Synthesis of novel naphthalene-chimonanthine scaffolds hybrids with potent antibacterial or antifungal activity

Abstract In this work, a total of 19 novel naphthalene hybrids with chimonanthine scaffolds were efficiently synthesised from indole-3-acetonitrile in good yields. The prepared compounds were evaluated for biological activity against Cryptococcus neoformans, Escherichia coli, Shigella spp, Candida albicans, Salmonella spp, and Staphylococcus aureus. The preliminary bioassays showed that most of the synthesised compounds exhibited significant antibacterial or antifungal activity. Notably, compound 8 showed potent activity against Cryptococcus neofonmans, Escherichia coli, Shigella spp, and Candida albicans than the positive control, all with the same MIC value of 3.53 µM. Compound 8 had a broad spectrum of antibacterial or antifungal activity, and will be studied further. Graphical Abstract


Introduction
Naphthalene derivatives are naturally occurring compounds present in Droseraceae, Nepenthaceae, Polygonaceae and many other carnivorous plants (Heubl et al. 2006;Altı ntop et al. 2016).The naphthalene ring system is a prominent structural motif found in a wide range of biologically active natural products, botanical pesticides, and materials ( Altı ntop et al. 2016;Ates-Alagoz et al. 2009;Ibrahim and Mohamed 2016;Gouhar et al. 2018;Wang et al. 2019). This scaffold emerges as a promising moiety in pesticide design due to its diverse biological activities generated from structural modifications. Various activities were reported for naphthalene-based compounds such as antibacterial (Voskien _ e et al. 2012), antifungal activity (Altı ntop et al. 2016), antiviral and cytotoxic.
On the other hand, molecular hybridisation is a useful strategy for structural modification of lead compound in botanical pesticides design and development based on the combination of pharmacophoric moieties of different bioactive substances to produce a new hybrid compound with improved affinity and efficacy. The botanical pesticides hybridisation could also generate new molecules with lower toxicity, improved affinity, and better biological activity (Abozeid et al. 2020).
In our previous work, calycanthaceous alkaloids derivatives were designed and synthesised. The biological testing showed that several of the synthesised compounds exhibited diverse and promising bioactivities, such as, compound a1 showed better against Verticillium dahlia compared with chlorothalonil, with a MIC value of 7.81 mg mL À1 ; compounds a2 and a3 revealed potent activity against acetylcholinesterase, with MIC values of 0.01 and 0.1 ng mL À1 , respectively ( Figure 1) (Zheng et al. 2016a, 2016b, 2017, 2018a, 2018b, 2019a, 2019bZhu et al. 2021).
As a continuation of our search, we report herein the design and synthesis of a hybrid scaffold by incorporating chimonanthine skeleton with naphthalene moiety in a single molecule. The synthesised compounds were tested for the antibacterial or antifungal activity.

Design and synthesis of hybrid compounds
The synthetic route of title compounds is given in Scheme 1. The novel naphthalenechimonanthine scaffolds hybrids were prepared using indole-3-acetonitrile as the starting material, DMSO as the solvent and the oxidising agent, with 37% HCl as a catalyst to form the compound b1 in 93% yield. Then, an excess of 2-(bromomethyl)naphthalene was reacted with compound b1 at the N-position and C-3 position, using NaH as a base, obtained the product b2, in 62% yield. The compound b2 was reduced with LiAlH 4 , and cyclised to form compound b3 in the yield of 58%. Compounds 1-19 were synthesised with the corresponding desired reagents based on the key intermediate b3. The synthesised naphthalene-chimonanthine scaffolds hybrids were characterised by 1 H-NMR, 13 C-NMR spectroscopy and ESI-MS.

Antimicrobial activity
The inhibitory effects of naphthalene-chimonanthine scaffolds hybrids against bacteria or fungi are listed in Table S1. The MIC values were evaluated with Streptomycin as a

Instruments
All reagents and solvents were reagent grade or purified according to standard methods before use. Analytical thin-layer chromatography (TLC) was performed with silica Scheme 1. Synthetic route to the title compounds 1-19.
gel plates using silica gel 60 GF 254 (Qingdao Haiyang Chemical Co., Ltd., Qingdao, China). The 1 H À NMR (400 MHz), and 13 C À NMR (100 MHz) were obtained on an AM À 500 FT À NMR spectrometer (Bruker Corporation, Switzerland) with CDCl 3 as the solvent and TMS as the internal standard. MS was recorded under ESI conditions using a LCQ Fleet instrument (Thermo Fisher, Waltham, MA, USA). Reaction yields were not optimised.

Synthesis of 2-(2-oxoindolin-3-yl)acetonitrile (b1)
Starting material indole-3-acetonitrile (1.56 g, 10 mmol) was added to hydrogen chloride (37% HCl) in anhydrous DMSO (50 mL). At first, the mixture was cooled in an ice bath for 30 min. Then the mixture was stirred at room temperature for 1.5 h. When TLC monitoring showed that the starting material had disappeared, the reaction mixture was evaporated under reduced pressure to remove the solvent to obtain the white solid b1 (1.47 g, 89% yield). The crude product b1 was used for the next step without further purification.

Synthesis of compounds 1-19
To a stirred solution of compound b3 in pyridine (10 mL) was added the corresponding desired reagent and the resulting mixture was heated under reflux for 2 h, then allowed to cool. When TLC monitoring showed that the compound b3 had disappeared, the solvent was removed under reduced pressure and the crude product was purified by flash column chromatography afforded the compounds 1-19 in yields from 82% to 91% (Characterisation data see supplementary materials).

Biological activity
The biological activity of naphthalene-chimonanthine scaffolds hybrids were measured according to the previously reported method (Zhang et al. 2009(Zhang et al. , 2013. The tested compounds dissolved in 5% dimethyl sulfoxide (DMSO), to a concentration of 1.02 mg/mL, 100 lL of the solutions were added to the first well and serially diluted from first well by taking 100 lL into second. This two-fold dilution was continued down the plate and 100 lL from the 8th column of the plated discarded. The 9th column of the plate was reserved for negative control wells (without inocula) and the 10th column, for the positive growth control wells. The concentrations were 256,128,64,32,16,8,4 and 2 lg/mL, respectively. The test plates were incubated aerobically at 37 C for 24 h, the test plates were incubated aerobically at 28 C for 48 h. The MICs were examined. All tests were performed in triplicate and repeated if the results differed.

Conclusions
A total of 19 novel naphthalene-chimonanthine scaffolds hybrids were prepared using indole-3-acetonitrile as the starting material, and their activities against bacteria or fungi were screened. The results of bioassays revealed that most of the title compounds manifested potent activities, which were more effective than the positive control. Notably, compound 8 in particular had a broad spectrum of antibacterial or antifungal activity, and might be a novel potential leading compound.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This work was supported by the zhoushan public welfare science and technology project (2019C31066).