## 4.2. Non-Linear Optical Properties

Push-pull dyes exhibiting a large ground state dipole moment usually have high molecular non-linear optical (NLO) efficiencies [347,348]. Considering this, indanedione-1,3-dione 4, by its electron-withdrawing ability, is an excellent candidate for the design of dyes exhibiting such a property [349,350]. To present large optical nonlinearities, molecules should be organized in a none-centrosymmetric arrangement, and different strategies have been developed to stabilizing the dipole orientation. Thus, introduction of indane-1,3-dione derivatives into polymer composites (498–500) [351] or formation of Langmuir–Blodgett films with amphiphilic derivatives (501–504) [352,353] proved to be effective strategies to address this issue (see Scheme 115). In this last case, introduction of an additional double bond did not significantly modify the structure of the Langmuir–Blodgett films. Highest hyperpolarizability was obtained with 501, indicating the high order of the LB film obtained with this molecule. Parallel to this, an increase in the number of layers enhanced the NLO signal. Covalent linkages to polymers proved as being another strategy to retain the molecular orientation obtained by electrical poling [354]. By heating the polymer 505 at a temperature higher than its T~g~ and upon application of an intense electric field, an orientation of the push-pull dyes connected to the polymer backbone could be obtained. While maintaining the polymer film at a temperature lower than the polymer T~g~, relaxation of the chromophore alignment could be efficiently slowed down, maintaining the molecular orientation of the dyes over time. Improvement of the molecular hyperpolarizability could also be obtained by improving the electron withdrawing of the acceptor, as exemplified with the dye 506 based on 2-methylidene-3-(dicyano-methylidene)-1-indanone [24]. Comparison with a reference compound, i.e., disperse red 1, revealed 243 to exhibit an hyperpolarizability as high as 1558 × 10⁻³⁰ esu·D, greatly higher than that of the reference compound (814 × 10⁻³⁰ esu·D). Most of the dyes prepared for NLO applications are synthesized by means of a Knoevenagel reaction, as exemplified by the selection of molecules 498–506 presented in Scheme 152.

## 4.3. Fluorescent Chemosensors and Chemodosimeters

The detection of metal cations, halide ions, cyanides and even of neutral species has been an active research field so that two types of detectors were developed [355]. The first category of fluorescent sensors are those comprising a binding site giving rise to an irreversible chemical reaction with ions, and these first types of compounds are named chemodosimeters. The second type of optical sensors are those capable of initiate a communication mechanism between the binding site and the ions, but in a reversible way. In this last case, these compounds are thus named fluorescent chemosensors. Indane-1,3-dione 4 has been at the origin of the elaboration of an efficient chemodosimeter for cyanide detection. Cyanides are extremely toxic anions so that a concentration as low as 2.7 µM is tolerated in drinking water [356,357]. In 2009, a chemodosimeter based on calix[4]pyrrole with an indane-1,3-dione unit at the β-pyrrolic position was reported by Lee and coworkers [213]. A dependence of the dye discoloration with the cyanide concentration was clearly evidenced, and a disappearance of the yellow color of 507 upon addition of cyanides could be easily detected with the naked eye. Chemodosimeter proved also to be highly selective since only cyanide ions were detected even when hidden within other ions. The remarkable selectivity and affinity for cyanide anions was explained by the fast equilibrium process involved in the complexation followed by the reaction induced by this extremely nucleophilic anion. After complexation of cyanide anions in the binding site of the calix[4]pyrrole, a nucleophilic addition at the β-position of the indane-1,3-dione group could occur, as shown in Scheme 151.
<figure><img src="image_278.png" /><figcaption>**Scheme 151.** Reaction occurring with cyanide anions.</figcaption></figure>

<figure><img src="image_279.png" /></figure>

<figure><img src="image_280.png" /></figure>

<figure><img src="image_281.png" /></figure>

<figure><img src="image_282.png" /><figcaption>**Scheme 152.** Chemical structures of dyes 498–506.</figcaption></figure>
Based on the same nucleophilic addition of cyanides onto the double bond of push-pull dyes developed with indane-1,3-dione, **508** could be used as a dosimeter enabling to combine two detection modes [358]. Thus, upon addition of cyanide anions on **508**, a clear discoloration of the dye could be detected with the naked eye. Parallel to this, a complete quenching of luminescence could be evidenced by photoluminescence measurements. If the detection of cyanides anions in THF was extremely fast, in water, kinetic of addition was considerably reduced, assigned to a solvation effect of water molecules around the cyanide anions (see Scheme 153). Here again, **508** proved to be selective for the detection of cyanide anion, and this is again related to the small size of cyanides but also to their averred nucleophilic character. A similar strategy was developed with diketopyrrolopyrrole **509** [359]. A dual mode of detection was also observed with this compound. In this last case, a mechanism of exciplex formation was proposed to support the fluorescence quenching observed experimentally (see Scheme 153 and Figure 3).

<figure><img src="image_283.png" /><figcaption>**Scheme 153.** Reaction occurring with cyanide anions.</figcaption></figure>
<figure><img src="image_284.png" /><figcaption>**Figure 3.** Cyanide ion detection with **509**: (a) optically with the naked eye; (b) by fluorescence changes. Reproduced with permission from Wang et al. [359].</figcaption></figure>

### 4.4. Solar Cells

Organic solar cells have received significant attention for the possibility to convert photons to electrons. Among the most widely studied electron-acceptors, [6,6]-phenyl C<sub>61</sub> butyric acid methyl ester (PC<sub>61</sub>BM) is the most popular one [99,360–362]. Due to a severe phase segregation upon aging of the device, the low solubility of PC<sub>61</sub>BM in most of the common organic solvents and non-fullerene acceptors has been identified as promising alternatives to address the segregation issue and the solubility issue. Furthermore, the photon-to-electron conversions remain low, lingering around 3–6% for non-fullerene acceptors [363–378], justifying the constant efforts to develop new structures. To create strong electron acceptors that could replace PC<sub>61</sub>BM, indane-1,3-dione 4 was notably combined with dibenzosilole [379]. A photon-to-current conversion efficiency (PCE) of 2.76% could be obtained for the solution-processed devices comprising a poly(3-hexylthiophene) (P3HT)/510 blend as the active layer. By replacing dibenzosilole by a naphthalimide unit, encouraging results were obtained with 511, with a PCE reaching 3.52% in the same conditions [380]. However, other authors also tested the opposite situation, using the dibenzosilole-based compounds as electron donors [381]. End-groups on 512 and 513 were determined as drastically affecting the morphology of the active layer, more than the photophysical properties of the dyes. Even after solvent vapor annealing, the power conversion efficiency of solar cells fabricated with the 512/PC<sub>61</sub>BM blend remained low, peaking around 0.5% contrarily to 6.6% for devices fabricated with the 513/PC<sub>61</sub>BM blend. The excellent electron-donating ability of 513 can also be assigned to the presence of the thiophene moiety, reported as improving both the optical and photovoltaic properties of the dyes comprising this group [382]. This trend was confirmed with 514 [11] and 515a [383] or 515b [384], with which a PCE of 2.4%, 6.46% and 8.22% were determined with solar cells of similar structures than that used for 513. In the case of 515b, improvement of the photovoltaic properties was assigned to the more balanced charge transportation in the devices and the use of copper isothiocyanate acting as a hole transport/injection layer. However, counterexamples exist, as exemplified with the asymmetric structure 516 [385]. Photovoltaic properties of 516 in bulk heterojunction solar cells remained poor, and a PCE of 1.7% was obtained for an active layer composed of a 1:1 515/PC<sub>61</sub>BM blend. This low efficiency was notably assigned to the low hole mobility of 516 and adverse charge transport within the active layer (see Scheme 154). Finally, a similar low power conversion efficiency was obtained with 517 and 518, still based on an asymmetric structure [386]. Here again, the low efficiency (0.24–0.33%) obtained with 517 and 518 was assigned to an unfavorable morphology of the active layer as well as its high roughness.
<figure><img src="image_285.png" /></figure>

510

<figure><img src="image_286.png" /></figure>

511

<figure><img src="image_287.png" /></figure>

512

<figure><img src="image_288.png" /></figure>

513

<figure><img src="image_289.png" /></figure>

514

<figure><img src="image_290.png" /></figure>

516

<figure><img src="image_291.png" /></figure>

515a: R = C<sub>8</sub>H<sub>17</sub>

515b: R = C<sub>8</sub>H<sub>13</sub>

517

518

<caption>**Scheme 154.** Chemical structures of 510–518.</caption>

**5. Biological Applications**

Indane-1,3-dione 4 has been the focus of intense research efforts in medicinal chemistry since the demonstration in the early 1930s of the bacteriostatic activity of indane-1,3-dione
and related derivatives, but also of many 1,3-diketo compounds showing interesting physiological activities [387,388]. This section provides an explicit overview of the importance of indane-1,3-dione 4 as a building block for the design of molecules with potential biological applications.

