Propyl Gallate

Abstract

The title compound, propyl gallate (III), is an important substance popularly used in the food, cosmetic and pharmaceutical industries. Current chemical syntheses of this compound are based on the acylation supported by thionyl chloride, DIC/DMAP or Fischer esterification using a range of homogenous and heterogenous catalysts. In this paper, an efficient, green, straightforward, and economical method for synthesizing propyl gallate using potassium hydrogen sulfate, KHSO₄, as the heterogenous acidic catalyst has been developed for the first time. In addition, this paper provides a comprehensive spectral dataset for the title compound, especially the new data on DEPT and 2D NMR (HSQC and HMBC) spectra which are not currently available in the literature.

1. Introduction

Propyl gallate (n-propyl gallate, n-propyl 3,4,5-trihydroxybenzoate, PG, compound III, CAS Registry number: 121-79-9) is an important synthetic substance widely used in cosmetics, foods, pharmaceuticals, and some other fields [1,2,3]. It is used as an effective antioxidant in cosmetics to stabilize vitamins, essential oils, perfumes, as well as fats and oils [1]. In foods, PG has been employed as an additive (E310) since 1948 to protect fats, oils, and fat-containing food from peroxide-induced rancidity [1]. At a concentration of up to 0.1%, PG is used as a strong preservative and stabilizer in various medicinal preparations approved by FDA [4,5]. In addition to its antioxidant activity, PG exhibits anti-inflammatory, anti-angiogenic, and anti-tumor properties [2,3,4,5,6].

PG is not a natural compound and can only be obtained via chemical synthesis. In practice, PG is prepared by either biological (enzymatic) or chemical methods [2,7,8,9,10,11,12,13]. The latter have been dominant, focusing on the reaction between gallic acid (I) and n-propanol (propan-1-ol, II) in different conditions as illustrated in Scheme 1 [1,2,3,14,15,16,17,18,19,20,21,22,23,24,25,26,27].

Experimentally, there are three main approaches to form PG from gallic acid and n-propanol. The first approach is via a typical Steglich esterification with N,N′-diisopropylcarbodiimide (DIC) as the coupling reagent and 4-dimethylaminopyridine (DMAP) as the catalyst [14]. However, urea generated by DIC as a by-product can sometimes be difficult to remove, especially when scaling up. In the second approach, thionyl chloride is used as an additive to convert gallic acid to galloyl chloride which in turn readily reacts in situ with n-propanol to form PG [15,16,17]. This approach requires rigorously anhydrous conditions, i.e., dried gallic acid, freshly distilled thionyl chloride and total exclusion of water or the use of anhydrous solvents. The third approach utilizes direct Fischer esterification in the presence of various homogenous and heterogenous catalysts, including concentrated sulfuric acid (H₂SO₄), p-toluenesulfonic acid (p-TsOH), p-toluenesulfonic acid and sulfamic acid (p-TsOH + H₂NSO₃H), perchloric acid (HClO₄), perchloric acid and sulfamic acid (HClO₄ + H₂NSO₃H), ionic liquid N-methyl-2-pyrrolidonium hydrogensulfate ([Hnmp]HSO₄), brominated sulfonic acid resin, mordenite (a zeolite mineral with orthorhombic structure containing calcium, sodium, potassium, aluminum, and silicate), and tetramethyl cucurbit[6]uril-phosphomolybdic acid (TMeQ [6]-PMA) (

Table 1. The reported chemical methods for the preparation of PG.

