STUDY OF THE SELECTIVE ACYLATION AND PHYSICOCHEMICAL PROPERTIES OF 6-BENZYLAMINOPURINE

ABSTRACT

This study is devoted to the in-depth investigation of the physicochemical properties and thermal stability of acyl derivatives of 6-benzylaminopurine. This compound is widely used as a plant growth-promoting phytohormone; however, its biological activity can be enhanced through modification of chemical stability, solubility, and thermal properties. Therefore, various acyl derivatives of 6-benzylaminopurine were synthesized, and their structures were confirmed using IR spectroscopy and spectrophotometry. The effect of acylation on the molecular structure, particularly the interaction of aromatic and aliphatic acyl groups with the purine core, was thoroughly analyzed. Physicochemical properties such as melting point, solubility, stability in dielectric media, and crystal structure were evaluated. This study helps to determine the relationship between the physicochemical characteristics of 6-benzylaminopurine derivatives and their bioactivity. The obtained results provide a scientific basis for the development of new growth stimulators, agrochemical formulations, and stable plant growth regulators.

АННОТАЦИЯ

Данное исследование посвящено глубокому изучению физико-химических свойств и термической стабильности ацильных производных 6-бензиламинопурина. Это соединение широко используется в качестве фитогормона для стимуляции роста растений, однако существует возможность повышения его биологической активности за счёт модификации химической стабильности, растворимости и термических свойств. В связи с этим были синтезированы различные ацильные производные 6-бензиламинопурина, и их структура была подтверждена с помощью ИК- и спектрофотометрических методов. Влияние ацильной замены на молекулярную структуру, особенно взаимодействие ароматических и алифатических ацильных групп с пуриновой системой, было тщательно проанализировано. Физико-химические свойства включали температуру плавления, растворимость, стабильность в диэлектрической среде и кристаллическую структуру. Исследование позволяет выявить связь между физико-химическими характеристиками ацильных производных 6-бензиламинопурина и их биоактивностью. Полученные данные могут служить научной основой для разработки новых стимуляторов роста, агрохимических препаратов и устойчивых фиторегуляторов.

Keywords: 6-benzylaminopurine, acylation, acyl derivatives, physicochemical properties, thermal stability, XRD, melting point, phytohormones, purine derivatives.

Ключевые слова: 6-бензиламинопурин, ацилирование, ацильные производные, физико-химические свойства, термическая стабильность, рентгеновская дифракция (XRD), температура плавления, фитогормоны, производные пурина.

INTRODUCTION

Functional modification of purine derivatives is one of the important directions in modern organic chemistry, as it allows control over their biological activity, chemical stability, and physicochemical properties. In particular, the purine core of 6-benzylaminopurine, a member of the cytokinin class, is chemically reactive, and the introduction of functional groups at different positions can lead to derivatives with entirely new bioactive properties. The N⁷ position of the purine ring is particularly significant as a nucleophilic center due to its electron density, aromatic character, and resonance stabilization. Therefore, acylation reactions occurring at the N⁷ position are of special scientific interest for studying the structure–activity relationships of purine derivatives [1, p. 788, 2, p. 258, 3.p.5, 4.p. 256, 5. p, 5].

However, in the 6-benzylaminopurine molecule, competitive reactions among the N⁷, N⁹, and N¹ centers complicate selective modification. Reaction selectivity strongly depends on factors such as solvent polarity, the type of catalyst, temperature, pH, and the reactivity of the acylating reagent used. Consequently, controlling acylation at the N⁷ position and distinguishing it from other positions is an important scientific task in organic synthesis.

N⁷-acylated derivatives of 6-benzylaminopurine introduce significant changes in the electron distribution of the purine system, affecting the molecule’s acid–base properties, lipophilicity, solubility, and thermal stability. Determining the relationship between their physicochemical parameters and biological activity provides the basis for the future development of effective plant growth regulators and a new class of agrochemical agents [6,p. 10, 7, p. 1, 8.p.452, 9.p. 6524, 10. p, 452,].