### 5.1. Indane-1,3-Dione as Antimicrobial Agent

Any agent that kills or slows down the growth of a micro-organism may be defined as an antimicrobial agent. The increasing demand of antimicrobial drugs resulting from a faster microbial resistance to drugs requires the development of new compounds of innovative structures. Thus, antiseptics have been designed with indane-1,3-dione 4 as soon as 1931 by Walker et al. and their biological activities improved one year later by Robinson et al., who developed 1,3-diketo systems exhibiting remarkable physiological properties In Robinson's work, the indane-1,3-dione core was formed by a Friedel-Craft reaction, enabling to introduce various *n*-alkyl groups or hydroxyl groups onto the aromatic core (compounds 521) (see Scheme 155). Notably, the presence of phenol groups was identified as improving the bacteriostatic effect of the resulting compounds [388,389].

<figure><img src="image_292.png" /><figcaption>**Scheme 155.** Synthesis of 4-hydroxyindan-1,3-diones 521 studied in the 1930s by Robinson et al. and Walker et al., first indanediones known for their antiseptic activities [388].</figcaption></figure>

All of these compounds were tested in vitro on Gram-positive bacteria: *Staphylococcus albus* Rosenbach (*S. epidermidis*), *Staphylococcus aureus* (*S. aureus*), (*Bacillus megatherium* (*B. megatherium*), *Bacillus subtilis* (*B. subtilis*), *Bacillus mycoides* (*B. mycoides*), Gram-negative bacteria: *Bacterium pyocyaneum* (*P. aeruginosa*), *Bacterium prodigiosum* (*B. Prodigiosum*) and *Escherichia coli* (*E coli*) but also on acid-fast bacteria such as *Mycobacterium phlei* (*M. phlei*) according to the Rideal-Walker method. Even though this method is not used anymore due to its lack of reliability on phenols, it was replaced by the McFarland protocol to furnish more reproducible data and to determine the minimum inhibitory concentration (MIC) [390]. This difference of protocols does not facilitate the comparison with recent research, but the activity of other phenol compounds still allows us to obtain a general picture of the antiseptic properties of this set of compounds. Notably, Robinson's team noticed that all the molecules they developed were particularly effective against Gram-positive bacteria but had small efficiency against Gram-negative bacteria. Concerning the acid-fast bacteria (*M. phlei*), some molecules could also kill these organisms but only at high concentrations. Finally, the authors demonstrated an improvement of the bactericidal activity by increasing the number of carbons in the side-chains toward Gram-positive bacteria but also on *Bacterium typhosum*, responsible for typhoid fever [391]. This effect reaches its maxima for three molecules (see Scheme 156: 522, 523 and 524), with an activity more than ten times higher than that of the methyl or p-n-octylphenol (expressed in equimolecular phenol coefficient of bactericidal power). Even if these molecules have not been used as antibiotics later, modern researchers may inspire for sure from these former studies to design modern drugs due to the rise in antibiotic resistance.
<figure><img src="image_293.png" /><figcaption>**Scheme 156.** Examples of hydroxyindanediones 522–524 with important antibacterial activities against *Bacterium typhosum*.</figcaption></figure>

The place of indane-1,3-dione in this field has clearly evolved. A structure related to indane-1,3-dione, namely indan-1-one, also showed interesting antimicrobial properties so that these two structures were often studied concomitantly for the design of antimicrobial compounds [1–3]. Considering that the biological activity of indane-1,3-dione **4** can be greatly improved by chemical engineering, indane-1,3-dione **4** has thus been extensively used as a building block in multicomponent reactions (MCRs) to combine the properties of indanones or indane-1,3-dione with that of other structures also displaying physiological properties.

A relevant example of this strategy has been reported in 2011 with a study devoted to the antimicrobial activity of 1,3-disubstituted indeno [1,2-c]pyrazoles [392]. Pyrazoles are an important class of pharmaceutical compounds. Considering that heterocyclic systems have been the focus of intense synthetic efforts during the last decades, indenopyrazoles have thus been extensively screened. In this study, the authors have notably synthetized 16 compounds possessing a 4-substituted thiazole moiety, all prepared with indane-1,3-dione as the starting material, and two different series were developed (see Scheme 157) in order to compare the impact of the fused indenopyrazole on the antimicrobial properties.

Several micro-organisms were tested such as Gram-positive bacteria (*S. aureus*, *B. subtilis*), Gram-negative-bacteria (*E. Coli*) and fungus (*Aspergillus niger*, *Candida albicans*). In this series of molecules, three molecules showed noticeable activities against *C. albicans*, *A. niger*, *S. aureus* and *E. Coli* (528h, 529d, and 529h). More precisely, antimicrobial activity of these molecules proved to be on par with that of the reference compound, i.e., Norfloxacin. Nevertheless, 528h was less effective against *B. subtilis*, whereas the two others only showed moderate minimum inhibitory concentration (MIC) with this micro-organism. Noticeably, an improved antimicrobial activity was found for all compounds once cyclized as indenopyrazoles, and a higher activity was also determined for all molecules substituted with 4-chlorophenyl groups.

After highlighting the importance of this structure, the same team published a second work in which the synthetic strategy was modified compared to the previous work and seventeen 3-aryl-1-heteroarylindeno [1,2-c]pyrazol-4(1H)-ones 532 could be prepared with this new synthetic method [393]. Indane-1,3-dione **4** was used to perform Knoevenagel condensations with various aldehydes 368, and the formation of the pyrazole cycle could be obtained by reaction of 2-hydrazinylbenzo[d]thiazole/2-hydrazinyl6-substituted benzo[d]thiazoles 530 and the Knoevenagel adduct 255 in stoichiometric amount, according to the reaction presented in Scheme 158. Despite mild reaction conditions, the reaction yields remained low, ranging between 25 and 41%, even after optimization. Nonetheless, almost all of these new synthetized molecules showed an activity against four bacteria, Gram-positive and Gram-negative, but also against fungi.
<figure><img src="image_294.png" /><figcaption>**Scheme 157.** The two families of indeno [1,2-c]pyrazoles examined for their antimicrobial activities.</figcaption></figure>

<figure><img src="image_295.png" /><figcaption>**Scheme 158.** Synthetic routes to 3-aryl-1-heteroarylindeno [1,2-c]pyrazol-4(1H)-ones 532.</figcaption></figure>
In 2018, another multicomponent reaction involving indane-1,3-dione **4** was reported by Alsharif et al. for the design of an antimicrobial agent comprising nitrogen-based heterocyclic compounds [394]. The four-component reaction involving indane-1,3-dione **4**, 9-ethyl-9H-carbazole-3-carbaldehyde **533**, ethyl acetoacetate **534** and ammonium acetate **535** in the presence of a catalytic amount of piperidine could furnish ethyl 4-(9-ethyl-9H-carbazol-3-yl)-2-methyl-5-oxo-4,5-dihydro-1H-indeno [1,2-*b*]pyridine-3-carboxylate (ECPC, **536**), following a procedure previously reported in the literature (see Scheme 159) [395]. This molecule was notably tested for its antibacterial activity, and for this study, *S. aureus Streptococcus pyogenes* (*S. pyogenes*) was selected as the Gram-positive bacteria and *E. Coli* as *Salmonella typhimurium* (*S. typhimurium*) as the Gram-negative bacteria. Biological activity of ECPC was also compared with that of tetracycline used as a standard for these tests. ECPC **536** showed an interesting antibacterial activity, since minimum inhibitory concentrations as low as 32 µg/mL were determined with the four bacteria, comparable to that required with the reference tetracycline.

