• Diego A. R. Zorio and
• Igor V. Alabugin

Beilstein J. Org. Chem. 2011, 7, 813–823, doi:10.3762/bjoc.7.93

• cytotoxicity) value of 1.49 × 10−7 M. Keywords: cancer cell proliferation assay; DNA alkylation; lysine conjugate; photocycloaddition; photo-DNA cleavage; plasmid relaxation assay; triplet excitation; Introduction Triggering chemical processes with light offers numerous practical advantages. Not only does

• alkylation remains a likely origin of the photodamaging ability of such alkynes. Such reactivity is consistent with the above-mentioned ability of alkynes to act as electrophilic alkylating agents toward electron-rich π-systems observed in triplet photocycloaddition of TFP-substituted diaryl acetylenes [53

• ]. The mechanism of triplet photocycloaddition involves a sequence of radical closures initiated by the formation of a triplet 1,4-diradical via the reaction of 1,4-CHD and the alkyne π,π*-triplet state. Although several plausible mechanistic pathways converge at the same homoquadricyclane product in

The arene–alkene photocycloaddition

• Ursula Streit and
• Christian G. Bochet

Beilstein J. Org. Chem. 2011, 7, 525–542, doi:10.3762/bjoc.7.61

• Ursula Streit Christian G. Bochet Department of Chemistry, University of Fribourg, Chemin du Musée 9, CH-1700 Fribourg, Switzerland 10.3762/bjoc.7.61 Abstract In the presence of an alkene, three different modes of photocycloaddition with benzene derivatives can occur; the [2 + 2] or ortho, the [3

• + 2] or meta, and the [4 + 2] or para photocycloaddition. This short review aims to demonstrate the synthetic power of these photocycloadditions. Keywords: benzene derivatives; cycloadditions; Diels–Alder; photochemistry; Introduction Photocycloadditions occur in a variety of modes [1]. The best

• known representatives are undoubtedly the [2 + 2] photocycloaddition, forming either cyclobutanes or four-membered heterocycles (as in the Paternò–Büchi reaction), whilst excited-state [4 + 4] cycloadditions can also occur to afford cyclooctadiene compounds. On the other hand, the well-known thermal [4

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

Photocycloaddition of aromatic and aliphatic aldehydes to isoxazoles: Cycloaddition reactivity and stability studies

• Axel G. Griesbeck,
• Marco Franke,
• Jörg Neudörfl and
• Hidehiro Kotaka

Beilstein J. Org. Chem. 2011, 7, 127–134, doi:10.3762/bjoc.7.18

• ; photochemistry; Introduction Photochemical [2 + 2] cycloadditions are among the most efficient photoreactions and are used in numerous synthetic applications due to the generation of highly reactive four-membered rings. An important example is the photocycloaddition of electronically excited carbonyl compounds

• of this reaction [6]. Previous publications have clearly demonstrated the versatility of the Paternò–Büchi reaction in various synthetic applications which gives rise to a multiplicity of different products. The photocycloaddition of furans to carbonyl compounds affords the corresponding β-hydroxy

• corresponding 1,3-diketones [13] or deaminated to yield Michael systems [14]. Thus, isoxazoles also appear to be important substrates for carbonyl–ene photocycloaddition due to possible applications in ring-opening transformations. Results and Discussion Synthesis of the isoxazole substrates The substrates

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

• -alkylthio-1-siloxy-2-oxatetramethylene 1,4-diradical in competition with the bond rotation was proposed [14]. Griesbeck et al. [15] observed substantial 2H-magnetic isotope effects on the diastereoselectivity of triplet photocycloaddition reactions. Weaker isotope effects on the diastereoselectivity were

Photocycloadditions and photorearrangements

• Axel G. Griesbeck

Beilstein J. Org. Chem. 2011, 7, 111–112, doi:10.3762/bjoc.7.15

• transfer modes. Photocycloaddition reactions can be performed in numerous ways using unsaturated substrates such as carbonyl compounds, Michael acceptors, monoalkenes, polyenes, or aromatic substrates leading to complex products that can be used in subsequent transformations. The carbonyl-ene

• photocycloaddition, for example, is an important route to oxetanes, products that have recently gained increasing attention as building blocks in organic synthesis as well as in materials science [1]. Photochemical rearrangements are impressive reactions with regard to the generation of complexity: 1,2- and 1,3-acyl

Total synthesis of (±)-coerulescine and (±)-horsfiline

• Mukund G. Kulkarni,
• Attrimuni P. Dhondge,
• Sanjay W. Chavhan,
• Ajit S. Borhade,
• Yunnus B. Shaikh,
• Deekshaputra R. Birhade,
• Mayur P. Desai and
• Nagorao R. Dhatrak

Beilstein J. Org. Chem. 2010, 6, 876–879, doi:10.3762/bjoc.6.103

• [31], dimethyldioxirane (DMDO) mediated oxidation [32], and by tandem intramolecular photocycloaddition–retro-Mannich reaction [33]. The Wittig olefination–Claisen rearrangement protocol [35] provides a ready access to 4-pentenals, which have served as versatile intermediates for the synthesis of a

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

• . In the same experiment with 1c, the MeO-group apparently tends to increase the efficiency in photodimerization vs mixed photocycloaddition, otherwise there is no obvious explanation for this result. Experimental 1. General. Acetals 1 were synthesized according to [8]. Both 1b, m.p. 60–62 °C, and 1c