Synthesis, crystal structures and properties of carbazole-based [6]helicenes fused with an azine ring

• Daria I. Tonkoglazova,
• Anna V. Gulevskaya,
• Konstantin A. Chistyakov and
• Olga I. Askalepova

Beilstein J. Org. Chem. 2021, 17, 11–21, doi:10.3762/bjoc.17.2

• are often difficult to separate. Another drawback of this method is the difficulty of scaling, since the reaction requires strong dilution to prevent the [2π + 2π] dimerization of the starting stilbene. Among other approaches to the carbazole-based helicenes are the Diels–Alder reaction of silyl enol

Changed reactivity of secondary hydroxy groups in C8-modified adenosine – lessons learned from silylation

• Jennifer Frommer and
• Sabine Müller

Beilstein J. Org. Chem. 2020, 16, 2854–2861, doi:10.3762/bjoc.16.234

• cycloaddition (CuAAC) became very popular [16]. A variant of this, the strain-promoted alkyne–azide cycloaddition (SPAAC) even offers the possibility of in cell application, as applies also to the inverse electron-demand Diels–Alder reaction (IEDDA) [17][18]. In vitro, often a combination of orthogonal methods

On the mass spectrometric fragmentations of the bacterial sesterterpenes sestermobaraenes A–C

• Anwei Hou and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2020, 16, 2807–2819, doi:10.3762/bjoc.16.231

• through reactions that are classified as σ-bond cleavages, α-fragmentations, inductive cleavages, McLafferty rearrangements [11], retro-Diels–Alder fragmentations [12][13], and the recently observed unusual radical-induced retro-Cope rearrangement (herein, “retro” indicates that the mass spectrometric

Ring-closing metathesis of prochiral oxaenediynes to racemic 4-alkenyl-2-alkynyl-3,6-dihydro-2H-pyrans

• Viola Kolaříková,
• Markéta Rybáčková,
• Martin Svoboda and
• Jaroslav Kvíčala

Beilstein J. Org. Chem. 2020, 16, 2757–2768, doi:10.3762/bjoc.16.226

• -2,7-diyne reacted chemo- and stereoselectively in a Diels–Alder reaction with N-phenylmaleimide affording the tricyclic products as a mixture of two separable diastereoisomers, the configuration of which was estimated by DFT computations. The reported enediyne metathesis paves the way to the

• enantioselective enyne metathesis yielding chiral building blocks for compounds with potential biological activity, e.g., norsalvinorin or cacospongionolide B. Keywords: Diels–Alder reaction; enediyne; enyne metathesis; ring-closing metathesis; ruthenium precatalyst; Introduction Among the plethora of metathetic

• analogous to the methallyl group containing oxaenediynes 9, i.e., no reaction was observed for G-I catalysis and G-II or HG-II catalysis under argon, or a complex mixture was obtained for G-II or HG-II catalysis using Mori conditions (Scheme 13). Diels–Alder reaction of RCEYM product 12b In general, the

Dawn of a new era in industrial photochemistry: the scale-up of micro- and mesostructured photoreactors

• Emine Kayahan,
• Mathias Jacobs,
• Leen Braeken,
• Leen C.J. Thomassen,
• Simon Kuhn,
• Tom van Gerven and
• M. Enis Leblebici

Beilstein J. Org. Chem. 2020, 16, 2484–2504, doi:10.3762/bjoc.16.202

• an ene reaction of β-citronellol and Diels–Alder reactions of α-terpinene and (5-methylfuran-2-yl)methanol [46] as well as the synthesis of cyclopent-2-enones from furans [47] and the synthesis of diverse γ-lactam scaffolds [48]. Conversions larger than 90% were achieved for all reactions [46][47][48

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

• coupling with TEMPO gives catalyst-bound iminium ion 261, which then cyclises with the nearby amine to produce the desired product 257 in good yields and enantioselectivities (9 examples, up to 97:3 er). Nicewicz et al. developed an enantioselective radical cation Diels–Alder reaction in both an

Hierarchically assembled helicates as reaction platform – from stoichiometric Diels–Alder reactions to enamine catalysis

• David Van Craen,
• Jenny Begall,
• Johannes Großkurth,
• Leonard Himmel,
• Oliver Linnenberg,
• Elisabeth Isaak and
• Markus Albrecht

Beilstein J. Org. Chem. 2020, 16, 2338–2345, doi:10.3762/bjoc.16.195

• David Van Craen Jenny Begall Johannes Grosskurth Leonard Himmel Oliver Linnenberg Elisabeth Isaak Markus Albrecht Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany 10.3762/bjoc.16.195 Abstract The stereoselectivity of a Diels–Alder reaction within the

