(5R,7R,11bR)-9-(di(1H-Indol-3-yl)methyl)-4,4,7,11b-tetramethyl-1,2,3,4,4a,5,6,6a,7,11,11a,11b-dodecahydrophenanthro[3,2-b]furan-5-yl Acetate

Abstract

The semi-synthesis of the (5R,7R,11bR)-9-(di(1H-indol-3-yl)methyl)-4,4,7,11b-tetramethyl-1,2,3,4,4a,5,6,6a,7,11,11a,11b-dodecahydrophenanthro[3,2-b]furan-5-yl acetate was performed via a pseudo-multicomponent reaction involving a double Friedel–Crafts alkylation between the natural product-derived aldehyde 6β-acetoxyvouacapane and the corresponding indole. The transformation was carried out under solvent-free mechanochemical conditions using mortar and pestle grinding, with ZnCl₂ as the catalyst. Structural elucidation of the target compound was accomplished using 1D and 2D NMR spectroscopy (¹H, ¹³C, COSY, HSQC, and HMBC), FT-IR, and high-resolution mass spectrometry (HRMS).

1. Introduction

Bis(3-indolyl)methane and 6β-acetoxyvouacapanes are heterocyclic natural products with significant biological and pharmacological properties [1,2,3,4]. Because of this one, their semi-synthetic derivatives have become attractive targets for synthetic chemists, representing an underexplored field within natural products chemistry. In particular, structural modification of these scaffolds through semi-synthetic approaches using powerful synthetic tools such as pseudo-multicomponent reactions may lead to enhanced biological activity or improved pharmacokinetic properties [5,6,7]. Moreover, their inherent chemical complexity offers multiple sites for functionalization, enabling the generation of novel molecular hybrids with potential therapeutic applications. Thus, by exploring these frameworks, one could contribute to discovering new hits or lead compounds in the drug discovery process [6]. To the best of our knowledge, semi-synthetic methodologies to access bis(3-indolyl)methane and 6β-acetoxyvouacapane derivatives remain scarce in the literature. Consequently, developing new synthetic strategies to obtain such compounds is important and of great interest.

In this context, mechanochemistry, which utilizes mechanical force to induce chemical transformations, has emerged as a valuable and sustainable alternative to traditional synthetic methods, significantly reducing or even eliminating the use of solvents. Among the mechanochemical techniques, liquid-assisted grinding (LAG)—which involves the manual or mechanical grinding of reactants in the presence of a minimal amount of liquid—has proven to be particularly efficient, enabling rapid, high-yielding reactions under mild conditions [8,9,10].

As part of our ongoing interest in the development of novel semi-synthetic derivatives of 6β-acetoxyvouacapane via multicomponent reactions such as the GBB reactions [11], we herein describe the semi-synthesis of a bis(3-indolyl)methane–6β-acetoxyvouacapane through a pseudo-multicomponent reaction which involves a double Friedel–Crafts alkylation between vouacapane-aldehyde 2 and the indole 3 under mechanochemical conditions, using a mortar and pestle for manual grinding in the presence of ZnCl₂ as the catalyst.

2. Results and Discussion

The synthesis of the target compound bis(3-indolyl)–6β-acetoxyvouacapane 4 is outlined in Scheme 1 and consists of a two-step reaction sequence. First, the natural product 6β-acetoxyvouacapane 1 was isolated and purified from the dichloromethane extract of Caesalpinia platyloba leaves. Its spectroscopic data were consistent with those previously reported by the del Río research group [11,12]. Subsequently, the purified 6β-acetoxyvouacapane was formylated at the C-2 position of the furan ring via a Vilsmeier–Haack reaction, using conditions previously developed by our group [8]. This transformation afforded the key intermediate, aldehyde 6β-acetoxyvouacapane 2, in 92% yield. The characterization of this intermediate matched the data reported by Servín-García et al. [11]. The final step involved a pseudo-multicomponent reaction, specifically a double Friedel–Crafts alkylation between aldehyde 6β-acetoxyvouacapane 2 and indole 3. This transformation was carried out under solvent-free mechanochemical conditions using mortar and pestle grinding in the presence of ZnCl₂ as a catalyst; reaction conditions were similar to those reported by the Ganesan and Tilve research group [13,14]. Thus, after 30–40 min of reaction, the desired product was obtained in 49% yield after purification by column chromatography.