№	Catalyst or Additive	T (°C)	Time (h)	Yield (%)	Organic Solvent	Purification Technique	Ref
1	DIC/DMAP	0	7	66.0	THF	Distillation, column	[14]
2	SOCl2	70	5–6	73.0	EtOAc	Distillation, extraction, column	[15]
		60–65	1	91.6	NU	Precipitation	[16]
		Reflux	5.5	NA	EtOAc	Extraction, crystallization	[17]
3	H2SO4	Reflux	4	NA	NU	Distillation, crystallization	[18]
		100	25	60.0	Isobutanol, n-butanol	Distillation, crystallization	[19]
		130	8	NA	CH2Cl2	Distillation, crystallization	[20]
		Reflux	8–12	74	Toluene	Distillation, column	[21]
		95–100	5	56.5	NU	Distillation, crystallization	[2]
		Reflux	Overnight	81.0	NU	Distillation	[22]
4	p-TsOH	Reflux	12	77.0	NA	NA	[23]
		107/MW	0.13	94.0	NA	NA	[23]
		80–90	14	63.4	Benzene	Distillation, crystallization	[24]
5	p-TsOH + H2NSO3H	80–90	15	76.0	Benzene	Distillation, crystallization	[24]
6	HClO4	80–90	14	63.9	Benzene	Distillation, crystallization	[24]
7	HClO4 + H2NSO3H	80–90	15	81.6	Benzene	Distillation, crystallization	[24]
8	[Hnmp]HSO4	95–105	5	89.8	NU	Distillation, crystallization	[2]
9	Brominated sulfonic acid resin	100	5	98.0	NU	Distillation, crystallization	[25]
10	Mordenite	70	5	96.2	NU	Distillation, crystallization	[26]
11	TMeQ[6]-PMA NCs	50–70	3-5	95.6	NU	Distillation	[27]

T: reaction temperature, Ref: reference, DIC: N,N′-diisopropylcarbodiimide, DMAP: 4-dimethylaminopyridine, THF: tetrahydrofurane, NA: not available, NU: not used, Hnmp: 1-methyl-2-oxopyrrolidin-1-ium, p-TsOH: p-toluenesulfonic acid, MW: microwave, TMeQ[6]-PMA: tetramethyl cucurbit[6]uril-phosphomolybdic acid, NCs: nanocubes.

) [18,19,20,21,22,23,24,25,26,27].

In recent years, the study and development of heterogenous catalysts has received significant interest in different areas of organic transformations including the synthesis of PG. Heterogenous catalysts obtain many noticeable advantages, including their reusability, higher reaction rate and selectivity, easy product/catalyst separation, and affordability [28,29,30,31]. Among them, potassium hydrogen sulfate (potassium bisulfate, KHSO₄) has emerged as an inexpensive, green, non-toxic, and easy to handle catalyst displaying high level of efficiency and reusability in many organic preparations [32]. However, the synthesis of PG using KHSO₄ as a catalyst has not been yet reported. In relation to the spectroscopic characterization of PG, the current data are not complete and restricted to MS, IR, ¹H, and ¹³C NMR data [33,34,35], while data on the pharmacological studies and physicochemical properties have been constantly updated [34,35,36].

Herein, we report an efficient, convenient, and economical synthetic procedure to obtain PG by Fischer esterification using KHSO₄ as the sole heterogenous catalyst. At the same time, we provide a more complete spectral data set updating missing data on DEPT and 2D NMR (HSQC and HMBC) spectra for this compound.

2. Results and Discussion

The target compound III was synthesized in 80.2% of yield from gallic acid monohydrate (I) and n-propanol (II) using potassium hydrogen sulfate as the heterogenous acidic catalyst (Scheme 2).

A thorough search in Reaxys and SciFinder databases has returned a range of different methods for the preparation of PG. These methods are classified according to the usage of chemical catalysts or additives and summarized in Table 1. Our synthetic procedure was based on [2,25,26] in entries 3, 9, and 10 of Table 1. These methods were straightforward and offered a green pathway to obtain the final products without a using additional organic solvents and column chromatography.