<figure><img src="image_296.png" /><figcaption>**Scheme 159.** Synthesis of ethyl 4-(9-ethyl-9H-carbazol-3-yl)-2-methyl-5-oxo-4,5-dihydro-1H-indeno [1,2-*b*]pyridine-3-carboxylate (ECPC **536**.</figcaption></figure>

In 2012, another series of heterocyclic molecules was synthetized using indane-1,3-dione **4** as the building block in an ionic liquid-catalyzed three-component condensation involving thiourea **537** and the appropriate aromatic aldehyde **368**, and the resulting molecules **538** were tested as antimicrobial agents (see Scheme 160 and Table 38) [396]. In this work, the authors focused their attention on the pyrimidine scaffold, which is present in numerous natural physiologically active substances. The authors improved the biological activity of indane-1,3-dione derivatives by attaching a thione moiety to the pyrimidine group, this group being also known for its antifungal, antibiotic and antibacterial activities. Among the fourteen 4,6-diaryl- and 4,5-fused pyrimidine-2-thiones synthesized in this work, only three of them were prepared with indane-1,3-dione, the other compounds being variously substituted tetrahydrobenzo[*h*]quinazoline-2-thiones and pyrimidine-2-thiones. Among the three indane-1,3-dione derivatives, only **538c** showed a better antibacterial activity toward *B. subtilis*, *S. aureus*, *Pseudomonas aeruginosa* (*P. aeruginosa*) and *E. coli* and a better antifungal activity toward *A. niger*, *C. albicans*, *A. fumigatus* than the variously substituted tetrahydrobenzo[*h*]quinazoline-2-thiones and pyrimidine-2-thiones. In addition, among the eleven other 4,6-diaryl- and 4,5-fused pyrimidine-2-thiones, only two of them showed similar antimicrobial properties than **538c**, even if all of them have a less important zone of inhibition than the reference compound, namely Ampicillin trihydrate.

<figure><img src="image_297.png" /><figcaption>**Scheme 160.** Synthesis of pyrimidine-2-thiones **538** starting from indane-1,3-dione **4** along with their corresponding antibacterial and antifungal activities.</figcaption></figure>
<table><caption>**Table 38.** Antibacterial and antifungal activities of **538** derivatives.</caption><thead><tr><th rowspan="3">Entry</th><th colspan="7">Zone of Inhibition/mm</th></tr><tr><th colspan="2">Gram-Positive</th><th colspan="3">Gram-Negative</th><th colspan="2">Fungi</th></tr><tr><th><i>B. subtilis</i></th><th><i>S. aureus</i></th><th><i>P. aeruginosa</i></th><th><i>E. coli</i></th><th><i>A. niger</i></th><th><i>C. albicans</i></th><th><i>A. fumigatus</i></th></tr></thead><tbody><tr><td><b>538a</b></td><td>9</td><td>10</td><td>6</td><td>8</td><td>8</td><td>10</td><td>11</td></tr><tr><td><b>538b</b></td><td>15</td><td>14</td><td>10</td><td>11</td><td>12</td><td>14</td><td>17</td></tr><tr><td><b>538c</b></td><td>21</td><td>24</td><td>12</td><td>14</td><td>19</td><td>17</td><td>19</td></tr></tbody></table>

Finally, in 2016, another research group designed a series of spiro[indolo-3,10'-indeno [1,2-b]quinolin]-2,4,11'-triones **540** by means of a three-component condensation of enam-inones **539**, isatin **231** and indane-1,3-dione **4** using a mixture of ethanol/water as the solvent and cerium ammonium nitrate (CAN) as the catalyst (see Scheme 161) [397]. As the main advantage of this approach, spiro[indolo-3,10'-indeno [1,2-b]quinolin]-2,4,11'-triones **540** could be obtained with short reaction times, in high yields and by using a simple synthetic protocol. From this viewpoint, spiro[indolo-3,10'-indeno [1,2-b]quinolin]-2,4,11'-triones **540** reported in this work could be obtained by a greener approach than that previously reported in the literature [300,398–401]. Antimicrobial activities of these compounds have been tested on Gram-positive bacteria (*S. aureus* and *B. subtilis*) and Gram-negative bacteria (*E. coli* and *P. aeruginosa*) as well as their antifungal activity in two yeasts: *Candida albicans* and *Saccharomyces cerevisiae*. Among the twenty-two molecules synthesized in this work, all of them showed good antibacterial and antifungal activities, but none of them were effective against *P. aeruginosa*. Nevertheless, the best results were obtained for **540-IVc**. Notably, **540-IVc** exhibited the lowest MIC of 16 mg/mL against *S. aureus*, a MIC of 8 µg/mL against *B. subtilis* and a MIC of 64 µg/mL against *E. coli*. In the case of yeast, **540-IVc**, **540-IVk** and **540-IVn** showed the best inhibition ability against *C. albicans* and *S. cerevisiae* with inhibition zones of 15 and 16 mm, respectively. Furthermroe, **540-IVc** and **540-IVn** were also determined as being more efficient against *B. subtilis* and *S. aureus*, the inhibition zone reaching 22 and 24 mm, respectively. However, despite these promising results, none of the new compounds could surpass the inhibition diameters obtained with the reference compound, i.e., ciprofloxacin (26, 24 and 25 nm against *S. aureus*, *B. subtilis* and *E. Coli*, respectively).

## 5.2. Indane-1,3-Diones as Anticancer Agents

Cancer is a generic name that regroups all diseases where the proliferation of abnormal cells is observed. Since only half of the cancer patients do not recover with systemic chemotherapy or obtain only a partial recovery, the development of anticancer agents is thus the focus of intense research efforts [402]. This research was also supported by the current incapacity to cure cancers combined with longer life expectancy but also by the fact that cancer is the second leading cause of death.

The combination of N-heterocycles and indenones has been widely studied during the last 40 years for the design of various compounds exhibiting anti-cancer properties [403–405]. Inspired by Onychine, which is a natural biologically active azafluorenone, numerous synthetic azafluorenonees have been prepared and identified as exhibiting promising cytotoxic, phosphodiesterase inhibitory, adenosine A2a receptor antagonistic, anti-inflammatory/antiallergic, coronary-dilating and calcium-modulating properties [406,407]. Following the course of these investigations, a series of indeno-heterocycles has been designed by an MCR involving indane-1,3-dione **4** as the building block, and a library of 33 indeno-heterocycles **541** could be obtained (see Scheme 162). All these molecules were evaluated for their potential cytotoxic and apoptosis-inducing properties [407]. As specificities, all molecules reported in this work exhibited the same indenopyridine moiety in order to maintain the planarity of the structure. Planar structures are required in order for these molecules to act as DNA intercalators and topoisomerase inhibitors. Even if a similar synthetic protocol was used for the synthesis of the 33 molecules, different structures were nevertheless ob-
tained. These compounds were notably tested for their cytotoxicity on Jurkat cells, a model for human T-cell leukemia, with Annexin-V/propidium iodide assay [408]. However, cytotoxicity of molecules **541** remained low since viability of the Jurkat cells was around 85–98% relative to the control experiments, and apoptosis induction was even lower, ranging between 1–6%. Aniline, pyrazole and triazole-based indeno-heterocycles **541** proved to be totally inactive for apoptosis induction. Only the pyrimidinedione-containing indeno-heterocycle showed a somewhat enhanced cytotoxicity, the cell viability being reduced to 80%. All attempts to mix the pyrimidinedione-containing indeno-heterocycle with the other compounds did not improve the cytotoxicity, evidencing that the pyrimidinedione moiety was the key-element to obtain an acceptable cytotoxicity. To obtain a deeper insight into the biological activity of this pyrimidinedione-containing indeno-heterocycle, this compound was compared to etoposide, which is an anti-cancer clinical drug agent known for its remarkable cytotoxic effect through topoisomerase II-dependent DNA cleavage. The two molecules could manifest good apoptosis-induction activities against Jurkat cells. Furthermore, the indeno-pyrimidine compound could demonstrate better cell-killing and apoptosis-induction activities than etoposide at low concentration (cytotoxic IC₅₀ = 3 µM). Nevertheless, clinical tests still need to be performed with this pyrimidinedione-containing indeno-heterocycle, and the authors also highlight the important drawback of poor water solubility of this compound, which may affect its future use as anti-cancer agent.

