Bioinspired tetraamino-bisthiourea chiral macrocycles in catalyzing decarboxylative Mannich reactions

• Hao Guo,
• Yu-Fei Ao,
• De-Xian Wang and
• Qi-Qiang Wang

Beilstein J. Org. Chem. 2022, 18, 486–496, doi:10.3762/bjoc.18.51

• . Only 5 mol % of the optimal macrocycle catalyst efficiently catalyzed the decarboxylative addition of a broad scope of malonic acid half thioesters to isatin-derived ketimines with excellent yields and good enantioselectivity. The rigid macrocyclic framework and the cooperation between the thiourea and

• report a systematic synthesis of tetraamino-bisthiourea chiral macrocycles and their performance in catalyzing the decarboxylative Mannich addition of malonic acid half thioesters (MAHTs) to isatin-derived ketimines. The macrocycle-enabled hydrogen-bonding activation network and the associated confined

• and activation ability. Catalytic reaction optimization The synthesized macrocycles were then applied as catalysts in the decarboxylative addition of malonic acid half thioesters (MAHTs) to isatin-derived ketimines [48]. The reaction between ketimine 6a and MAHT 7a was initially performed in THF at

Unexpected chiral vicinal tetrasubstituted diamines via borylcopper-mediated homocoupling of isatin imines

• Marco Manenti,
• Leonardo Lo Presti,
• Giorgio Molteni and
• Alessandra Silvani

Beilstein J. Org. Chem. 2022, 18, 303–308, doi:10.3762/bjoc.18.34

• diffraction analysis and proved to be single diastereoisomers and atropisomers. A plausible mechanism for the one-pot Cu(II)-catalyzed Bpin addition to the isatin-derived ketimine substrate and subsequent homocoupling is described. Keywords: atropoisomer; bis(pinacolato)diboron; 3,3′-bisoxindole; N-tert

• intramolecular transformation [14], the insertion of a boron atom into chiral oxindoles is scarcely reported. Continuing with such previous project, next we looked at the copper-mediated reaction of isatin-derived, optically pure sulfinyl ketimines with bis(pinacolato)diboron, as a potential way to access

• described by Ellman and co-workers, but the expected α-aminoboronate could not be isolated. Extensive hydrolysis of the starting ketimine occurred, allowing only the recovery of the 1-methyl-isatin precursor. In the light of this initial outcome, we started a detailed screening of reaction conditions (Table

Bifunctional thiourea-catalyzed asymmetric [3 + 2] annulation reactions of 2-isothiocyanato-1-indanones with barbiturate-based olefins

• Jiang-Song Zhai and
• Da-Ming Du

Beilstein J. Org. Chem. 2022, 18, 25–36, doi:10.3762/bjoc.18.3

• construction of bispirobarbiturates [30][31]. In 2019, for example, An and co-workers reported an asymmetric Michael/Mannich [3 + 2] cycloaddition reaction between N-(2,2,2-trifluoroethyl)isatin ketimines and barbiturate-based olefins (Scheme 1a) [32]. Based on the current knowledge, the construction of

Recent advances in the asymmetric phosphoric acid-catalyzed synthesis of axially chiral compounds

• Alemayehu Gashaw Woldegiorgis and
• Xufeng Lin

Beilstein J. Org. Chem. 2021, 17, 2729–2764, doi:10.3762/bjoc.17.185

• also achieved the first catalytic asymmetric construction of axially chiral 3,3’-bisindole scaffolds 49 bearing both axial and central chirality by employing the CPA-14-catalyzed asymmetric addition reaction of 2-substituted 3,3’-bisindoles 47 to isatin-derived 3-indolylmethanols 48. The isatin-derived

• nucleophilic addition reactions using isatin-derived imines 50 as electrophiles was reported by Tan, Shi and co-workers in 2020. Enantioenriched 3,3'-bisindoles with a biologically essential chiral 3-aminooxindole unit can be readily prepared by these methods. CPA 9 utilizes hydrogen bonding to activate both

• reactants, resulting in enantioenriched 3,3'-bisindole derivatives with multiple chirality. In the presence of the chiral phosphoric acid catalyst CPA 9, various groups of 2-substituted 3,3'-bisindoles and isatin-derived imines were tolerated and gave good to high yields (up to 80%) and moderate to

A novel methodology for the efficient synthesis of 3-monohalooxindoles by acidolysis of 3-phosphate-substituted oxindoles with haloid acids