Using traditional catalysts such as concentrated sulfuric acid (H₂SO₄), p-toluenesulfonic acid (p-TsOH), and perchloric acid (HClO₄), its mixture with sulfamic acid (H₂NSO₃H) afforded PG in moderate to excellent (56.5–94.0%, entries 3–7, Table 1) [18,19,20,21,22,23,24]. However, these reactive catalysts can cause equipment corrosion problems, especially on a large scale. In addition, their use was usually accompanied by toxic and expensive solvents such as dichloromethane and benzene, as well as column chromatography to purify final products [19,20,21,24]. In [2], ionic liquid N-methyl-2-pyrrolidonium hydrogensulfate ([Hnmp]HSO₄) was used as an efficient catalyst to afford PG in 89.8% of yield (entry 8, Table 1). However, the preparation of this ionic liquid is quite complicated, and its regeneration requires high energy to evaporate water from the filtrate. Recently, three [25,26,27] have demonstrated the synthesis of PG with excellent yield (up to 98.0%) and purity (up to 99.95%) using heterogenous catalysts, i.e. modified sulfonic acid resin, mordenite, and tetramethyl cucurbit[6]uril-phosphomolybdic acid (entries 9–11, Table 1). However, these catalysts needed to be activated before any reaction using hard conditions such as chlorination, bromination [25], calcination at 400–500 °C under inert atmosphere [26], or fabricated into nanocubes [27].

Although KHSO₄ has been widely used in various organic transformations [32,37,38], there are few publications on its application for Fischer esterification. One of the most related example was mentioned in [39], in which it was used as an efficient catalyst for the preparation of butyl paraben in 92.9% yield from p-hydroxybenzoic acid and n-butyl alcohol.

Our experiments have showed that KHSO₄ can be used as an inexpensive, eco-friendly, and easy to handle catalyst for synthesizing PG under mild conditions. The optimized reaction conditions were investigated and presented in

Table 2. Effect of the reaction temperature on the yield of propyl gallate ^a.

Entry	Reaction Temperature (°C)	Isolated Yield of PG (%) b
1	70	51.2
2	85	60.5
3	100	68.1
4	>105	62.9

^a Reaction conditions: Time (14 h), gallic acid (10.0 mmol), n-propanol (100.0 mmol), KHSO₄ (2.0 mmol); ^b Purity was confirmed by TLC and melting point.

,

Table 3. Effect of the reaction time on the yield of propyl gallate ^a.

Entry	Reaction Time (h)	Isolated Yield of PG (%) b
1	10	64.0
2	11	65.1
3	12	67.9
4	13	68.1
5	14	68.1

^a Reaction conditions: Temperature (100 °C), gallic acid (10.0 mmol), n-propanol (100.0 mmol), KHSO₄ (2.0 mmol); ^b Purity was confirmed by TLC and melting point.

,

Table 4. Effect of mole ratio of reactants on the yield of propyl gallate ^a.

Entry	n-Propanol: Gallic Acid (Mole Ratio)	Isolated Yield of PG (%) b
1	9.0:1	64.2
2	10.0:1	67.9
3	11.0:1	69.0
4	12.0:1	72.2
5	13.0:1	72.0
6	14.0:1	71.6

^a Reaction conditions: Temperature (100 °C), time (12 h), gallic acid (10.0 mmol), KHSO₄ (2.0 mmol); ^b Purity was confirmed by TLC and melting point.

,

Table 5. Effect of catalyst dosage on the yield of propyl gallate ^a.

Entry	KHSO4: Gallic Acid (Mole Ratio)	Isolated Yield of PG (%) b
1	0.1:1	68.3
2	0.2:1	72.2
3	0.3:1	79.8
4	0.4:1	80.2
5	0.5:1	80.2

^a Reaction conditions: Temperature (100 °C), Time (12 h), Gallic acid (10.0 mmol), n-Propanol (120.0 mmol); ^b Purity was confirmed by TLC and melting point.

and

Table 6. Reusability of KHSO₄ ^a.

Entry	Recycling Times (n)	KHSO4 Recovered (%) b
1	1	85.0
2	2	72.4
3	3	62.5
4	4	50.0

^a Reaction conditions: Temperature (100 °C), time (12 h), gallic acid (10.0 mmol), n-propanol (120.0 mmol), initial KHSO₄ (4.0 mmol); ^b Percent was calculated based on the initial amount of KHSO₄.

.