Figure 1. Scheme

From this perspective, the study of selective acylation at the N⁷ position of 6-benzylaminopurine, elucidation of the reaction mechanism, identification of structural changes, and evaluation of the physicochemical and thermal properties of the resulting derivatives define the relevance and significance of the present research.

MATERIALS AND METHODS

In this study, high-purity 6-benzylaminopurine was used as the main substrate. Acetic anhydride served as the acylating reagent. Aprotic, polar solvents—dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and acetonitrile—were selected as the reaction media because they enhance selective acylation at the N⁷ position. Mild bases such as triethylamine and pyridine were used as catalysts to facilitate proton abstraction and activate the electrophilic reagent. Column chromatography on silica gel was employed to purify the product and separate fractions.

6-Benzylaminopurine interacts with various electrophilic reagents in acylation reactions; among them, acetic anhydride is the simplest, most effective, and widely used. Its high electrophilicity and reactivity redistribute the electron density in the purine ring, creating favorable conditions for selective acylation at the N7 nitrogen atom.

Reactions carried out in aprotic polar solvents—DMF, DMSO, and acetonitrile—further enhance the electrophilicity of acetic anhydride and increase the nucleophilicity of the N7 center. Since these solvents do not donate protons, the amino group remains unprotonated, minimizing acylation at the –NH₂ site and directing selectivity predominantly to N⁷.

The reaction was conducted at 95°C in DMF and DMSO, and at 80–85°C in acetonitrile. Elevated temperatures increase the kinetic energy of molecules, lowering activation barriers and accelerating the electrophilic attack of acetic anhydride on the purine ring. At boiling temperature, acetic anhydride initially forms a transient N⁷–acylium complex with the purine, which then stabilizes to the N7-acetyl derivative through proton elimination. This process proceeds via an addition–elimination mechanism.

The presence of mild bases such as triethylamine or pyridine enhances the activity of acetic anhydride by binding the formed acetate anion and accepting protons. This ensures complete acylation at the N⁷ center while significantly reducing side reactions, such as acylation of the amino group.

A result, acylation with acetic anhydride produces N⁷-acetyl-6-benzylaminopurine with high selectivity. Under these conditions, the yield of the acylated products typically ranges from 75–90%. The resulting derivatives exhibit stable physicochemical properties, and their thermal resistance is relatively higher compared to other acylated purine derivatives. IR and spectrophotometric analyses confirmed the presence of the acetyl group specifically at the N⁷ position.

RESULTS AND DISCUSSION

The acylation of 6-benzylaminopurine at the N7 position using acetic anhydride was carried out in various aprotic, polar solvents, and the effect of each solvent on selectivity, yield, and physicochemical properties was evaluated. Reactions in DMF were conducted at 95°C. The aprotic and polar nature of DMF selectively activated the N⁷ center, minimizing side reactions and ensuring a high yield of the product.

As a result, N⁷-acetyl-6-benzylaminopurine was obtained with high selectivity, and the yield reached 82%. The product was isolated as yellow crystalline material. IR and NMR analyses confirmed the attachment of the acetyl group specifically at the N⁷ position.

Figure 2. Results

Reactions in DMSO proceeded with slightly lower selectivity compared to DMF, although the thermal stability of the product was preserved at a higher level. Selectivity was observed around 78%. The polar, aprotic environment enhanced the activity of the electrophilic reagent, promoting acylation at the N7 position. In the IR spectrum, C=O stretching bands appeared in the 1685–1715 cm⁻¹ range, and in the NMR spectrum, signals corresponding to CH₃–CO and N⁷-acyl protons confirmed N7 acylation. Additionally, DMSO formed a homogeneous reaction mixture, maintaining the crystal morphology and physicochemical properties of the product.

Acetonitrile (ACN) showed lower selectivity compared to DMF and DMSO, approximately 75%. The reaction rate was relatively slower. Due to the aprotic nature of ACN, the amino group was protected, but the lower solubility and reduced homogeneity of the reaction mixture, compared to DMF and DMSO, led to a slightly decreased yield. Nevertheless, ACN maintained N⁷ selectivity and minimized side reactions. Column chromatography was used for purification and crystallization, and IR and NMR analyses confirmed the structure of the product.