<figure><img src="image_298.png" /><figcaption>22 compounds, 16 tested</figcaption></figure>

<table><thead><tr><th>Product</th><th>R1</th><th>R2</th><th>R3</th><th>Tested</th></tr></thead><tbody><tr><td>IVa</td><td>CH3</td><td>C6H5</td><td>H</td><td>No</td></tr><tr><td>IVb</td><td>CH3</td><td>4-BrC6H4</td><td>H</td><td>No</td></tr><tr><td>IVc</td><td>CH3</td><td>4-CIC6H4</td><td>H</td><td>Yes</td></tr><tr><td>IVd</td><td>CH3</td><td>4-CH3C6H4</td><td>H</td><td>Yes</td></tr><tr><td>IVe</td><td>CH3</td><td>4-(OCH3)C6H4</td><td>H</td><td>Yes</td></tr><tr><td>IVf</td><td>H</td><td>4-CIC6H4</td><td>H</td><td>Yes</td></tr><tr><td>IVg</td><td>H</td><td>C6H5</td><td>H</td><td>Yes</td></tr><tr><td>IVh</td><td>H</td><td>4-(OCH3)C6H4</td><td>Br</td><td>Yes</td></tr><tr><td>IVi</td><td>H</td><td>4-CH3C6H4</td><td>Br</td><td>Yes</td></tr><tr><td>IVj</td><td>H</td><td>4-CH3C6H4</td><td>NO2</td><td>Yes</td></tr><tr><td>IVk</td><td>H</td><td>4-CIC6H4</td><td>NO2</td><td>Yes</td></tr><tr><td>IVl</td><td>H</td><td>C6H5</td><td>NO2</td><td>Yes</td></tr><tr><td>IVm</td><td>H</td><td>C6H5</td><td>Br</td><td>Yes</td></tr><tr><td>IVn</td><td>CH3</td><td>C6H5</td><td>NO2</td><td>Yes</td></tr><tr><td>IVo</td><td>H</td><td>4-BrC6H4</td><td>NO2</td><td>No</td></tr><tr><td>IVp</td><td>CH3</td><td>4-CIC6H4</td><td>NO2</td><td>Yes</td></tr><tr><td>IVq</td><td>H</td><td>4-BrC6H4</td><td>Br</td><td>No</td></tr><tr><td>IVr</td><td>CH3</td><td>n-butyl</td><td>Br</td><td>Yes</td></tr><tr><td>IVs</td><td>H</td><td>Cyclohexyl</td><td>Br</td><td>Yes</td></tr><tr><td>IVt</td><td>CH3</td><td>4-CIC6H4</td><td>Br</td><td>Yes</td></tr><tr><td>IVu</td><td>CH3</td><td>2-Furfuryl</td><td>Br</td><td>No</td></tr><tr><td>IVv</td><td>CH3</td><td>2-Furfuryl</td><td>NO2</td><td>No</td></tr></tbody></table>

<figure><img src="image_299.png" /><figcaption>**Scheme 161.** Strategy used for the synthesis of spiro[indolo-3,10'-indeno[1,2-b]quinolin]-2,4,11'-triones **540**-(IVa-IVv) along with their graphical representations of the diameter of growth of inhibition (mm) against bacteria strains. Reproduced with permission from Ref. [397].</figcaption></figure>
<figure><img src="image_300.png" /><figcaption>**Scheme 162.** MCR leading to two different structures of indeno-heterocycles **541**. Apoptosis properties were evaluated at 25 µM.</figcaption></figure>

Three years later, in the continuation of the pioneering work initiated by Manpadi et al., another series of 35 pentacycle-based indeno-heterocycles **544** and **545** was prepared in order to investigate the cell-killing and the apoptosis mechanism toward a panel of human cancer cell lines [409]. All these structures were inspired by camptothecin, which is extensively used in traditional Chinese medicine. As the main motivation of this study, the authors suggested the drug action of these molecules to operate via an intracellular oxidation of the dihydropyridine cycle of **544**, leading to the formation of the pyridine moiety **545** in situ (see Scheme 163). To demonstrate this, all compounds were synthesized with a hydrogenated (“h”) and an aromatized (“a”) version of the indenopyridine core using the same three-component synthesis of that used by Manpadi. When no spontaneous oxidation was observed, an additional oxidation step with chloranil in DMF was required to obtain the aromatized structure. These molecules were further tested for their antiproliferative properties on a panel of human cancer cell lines (HeLa, Jurkat, MCF-7, A-549, Lovo, U373, SKMEL, PC3, MG-MID), representing many species of cancer such as T-cell leukemia, cervical, breast, or lung cancers. The authors demonstrated that the modification of the R group on the aromatic ring did not significantly modify the biological activity of the molecules, except that the molecule bearing R=H showed no cytotoxic activity. The biological tests also revealed that the dihydropyridines **544** do not have cytotoxic activities contrarily to the pyridine analogues **545**, supporting an intracellular oxidation process of the dihydropyridines **544** to the corresponding pyridines **545**.

Sulfonamides are well known to act as inhibitors of numerous human α-carbonic anhydrases [410]. Carbonic anhydrases are essential in humans, as these anhydrases are capable of catalyzing the reversible hydration of CO₂ and allow for the respiration and the transportation of carbon dioxide within the human body. In 2012, the idea of Ghorab et al. was to combine the remarkable properties of sulfonamides to that of indenopyridines, which also possess anti-cancer properties [411]. To obtain these structures (**549**), a two-step synthesis involving indane-1,3-dione **4**, 4-aminobenzenesulfonamide **546** and an aromatic aldehyde **368** as the starting materials was performed, the two steps being both realized in EtOH as the solvent. Anti-cancer activity of the resulting 18 compounds **549a-549q** was tested in vivo against breast cancer cell-line (MCF 7) and their biological activities compared to that of doxorubicin, a reference drug. Among all compounds, **549d** exhibited an IC₅₀ of 4.34 µM, lower than that of the reference doxorubicin (5.40 µM), evidencing the
pertinence of the approach. Furthermore, **549d**, which is specifically substituted with a hydroxyphenyl group at the 4-position and a cyano group at the 3-position exhibited an increased cytotoxic activity compared to the other compounds, and this specific substitution is certainly at the origin of the higher potency of this molecule (see Scheme **164**).

Related to indenopyridines, indenopyrazoles are another family of structures showing important inhibiting tyrosine kinase activities, but also anti-cancer or anti-proliferative properties [412]. Nevertheless, for this purpose, indane-1,3-dione **4** was not directly used for their synthesis but a derivative containing a close scaffold: 2-(4-methoxybenzoyl)indan-1,3-dione **552**. Its synthesis is similar to that of compounds **4** and **67** described in Section 1 except that no heating was required for the last step to obtain the triketone molecule and, in a second step, the intramolecular cyclization of **552** with hydrazine hydrate could furnish the targeted indenopyrazoles **553**, as shown in Scheme **165** [413]. In 2002, another library of indenopyrazoles mainly substituted at the 5-position of aniline and at the 3-position with various groups was examined as Cyclin-dependent kinase (CDK) inhibitors [414]. Ccln-dependent kinases play a key role in the cell cycle regulatory machinery and the replication process. Considering that the different indenopyrazoles can regulate the development of proliferative tumors such as cancers, these molecules have notably been tested against various cancer cell lines (HCT116, NCI-H460, PC-3, MiaPaCa-2, HT-29, HT-1080 and B16-F0). After selecting **553-k** for its good activity and selectivity against kinase targets (CDK2/E: IC₅₀ = 13 nM, CDK1/B: IC₅₀ = 44 nM), this molecule exhibited good cytotoxicity, particularly for HCT116 cells, which died not only by CDK inhibition but also by activation of the apoptotic machinery and by inhibition of Rb phosphorylation. This strong activity combined with a remarkable selectivity of cells offers very promising therapeutic applications, explaining why several patents have been established for structures close to that of **553-k** [412,415,416].