• Li Liu,
• Yue Li,
• Tiao Huang,
• Dulin Kong and
• Mingshu Wu

Beilstein J. Org. Chem. 2021, 17, 2321–2328, doi:10.3762/bjoc.17.150

• University, Haikou 571199, Hainan Province, P. R. China 10.3762/bjoc.17.150 Abstract A novel method for the synthesis of 3-monohalooxindoles by acidolysis of isatin-derived 3-phosphate-substituted oxindoles with haloid acids was developed. This synthetic strategy involved the preparation of 3-phosphate

• -substituted oxindole intermediates and SN1 reactions with haloid acids. This new procedure features mild reaction conditions, simple operation, good yield, readily available and inexpensive starting materials, and gram-scalability. Keywords: acidolysis; haloid acids; isatin; 3-monohalooxindole; 3-phosphate

• recently as precursors in Friedel–Crafts reactions of arenes [30][31] and cross-coupling reactions of arylboronic reagents [32]. However, the direct SN1 reaction of such isatin-derived 3-phosphate-substituted oxindoles by halide ions as nucleophiles has not been developed yet and remains an unsolved

Asymmetric organocatalyzed synthesis of coumarin derivatives

• Natália M. Moreira,
• Lorena S. R. Martelli and
• Arlene G. Corrêa

Beilstein J. Org. Chem. 2021, 17, 1952–1980, doi:10.3762/bjoc.17.128

• recently, the strategy via introducing secondary interactions for the design of the bifunctional catalysts achieved wide application in asymmetric reactions [74]. Wu et al. described a Mannich asymmetric addition of cyanocoumarins 39 to isatin imines 112 catalyzed by an amide-phosphonium salt 114. This

• of 2-hydroxycinnamaldehydes 109 with 4-hydroxycoumarins 1. Mannich asymmetric addition of cyanocoumarins 39 to isatin imines 112 catalyzed by the amide-phosphonium salt 114. Enantioselective total synthesis of (+)-scuteflorin A (119). Funding The authors gratefully acknowledge FAPESP (grants 2013

N-tert-Butanesulfinyl imines in the asymmetric synthesis of nitrogen-containing heterocycles

• Joseane A. Mendes,
• Paulo R. R. Costa,
• Miguel Yus,
• Francisco Foubelo and
• Camilla D. Buarque

Beilstein J. Org. Chem. 2021, 17, 1096–1140, doi:10.3762/bjoc.17.86

• the stereoselective synthesis of spiro β-lactam 57 from chiral (RS)-N-tert-butanesulfinyl isatin ketimine 53 (R1 = H), with a bulky trityl protecting group bonded to the nitrogen indolic atom (Tr = triphenylmethyl), and ethyl bromoacetate. The Zn/Cu-mediated Reformatsky-type reaction furnished

Microwave-assisted multicomponent reactions in heterocyclic chemistry and mechanistic aspects

• Shivani Gulati,
• Stephy Elza John and
• Nagula Shankaraiah

Beilstein J. Org. Chem. 2021, 17, 819–865, doi:10.3762/bjoc.17.71

• ). Similarly, the same group extended the work by illustrating [51] a three and four-component microwave-assisted base and catalyst-free reaction for the synthesis of substituted spirooxindoles 40. The three-component reaction involved the reaction between substituted isatin 35, but-2-ynedioates 39 and amino

• acids 37. Likewise, the four-component reaction comprised of isatin 35, but-2-ynedioates 39, amino acids 37 and phenacyl bromides 41 to yield the N-acylated spirooxindoles 42 in good yields (Scheme 13). The reaction effectively explored the 1,3 dipolar compound generated with isatin and amino acids

• plausible mechanism shown in Scheme 14 explains the formation of azomethine ylide B by condensation of isatin with amino acid followed by release of a molecule of CO2 via A. The imine B undergoes 1,3-dipolar cycloaddition with the dipolarophiles 39. The cyclization yields the desired product 40 of the three

Synthesis of (Z)-3-[amino(phenyl)methylidene]-1,3-dihydro-2H-indol-2-ones using an Eschenmoser coupling reaction

