Study on Diels–Alder Reaction of Spilanthol ^†

Abstract

This study explores the Diels–Alder reaction using spilanthol, a natural diene isolated from Heliopsis longipes roots, to synthesize potentially bioactive compounds. Spilanthol was purified through silica gel column chromatography, yielding 16 g/kg of dried roots, and characterized by ¹H NMR spectroscopy. Among the dienophiles tested, only p-anisaldehyde reacted efficiently in the presence of BF₃·OEt₂ as a Lewis acid catalyst. A cyclic adduct was obtained with yields of 9.72% (endo) and 24.32% (exo). ¹H NMR analysis confirmed the formation of a pyran ring, demonstrating the viability of this synthetic pathway for producing functionalized cyclic compounds with potential biological activity.

1. Introduction

Spilanthol 1 (Scheme 1), N-isobutyl-(2E,6Z,8E)-deca-2,6,8-trienamide, is an alkylamide isolated from the roots of Heliopsis longipes, a medicinal plant endemic to regions of Mexico such as the Sierra Álvarez and Sierra Gorda [1]. Its architecture—an unsaturated amide appended to a conjugated diene—renders spilanthol 1 a competent diene for Diels–Alder cycloadditions [2]. Beyond its synthetic utility, spilanthol 1 displays analgesic, anti-inflammatory, antimicrobial, and antioxidant activities, underscoring its pharmacological promise [3]. Leveraging these features, spilanthol 1 can serve as a renewable scaffold for the sustainable construction of bioactive heterocycles.

In this study, we assess the feasibility and selectivity of the Diels–Alder reaction between spilanthol 1 and p-anisaldehyde. We further examine the influence of different catalysts on yield and selectivity, aiming to optimize conditions and advance greener, more efficient synthetic methodologies.

2. Materials and Methods

2.1. Extraction and Identification of Spilanthol 1

Ten grams of ground root of H. longipes were macerated in absolute ethanol (1:10 w/v) for one week, and this was repeated three times with fresh solvent. The extract was filtered, evaporated, and analyzed via thin-layer chromatography using hexane/ethyl acetate (3:2), identifying the target compound with an Rf = 0.3. Structural identification was confirmed by proton nuclear magnetic resonance spectroscopy (¹H-NMR, 400 MHz, CDCl₃), comparing chemical shifts with the reported literature data [4,5].

Spilanthol 1. IR (film) ν_max: 3276, 2958, 2928, 2871, 1669, 1629, 1552, 1247, 1159, 980, 947, 820 cm⁻¹.¹HNMR (CDCl₃, 400 MHz) δ_H: 6.85 (dt, 1H, J₁ 15.0 Hz, J₂ 6.6 Hz, H-3), 6.30–6.26 (m, 1H, H-8), 5.97 (dd, 1H, J₁ = J₂ = 10.8 Hz, H-7), 5.80 (dt, 1H, J₁ 15.6 Hz, J₂ 1.8 Hz, H-2), 5.70 (dq, 1H, J₁ 12.0, J₂ 6.0, H-9), 5.52 (br, s, 1H, NH), 5.26 (dt, 1H, J₁ 10.8 Hz, J₂ 7.2 Hz, H-6), 3.15 (dd, 2H, J₁ = J₂ 6.3 Hz, H-1′), 2.36–2.22 (m, 4H, H-4 and H-5), 1.82–1.77 (m, 2H, H-2′ and H-10), 0.92 (d, 1H, J 6.0 Hz, H-3′ and H-4′) ppm; ¹³CNMR (CDCl₃, 100 MHz) δ_C: 165.9 (C=O), 143.4 (C-3), 129.9 (C-9), 129.4 (C-7), 127.8 (C-6), 126.6 (C-8), 124.1 (C-2), 46.8 (C-1′), 32.1 (C-4), 28.6 (C-2′), 26.4 (C-5), 20.1 (C-3′ andC-4′), 18.3 (C-10) ppm; EIMS m/z (%) 221 (M⁺, 6), 141 (61), 126 (17), 98 (18), 81 (100), 79 (19), 41 (31); HREIMS m/z 221.1801 (calcd for C₁₄H₂₃NO 221.1779).

2.2. Diels-Alder Reaction

A solution of spilanthol 1 (1 mmol), the corresponding dienophile (1 mmol), and 5 mol% of catalyst in dry tetrahydrofuran (THF, 0.2 M) was stirred for 24 h under two conditions—at room temperature and under reflux —while monitoring progress by TLC and visualized with potassium permanganate; upon completion, distilled water was added, and the mixture was extracted with dichloromethane (3 × 25 mL), after which the combined organic layers were dried, filtered, and concentrated under reduced pressure. The crude product was purified by silica gel column chromatography using a hexane/ethyl acetate gradient (100:0 to 80:20), affording a purified adduct that was analyzed by ¹H-NMR to determine chemical structure.