We first screened the influence of temperature on PG yield (Table 2). Increasing temperature from 70 to 100 °C resulted in an increase of yield from 51.2 to 68.1%. However, temperatures above 100 °C seemed to have a detrimental effect on the yield (62.9%, entry 4, Table 2). We thus maintained the temperature at 100 °C for further reaction optimization. Second, in relation to the reaction time, we found that there was a slight change in product yield when running the reaction for 10 to 12 h (64.0–67.9%. entries 1–3, Table 3). Longer reaction time of up to 13 and 14 h returned 68.1% of product (entries 4 and 5, Table 3). This indicated that the reaction has reached a plateau and longer reaction time does not improve the yield. As there was only a slight change in product yield between 12 and 13 h reacting time (67.9% vs. 68.1%, entries 3 and 4, Table 3), and no starting material was detected by TLC after 12 h, we decided that 12 h was the standard reacting time for our protocol. We then investigated the suitable mole ratio between n-propanol and gallic acid, and found that the ratio (12.0:1, entry 4, Table 4) offered the best product yield of 72.2%. Finally, we screened for the suitable equivalent of KHSO₄ to be used in this reaction, and identified the mole ratio of 0.4:1 (KHSO₄: gallic acid, entry 4, Table 5) to be the one affording the best product yield of 80.2%.

Notably, the recovery of the KHSO₄ was straightforward by filtration or decantation following by drying at 100 °C for 3 h. Up to 85.0% of dried KHSO₄ was recovered after the first use and it was further reused for three consecutive times with only slight variation in the yields of the final product (data not shown). The reaction work-up was performed as indicated in [2,18,25,26], including distillation (under or without vacuum condition), crystallization/precipitation in water and subsequent filtration to remove water-soluble impurities (starting gallic acid and a small amount of the catalyst dissolved in the filtrate). Using this protocol at a larger scale of 26.58 mmol gallic acid returned the product with similar yield (80.2%). This yield is comparable to those reported in the literature using traditional catalysts [2,18,19,21,22,23,24]. The purity of the product was confirmed by differential scanning calorimetry (DSC) as one of the most widely used thermal analysis technique in the chemical and pharmaceutical industries. The purity was 99.60% without an additional refining, which both met the requirements of US Pharmacopeia (98.0–102.0%) [40] and the European Pharmacopoeia (97.0–103.0%) [41].

MS, FT-IR, ¹H- And ¹³C-NMR data for compound III were in entire accordance with those reported in the literature [2,14,15,21,23,27,33]. HR-MS data showed a molecular ion peak at m/z 211.0622 ([M-H]⁻), which indicated molecular formula of PG C₁₀H₁₂O₅. The absorption band on the infrared spectrum, 1687 cm⁻¹, demonstrated the presence of ester C=O group in compound III. A characteristic triplet (H-3’, δ 0.95 ppm), a multiplet (H-2’, δ 1.67 ppm) and a triplet (H-1’, δ 4.11 ppm) splitting pattern and integration indicated that propyl esterification had successfully taken place. The signals of three hydroxyl protons were overlapped as a broad and unsymmetrical singlet at downfield 9.19 ppm. One aromatic singlet at 6.96 ppm with the integral of 2 demonstrated two characteristic symmetrical aromatic protons (H-2 and H-6). The results were also supported by the ¹³C signals at 165.9 (C-1’), 65.4 (C-1’), 21.7 (C-2’), and 10.4 ppm (C-3’), which verified the success of the esterification.

All new obtained data by DEPT and 2D NMR experiments are in entire agreement with the structure of the title compound III (

Table 7. Two-dimensional (2D) NMR (HSQC and HMBC) correlations for the title compound III.

	Proton	H-2	H-6	H-1’	H-2’	H-3’	H-O
Carbon	ppm	6.96	6.96	4.11	1.67	0.95	9.19
C-1	119.6	α	α
C-2	108.5	x	β
C-3	145.5	α
C-4	138.3	β	β
C-5	145.5		α
C-6	108.5	β	x
C-1’	165.9	β	β	β
C-1’	65.4			x	α	β
C-2’	21.7			α	x	α
C-3’	10.4			β	α	x

α: Due to a two-bond coupling (²J_CH); β: Due to a three-bond coupling (³J_CH); x: Coupling by HSQC.

). In addition, powder X-ray diffraction (PXRD) data were also collected as an effort to provide a comprehensive spectral data set for a polymorphic form of the title compound III obtained by our method (

Table 8. PXRD data for the title compound III.

