2,2′-(Methylenebis(3,4-Dimethoxy-6,1-Phenylene))diacetic Acid

1. Introduction

Side products in chemical synthesis are substances formed alongside the target products of the reaction [1]. Their formation is often unavoidable, necessitating the development of strategies to remove them from the reaction mixture and reduce impurities in the final product. In many cases, however, side products prove to be valuable species with diverse applications or serve as precursors and raw materials for further transformations [2]. The study of side products is therefore of considerable importance. It not only contributes to the optimization of synthetic processes and improves the yield and purity of target compounds, but also enables the identification of previously unknown structures with potential applications across manufacturing, pharmaceutical, cosmetic, and other industrial sectors.

Homophthalic anhydride and its substituted derivatives are important precursors in the synthesis of a wide range of compounds [3,4,5,6,7,8,9,10,11,12,13]. One common approach to their preparation involves a four-step synthetic procedure starting from appropriately substituted phenylacetic acids [14,15]. The initial step in this pathway is an oxa-Pictet–Spengler reaction — a variation of the classical Pictet-Spengler reaction — which yields the corresponding isochroman-3-one as the principal product.

Although the reaction has been known in the literature for a long time, the main side product obtained – 2,2'-(methylenebis(3,4-dimethoxy-6,1-phenylene))diacetic acid – has not been studied in detail to date, which set the goal of the studies described below.

2. Results and Discussion

The Oxa-Pictet-Spengler reaction is a variant of the Pictet-Spengler reaction where an oxygen atom acts as the nucleophile instead of nitrogen. Using phenylacetic acid and its aryl-substituted analogs facilitates the formation of isochroman-3-one structures, as shown in Scheme 1. This transformation is widely used not only to create the isochromane backbone but also as a crucial step in synthesizing substituted homophthalic anhydrides, which are important intermediates in the production of various compounds [7,12,13].

This well-documented reaction is usually carried out in the presence of an acid. Under standard conditions, TLC analysis of the reaction mixture revealed, in addition to the target isochroman-3-one, a secondary product. Workup revealed that this side product was acidic in nature, as it was selectively extracted with a 5% aqueous NaHCO₃. Acidifying the aqueous layer and extracting with ethyl acetate yielded the desired compound, which was then purified by recrystallization. The isolated product was characterized by ¹H, ¹³C and DEPT-135 NMR, FTIR, UV-Vis and HRMS analyses, as detailed below.

The recorded NMR spectra clearly indicate the presence of an aromatic system in the structure, as well as a strongly negative heteroatom. Overall, the spectra closely resemble those reported in the literature for the starting acid [16]. In the ¹H-NMR spectrum, two singlets at 3.87 and 3.71 ppm are observed, corresponding to methoxy groups. Additionally, two singlets at 6.72 and 6.43 ppm are assigned to aromatic protons, while the singlet at 3.56 ppm is attributed to a methylene group - in this case, does adjacent to the carboxylic group. A key distinction between the isolated product and the starting phenylacetic acid derivative is the absence of one aromatic proton, accompanied by the appearance of a new singlet signal at 3.91 ppm (Figure 1). This signal, according to the area ratio and chemical shift, can be attributed to the methylene group linking the two aromatic rings. This assignment aligns with the literature on diphenylmethane derivatives, which reports the methylene protons to appear as a singlet at 4.11 ppm in ¹H-NMR spectrum [17].

The ¹³C-NMR spectrum, together with DEPT-135 analysis, confirms the presence of a carboxyl group, evidenced by a signal at 179 ppm. In addition, six distinct aromatic carbon signals are observed, comprising two tertiary and four quaternary carbons. The spectra also reveal two methylene groups, with an approximate intensity ratio of 1:2. The ¹H, ¹³C and DEPT-135 NMR spectra are presented in Figures S1, S2 and S3 in the Supplementary Materials, respectively.

The FTIR spectrum of the compound exhibits several characteristic absorption bands. Broad signals around 3000 cm⁻¹ (stretching vibrations, υ_O-H) and a strong band near 1700 cm⁻¹ (stretching vibrations, υ_C=O) are consistent with the presence of a carboxyl group. Absorptions at 1610 and 1510 cm⁻¹ correspond to C=C stretching vibrations of the aromatic ring. Additionally, strong bands at 1270, 1230, and 1030 cm⁻¹ are attributed to C-O stretching vibrations of the methoxy groups, as well as the C-O-H functionality of the carboxylic acid. The FTIR spectrum of the compound is presented in Figure S4 in the Supplementary Materials.