Compound 2: ¹H NMR (400 MHz, CDCl₃) δ ¹H NMR (400 MHz, CDCl₃) δ 7.40 (m), 6.80 (m), 5.90 (d), 5.75 (m), 5.58 (Br s), 5.35 (m), 4.20 (m), 2.40 (m, 1H), 1.95 (m, 2H), 1.90 (s, 3H), 1.01 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 166.0, 145.9, 138.6, 132.9, 129.0, 128.9, 128.4, 128.2, 126.1, 124.1, 122.4, 82.9, 77.0, 47.1, 36.4, 35.7, 28.7, 26.9, 20.4, 20.3, 15.2; EIMS m/z (%) 327 (M⁺, 16), 328 (23), 133 (17), 98 (18), 81 (100); HREIMS m/z 327.2215 (calcd for C₂₁H₂₉NO₂ 327.2201).

3. Results and Discussion

3.1. Isolation and Characterization of Spilanthol 1

The isolated compound, purified from the ethanolic extract of Heliopsis longipes, was then subjected to ¹H NMR analysis. The spectrum exhibited characteristic chemical shift signals corresponding to specific proton environments. Three methyl group signals (–CH₃) were observed at δ_H 0.93 and 1.74 ppm, along with a methine proton (–CH–) at δ_H 1.76 ppm. Three methylene group signals (–CH₂–) were detected in the δ_H 2.35–2.40 ppm region. Additionally, five olefinic protons (C=CH) were identified in the δ_H 5.59–6.00 ppm range. The proton signals were consistent with those previously reported in the literature [6], supporting the structural identification of the compound. The extraction yielded approximately 16 g of Spilanthol 1 per kilogram of dried root material.

3.2. Diels-Alder Reaction Between Spilanthol 1 and p-Anisaldehyde

The Diels-Alder reaction between spilanthol 1 and p-anisaldehyde was examined to probe diene–dienophile reactivity under thermal conditions and in the presence of Lewis acids (Scheme 2). Reactions were first run without a catalyst in dry THF, exploring two temperature regimes to distinguish kinetic control (favouring the endo adduct) from thermodynamic control (favouring the more stable exo isomer) [7].

Under catalyst-free conditions, no appreciable cycloaddition was detected, indicating that activation of the dienophile is required. This lack of reactivity is consistent with insufficient orbital overlap between the HOMO of spilanthol 1 and the LUMO of p-anisaldehyde and with an unfavourable HOMO–LUMO gap that hampers formation of the cycloaddition transition state [8].

To promote activation, several Lewis acids were screened—AuCl₃, AlCl₃, AgSbF₆, and BF₃·OEt₂ (

Table 1. Catalysts used in the synthesis.

Lewis Acid	Observation	Endo:Exo	Yield %
AuCl3	No conversion	-	0
AlCl3	No observable reaction	-	0
Ag[SbF6]	No clear conversion	-	0
BF3·OEt2		1:5	75

). Of these, only BF₃·OEt₂ effectively delivered the Diels–Alder adducts, attributable to coordination at the aldehyde carbonyl, which stabilizes the dienophile LUMO and narrows the HOMO–LUMO gap, thereby facilitating the transition state [9].

The purified compound obtained from the Diels–Alder reaction was characterized by ¹H-NMR to determine its structural identity, as well as the spectral purity and integrity of the isolated cycloadduct. Figure 1 shows the ¹H-NMR spectrum of the compound recorded in chloroform at 400 MHz.

In the aliphatic region, signals were observed at δH 1.01 and 1.90 ppm, corresponding to methyl protons (–CH₃). An additional signal at δH 1.95 ppm was attributed to methylene protons (–CH₂–), while the resonance at δH 2.40 ppm was assigned to a methine proton (–CH–). Signals at δH 5.58, 5.75, 5.90, and 6.80 ppm were characteristic of olefinic protons (C=C). Finally, the signal at δH 7.40 ppm indicated the presence of aromatic protons. Collectively, these resonances support the proposed structure of the compound 2 as a pyran, a six-membered heterocyclic system composed of five carbon atoms and one oxygen atom, bearing an unsaturation (double bond) [10].

These findings are relevant for the synthesis of functionalized cyclohexene-type scaffolds, with potential applications in the development of bioactive compounds. Moreover, the fact that the reaction proceeds under mild conditions and employs acceptable solvents positions it as a strategy aligned with the principles of green chemistry, by enabling the formation of new molecular architectures without the need for intermediate steps or protecting groups [11].

4. Conclusions

Spilanthol 1 was successfully isolated and purified from the dried roots of Heliopsis longipes, and 1H-NMR confirmed its structural identity. Its reactivity as a diene in the Diels–Alder reaction was evaluated, and it was found that only p-anisaldehyde, in the presence of the Lewis acid catalyst BF₃·OEt₂, enabled the formation of the desired cyclic adduct.

The study confirmed that the reaction proceeds through a hetero-Diels–Alder mechanism, as evidenced by the formation of a six-membered oxygen-containing ring, consistent with a pyran structure. Furthermore, both the yield and the stereoselectivity of the reaction were strongly influenced by the reaction conditions and the nature of the catalyst used.

These findings validate the use of Spilanthol 1 as a natural diene in cycloaddition reactions with synthetic potential, under conditions compatible with the principles of green chemistry. The developed methodology provides a solid foundation for synthesising structurally complex derivatives with potential biological activity.