Thus, aprotic and polar solvents such as DMF, DMSO, and ACN provide a favorable environment for selective acylation at the N⁷ position. DMF ensured the highest yield and selectivity, DMSO improved thermal stability, and ACN preserved selectivity while protecting the amino group. These results demonstrate that the choice of solvent directly influences the efficiency of the N⁷-acylation reaction and the properties of the resulting product (Table 1)

Table 1.

Results

REACTION OF 6-BENZYLAMINOPURINE WITH ACETIC ANHYDRIDE: ANALYSIS OF THE OPTICAL DENSITY SPECTRUM

The optical density (absorbance) spectrum of the compound formed from the reaction of 6-benzylaminopurine with acetic anhydride was measured in the 300–800 nm range. The overall spectrum shows that the absorbance values reach a maximum of 3.5 and decrease significantly toward the long-wavelength region (500–800 nm). This type of spectrum reflects the specific electronic conductivity of the aromatic purine ring and the N6-amino group resulting from acetylation.

The main peaks observed in the spectrum are described as follows. The first and most pronounced peak appears in the 300–310 nm range (≈1.8–2.0 Abs). This peak is associated with π→π* electronic transitions in the purine ring, which are slightly reduced due to the acetylation of the N6 amino group. A second, smaller peak is observed in the 420–450 nm range (≈0.7–0.8 Abs). This corresponds to n→π* transitions and demonstrates the bathochromic effect of the compound formed after N6 amino group acetylation. This peak also correlates with the molecule’s yellowish or pale-crystalline appearance.

Additionally, weaker peaks in the 350–400 nm range (≈0.2–0.5 Abs) are observed, which are attributed to electronic delocalization and resonance effects. Changes in the electron density at the N⁷ and N⁹ positions slightly broaden the spectrum.

In the long-wavelength region (500–800 nm), the absorbance drops below 0.1, indicating that the molecule does not possess significant electronic transitions in this range. At the same time, this confirms that the product was isolated with high selectivity and that side reactions were minimal (fig-1).

Figure 1. Reaction of 6-benzylaminopurine with acetic anhydride

From a chemical perspective, the reaction of 6-benzylaminopurine with acetic anhydride results in acetylation of the N6 amino group. This modification leads to a bathochromic shift and the appearance of new peaks in the optical spectrum. Aprotic and polar solvents, such as DMF, activate the reaction centers and minimize side reactions at the N⁷ position.

The main peaks in the spectrum are located in the 300–310 nm and 420–450 nm regions. The electronic structure and resonance effects of the resulting N6-acetyl-6-benzylaminopurine dictate the formation of these spectral features. The decrease in absorbance at longer wavelengths indicates molecular stability and minimal side reactions. Spectral studies thus play a crucial role in confirming the structure of the compound and assessing its potential biological activity.

CONCLUSION

This study focused on the comprehensive investigation of the physicochemical properties of various acylated derivatives of 6-benzylaminopurine. The structures of the synthesized derivatives were confirmed using IR and spectrophotometric methods, allowing the evaluation of the interactions between the acyl groups and the purine core. The results demonstrated that acylation significantly affects the molecular structure, with both aromatic and aliphatic acyl groups interacting with the purine ring through resonance and electronic conjugation.

The physicochemical properties examined included melting point, solubility, stability in dielectric media, and crystal structure. These parameters enable the prediction of structure–property relationships and the biological activity of the derivatives. Optimization of these properties provides the potential to enhance the efficiency of derivatives as plant growth stimulators.

Overall, the results provide a scientific basis for the development of new growth stimulators, agrochemical agents, and stable phyto-regulators. By correlating the physicochemical parameters of 6-benzylaminopurine derivatives with their biological activity, this study contributes to the rational design of more effective plant growth regulators.

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