Introduction

The polyethylene glycol family (PEGs) is widely used in the fields of biomedicine and materials science due to its excellent biocompatibility and low toxicity. Among them, Tetraethylene Glycol (OEG4) with a low degree of polymerization plays a unique dual role: it is not only a "Lego building block" for the synthesis of high-end pharmaceutical excipients, but also an impurity indicator that needs to be strictly controlled in polyethylene glycol raw materials [1-2]. This article will explain the value of tetraethylene glycol from two dimensions: synthetic application and analytical detection.

Application of tetraethylene glycol in drug improvement

Curcumin is a natural active substance with potential therapeutic effects on various diseases such as cancer and diabetes, but its extremely low water solubility severely limits clinical application [1]. The researchers found that introducing hydrophilic oligoethylene glycol chains into the curcumin molecule can significantly improve its solubility.

Shi Liwang and others designed an efficient synthesis route using cheap and easily available dihydroxytetraethylene glycol as the starting material. They first converted the hydroxyl groups at both ends of tetraethylene glycol into azide groups and tert-butyl ester groups respectively, obtained the octaethylene glycol skeleton through Williamson ether-forming reaction, and finally coupled it with curcumin to prepare carboxy octaethylene glycol curcumin (C-OEG?-Cur) [1]. The optimized reaction conditions are: using tetrahydrofuran as the solvent, NaH as the base, reaction at 25°C, and the yield is as high as 95%. The solubility of the final product in pH=7.4 buffer reaches 106.14 μg/mL, which is more than 9 times that of curcumin original drug (11.70 μg/mL) [1]. This shows that the tetraethylene glycol derivatization strategy can effectively improve the bioavailability of poorly soluble drugs.

Synthesis and derivatization pathways of tetraethylene glycol

In the above work, the heteromeric functional modification of tetraethylene glycol is a key step. Researchers have synthesized two derivatives, azide-p-toluenesulfonyltetraethylene glycol and single-ended triphenyltetraethylene glycol, which undergo Williamson ether-forming reaction under strong alkaline conditions, extending the chain length from tetraethylene glycol to octaethylene glycol [1]. This strategy avoids the instability problem of tert-butyl ester-containing substrates under ether-forming conditions and lays the foundation for the subsequent introduction of carboxyl and amino functional groups.

Detection method of tetraethylene glycol: gas chromatography

Tetraethylene glycol is not only a synthesis intermediate, but also a common by-product impurity in polyethylene glycol 200. Kong Xiangxiang et al. established a gas chromatography-hydrogen ion flame method (GC-FID) method for the simultaneous determination of the contents of ethylene glycol, diethylene glycol, triethylene glycol and tetraethylene glycol in polyethylene glycol 200 [2]. An HP-INNOWAX column (30 m × 0.25 mm × 0.25 μm) was used. The programmed temperature rise condition was 100°C for 1 min, and then raised to 200°C for 15 min at a rate of 10°C/min. Under these conditions, the retention time of tetraethylene glycol is approximately 23.5 minutes, the linear range is 50-1000 mg/L, the linear correlation coefficient reaches 0.99882, the detection limit is 4.79 mg/L, the quantification limit is 15.96 mg/L, the standard recovery rate is 93.9%-116.9%, and the relative standard deviation is 0.95% [2]. This method has simple pre-treatment and only needs to be diluted with water before direct injection, and is suitable for quality control of polyethylene glycol raw materials.

Things to note

1. Synthesis operation: Williamson's ether-forming reaction is sensitive to water vapor, so freshly dehydrated tetrahydrofuran needs to be used and carried out under nitrogen protection. The choice of base is crucial, and NaH is far more effective than NaOH or K?CO?[1].

2. Impurity control: According to the "Safety Standards for Cosmetics" (2022 Edition), diethylene glycol is listed as a banned substance. When polyethylene glycol is used as an excipient, the oligomer impurities such as ethylene glycol and diethylene glycol need to be strictly monitored. Although there is no clear separate limit for tetraethylene glycol, its content can reflect the stability of the polymerization process and the purity of the product [2].

3. Safe storage: Tetraethylene glycol is hygroscopic and should be sealed and stored in a cool, dry place to avoid contact with strong oxidants.

References

[1] Shi Liwang, Zhang Nan, Du Chao, et al. Preparation of amino tert-butyl octaglycol and improvement of curcumin solubility [J]. petrochemical technology, 2023, 52(12): 1669-1675.

[2] Kong Xiangxiang, Li Jianbing, Chen Dong, et al. Determination of ethylene glycol，diethylene glycol，triethylene glycol and tetraethylene glycol in polyethylene glycol 200 by gas chromatography [J]. quality safety inspection and testing, 2024.