• Lukáš Marek,
• Lukáš Kolman,
• Jiří Váňa,
• Jan Svoboda and
• Jiří Hanusek

Beilstein J. Org. Chem. 2021, 17, 527–539, doi:10.3762/bjoc.17.47

• with thiobenzamide (2a) did not proceed at all and only the formation of isoindigo 9a was observed. However, a very easy synthesis of diethyl (2-oxoindolin-3-yl)phosphate (1g) from isatin has recently appeared [42] which encouraged us to submit this reagent to the Eschenmoser reaction. Unfortunately

• ) 3-hydroxyoxindole and thiobenzamide (2a). We partly succeeded because we isolated the coupling product 5aa in 65% yield (48% from isatin) which is still less than when starting from isatin through 1a (57%). For a further two representatives, namely 4’-trifluoromethylthiobenzanilide (3d) and N,N

• derivative. Such nitrogen protection/deprotection (e.g., acetylation/deacetylation) is often used in the other synthetic strategies described in Scheme 1. Therefore, the starting isatin was acetylated first with acetic anhydride [47] in 95% yield and then reduced with various complex borohydrides (e.g

Bifurcated synthesis of methylene-lactone- and methylene-lactam-fused spirolactams via electrophilic amide allylation of γ-phenylthio-functionalized γ-lactams

• Tetsuya Sengoku,
• Koki Makino,
• Ayumi Iijima,
• Toshiyasu Inuzuka and
• Hidemi Yoda

Beilstein J. Org. Chem. 2020, 16, 2769–2775, doi:10.3762/bjoc.16.227

• -lactone derivatives spiro-fused to an oxindole (A), one of which exhibited a potent cytotoxic activity [9]. Our research group succeeded the enantiopure synthesis of these compounds, in which enantioselective allylation of an isatin derivative with an amido-functionalized allylstannane was employed as a

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

• testing a variety of reaction conditions. More complex biorelevant examples such as isatin derivatives were also suitable substrates, affording the corresponding spirocyclic derivatives, albeit in low yields (Scheme 21) [59]. Unfortunately, thioether and sulfone-based linkers were unsuitable in this

Recent synthesis of thietanes

• Jiaxi Xu

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

• ). Also isatins 407 reacted with chloromethylthiirane (398a) to afford N-(thietane-3-yl)isatin derivatives 408 in moderate yields [112] (Scheme 87). When weakly nucleophilic nitrophenols 409 were used as nucleophiles, the ring-expansion reaction of chloromethylthiirane (398a) yielded the corresponding 3

Copper catalysis with redox-active ligands

• Agnideep Das,
• Yufeng Ren,
• Cheriehan Hessin and
• Marine Desage-El Murr

Beilstein J. Org. Chem. 2020, 16, 858–870, doi:10.3762/bjoc.16.77

• reports on copper complexes of the redox-active Schiff-base isatin bearing various functional groups (Scheme 4) [19]. The main feature of these ligands is their ability to form four distinct and well-defined types of complexes depending on the reaction conditions (i.e., the number of equivalents of metal

• oxidation of alcohols and representative substrate scope. Introduction of H-bonding network in the ligand coordination sphere. Well-defined isatin copper complexes. Catalyst control in the biomimetic phenol ortho-oxidation. Structural diversity accessible by direct functionalization. Copper-catalyzed

A heteroditopic macrocycle as organocatalytic nanoreactor for pyrroloacridinone synthesis in water

• Piyali Sarkar,
• Sayan Sarkar and
• Pradyut Ghosh

Beilstein J. Org. Chem. 2019, 15, 1505–1514, doi:10.3762/bjoc.15.152

• pyrroloacridinones The organocatalytic activity of the BATA-MC nanoreactor was then explored in the synthesis of pyrroloacridines. To optimize the reaction conditions, we chose the synthesis of 4a as the model reaction. Thus, we performed the reaction of isatin (1a, 1 mmol), 5,5-dimethylcyclohexane-1,3-dione (2, 1

• nanoreactor which is a single molecule as catalyst in the earliest steps of the mechanistic pathway (Figure 6). This multicomponent reaction is expected to comprise a series of condensations and subsequent ring-closure cascade reactions (Scheme 3). To demonstrate the mechanistic pathway, dimedone (2), isatin

• taken from the reaction mixture after 10 minutes and analyzed by ESIMS. The observed peak at m/z 801.4 corresponding to [BATA-MC + 2 + 3a + Na]+ confirmed the involvement of BATA-MC as nanoreactor (Supporting Information File 1, Figure S1). When we investigated the interaction of isatin with BATA-MC, we