Peak n	2θ (°)	d (Å)	I/Imax (%)	Peak n	2θ (°)	d (Å)	I/Imax (%)
1	4.86	18.1830	79.91	26	24.36	3.6540	77.34
2	6.14	14.3950	179.48	27	25.06	3.5535	46.74
3	8.20	10.7827	25.85	28	25.68	3.4691	73.58
4	9.52	9.2904	75.04	29	26.24	3.3963	700.96
5	11.42	7.7486	21.16	30	26.74	3.3339	1000
6	11.86	7.4621	70.31	31	27.42	3.2528	468.64
7	12.94	6.8416	172.34	32	28.04	3.1823	75.05
8	15.22	5.8215	149.44	33	28.98	3.0811	68.16
9	16.44	5.3921	66.88	34	29.78	3.0002	37.78
10	16.72	5.3025	48.17	35	30.38	2.9423	40.11
11	17.14	5.1735	46.44	36	31.24	2.8632	20.94
12	17.52	5.0621	30.97	37	31.96	2.8003	20.13
13	17.86	4.9665	42.91	38	32.22	2.7783	28.01
14	18.10	4.9012	51.99	39	32.58	2.7484	45.13
15	18.42	4.8167	34.90	40	33.82	2.6505	37.26
16	18.98	4.6759	37.43	41	34.44	2.6042	22.61
17	19.42	4.5709	24.62	42	34.90	2.5709	38.76
18	19.86	4.4706	76.18	43	36.02	2.4935	20.77
19	20.58	4.3158	36.27	44	36.92	2.4347	42.26
20	21.06	4.2185	60.30	45	37.26	2.4133	40.24
21	21.60	4.1143	33.58	46	38.24	2.3537	29.33
22	22.12	4.0187	73.70	47	39.64	2.2737	27.41
23	23.02	3.8636	110.09	48	40.76	2.2138	21.06
24	23.26	3.8243	114.18	49	43.92	2.0616	40.91
25	23.86	3.7294	156.29	50	44.22	2.0483	27.17

).

This method of obtaining PG can be useful for extensive research on the application of potassium hydrogen sulfate as a catalyst in Fischer esterification of a wide range of carboxylic acids, especially at industrial scale, due to its feasibility, environment friendliness, economy and affordability. In addition, the updated 2D-NMR and PXRD data can support studies on PG metabolites in various medical and pharmaceutical studies of this compound.

All the mass, FT-IR, ¹H-NMR, ¹³C-NMR, DEPT, HSQC, HMBC, PXRD spectra, and DSC data are presented in the Supplementary Material File.

3. Materials and Methods

3.1. General Information

Gallic acid monohydrate (compound I, 98.5%) was purchased from Shanghai Zhanyun Chemical Co., Ltd. (Shanghai, China). Propan-1-ol (99.0%) was obtained from Samchun (Gyeonggi-do, Korea). Ethyl acetate (99.5%), n-hexane (95.0%), potassium hydrogen sulfate (99.0%), and sulfuric acid (95.0–98.0%) were used from Xilong Scientific Co., Ltd. (Guangdong, China). Formic acid (88.0%) was obtained from Guangdong Guanghua Sci-Tech Co., Ltd. (Guangdong, China).