<figure><img src="image_301.png" /><figcaption>**Scheme 163.** Camptothecin-inspired pentacycle-based indeno-heterocycles **544** and **545**.</figcaption></figure>
<figure><img src="image_302.png" /></figure>

17 examples

549a : Ar = C₆H₅

549b : Ar = p-C₆H₄CH₃

549c : Ar = o-C₆H₄OH

549d : Ar = p-C₆H₄OH

549e : Ar = p-C₆H₅CH=CH

549f : Ar = o-C₆H₄OCH₃

549g : Ar = p-C₆H₄OCH₃

549h : Ar = pipronyl

549i : Ar = p-C₆H₄N(CH₃)₂

549j : Ar = m-C₆H₄NO₂

549k : Ar = p-C₆H₄NO₂

549l : Ar = m-C₆H₄Cl

549m : Ar = o,p-C₆H₃Cl₂

549n : Ar = 2-C₁₀H₆OH

549o : Ar = 2-C₁₀H₆OCH₃

549p : Ar = 4-C₁₀H₆OH₃

549q : Ar = 2-thienyl

<figure><img src="image_303.png" /><figcaption>**Scheme 165.** Synthetic routes to indenopyrazoles and the best candidate **553-k**.</figcaption></figure>

5.3. *Indane-1,3-Dione as Building Block for Bioimaging Agents*

Fluorescent imaging techniques play an important role in medicinal chemistry by enabling to localize tumors at the cellular level and in this aim; extensive works are performed to develop fluorescent dyes capable of target specific cells or ions [417]. With the aim at gaining a better understanding of all these complex diseases, multiple recent techniques have been developed to visualize tumors, and numerous fluorescent agents have been proposed with indane-1,3-dione, which remains a cheap and versatile building-block in chemistry.

One of the most popular imaging techniques is named aggregation-induced emission (AIE). As specificity, dyes exhibiting AIE properties are often weakly emissive in solution but highly emissive in the solid state due to a restriction of the intramolecular rotations that constitute non-radiative deactivation processes. In addition to enhancing the fluorescence intensity of dyes, AIE is also extensively used for bioimaging due to its biocompatibility, its high-fidelity imaging, high brightness and long-living excited state favorable for an efficient
in-situ imaging [418]. In 2020, an interesting study demonstrated that beyond imaging, AIE dyes could also be used for the generation of reactive oxygen species (ROS), enabling to combine bioimaging and photodynamic therapy (PDT) [419]. However, the key point of this strategy relies in a perfect targeting of the infected lysosomes by AIE dyes. To achieve this goal, three molecules based on triphenylamines were designed, bearing morpholines as pendant groups. In these structures, triphenylamine was used as the electron donor for the design of the different push-pull dyes and indane-1,3-dione, malononitrile or the formyl group were used as the electron acceptors (see **MPAA 554**, **MPAN 555** and **MPAT 556** in Scheme 166).

<figure><img src="image_304.png" /><figcaption>**Scheme 166.** "Push-pull" effect in IND-TPA with TICT schematisation used in this study.</figcaption></figure>

Among the three dyes, **MPAT 556** exhibited the most red-shifted absorption and emission of the series so that this dye was the focus of extensive works. The three dyes showed a remarkable fluorescence stability since upon irradiation with a laser for 20 min., a decrease of less than 10% of the initial fluorescence intensity was determined, far from the loss of 50% determined with the benchmark LysoTracker Green DND-26 [420]. Examination of the fluorescence properties of **MPAA 554**, **MPAN 555** and **MPAT 556** in water revealed these dyes to exhibit high stoke shifts ranging between 187, 160 and 188 nm for **MPAA 554**, **MPAN 555** and **MPAT 556**, respectively. For **MPAT 556** displaying the most red-shifted emission, fluorescence maxima at 673 nm in water and 675 nm in the solid state could be determined. Conversely, for the other dyes, emissions located at 530 and 614 nm could be
respectively determined in water for **MPAA 554** and **MPAN 555**. For these reasons, the indane-1,3-dione adduct **MPAT 556** was selected for the biological tests in A549, HeLa and HepG2 cells. No significant decrease for the cells' viability was detected after 24 h of incubation, even at high concentration (100 µM), demonstrating that **MPAT 556** can be used for long-term monitoring of cells without inducing cells apoptosis. In vivo experiments in zebrafish also furnished nice lysosome images with a good dot distribution during imaging of vertebrates. Finally, the ability of **MPAT 556** to promote ROS production was evidenced while using H2DCF-DA as the probe. In PBS buffer solutions, a 36-fold enhancement of the photoluminescence of H2DCF-CA was evidenced, demonstrating the production of ROS by **MPAT 556** upon green light irradiation. In HeLa cells, cell viability showed a remarkable decline, the cell viability reducing to 19% at 40 µM MPAT and after 10 min. of irradiation with a green light (see Figure 4).

<figure><img src="image_305.png" /><figcaption>**Figure 4.** (A) Generation of ROS in PBS buffer. (B) Cell viability of HeLa living cells, stained with MPAT, upon irradiation with a green light for 10 min. Reproduced with permission from Ref. [419].</figcaption></figure>

In 2017, Gao et al. used AIE dyes for a completely different application, namely the lipid droplet-specific (LD) imaging and the dynamic movement tracking [421]. Lipid droplets (LDs) are involved in many metabolic processes. However, conventional dyes used to stain LDs suffer from numerous drawbacks such that new fluorophores are actively researched. Several families of molecules have been tested over the years, but the different fluorophores exhibited numerous limitations such as aggregation-caused quenching (ACQ), low stoke shift (40 nm), high background noise or even difficulty of preparation [422–430]. Consequently, the molecule selected in this study was a classical push–pull dye made of triphenylamine, as the electron-donating group was connected to indane-1,3-dione acting as the electron-withdrawing group and prepared by means of a Knoevenagel reaction. In addition of its AIE properties, **557** also exhibits what is named a twisted intramolecular charge transfer (TICT), resulting from restricted intramolecular rotations in the solid state. Based on both AIE and TICT properties, **557** was thus identified as a promising candidate for LD-specific imaging and movement tracking due to its high two-photon absorption cross-sections in the near infrared (NIR) range (see Scheme 167). For this reason, **557** was notably used as fluorescent dye for near infrared (NIR) two-photon excited fluorescence (TPEF).
<figure><img src="image_306.png" /><figcaption>**Scheme 167.** Chemical structure of 557 and the different advantages of this AIE dye.</figcaption></figure>

To evidence the AIE effect in **557**, UV-visible absorption and emission spectra were recorded in a THF/water mixture. A raise in the water fraction in THF from 0 to 70% led to a decrease in the fluorescence but also to a significant redshift not only of the emission spectrum (from 594 to 609 nm) but also of the absorption spectrum (from 478 to 489 nm), which can be attributed to TICT effects. A further increase in the water fraction in THF up to 99% induced a further redshift of the emission maximum up to 612 nm, resulting from the formation of nanoaggregates of average diameter of 119.6 nm, measured by light scattering analyses. Finally, **557** was tested on HCC827 and A549 cells. As shown in Figure 5, **557** could efficiently stain the cells with high signal-to-noise ratios. From the different tests, several advantages were determined for **557** such as:

* A fast permeability;

* An excellent selectivity for LD;

* A high photostability;

* A low cytotoxicity.

<figure><img src="image_307.png" /><figcaption>**Figure 5.** CLSM images of HCC827 (A–E) and A549 (F–J) cells after incubation with 557 (5 mM) and BODIPY493/503 (100 nM) at 37 °C for 15 min. (A,F) Bright-field images. (B,G) Fluorescence image from 557 and from BODIPY493/503. (D,I) The merged images. (E,J) The intensity profile of ROI lines. Scale bar = 20 mm. Reproduced with permission of Gao M, Su H, Lin Y, Ling X, Li S, Qin A, and Zhong Tang B. Photoactivatable aggregation-induced emission probes for lipid droplets-specific live cell imaging. Reproduced with permission from Ref. [421].</figcaption></figure>
Finally, photostability of 557 was examined upon irradiation at 514 nm with a laser (7% power) for 10 min. As shown in Figure 6, the fluorescence intensity loss was less than 20%, evidencing the good photostability of this dye.