The structural resemblance between the studied compound and 3,4-dimethoxyphenylacetic acid suggests similar UV-VIS spectral features. The methylene group between the aromatic ring and the carboxyl group precludes conjugation, so the spectrum is dictated solely by the benzene ring and its two methoxy substituents. The positive mesomeric effect (+M) of the methoxy groups outweighs their negative inductive effect (-I), causing a bathochromic shift of the benzene absorption bands. Accordingly, two maxima are observed — at 232 nm and a lower-intensity band at 286 nm. Measurements were performed at a concentration of 25 μM in ethanol. The corresponding UV-VIS spectrum is presented in Figure S5 in the Supplementary Materials.

Due to the presence of a carboxyl group in the compound, accurate mass determination was performed in negative ion scan mode. The mass spectrum exhibited a peak at m/z 403.13838, which corresponds to the deprotonated molecular ion ([M-H]^-) of the studied compound. The HRMS spectrum of the compound is presented in Figure S6 in the Supplementary Materials.

The results of these studies confirm that the structure of the reaction’s side product is consistent with those proposed in Scheme 1, resembling a dimer of the 3,4-dimethoxysubstituted phenylacetic acid bridged with an methylene group.

3. Materials and Methods

3.1. General

All chemicals used were purchased from Sigma-Aldrich (FOT, Sofia, Bulgaria ) and used without further purification. TLC was performed on silica gel coated aluminum sheets (Alugram® SIL G/UV254, Macherey-Nagel, Merck, Darmstadt, Germany). NMR spectra were recorded on a Bruker Avance III HD (500 MHz and 126 MHz for ¹H and ¹³C, respectively) using TMS as an internal standard and CDCl₃ as a solvent. The chemical shifts (δ) are given in ppm and J values are reported in Hz. FTIR spectra were recorded in solid state on a Nicolet iS5 FT-IR Spectrometer equipped with iD5 ATR Accessory (Thermo Scientific, ACM 2, Sofia, Bulgaria). UV-Vis spectra were recorded on Evolution 60S UV-Visible Spectrophotometer (Thermo Scientific, ACM 2, Sofia, Bulgaria) using quartz cuvette. High-Resolution Mass Spectra (HRMS) were obtained on a Shimadzu LCMS-9050 (Shimadzu Handels GmbH., Korneuburg, Austria).

3.2. Procedure

In a round-bottom flask 3,4-dimethoxyphenylacetic acid (8.9 g, 0.045 mol) and glacial acetic acid — AcOH, (25 mL, 0.437 mol) were heated under reflux with continuous stirring until reaching 80 °C. At this temperature, conc. hydrochloric acid — HCl, (15 mL 0.485 mol) and 37% aqueous formaldehyde solution — HCHO (15 mL, 0.20 mol) were added, after which the temperature was raised to 90 °C and maintained for an hour with continuous stirring under reflux. After cooling, 50 mL of water was added to the reaction mixture and extracted with chloroform. The organic layer was washed sequentially with 5% NaHCO₃ until the presence of the studied compound was no longer detected in the organic layer (TLC). The aqueous extracts were combined, acidified with HCl to pH = 3 and extracted with EtOAc. The organic layer was washed with water until pH = 7 and dried with sodium sulfate — Na₂SO₄. After evaporation to dryness, the resulting solid was recrystallized from EtOAc to give 2,2'-(methylenebis(3,4-dimethoxy-6,1-phenylene))diacetic acid as white crystals (1.03 g), m.p. = 185-186 °C . R_f = 0.31 (acetone: heptane = 1:4); UV-Vis (ethanol): λmax = 232 nm (ε = 27840 L mol⁻¹ cm⁻¹); 286 nm (ε = 10080 L mol⁻¹ cm⁻¹); FTIR (solid) cm⁻¹: ν = 2938 (w, O=H), 1697 (s, C=O), 1607 (w, C=C), 1513 (s, C=C), 1275 (s, C-O), 1231 (s, C-O), 1033 (m, C-O); ¹H-NMR (500 MHz, CDCl₃): δ = 6.72 (2H, s, Ar-H), 6.43 (2H, s, Ar-H), 3.91 (2H, s, CH₂), 3.87 (6H, s, OCH₃), 3.71 (6H, s, OCH₃), 3.56 (4H, s, CH₂). ¹³C-NMR (126 MHz, CDCl₃): δ = 179.05 (C, COOH), 148.53 (C), 147.58 (C), 131.24 (C), 124.41 (C), 114.26 (CH), 113.05 (CH), 56.07 (CH₃, OCH₃), 56.01 (CH₃, OCH₃), 38.07 (CH₂), 35.80 (CH₂). HRMS (ESI): m/z calculated for [M-H]^- C₂₁H₂₃O₈^-: 403.13984; found: [M-H]^- 403.13838.