• on the results of the stoichiometric reaction, a secondary amine-catalyzed nitro-Michael reaction is performed as well which afforded reasonable diastereoselectivities. Keywords: Diels–Alder reaction; enamine catalysis; hierarchical helicates; remote-control; stereoselectivity; Introduction Carbon

• providing the stereoinformation necessary for induction during the C–C bond formation. Catalytic approaches for C–C bond-forming reactions even found their way into the relatively young field of supramolecular chemistry, e.g., regioselective Diels–Alder reactions within supramolecular hosts as described by

Formation of an exceptionally stable ketene during phototransformations of bicyclo[2.2.2]oct-5-en-2-ones having mixed chromophores

• Asitanga Ghosh

Beilstein J. Org. Chem. 2020, 16, 2297–2303, doi:10.3762/bjoc.16.190

• groups were bulkier than the earlier reported one. Results and Discussion 5,6-Dibenzoyl-4-phenylbicyclo[2.2.2]oct-5-en-2-one (7a) and 5,6-dibenzoyl-4-isopropenylbicyclo[2.2.2]oct-5-en-2-one (7b) have been prepared through Diels–Alder reaction between substituted silyloxycyclohexa-1,3-dienes 8a and 8b [17

Synthetic approaches to bowl-shaped π-conjugated sumanene and its congeners

• Shakeel Alvi and
• Rashid Ali

Beilstein J. Org. Chem. 2020, 16, 2212–2259, doi:10.3762/bjoc.16.186

• , an excess of sodium thioalkoxide in 1,3-dimethyl-2-imidazolidinone 98 reacted with 94 at 100 °C to generate the required sumanene derivatives 99a–c. In another report, Sakurai’s group in situ generated sumanyne and treated it with different dienes (101a–d) in a Diels–Alder (DA) manner to produce the

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

• iminium salts. Herein we present several reactions which are based on the electron-poor acetylenic bond and on the high electrophilicity of the CF3-substituted iminium group. These salts were found to be highly reactive dienophiles in Diels–Alder reactions with cyclopentadiene, 2,3-dimethylbutadiene and

• group, which has both an electronic (electron-withdrawing) and steric (e.g., CF3 vs a phenyl substituent [28]) component. Moreover, a comparison with the thermal conditions of the Diels–Alder reaction of 4-phenyl-1,1,1-trifluorobut-3-yn-2-one and cyclopentadiene [29] confirms the expected accelerating

• effect of the iminium activation. The Diels–Alder reaction of alkyne 1a with 2,3-dimethylbutadiene also occurred under very mild conditions and yielded the iminium-substituted 1,4-cyclohexadiene 4-Ch (Scheme 2), which, due to its high sensitivity toward moisture, was not purified but was further

One-pot and metal-free synthesis of 3-arylated-4-nitrophenols via polyfunctionalized cyclohexanones from β-nitrostyrenes

• Haruyasu Asahara,
• Minami Hiraishi and
• Nagatoshi Nishiwaki

Beilstein J. Org. Chem. 2020, 16, 1830–1836, doi:10.3762/bjoc.16.150

• School of Pharmaceutical Sciences, Osaka University, Yamadaoka 1-6, Suita, Osaka 565-0871, Japan 10.3762/bjoc.16.150 Abstract β-Nitrostyrenes underwent a Diels–Alder reaction with Danishefsky’s diene to afford cyclohexenes together with the corresponding hydrolyzed products, 3-arylated-5-methoxy-4

• with Danishefsky’s diene could be conducted in one pot to directly afford the corresponding nitrophenols. Moreover, a heteroaryl group, e.g., a thienyl group could be introduced into the nitrophenol framework. Keywords: 3-arylated-4-nitrophenol; Danishefsky’s diene; Diels–Alder reaction; nitroalkene

• method toward the preparation of 3-arylated-4-nitrophenols. On the other hand, nitroalkenes possess an electron-deficient double bond, and hence, they serve as an excellent Michael acceptor and dienophile [12][13][14][15][16][17][18][19]. When β-nitrostyrene (1, Ar = Ph) is subjected to the Diels–Alder

Pauson–Khand reaction of fluorinated compounds

• Jorge Escorihuela,
• Daniel M. Sedgwick,
• Alberto Llobat,
• Mercedes Medio-Simón,
• Pablo Barrio and
• Santos Fustero

