## 1 Introduction  
\(\beta\)-Glucuronidase is a well-known enzyme that hydrolyzes conjugated compounds, including \(\beta\)-glucuron acid, into their derivatives and free glucuronic acid [1]. \(\beta\)-Glucuronidase is frequently found in unicellular microorganisms, such as _Escherichia coli_ and _Peptostrophe-tococus_ species, and multicellular organisms, including plants and mammals [2, 3]. In particular, this enzyme can be detected in several human body organs, such as the kidney, liver, lung, and digestive system [4]. In 2010, [PERSON] et al. first reported the crystal structure of _E. coli_\(\beta\)-glucuronidase [5]. The \(\beta\)-glucuronidase asymmetric unit (139 kDa) comprises two monomers (597 residues) and is organized into the following three areas: the N-terminal contains 180 residues and is similar to the carbohydrate-binding domain of the glycoside hydrolase 2 members; the C-terminal contains residue 274 to residue 603 and comprises an \(\alpha\beta\) loop; and the region between terminals N and C contains an Ig-like \(\beta\)-sandwich domain, as is the case with other members of the glycoside hydrolase 2 family [5, 6]. Inhibiting \(\beta\)-glucuronidase can be beneficial for preventing and treating various diseases [7]. \(\beta\)-Glucuronidase is generated in the synovial fluid under inflammatory conditions, such as rheumatoid arthritis [8]. Moreover, an increased risk of colon cancer has been associated with the role of \(\beta\)-glucuronidase in the disease, as well as enhanced intestinal enzyme levels [9]. Increased levels of \(\beta\)-glucuronidase in the blood, owing to liver injury, can lead to liver cancer [7; 10]. Thus, the discovery and development of \(\beta\)-glucuronidase inhibitors may be valuable in reducing these carcinogenic risks. _Acer_ L., commonly known as maple, is a diverse genus in the Aceraceae family, which comprises over 125 species with several infraspecific taxa, and is widely distributed in northern temperate regions worldwide [11]. Many _Acer_ species provide products of economic value, including furniture, lumber, horticultural plants, and herbal medicines, especially gamma-linolenic acid, a dietary supplement that can help treat cancer and cardiovascular diseases [12]. _A. mono_ Maxim., a deciduous tree of the _Acer_ genus, is commonly found in Korea, Japan, and Northeast China [13; 14]. Over the years, this plant has attracted considerable attention from both chemists and pharmacists, given its noteworthy traditional uses and pharmacological activities. In Korea, the leaves of _A. mono_ Maxim. have long been employed as a material for hemostasis, whereas the roots have been used in traditional Korean medicine to treat arthralgia and cataclassis [13; 14; 15]. The sap of _A. mono_ Maxim. has been used to treat gout, neuralgia, urinary hesitancy, constipation, and other gastroenteric conditions [15; 16]. _A. mono_ Maxim. reportedly possesses various pharmacological properties, including hepatoprotective, antioxidant, and osteoporotic inhibitory effects [13; 14]. Phytochemical constituents of _A. mono_ Maxim. include stilbenes, flavonoids, and their derivatives [14]. Among them, 5-\(O\)-methyl-(\(E\))-resveratrol 3-\(O\)-\(\beta\)-D-glucopyranoside and 5-\(O\)-methyl-(\(E\))-resveratrol 3-\(O\)-\(\beta\)-D-apiofuranosyl-(1-\(\to\)6)-\(\beta\)-D-glucopyranoside, two still-bene glycosides isolated from the leaves of _A. mono_ Maxim., could reduce the levels of DPPH radicals at the concentrations of 100 \(\upmu\)M, thereby exhibiting significant free-radical scavenging effects, with IC\({}_{50}\) values of 103.6 and 80.5 \(\upmu\)M, respectively [13]. As part of our ongoing investigation of the secondary metabolites and respective biological effects from herbal and medicinal plants in Korea, we isolated and structurally elucidated eight compounds, including two isocoumarins and six coumarins, from the stems and branches of _A. mono_ Maxim. in the present study. To the best of our knowledge, this is the first report regarding the isolation of isocoumarins from the _Acer_ genus.