The melting point (M.p) was measured by using the capillary tube method with an SRS EZ-Melt apparatus (Stanford Research Systems, Sunnyvale, CA, USA) and was uncorrected. Differential Scanning Calorimetry (DSC) measurements were performed with Shimadzu DSC-60 (Kyoto, Japan) from 140 °C to 160 °C with a heating rate of 2 °C/min after pre-heating with the average rate of 10 °C/min. An empty aluminum pan was used as the reference. HR-MS was performed at an SCIEX X500 QTOF system (AB Sciex Pte. Ltd., Woodlands Central Indus. Estate, Singapore) in an electrospray ionization (ESI) mode. The FT-IR spectrum was recorded by a Shimadzu spectrometer (Kyoto, Japan). Nuclear magnetic resonance (¹H, ¹³C, DEPT, HSQC, and HMBC) experiments were measured on a Bruker Ascend spectrometer (Billerica, MA, USA) at 500 MHz for proton and 125 MHz for carbon-13 using DMSO-d₆ as the solvent and tetramethylsilane (TMS) as an internal standard. Powder X-ray diffraction (PXRD) was performed on a D8-Advance Bruker AXS diffractometer (Karlsruhe, Germany) with CuKα radiation (λ = 1.541874 Å) at room temperature (25 °C), 40 mA and 40 kV (Göbel mirror; θ-2θ scan; 2θ = 2–80°; step size = 0.020°; scan speed = 1.0 s/step). The PXRD data were analyzed by Match! version 3.11.1.183 64-bit (Crystal Impact, Bonn, Germany, author: Dr. Holger Putz, serial number: 7.3.9.2015001.0001). The reaction mixtures were monitored, and the purity of the compounds was checked by thin-layer chromatography (TLC) on silica gel 60 F₂₅₄ plates (Merck, Darmstadt, Germany).

3.2. Synthetic Procedure

Propyl Gallate (PG, III)

The synthetic procedure for compound III was based on [2,25,26] with some modifications.

Gallic acid monohydrate (compound I, 5.00 g, 26.58 mmol, 1 eq), n-propanol (compound II, 24.0 mL, 399.33 mmol, 12.0 eq.) and potassium hydrogen sulfate (1.45 g, 10.65 mmol, 0.4 eq.) were successively charged into a round bottom flask immersed in an oil bath and fitted with reflux condenser. The mixture was stirred, heated to 100 °C and kept at this temperature for 12 h under anitrogen atmosphere. After completion of the reaction as indicated by TLC (12 h), the reaction mixture was cooled and filtered to recover the solid KHSO₄ (which was dried to recycle for the next esterification). The filtrate was distilled by a rotary vacuum to remove water and unreacted n-propanol. The concentrate was then diluted with water (100 mL) under stirring at room temperature and the precipitate was collected by vacuum filtration, washed three times with water (10 mL each time).

The reaction product was dried at 60 °C for 3 h to obtain propyl gallate (compound III) as a white crystalline solid (4.52 g, 80.2%). Purity 99.60% (DSC). M.p 147.7–150.7 °C. TLC R_f 0.48 (n-hexane/ethyl acetate/formic acid, 3.0:7.0:0.1). HR-MS (ESI⁻, MeOH), m/z: Calculated for C₁₀H₁₂O₅ [M-H]⁻: 211.0607, found: 211.0622; [2M-H]⁻: 423.1291, found: 423.1316. FT-IR (KBr), ν_max (cm⁻¹): 3454, 3309 (OH); 3161 (C-H sp²); 2970, 2879 (C-H sp³); 1687 (C=O); 1608, 1537 (C=C); 1298, 1242, 1195 (C-O). ¹H-NMR (DMSO-d₆), δ (ppm): 9.19 (br.s, 3 H, phenolic OH); 6.96 (s, 2 H, H-2’, H-6’); 4.11 (t, J = 7.0 Hz, 2 H, H-1’); 1.67 (sextet, J = 7.0 Hz, 2 H, H-2’), 0.95 (t, J = 7.0 Hz, 3 H, H-3’). ¹³C-NMR (DMSO-d₆), δ (ppm): 165.9 (C-1’); 145.5 (C-3, C-5); 138.3 (C-4); 119.6 (C-1); 108.5 (C-2, C-6); 65.4 (C-1’); 21.7 (C-2’); 10.4 (C-3’).

4. Conclusions

In summary, an efficient synthesis of propyl gallate was developed for the first time from gallic acid and n-propanol by Fischer esterification using solid potassium hydrogen sulfate as heterogenous acidic catalyst. This is a green, inexpensive and straightforward method which can be applied for a wide range of carboxylic acids in Fischer esterification, especially at large scale. The target compound has been fully characterized by the melting point, high-resolution mass spectrometry, FT-IR, ¹H-NMR, ¹³C-NMR, DEPT, HSQC, HMBC spectroscopies, PXRD, and DSC analysis.