Novel (2-amino-4-arylimidazolyl)propanoic acids and pyrrolo[1,2-c]imidazoles via the domino reactions of 2-amino-4-arylimidazoles with carbonyl and methylene active compounds

• Victoria V. Lipson,
• Tetiana L. Pavlovska,
• Nataliya V. Svetlichnaya,
• Anna A. Poryvai,
• Nikolay Yu. Gorobets,
• Erik V. Van der Eycken,
• Irina S. Konovalova,
• Svetlana V. Shiskina,
• Alexander V. Borisov,
• Vladimir I. Musatov and
• Alexander V. Mazepa

Beilstein J. Org. Chem. 2019, 15, 1032–1045, doi:10.3762/bjoc.15.101

• considered as the analogues of both 3,3’-spirooxindole and 2-aminoimidazole marine sponge alkaloids. Keywords: 2-amino-4-arylimidazole; (2-amino-4-arylimidazolyl)propanoic acid; isatin; Meldrum’s acid; multicomponent reactions; pyrrolo[1,2-c]imidazole; 3,3’-spirooxindoles; Introduction Heterocyclic

• zwitter-ionic form and to consider the structure of 16a as superposition of two resonance structures (Scheme 5). In the next step of our research we have involved isatin 18 as the compound bearing a carbonyl group, as well in this case the pyrolo[1,2-c]imidazole moiety will be spiro-fused with the

• described in the literature [38][39][40], the authors recognized the importance of protecting the amide fragment of isatin, since it affects the reactivity and, in some cases, the enantioselectivity of processes. In order to prevent undesirable side reactions in the future, three-component condensations

Annulation of 1H-pyrrole-2,3-diones by thioacetamide: an approach to 5-azaisatins

• Aleksandr I. Kobelev,
• Ekaterina E. Stepanova,
• Maksim V. Dmitriev and
• Andrey N. Maslivets

Beilstein J. Org. Chem. 2019, 15, 364–370, doi:10.3762/bjoc.15.32

• developed via an unprecedented annulation of pyrrolo[2,1-c][1,4]benzoxazine-1,2,4-triones by thioacetamide. A new way of C–H functionalization of thioacetamide has been discovered. The reaction proceeds under green catalyst-free conditions. Keywords: annulation; domino reactions; isatin; nitrogen

• heterocycles; thioamides; Introduction Omnipresent among natural compounds and drugs, the indole core is a privileged scaffold for library design and drug discovery [1]. Its dioxo derivative, isatin (1H-indole-2,3-dione), is an important building block for the construction of diverse indole-based compounds

• chromatography. Moreover, the resulting compounds may be of pharmaceutical interest and can be starting materials for the synthesis of various aza analogs of isatin-based heterocycles. It also should be mentioned that we have observed an interesting thioacetamide-based reactive intermediate that can possibly

An overview on recent advances in the synthesis of sulfonated organic materials, sulfonated silica materials, and sulfonated carbon materials and their catalytic applications in chemical processes

• Hashem Sharghi,
• Pezhman Shiri and
• Mahdi Aberi

Beilstein J. Org. Chem. 2018, 14, 2745–2770, doi:10.3762/bjoc.14.253

• these catalysts for extensive reactions, including the synthesis of bis(indolyl)methane derivatives, β-amino carbonyl compounds, 14H-dibenzo[a,j]xanthene derivatives, 1,8-dioxodecahydroacridine derivatives, xanthene derivatives, pyrimido[4,5-b]quinoline derivatives, spiro-isatin derivatives, spiro

• ), and 1-naphthol (34d)), and malononitrile (35) in water at 90 °C. Isatin (32) and acenaphthenequinone (33) were reacted with C–H activated acids 20a,b, 23, and 34a–c and malononitrile (35) to form the corresponding spiro-isatin derivatives 36 and spiro-acenaphthenequinone derivatives 37 under mild and

• homogeneous conditions (Scheme 5). After this successful application, catalyst 31 was tested in the synthesis of bis(2-amino-4H-pyran) derivatives 39–44 via a one-pot multicomponent reaction of dialdehydes 38 (instead of isatin and acenaphthenequinone substrates), a variety of C–H activated acids 20a,b, 23

Synthesis of chiral 3-substituted 3-amino-2-oxindoles through enantioselective catalytic nucleophilic additions to isatin imines