The structure of the target compound was elucidated by ¹H and ¹³C NMR, including APT, and supported by 2D NMR experiments (COSY, HSQC, and HMBC), which were instrumental in the assignment of key signals. In the ¹H NMR spectrum, the most relevant signals were observed, such as a doublet at δ 7.99 ppm (J = 3.1 Hz, 2H), which corresponds to the NH protons of both indole rings. A distinctive doublet of doublets at δ 6.88 ppm (J = 7.5, 2.4, 1.0 Hz, 2H) was assigned to the protons at position 2 of the indole moieties. The remainder of the aromatic region (δ 7.55–7.03 ppm) displayed multiplets corresponding to the other indole protons. A singlet at δ 5.87 ppm integrating for one proton was attributed to the benzylic methine hydrogen, while a singlet at δ 5.85 ppm was assigned to the proton at position 3 of the furan ring. Additionally, a doublet at δ 5.52 ppm (1H) was attributed to the proton at the α-position to the acetate group in the 6β-acetoxyvouacapane unit. The remaining signals were consistent with those previously reported by our research group for related vouacapane derivatives.

In the ¹³C NMR spectrum, the key signals were as follows: the carbonyl carbon of the acetate appeared at δ 170.7 ppm. The furan ring carbons were observed at δ 154.7 ppm (C-2), 148.0 ppm (C-5), 122.6 ppm (C-4), and 106.5 ppm (C-3). Signals at δ 136.5 ppm and 123.1 ppm were assigned to the indole carbons at positions 9 and 2, respectively. The tertiary carbon bearing the acetate group resonated at δ 69.8 ppm, and the benzylic methine carbon appeared at δ 34.2 ppm. The complete assignments were supported by HSQC and HMBC correlations. The remaining signals matched the carbon chemical shifts previously reported by del Río and coworkers for vouacapane-type compounds. High-resolution mass spectrometry (HRMS, ESI⁺) further confirmed the molecular formula: m/z calcd. for C₃₉H₄₅N₂O₃ [M+H]⁺: 589.3430; found: 589.3438.

3. Materials and Methods

3.1. General Information

All reagents, reactants, and solvents were purchased from Merck (Darmstadt, Germany) and were used as received. The reaction progress was monitored by thin layer chromatography (TLC) using silica gel 60 F₂₅₄ from Merck, and the spots were visualized under UV light at 254 or 365 nm. The melting points were determined on a Fisher–Johns melting point apparatus (Thermo Scientific, Vernon Hills, IL, USA) and are uncorrected. Column chromatography was performed using silica gel (230–400 mesh). Chemical structures and names were generated using ChemDraw Professional (version 19.1.1.21). NMR spectra were recorded in a Bruker AMX Advance III spectrometer (500 MHz) (Bruker Daltonics, Bremen, Germany). Chemical shifts were reported as δ values (ppm). Coupling constants J are reported in Hertz (Hz). Internal reference for NMR spectra is in respect to TMS at 0.0 ppm. Multiplicities are reported, using the standard abbreviations, as follows: singlet (s), doublet (d), triplet of doublets (td), doublet of doublets (ddd), multiplet (m). Spectral analysis was performed using MestreNova software (version 14.1.0-24037). IR spectra were recorded using a Thermo Scientific NICOLET Is10 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) via the ATR method with neat compounds. Wavelengths are reported in reciprocal centimeters (ν/cm⁻¹). High-resolution mass spectra (HRMS) were acquired using a Bruker MicroTOF-II spectrometer (Bruker Daltonics, Bremen, Germany).

3.2. Plant Material, Extraction, and Isolation

6β-Acetoxyvouacapane 1 was extracted from Caesalpinia platyloba specimens collected in Los Charcos, municipality of Buenavista Tomatlán, Michoacán, Mexico, in September 2020. The plant material was identified by Prof. Xavier Madrigal Sánchez at the Facultad de Biología, Universidad Michoacana de San Nicolás de Hidalgo, where a voucher specimen (No. 2401) is preserved. The isolation and purification of the compound were carried out following the procedure described by Servín-García et al. [8]

3.3. Synthesis of (5R,6aS,7R,11aS,11bR)-9-Formyl-4,4,7,11b-tetramethyl-1,2,3,4,4a,5,6,6a,7,11,11a,11b-dodecahydrophenanthro[3,2-b]furan-5-yl Acetate

Synthesis of (5R,6aS,7R,11aS,11bR)-9-formyl-4,4,7,11b-tetramethyl-1,2,3,4,4a,5,6,6a,7,11,11a,11b-dodecahydrophenanthro[3,2-b]furan-5-yl acetate or aldehyde 6β-acetoxyvouacapane 2 was performed according to the methodology reported by the Cortés-García research group.