Beilstein J. Org. Chem. 2020, 16, 1662–1682, doi:10.3762/bjoc.16.138

• concomitant detrifluoromethylation. Furthermore, the Lewis acid-mediated retro Diels–Alder reaction was carried out uneventfully on product 61, affording the corresponding cyclopentenone 62 in moderate yield (Scheme 33). In order to rationalize the unexpected loss of the trifluoromethyl group upon

• conjugated addition of trimethylaluminum to form 66, followed by Lewis acid-mediated retro Diels–Alder reaction (Scheme 38). In another study by Riera and co-workers, they studied the influence of the olefin counterpart on the regioselectivity of the reaction [79]. Two olefins other than norbornadiene were

• Diels-Alder reactions [70]. Tentative mechanism for the nucleophilic addition/retro-aldol reaction sequence. Catalytic PKR with norbornadiene [70]. Scope of the PKR of trifluoromethylalkynes with norbornadiene [72]. DBU-mediated detrifluoromethylation [72]. A simple route to enone 67, a common

Models of necessity

• Timothy Clark and
• Martin G. Hicks

Beilstein J. Org. Chem. 2020, 16, 1649–1661, doi:10.3762/bjoc.16.137

• instance, an MO picture of the SN2 reaction is attractive but less common than the compact and informative “curly arrow” picture. The Diels–Alder reaction or aromaticity, on the other hand, cannot really be treated adequately within the Lewis picture without resorting to the Dewar–Zimmerman rules [80][81

An overview on disulfide-catalyzed and -cocatalyzed photoreactions

• Yeersen Patehebieke

Beilstein J. Org. Chem. 2020, 16, 1418–1435, doi:10.3762/bjoc.16.118

• . Huang and co-workers proposed a polar radical crossover cycloaddition mechanism for this Diels–Alder cycloaddition (Scheme 6). The electron transfer from the electron-rich styrene 14 to the activated acridinium photocatalyst 15 oxidizes the styrene 14 to form the styrene radical 16 and the acridine

• benzyloxymethylenetriphenylphosphane to give the dienes 71 and 72 as a 3:1 mixture. At room temperature, the resulting (E,E)-isomer 71 is quantitatively converted into the desired oxatricyclic system 74 via a diastereoselective Diels–Alder cycloaddition process. However, the (Z,E)-isomer 72 remains unreactive under these reaction

Recent synthesis of thietanes

• Jiaxi Xu

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

• (trifluoromethyl)-1,3-dithietane (363) and styrenes 383 produced the [2 + 2] adducts 4-aryl-2,2-bis(trifluoromethyl)thietanes 384 and Diels–Alder adducts 385, which further reacted with another molecule of bis(trifluoromethyl)thioketone (381) to yield the double Diels–Alder adducts 385 and thiochromane derivatives

• 386, respectively, through another Diels–Alder reaction and an ene reaction [101] (Scheme 79). The reaction of 2,2,4,4-tetrakis(trifluoromethyl)-1,3-dithietane (363) and quadricyclane (218) gave the bridged-thietane derivative 364 quantitatively in DMSO at 50 °C within 30 min [101] (Scheme 80). 3.3

Highly selective Diels–Alder and Heck arylation reactions in a divergent synthesis of isoindolo- and pyrrolo-fused polycyclic indoles from 2-formylpyrrole

• Carlos H. Escalante,
• Eder I. Martínez-Mora,
• Carlos Espinoza-Hicks,
• Alejandro A. Camacho-Dávila,
• Fernando R. Ramos-Morales,
• Francisco Delgado and
• Joaquín Tamariz

Beilstein J. Org. Chem. 2020, 16, 1320–1334, doi:10.3762/bjoc.16.113

• - and pyrrolo-fused polycyclic indoles is herein described, starting from 2-formylpyrrole and employing Diels–Alder and Heck arylation reactions. 3-(N-Benzyl-2-pyrrolyl)acrylates and 4-(pyrrol-2-yl)butenones underwent a highly endo-Diels–Alder cycloaddition with maleimides to furnish octahydropyrrolo

• aromatization of the polycyclic intermediates provides the corresponding polycyclic pyrrolo-isoindoles and isoindolo-pyrrolo-indoles. A theoretical study on the stereoselective Diels–Alder reactions, carried out by calculating the endo/exo transition states, revealed the assistance of non-covalent interactions

• in governing the endo stereocontrol. Keywords: endo-Diels–Alder stereocontrol; 2-formylpyrrole; intramolecular Heck arylation reaction; non-covalent interactions; pyrroloindoles; pyrroloisoindoles; Introduction Pyrrolizines [1][2] and pyrrolizidines [3][4], abundant in nature [4][5], are among the