• Hélène Pellissier

Beilstein J. Org. Chem. 2018, 14, 1349–1369, doi:10.3762/bjoc.14.114

• Helene Pellissier Aix-Marseille Univ, CNRS, Centrale Marseille, iSm2, Marseille, France 10.3762/bjoc.14.114 Abstract This review collects the recent developments in the synthesis of chiral 3-substituted 3-amino-2-oxindoles based on enantioselective catalytic nucleophilic additions to isatin

• imines published since the beginning of 2015. Keywords: asymmetric synthesis; chiral 3-amino-2-oxindoles; chirality; isatin imines; nucleophilic addition; Introduction Chiral oxindoles represent an important class of products widely present in nature and exhibiting many biological activities. Among

• organocatalyzed methodologies [10][13]. The simplest method to prepare chiral quaternary 3-amino-2-oxindoles is based on enantioselective catalytic nucleophilic additions to isatin imines. This is not only because of the easy access to isatin imines, but also by the possibility of using a wide range of

Development of novel cyclic NGR peptide–daunomycin conjugates with dual targeting property

• Andrea Angelo Pierluigi Tripodi,
• Szilárd Tóth,
• Kata Nóra Enyedi,
• Gitta Schlosser,
• Gergely Szakács and
• Gábor Mező

Beilstein J. Org. Chem. 2018, 14, 911–918, doi:10.3762/bjoc.14.78

• , 0.1 M HOBt in DMF (4 times; 2 + 2 + 5 + 10 min), (iii) DMF washing (10 × 0.5 min), (iv) coupling of Fmoc-protected amino acid derivative: DIC:HOBt in DMF (4 equiv each) (1 × 60 min), (v) DMF washing (3 × 0.5 min), (vi) DCM washing (2 × 0.5 min), (vii) ninhydrin or isatin test. The cleavage of Mtt

Methyl isocyanide as a convertible functional group for the synthesis of spirocyclic oxindole γ-lactams via post-Ugi-4CR/transamidation/cyclization in a one-pot, three-step sequence

• Amarendar Reddy Maddirala and
• Peter R. Andreana

Beilstein J. Org. Chem. 2018, 14, 875–883, doi:10.3762/bjoc.14.74

• ]. Significant efforts have been made to design creative synthetic strategies for spirocyclic oxindole molecules, of which, isatin-based domino reactions [24][25][26][27][28][29][30] have proved to be very versatile [31] and readily achievable [32][33][34][35][36][37]. However, finding a simple and efficient

Recent developments in the asymmetric Reformatsky-type reaction

• Hélène Pellissier

Beilstein J. Org. Chem. 2018, 14, 325–344, doi:10.3762/bjoc.14.21

• -butylsulfinyl)amino]-2-oxoindolin-3-yl}acetates 33a–m exhibiting a quaternary stereogenic center which was based on the zinc-mediated diastereoselective aza-Reformatsky reaction of isatin-derived chiral N-sulfinyl ketimines 34a–m [30]. The Reformatsky reagent was in situ generated from ethyl bromoacetate (9b

• between chiral α-trifluoromethyl N-tert-butylsulfinyl hemiaminals and α-bromoesters. In the same area, diastereoselectivities of up to >90% de were achieved in the Zn-mediated aza-Reformatsky reactions of chiral sulfinyl imines with fluorobromoesters/ketones. Moreover, isatin-derived chiral N-sulfinyl

5-Aminopyrazole as precursor in design and synthesis of fused pyrazoloazines

• Ranjana Aggarwal and
• Suresh Kumar

Beilstein J. Org. Chem. 2018, 14, 203–242, doi:10.3762/bjoc.14.15

• -aminopyrazole 16 and 4-hydroxycoumarin (50) with isatin 54 under ultrasound irradiation in water (Scheme 11). Solvent and catalytic screening for the reaction have shown that water in presence of p-TSA at 60 °C on heating for 6 hours provide best results with excellent yields. Recently, Wang et al. [57] also

• described the construction of spiro[indoline-3,4'-pyrazolo[3,4-b]pyridine] derivatives 57 from the multicomponent reaction of 5-amino-3-hydroxy-1-phenyl-1H-pyrazole (46), ketones 56 and isatin 54 in water/acetic acid (3:1) at 90 °C (Scheme 12). Quiroga et al. [58] reported the synthesis of the pyrazolo[3,4