<figure><img src="image_308.png" /><figcaption>**Figure 6.** I/I₀ (%) of fluorescence intensity of HCC827 cells colored with 557 (5 µM) with increasing time of irradiation at 514 nm with 7% laser power. Inset: Fluorescence images of HCC827 cells with increasing time of irradiation. Reproduced with permission from Gao M, Su H, Lin Y, Ling X, Li S, Qin A, Zhong Tang B. Photoactivatable aggregation-induced emission probes for lipid droplets-specific live cell imaging. Reproduced with permission from Ref. [421].</figcaption></figure>

As we saw previously, the selectivity of dyes to target the right cells is a primordial need, especially when photodynamic therapy (PDT) is envisioned in complement to cell visualization. In 2019, Mondal et al. performed an MCR using indane-1,3-dione 4 as a component to elaborate a series of spiro-heterocyclic compounds 561–564 (see Scheme 168) that was subsequently tested as chromophores for the selective detection of Zn²⁺ ion and cell imaging [431]. Regarding specificity, this synthesis was realized by means of a microwave-assisted multi-component reaction (MWAMCRs); the reaction was carried out in EtOH, and the products could be obtained in pure form without chromatography, making this approach highly biocompatible.

Nine of the compounds were selected for further tests aiming at evaluating the possibility to coordinate Zn²⁺ ion, determining the selectivity toward other ions other than Zn²⁺ or the fluorescence emission at physiological pH. Among the nine dyes, one of them (i.e., **564f**) was even selected for further in vivo tests on human hepatocellular liver carcinoma cells (HepG2 cells). This chemosensor could be successfully used as an intracellular Zn²⁺ imaging agent due to its remarkable cell permeability properties, where this molecule exhibited a fluorescence comportment only when coordinated with Zn²⁺ even in vivo. Cytotoxicity tests also revealed **564f** to be weakly toxic since the cell viability was higher than 90% at 10 µM. Thus, **564f** can be effectively used in vivo as chemosensors for the detection of Zn²⁺ cations. A mechanism supporting the enhancement of fluorescence upon coordination to Zn²⁺ cations was also proposed by the authors and is depicted in Scheme 169.
<figure><img src="image_309.png" /></figure>

<figure><img src="image_310.png" /></figure>

561a : R₁= phenyl; X=H; Yield=89%

561b : R₁= 3-methylbenzene; X=H; Yield= 85%

561c : R₁= 4-methylbenzene; X=H; Yield= 90%

561d : R₁= 4-methoxybenzene; X=H; Yield= 95%

561e : R₁= 4-chlorobenzene; X=H; Yield= 88%

561f : R₁= 4-bromobenzene; X=H; Yield= 87%

561g : R₁= butyl; X=H; Yield= 76%

561h : R₁= phenyl; X=Cl; Yield=88%

561i : R₁= 3,4-bismethylbenzene; X=Cl; Yield= 92%

561j : R₁= phenyl; X=Br; Yield=89%

561k : R₁= 4-methylbenzene; X=Br; Yield= 91%

<figure><img src="image_311.png" /></figure>

562a : R₁= 4-bromobenzene; R₂= -CH₃; X=H; Yield= 90%

562b : R₁= 4-Chlorobenzene; R₂= -CH₂CH₃; X=H; Yield= 88%

562c : R₁= 4-methoxybenzene; R₂= -CH₂CH₂CH₃; X=H; Yield= 92%

562d : R₁= 3,4-bismethylbenzene; R₂= -CH₂CH₂CH₂CH₃; X=H; Yield= 86%

562e : R₁= phenyl; R₂= -CH₂C₆H₅; X=H; Yield= 75%

562f : R₁= 4-methoxybenzene; R₂= -CH₂C₆H₅; X=H; Yield= 85%

562g : R₁= 4-methylbenzene; R₂= -CH₂CH₂CH₃; X=Cl; Yield= 80%

562h : R₁= 4-methylbenzene; R₂= -CH₂CH₂CH₂; X=Cl; Yield= 74%

562i : R₁= phenyl; R₂= -CH₂C₆H₅; X=Br; Yield= 75%

<figure><img src="image_312.png" /></figure>

563a : R₁= 4-methylbenzene; X=H; R₂=CH₃; Yield=88%

563b : R₁= 4-chlorobenzene; X=H; R₂=CH₃; Yield=83%

563c : R₁= butyl; X=H; R₂=CH₃; Yield=75%

563d : R₁= 4-methoxybenzene; X=Cl; R₂=CH₃; Yield= 90%

563e : R₁= 3,4-bismethylbenzene; X=Br; R₂=CH₃; Yield= 89%

563f : R₁= 4-bromobenzene; X=NO₂; R₂=CH₃; Yield= 80%

563g : R₁= 4-methylbenzene; X=H; R₂=C₆H₅; Yield=78%

<figure><img src="image_313.png" /></figure>

564a : R₁= phenyl; Yield=84%

564b : R₁= 3-methylbenzene; Yield= 92%

564c : R₁=3,4-bismethylbenzene; Yield= 91%

564d : R₁= 4-isopropylbenzene; Yield= 92%

564e : R₁= 4-terbutylbenzene; Yield= 90%

564f : R₁= 4-methoxybenzene; Yield= 96%

564g : R₁= 4-chlorobenzene; Yield= 82%

564h : R₁= 4-bromobenzene; Yield=85%

564i : R₁= 2-iodobenzene; Yield= 80%

<figure><img src="image_314.png" /><figcaption>**Scheme 169.** Plausible mechanism of the above molecules in the presence of Zn²⁺ cations.</figcaption></figure>

### 5.4. Indane-1,3-Dione in Neurology Drugs

The brain is the most complex organ in humans, and its working principle is still not fully understood yet, which explains the lack of medicines specific to this organ, despite all the research performed in this field. Moreover, the eldering of the populating on Earth
will cause a significant increase in the neurodegenerative diseases that usually occur after 65 years, so that the demand on treatments for these illnesses will increase in the coming decades [432,433]. Parkinson's disease (PD) is one of the most important neurodegenerative diseases and causes an important loss of dopamine in the brain, altering motor function cognition and the mood of the patient. Common treatments to PD are antagonists of two brain receptors, namely A₂a and A₁. However, it is still unknown yet if either one of the two brains receptors plays a more important role than the other in the disease [434].

In this context, Shook's team published a series of arylindenopyrimidines **565** that were developed as potential dual A₂A/A₁ antagonists and that could be potentially used for the treatment of AD [435]. These arylindenopyrimidines **565** were synthetized through a two-step synthesis using indane-1,3-dione **4** as the starting material (see chemical structures in Scheme 170). In vitro and in vivo tests performed with eight molecules revealed unequal results depending on the substitution pattern. Notably, the different tests revealed the necessity to let the NH₂ group be unsubstituted, whereas the substitution of the pendant aromatic group had almost no incidence on the in vivo and in vitro activity. Furthermore, molecules substituted with morpholine or pyrrolidine groups showed the highest in vivo activity in mouse catalepsy.

<figure><img src="image_315.png" /><figcaption>**Scheme 170.** Synthetic route to arylindenopyrimidines **565**.</figcaption></figure>

To improve the biological activity, two different syntheses were developed to obtain substitutions on the heterocycle next to the indane group. The 12-step synthesis developed by the authors could give access to twelve compounds **577** substituted at the 9-position, and the seven-step synthesis developed in parallel could furnish 12 other compounds **584** substituted at the 8-position (see Scheme 171).

Comparison between the two series of analogs **577** and **584** revealed an improved biological activity for all molecules substituted at the 8-positions compared to those bearing a substitution at the 9-positions. Notably, a difference in the mouse catalepsy as high as 0.2 and 10.7 mg/kg for the ED₅₀ for the regioisomers **577-(1)** and **584-(1)** bearing a piperidine substituent could be determined. Furthermore, several adducts, particularly those containing cyclic amines and heterocyclic functions attached with an alkyl secondary amine showed ED₅₀ in the micro-scale for the mouse catalepsy. These adducts also showed decent in vitro activity so that these structures can be cited as promising candidates for AD treatments (see Scheme 172). Nevertheless, the lack of the commercially available substituted indane-1,3-diones allowing access to these structures is relatively complex and costly.

Furthermore, these structures remain promising, as exemplified with JNJ-40255293 (**585**) for which preclinical tests have been carried out (see Scheme 173) [436]. These additional tests revealed a better selectivity of the JNJ-40255293 antagonist toward the A₂a receptors than the A₁ ones. Unfortunately, preclinical tests were rapidly abandoned due to genotoxicity, neuronal necrosis and edema observed in several animals [437].