The charge-assisted hydrogen-bonded organic framework (CAHOF) self-assembled from the conjugated acid of tetrakis(4-aminophenyl)methane and 2,6-naphthalenedisulfonate as a new class of recyclable Brønsted acid catalysts

• Svetlana A. Kuznetsova,
• Alexander S. Gak,
• Yulia V. Nelyubina,
• Vladimir A. Larionov,
• Han Li,
• Michael North,
• Vladimir P. Zhereb,
• Alexander F. Smol'yakov,
• Artem O. Dmitrienko,
• Michael G. Medvedev,
• Igor S. Gerasimov,
• Ashot S. Saghyan and
• Yuri N. Belokon

Beilstein J. Org. Chem. 2020, 16, 1124–1134, doi:10.3762/bjoc.16.99

• , including the ring opening of epoxides by water and alcohols. A Diels–Alder reaction between cyclopentadiene and methyl vinyl ketone was also catalyzed by F-1 in heptane. Depending on the polarity of the solvent mixture, the CAHOF F-1 could function as a purely heterogeneous catalyst or partly dissociate

• , providing some dissolved F-1 as the real catalyst. In all cases, the catalyst could easily be recovered and recycled. Keywords: Brønsted acid catalyst; charge-assisted hydrogen-bonded framework; Diels–Alder; epoxide ring opening; heterogeneous catalyst; Introduction Tremendous successes in homogeneous

• catalyst for the Brønsted acid-catalyzed ring opening reactions of epoxides with alcohols and water, with the latter reaction occurring in a three-phase medium. In addition, a Diels–Alder reaction was promoted by F-1 in heptane. Results and Discussion Mixing together aqueous solutions of two equivalents of

Diversity-oriented synthesis of 17-spirosteroids

• Benjamin Laroche,
• Thomas Bouvarel,
• Martin Louis-Sylvestre and
• Bastien Nay

Beilstein J. Org. Chem. 2020, 16, 880–887, doi:10.3762/bjoc.16.79

• Paris, Palaiseau Cedex, France 10.3762/bjoc.16.79 Abstract A diversity-oriented synthesis (DOS) approach has been used to functionalize 17-ethynyl-17-hydroxysteroids through a one-pot procedure involving a ring-closing enyne metathesis (RCEYM) and a Diels–Alder reaction on the resulting diene, under

• the propargylic alcohol, the resulting enyne is a good substrate for ring closing enyne metathesis (RCEYM) towards new diene substrates [40][41][42], which can be employed in Diels–Alder reactions with a variety of dienophiles (Figure 1) [43][44][45][46][47][48][49][50][51][52][53][54]. This strategy

• ) and lynestrenol (2). By using the sequence of RCEYM/Diels–Alder reaction, we rapidly synthesized "17-spirosteroid" compounds, orthogonally linking a non-steroidal polycyclic moiety to the steroid part at position 17. Spirocyclic systems play an important role in current medicinal chemistry thanks to

Combining enyne metathesis with long-established organic transformations: a powerful strategy for the sustainable synthesis of bioactive molecules

• Valerian Dragutan,
• Ileana Dragutan,
• Albert Demonceau and
• Lionel Delaude

Beilstein J. Org. Chem. 2020, 16, 738–755, doi:10.3762/bjoc.16.68

• to build the desired 1,3-dienic units. The advantageous association of this approach with name reactions like Grignard, Wittig, Diels–Alder, Suzuki–Miyaura, Heck cross-coupling, etc. is illustrated. Examples unveil the generality of such tandem reactions in providing not only the intricate structures

• ][39][40][41][42][43]. In ingeniously elaborated procedures, olefin metathesis has been frequently employed as such or associated with name reactions like Grignard, Wittig, Diels–Alder, Suzuki–Miyaura, Heck, etc., resulting in the assembly of diverse intricate building blocks of the targeted structures

• subsequent Eu(fod)3-catalyzed intermolecular Diels–Alder cycloaddition and epoxidation reactions (Scheme 5) [69]. In this stereoselective synthesis, the last biomimetic step was critical to obtain the proper enantiomer of the tetracyclic core of nanolobatolide. Amphidinolide macrolides Amphidinolides

Recent developments in photoredox-catalyzed remote ortho and para C–H bond functionalizations