• -b]pyridine-spiroindolinone nucleus 59 with a high degree of regioselectivity without formation of the regioisomeric pyrazolo[1,5-a]pyrimidine 60 involving three-component reaction of 5-aminopyrazole 16, isatin 54 and cyclic β-diketones 58 in aqueous ethanol with p-TSA as catalyst (Scheme 13

An effective one-pot access to polynuclear dispiroheterocyclic structures comprising pyrrolidinyloxindole and imidazothiazolotriazine moieties via a 1,3-dipolar cycloaddition strategy

• Alexei N. Izmest’ev,
• Galina A. Gazieva,
• Natalya V. Sigay,
• Sergei A. Serkov,
• Valentina A. Karnoukhova,
• Vadim V. Kachala,
• Alexander S. Shashkov,
• Igor E. Zanin,
• Angelina N. Kravchenko and
• Nina N. Makhova

Beilstein J. Org. Chem. 2016, 12, 2240–2249, doi:10.3762/bjoc.12.216

• -triazine moieties has been developed. The method relies on a 1,3-dipolar cycloaddition of azomethine ylides generated in situ from isatin derivatives and sarcosine to 6-benzylideneimidazo[4,5-e]thiazolo[3,2-b]-1,2,4-triazine-2,7-diones. Keywords: azomethine ylides; cycloaddition; diastereoselectivity

• 4,5-dihydroxy-4,5-diphenylimidazolidine-2-one [42] and thiosemicarbazide in 96% yield [43]. To optimize the 1,3-dipolar cycloaddition reaction conditions, 4-bromobenzylidene derivative 1a was chosen as a model substrate in the reaction with sarcosine and isatin 3a. The solvent, reaction time, and

• derivatives 1b,c were subjected to the reaction with isatins 3a–e and sarcosine under the optimized conditions to afford the dispiro compounds 4f–o in 52–83% yields. As shown in Figure 2, the best yields of the cycloadducts 4 were observed in the reaction of unsubstituted isatin (3a) as the carbonyl component

Enantioselective addition of diphenyl phosphonate to ketimines derived from isatins catalyzed by binaphthyl-modified organocatalysts

• Hee Seung Jang,
• Yubin Kim and
• Dae Young Kim

Beilstein J. Org. Chem. 2016, 12, 1551–1556, doi:10.3762/bjoc.12.149

• few decades [20][21][22]. General approaches for the synthesis of chiral 3-substituted-3-aminooxindole derivatives include the amination of various 3-monosubstituted oxindoles [23][24][25][26][27] and the nucleophilic addition to ketimines derived from isatin derivatives [28][29][30][31][32][33][34

• reaction time, and low temperature required for good enantioselectivity. Thus, new approaches for the organocatalytic enantioselective addition of diphenyl phosphonate to isatin imines are highly desired. In connection with our ongoing research program on the design and application in asymmetric catalysis

• ). The reaction of diphenyl phosphonate (2) with N-allylated and 5-halo-N-allylated isatin imines provided adducts 3a–d in good yields (80–94%) with excellent enantioselectivities (93–97% ee, Table 2, entry 1–4). The addition of diphenyl phosphonate (2) to 5-chloro-N-substituted isatin imines 1e and 1f

One-pot synthesis of tetracyclic fused imidazo[1,2-a]pyridines via a three-component reaction

• Bo Yang,
• Chuanye Tao,
• Taofeng Shao,
• Jianxian Gong and
• Chao Che

Beilstein J. Org. Chem. 2016, 12, 1487–1492, doi:10.3762/bjoc.12.145

• aromatic imidazole. Therefore, we envisioned that a ketone-involved reaction could proceed, if the [4 + 1] cycloaddition adduct can further rearrange to form an aromatic imidazole. With this idea in mind, we started to study an isatin-involved GBB reaction [38][39], because the resulting [4 + 1

• ] cycloaddition adduct could further proceed with a retro-aza–ene reaction via a concerted [1,5]-hydride shift. We first explored the reaction of 2-aminopyridine (2a), isatin (3a) and tert-butyl isocyanide (4a). Under the classical reaction conditions (entry 1, Table 1), the product 1a was isolated only in 4

• HOAc were also screened, and the results indicated that the use of PTSA led to a slightly decreased yield, and weaker acids failed to promote this process. It is worth noting that the reaction proceeded incompletely and significant amounts of isatin (3a) were recovered. When increased amounts (1.35