In 2002, more promising results were obtained with another series of indeno-fused structures, namely arylindenopyridines **588** and **593** that were patented for AD treatments (see Scheme 174) [438]. To support the biological efficiency, the authors suggested a binding of arylindenopyridines onto the A₂a receptors. Anti-inflammatory activity was also identified, with phosphodiesterase (PDE)-inhibiting activity. The starting materials for these syntheses were various 1,3-indanedione adducts (**586** or **590**), such as 5-nitroindane-1,3-dione (see Scheme 138). The MCR used to access to these structures involved in the first step a Knoevenagel reaction between the indane-1,3-dione derivative **586** or **590** with
the appropriate aldehyde **368**, followed by a condensation of guanidine carbonate **588** to the push-pull compound **587** to form the indenopyrimidine **589**. In the case of **491**, an aromatization with DDQ was required, providing **593**. The selectivity toward the A₂a receptor and A₁ were tested in vitro, and it turned out that some of these molecules could exhibit Kᵢ lower than 50 nM, particularly with the A₂a binding. Despite these interesting preliminary results, no in vivo tests were performed with these molecules.

The 12-steps synthesis

<figure><img src="image_316.png" /><figcaption>**Scheme 171.** *Cont.*</figcaption></figure>
The 7-steps synthesis

<figure><img src="image_317.png" /><figcaption>**Scheme 171.** Summary of the synthetic strategy developed to access compounds 577 and 584 substituted at the 8- and 9-positions.</figcaption></figure>

<figure><img src="image_318.png" /><figcaption>**Scheme 172.** Comparisons between six compounds substituted at the 8- and the 9-positions for their in vitro and in vivo activities. In vitro activity for A2a and A1 functional assays and in vivo results for mouse catalepsy at 10 mg/kg, po.</figcaption></figure>
JNJ-40255293 (585)

<figure><img src="image_319.png" /></figure>

A2a Kᵢ = 6.5 nm

A1 Kᵢ = 48.2 nm

ED₅₀ (mouse Catalepsy) = <0.1 mg/kg p.o.

<figure><img src="image_320.png" /><figcaption>**Scheme 174.** Strategy employed for the synthesis of arylindenopyridines 589 and 593 in the patents.</figcaption></figure>

In 2011, a series of tricyclic 3,4-dihydropyrimidine-2-thiones **595** was proposed as a potential A₁ receptor and TRPA1 antagonists. The TRPA1 channel is implicated in numerous inflammatory or neuropathic pains [439]. These molecules were notably prepared via a Biginelli reaction with indane-1,3-dione 4 due to which the effects of the substitution of the phenyl groups along with the replacement of urea by thiourea in the cyclization reaction were studied. According to the in vitro tests performed on human and rats cells, experimental results revealed that all compounds prepared with thiourea were more biologically active than those prepared with urea. Influence of the stereochemistry is also clearly evidenced, as shown for compounds **595-(3)** and **595-(4)**. Thus, for **595-(2)**, a half maximal inhibitory concentration hTRPA1 IC₅₀ of 0.075µM was determined for the S,R-stereoisomer, whereas for the S,S-stereoisomer **595-(3)**, an hTRPA1 IC₅₀ higher than 10 µM was determined (see Scheme 139). The *meta*-substitution on the phenyl ring could induce a significant increase in the activity (see Scheme 175).

Nevertheless, only in vitro tests were performed with these molecules, and the authors highlighted some drawbacks such as a low solubility, a poor metabolic stability and the potential toxicity of thiourea used to prepare these molecules [440].

In 2015, Ahmed et al. described a solvent-free three-component synthesis based on 1,3-indanedione as the starting material and developed a series of 14 potential anticonvulsants compounds with this strategy [243]. Seizure is a transient occurrence of signs and/or symptoms due to an abnormal excessive or synchronous neuronal activity in the brain [441]. This condition is often affiliated with epilepsy, which concerns about 0.5–1% of the population. Moreover, serious improvement needs to be performed concerning their efficiency but also to address the important issue of side effects [442]. The authors principally acted on one factor, namely the substitution on the pendant phenyl group. After synthesis, each molecule was tested via an anticonvulsant evaluation with the maximal electroshock (MES) method and compared with the results obtained with a standard drug, i.e., phenytoin, along with their toxicity with the Rotorod test. Most of the compounds
designed in this study seemed to have positive effects on seizures ranging from moderate to good activity, particularly **222d**, **222e**, and **222j** (see Scheme 176), which proved to be significantly active at a dose of 40 mg/kg. Moreover, the Rotarod tests showed no toxicity for all the compounds tested. However, none of them were as efficient as the reference compound phenytoin.

Among the neurodegenerative diseases, Alzheimer's disease (AD) is the most common one, but this disease is also badly treated. During the last 25 years, only five medicines were approved for AD treatment with two different strategies of action. Thus, cholinesterase inhibitors (ChEIs) such as tacrine (1), donepezil (2), rivastigmine (3) and galantamine (4) were proposed in parallel to N-methyl-D-aspartate (NMDA) antagonists such as memantine (5) [443]. Nonetheless, none of them can repair the damage or delay the disease progression, which could constitute a major improvement in this field. The aggregation of a small peptide named amyloid β (Aβ) is associated with AD, and the inhibition of both cholinesterase sites (acetylcholinesterase, AChE, and butyrylcholinesterase, BChE) may prevent this aggregation [444].

<figure><img src="image_321.png" /></figure>

<figure><img src="image_322.png" /></figure>

<table><thead><tr><th></th><th>Compounds</th><th>595-(1)</th><th>595-(2)</th><th>595-(3)</th><th>595-(4)</th></tr></thead><tbody><tr><td>A</td><td></td><td>S</td><td>S</td><td>S</td><td>O</td></tr><tr><td>Stereochemistry</td><td></td><td>RS</td><td>R</td><td>S</td><td>RS</td></tr><tr><td>hTRPA1 IC<sub>50</sub> (µM)<sup>a</sup></td><td></td><td>0.13</td><td>0.075</td><td>&gt;10</td><td>3.9</td></tr><tr><td>rTRPA1 IC<sub>50</sub> (µM)<sup>b</sup></td><td></td><td>0.02</td><td>0.012</td><td>&gt;10</td><td>4.7</td></tr></tbody></table>

<figure><img src="image_323.png" /></figure>

<table><caption>**Scheme 175.** Synthesis of tricyclic 3,4-dihydropyrimidine derivatives **595** via Biginelli reaction along with the most promising compounds. <sup>a</sup> human TRPA1 antagonism <sup>b</sup> rat TRPA1 antagonism.</caption><thead><tr><th>595-(5)</th><th>595-(6)</th><th>595-(7)</th><th>595-(8)</th></tr></thead><tbody><tr><td>S</td><td>S</td><td>S</td><td>O</td></tr><tr><td>RS</td><td>R</td><td>S</td><td>RS</td></tr><tr><td>0.05</td><td>0.013</td><td>&gt;10</td><td>1.7</td></tr><tr><td>0.011</td><td>0.004</td><td>&gt;10</td><td>0.5</td></tr></tbody></table>
<figure><img src="image_324.png" /></figure>

4

<figure><img src="image_325.png" /></figure>

368

<figure><img src="image_326.png" /></figure>

596

<figure><img src="image_327.png" /></figure>

<figure><img src="image_328.png" /></figure>

222

<figure><img src="image_329.png" /></figure>

**222d:**
MES duration of hind limb
extentionª:
4.7±1 S
Rotarod testᵇ: 0/4

<figure><img src="image_330.png" /></figure>

**222e:**
MES duration of hind limb
extentionª:
6.3±1 S
Rotarod testᵇ: 0/4

<figure><img src="image_331.png" /></figure>

**222j:**
MES duration of hind limb
extentionª:
7.3±1 S
Rotarod testᵇ: 0/4

**Scheme 176.** MCR for the synthesis of **222** with three examples having decent anticonvulsant activity.ª, Values represent means SEM (n = 3).ᵇ, Rotarod toxicity (number of animals exhibiting toxicity/number of animals tested).

In 2018, Tanoli et al. developed a series of tricyclic fused ring systems exhibiting activity against both acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). Precisely, this study was supported by previous works reported in the literature, demonstrating that other tricyclic structures based on pyrimidine or quinazoline could act as AChE and BChE inhibitors [445]. In order to access the pyrimidine-fused rings, indane-1,3-dione **4** was used in a Biginelli reaction along with thiourea **597** and benzaldehyde **368** (Scheme **177**).