3.4. Synthesis of (5R,7R,11bR)-9-(di(1H-Indol-3-yl)methyl)-4,4,7,11b-tetramethyl-1,2,3,4,4a,5,6,6a,7,11,11a,11b-dodecahydrophenanthro[3,2-b]furan-5-yl Acetate

In an agate mortar, aldehyde 6β-acetoxyvouacapane 2 (50 mg, 1.0 equiv), indole (31.44 mg, 2.0 equiv), and ZnCl₂ (0.05 equiv) were added. Manual grinding was performed using an agate pestle, with the addition of 2–3 drops of dichloromethane (DCM) to assist mixing. The reaction was carried out under solvent-assisted conditions for 30–40 min, monitoring its completion via TLC. After completion, the reaction mixture was purified by column chromatography using a hexane:ethyl acetate mixture (8:2, v/v), affording bis(3-indolyl)-6β-acetoxyvouacapane 4 as a red solid (38.6 mg, 49%). mp = 139–142 °C. R_f = 0.32 (Hexane-AcOEt 8:2 v/v); ¹H NMR (400 MHz, CDCl₃): δ = 7.99 (d, J = 3.1 Hz, 1H), 7.54 (td, J = 8.1, 1.0 Hz, 2H), 7.35–7.33 (m, 2H), 7.19–7.16 (m, 2H), 7.05 (dddd, J = 8.0, 7.0, 4.7, 1.0 Hz, 2H), 6.88 (ddd, J = 7.5, 2.4, 1.0 Hz, 2H), 5.87 (s, 1H), 5.85 (s, 1H), 5.52 (d, J = 2.3 Hz, 1H), 2.59–2.43 (m, 3H), 2.06–2.03 (m, 4H), 1.83 (dt, J = 14.5, 3.6 Hz, 1H), 1.68–1.65 (m, 1H), 1.61 (dt, J = 13.4, 3.0 Hz, 1H), 1.55–1.50 (m, 2H), 1.49–1.47 (m, 1H), 1.46–1.43 (m, 1H), 1.42–1.40 (m, 1H), 1.29 (s, 1H), 1.20 (s, 3H), 1.08 (d, J = 1.8 Hz, 1H), 1.04 (s, 3H), 1.00 (s, 3H), 0.91 (d, J = 7.0 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃): δ = 170.7, 154.7, 148.0, 136.5, 126.9, 123.1, 119.8, 119.1, 117.5, 111.0, 106.5, 69.8, 55.4, 45.7, 43.7, 42.2, 38.0, 36.3, 34.2, 33.7, 31.1, 30.9, 29.7, 23.5, 21.9, 21.8 18.8, 17.5, 17.2. FT-IR (ATR) ν_max/cm⁻¹ 3415, 2962, 2928, 2854, 1708, 1458, 1370, 1245, 1024. HRMS (ESI⁺): m/z: Calcd. for C₃₉H₄₅N₂O₃[M+H]⁺: 589.3430; Found: 589.3438.

4. Conclusions

A semi-synthetic strategy to obtain the bis(3-indolyl)-6β-acetoxyvouacapane 4 was developed via a pseudo-multicomponent reaction under mechanochemical conditions. The approach, which involved a double Friedel–Crafts alkylation catalyzed by ZnCl₂, proved to be an efficient and environmentally friendly method, affording the target compound in moderate yield. Structural elucidation was achieved through comprehensive spectroscopic techniques, including 1D and 2D NMR, FT-IR, and HRMS. This study highlights the potential of mechanochemistry and multicomponent strategies in the functionalization of complex natural product scaffolds, opening new ways for generating semi-synthetic derivatives with potential pharmacological relevance. Further in vitro studies, guided by in silico predictions, will be undertaken to explore the biological activity of the synthesized compound.