• Rafia Siddiqui and
• Rashid Ali

Beilstein J. Org. Chem. 2020, 16, 248–280, doi:10.3762/bjoc.16.26

• temperature (Scheme 4) [94]. In their study, they observed that electron-donating groups provided better yields as compared to electron-withdrawing groups. The earlier reported methods for the synthesis of fluorenones, viz, Friedel–Craft acylations [95][96], oxidations of fluorenes [97][98], and Diels–Alder

Combination of multicomponent KA² and Pauson–Khand reactions: short synthesis of spirocyclic pyrrolocyclopentenones

• Riccardo Innocenti,
• Elena Lenci,
• Gloria Menchi and
• Andrea Trabocchi

Beilstein J. Org. Chem. 2020, 16, 200–211, doi:10.3762/bjoc.16.23

• [39][40], consisting of a [2 + 2 + 1] cycloaddition between an olefin, alkyne, and carbon monoxide. This reaction has been also applied in cascade approaches [41], and in combination with RCM [42], Diels–Alder [43] and Staudinger [44] reactions to produce novel structurally complex chemical entities

Bacterial terpene biosynthesis: challenges and opportunities for pathway engineering

• Eric J. N. Helfrich,
• Geng-Min Lin,
• Christopher A. Voigt and
• Jon Clardy

Beilstein J. Org. Chem. 2019, 15, 2889–2906, doi:10.3762/bjoc.15.283

• analogy to nature’s biosynthetic strategies outlined above, cationic, Diels–Alder, oxidative, and radical cyclization strategies have been successfully applied in total syntheses of terpenoids [53]. In addition, chemists are currently creatively exploring means to mimic classical terpene cyclases and

Synthetic terpenoids in the world of fragrances: Iso E Super^® is the showcase

• Alexey Stepanyuk and
• Andreas Kirschning

Beilstein J. Org. Chem. 2019, 15, 2590–2602, doi:10.3762/bjoc.15.252

• ]. Following this invention, many derivatives were produced to find new viable targets. These studies mostly focused on Diels–Alder cycloadditions to create structures that resemble terpenoids readily available from easily accessible and affordable starting materials like myrcene (1). One of the newly found

• ) (Scheme 3) [17][18][19][20][21]. To produce Ambrelux (32), myrcene (1) is reacted with dienophile (31) in a Diels–Alder cycloaddition promoted under Lewis-acidic conditions. In order to obtain Iso E Super® (33), Brønstedt acid-mediated cyclisation, similar to the one utilised for the first synthesis of

• conditions for the synthesis of all Iso E Super® related compounds vary slightly. The main difference lies in a prolonged isomerisation process of the Diels–Alder product 32 before and after the second cyclisation step. Georgywood® (35) named after Georg Fráter is industrially produced with, e.g., methanol

Synthesis of acremines A, B and F and studies on the bisacremines

• Nils Winter and
• Dirk Trauner

Beilstein J. Org. Chem. 2019, 15, 2271–2276, doi:10.3762/bjoc.15.219

• both secondary allylic alcohols and afforded 2 in good overall yield (Scheme 3). Bisacremine E (7) was proposed to be formed in nature via [4 + 2] cycloaddition involving two acremine F (5) units [4]. Although dimerization of 5 through a Diels–Alder cycloaddition is not electronically favorable, we

• undergo Diels–Alder reactions with electron-rich dienophiles [13][14][15][16][17], treatment of 5 with Fukuzumi’s catalyst [18] under illumination with blue light only led to decomposition of the starting material (Table 1, entry 15). Notably, a photoredox catalyst with a lower oxidation potential could

An overview of the cycloaddition chemistry of fulvenes and emerging applications

• Ellen Swan,
• Kirsten Platts and
• Anton Blencowe

Beilstein J. Org. Chem. 2019, 15, 2113–2132, doi:10.3762/bjoc.15.209

• Diels–Alder transition state, whilst those which were electron-withdrawing (e.g., NO2, CN, NMeAc) decreased the reaction rate. Interestingly, strong EDG (e.g., OMe, NMe2) exhibited a slower reaction rate than predicted, but this is likely due to the increased stabilisation of the reactant, rather than

• spectroscopically at −20 °C [4]. There have been numerous reports of pentafulvenes undergoing dimerization via Diels–Alder cycloadditions (DACs) (Scheme 8) at room temperature [6][66][108][109][114][115][117][119]. In some cases, the resulting dimers can undergo subsequent cycloadditions to form trimers via [6 + 4

• , when the fulvene has an EWG attached, it is more likely to function as a dienophile in an inverse electron-demand Diels–Alder (iEDDA) reaction [153][154][156]. This requires the other reactant to have strong EDGs in order to function as a diene, otherwise fulvene dimerization becomes the preferred