<figure><img src="image_332.png" /></figure>

4

+

<figure><img src="image_333.png" /></figure>

368

+

<figure><img src="image_334.png" /></figure>

597

<figure><img src="image_335.png" /></figure>

<figure><img src="image_336.png" /></figure>

598

<figure><img src="image_337.png" /></figure>

599

<figure><img src="image_338.png" /></figure>

<figure><img src="image_339.png" /></figure>

600

**Scheme 177.** Synthetic route to dihydropyrimidine.
The dihydropyrimidine (DHPM) scaffold is a potential inhibitor of cholinesterases so that in vitro tests were carried out. Interesting results were obtained with these molecules since the introduction of the indanone-pyrimidine fused ring could lead to an improved inhibition activity compared to tacrine and donepezil for both electric eel AChE (IC₅₀ = 0.09 µM) and equine serum BChE (IC₅₀ = 1.04 µM) assays. These results are close to that obtained with the commercially available Donepezil with IC₅₀ eeAChE = 0.05 µM and IC₅₀ eqBChE = 5.4 µM. Docking studies on this molecule have also exposed the well accommodation of this molecule into the bottom of the gorge and the importance of the hydrogen bonding with its environment, such as Ser200, or even the phenyl ring at the 4-position, which forms π-π stacking interactions with trp84 (see Figure 7).

<figure><img src="image_340.png" /><figcaption>**Figure 7.** Close-up depiction of the lowest-energy three-dimensional (3-D) docking poses of 600 into the binding site of Torpedo californica acetylcholinesterase TaAChE. Reproduced with permission from Ref. [445].</figcaption></figure>

In 2010, the good activity of 2-[(2-(4-chlorophenyl)hydrazinyl) methylene]-1H-
-indene-1,3(2H)-dione **605f** as a novel inhibitor capable of prevent amyloid aggregation
(IC₅₀ = 23µM) [446] motivated the group of Campana to obtain a deeper insight into the bio-
logical activity of this molecule and in this aim a series of indane-2-arylhydrazinylmethylene
-1,3-diones **605a–f** and indol-2-aryldiazenylmethylene-3-ones **608a–m** (see Scheme 178) [447].

In this work, the authors clearly highlight a significant increase in the biological activ-
ity with the molecules prepared with indane-1,3-dione compared to those prepared with
1,3-cyclopentanedione. Nonetheless, even if all the compounds exhibited decent to good AB
aggregation processes, none of them could overcome the reference quercetin (IC50 = 0.8 µM).

5.5. *Indane-1,3-Dione as Anticoagulant Drugs*

One of the oldest utilizations of indanedione derivatives in medicine concerns the anticoagulant properties. Since the 1940s, several authors reported the insecticidal properties of acylated indane-1,3-diones (see Scheme 179), especially for houseflies [154]. Later on, this toxicity has been attributed to an anticoagulant property, especially marked for acylated indane-1,3-diones (see Scheme 179) [448]. However, this activity can also be used as treatment for vitamin K antagonists (VKAs), as observed for phenindione derivatives (see Scheme 179) [449]. However, pheindione is not so much used anymore and was replaced by its homologue, fluindione, or other coumarins VKA due to side effects such as hypersensitivity identified for pheindione [450]. However, despite a proven efficiency as vitamin K epoxide reductase, fluindione **609b** is used almost only in France because of a lack of study data on old people [451].
<figure><img src="image_341.png" /><figcaption>**Scheme 178.** Chemical structures of indane-2-arylhydrazinylmethylene-1,3-diones 605a–f and indol-2-aryldiazenylmethylene-3-ones 608a–m.</figcaption></figure>

<figure><img src="image_342.png" /><figcaption>**Phenindione derivatives**</figcaption></figure>

609a : R₁=H : Pindione

609b : R₁=F : Fluindione

<figure><img src="image_343.png" /><figcaption>**A cylated indane-1,3-dione**</figcaption></figure>

<figure><img src="image_344.png" /></figure>

610a : R₂= : Diphacione

610c : R₂= : Pindone

<figure><img src="image_345.png" /></figure>

610d : R₂= : Valone

<figure><img src="image_346.png" /><figcaption>**Hydroxycoumarins derivatives**</figcaption></figure>

<figure><img src="image_347.png" /></figure>

611a : R₃= : Brodifacoum

<figure><img src="image_348.png" /></figure>

611c : R₃= : Warfarin

<figure><img src="image_349.png" /></figure>

611b : R₃= : Difenacoum

<figure><img src="image_350.png" /></figure>

611d : R₃= : Bromadiolon

<caption>**Scheme 179.** Chemical structures of various indane-1,3-dione derivatives 609–611 with anticoagulant properties.</caption>
The synthesis of these indane-1,3-dione derivatives is actually not performed anymore starting from the unsubstituted indane-1,3-dione, but a recent study uses a similar structure with indane-1,3-dione as the starting material [452]. Nonetheless, this molecule is only an intermediary of reaction for the synthesis of 2,3-disubstituted indoles.

Concerning the synthesis of acylindanediones **613**, the standard synthetic strategy is quite similar to that used for indane-1,3-dione **4** since it consists in a Claisen condensation using an alcoolate as the base (see Scheme **180**) [453]. However, an alternative was proposed in 2018 by Larsen et al. involving indane-1,3-dione **4** as the starting material with the possibility to afford 25 different C2 acylated 1,3-indandiones **616** (see Scheme **181**) [454]. This strategy, which is quite efficient, makes use of an easy set up and producing the molecules in high yields using 1-ethyl-(3-(3-dimethylamino)propyl)-carbodiimide (EDCI) and 4-dimethylaminopyridine (DMAP) as the coupling agents. Furthermore, all tests made with this new strategy have only been performed at a small scale compared to the initial synthesis.

<figure><img src="image_351.png" /><figcaption>**Scheme 180.** The different synthetic routes to acylindane-1,3-diones **613**.</figcaption></figure>

<figure><img src="image_352.png" /><figcaption>**Scheme 181.** New synthetic route developed by Larsen et al. to access to acylindanediones **616**.</figcaption></figure>

These type of indanes are more commonly used as rodenticides than as for pharmaceutical purposes. All commercially available rodenticides are either 4-hydroxycoumarins or indane-1,3-diones and even if coumarins are more widely used, a resistance has recently been evidenced toward derivatives such as Warfarin. At present, less resistances have been identified toward indane derivatives. Chlorophacinone, which belongs to the first generation of rodenticides, is still commonly used in many countries. Even if the second generation is more effective against rodents, their impact on the environment is quite significant with multiple poisoning detected in cats, dogs or even otters [455,456].

**6. Conclusions-Perspectives**

Indane-1,3-dione is a versatile molecule that has found applications in numerous research fields. Directly related to the broadness of applications, numerous synthetic routes have been examined first to prepare indane-1,3-dione, the corresponding derivatives bearing various substituents. If indane-1,3-dione is an excellent electron acceptor for the design of various push-pull dyes, the most popular use of indane-1,3-dione is undoubtedly in multicomponent or domino reactions to prepare polycyclic structures for biological activities. Considering the remarkable biological activity of indane-1,3-dione derivatives,
future work will certainly be more focused on these biological applications than on the optoelectronics applications. Numerous electron acceptors have already been prepared with this scaffold, limiting the possibility of new developments.

**Author Contributions:** Conceptualization, C.P., D.B. and F.D.; methodology, C.P., D.B. and F.D.; software, C.P., D.B. and F.D.; validation, C.P., D.B. and F.D.; formal analysis, C.P., D.B. and F.D.; investigation, C.P., D.B. and F.D.; resources, F.D.; data curation, F.D.; writing—original draft preparation, C.P., D.B. and F.D.; writing—review and editing, C.P., D.B. and F.D.; visualization, C.P., D.B. and F.D.; supervision, F.D.; project administration, F.D.; funding acquisition, F.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by The Agence Nationale de la Recherche (ANR), grant number ANR-17-CE08-0010 DUALITY project for the PhD grant of Corentin Pigot. This research was funded by the Agence Innovation Defense (AID) through the PhD grant of Damien Brunel. The research was funded by Aix Marseille University and The Centre National de la Recherche Scientifique (CNRS), under the frame of permanent fundings.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** No data available.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are not available from the authors.

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