## 1 Introduction  
In 1936, [PERSON] introduced lignans as a group of plant-based compounds consisting of \(\beta\)-\(\theta^{\prime}\)-linked dimeric phenylpropane units (Figure 1) [1]. Similar to other natural phenolics such as stilbenes and flavonoids, lignans are formed through the shikimic acid pathway [2]. Lignans have attracted great interest in research as they have a number of health benefits, such as being strong antioxidants, lowering the risk of coronary heart disease, and having neuroprotective effects and anticancer properties [3; 4; 5; 6]. Lignans are also found in foods, for example, flaxseed, sesame, chickpeas and cereals (rye, oat, barley), and red wine contains high levels of lignans. [7]  
Figure 1: The structure of lignan and 9-norlignan and norlignan. A subclass of lignans called norlignans are dimers composed of a phenylpropane unit and a phenylethane unit coupled through a \(\beta\)-\(\beta^{\prime}\) linkage (Figure 1). The prefix \"nor\" indicates the lack of one or more carbon in the parent skeleton, and the term 9-norlignan is used when the lignan structure lacks C-9 (or \(\gamma\) carbon). As lignans, norlignans are also found in plants, having a range of biological activity [8; 9]. The lignan 7-hydroxymatairesinol (HMR) can be isolated in large-scale from the knotwood of Norway spruce (_Piceu abies_) as it constitutes up to 24% of the dry weight [10]. HMR can be used as raw material for the semi-synthesis of a range of different lignans, among others are a family of norlignans belonging to the imperanene family [9]. Some of these semi-synthetic and unsaturated 9-norlignans were used as substrates in this study (Figure 2, substrates **1-4**). The substrates were chosen for two reasons. Firstly, they were chosen to study the selectivity in the oxidation reactions (epoxidation) with substrates containing free phenolics (**1** and **3**) compared to protected (methoxylated) derivatives (**2** and **4**). Secondly, we wanted to study the outcome of the epoxidation reaction in the presence of a primary hydroxyl (**1-2**), carboxylic acid (**3**) and methyl ester (**4**). Epoxidation reactions are undoubtedly important in organic chemistry and several methods have been developed to perform these reactions [11; 12; 13; 14]. However, to our knowledge, no epoxidation reactions have previously been reported for substrates **1-4**. As part of our work on oxidative transformation of lignans and norlignans, we wanted to investigate how these substrates react upon epoxidation. We chose two different methods for this study. First, the stoichiometric \(m\)CPBA-mediated epoxidation method known as the Prilezhaev reaction was studied. Previously, \(m\)CPBA has been used for Baeyer-Villiger oxidation of lignans containing ketone or aldehyde groups, for epoxidation of lignan structures containing double bonds, and for epoxidation of dihydro-naphthalene model compounds having a norlignan-type skeleton [15; 16]. Because of the simplicity of the Prilezhaev reaction and the average high product yields for the reaction, \(m\)CPBA was a good epoxidation agent for the purpose of our study. As we wanted to study more environmentally friendly alternatives for epoxidation, our attention turned to catalytic reactions using H\({}_{2}\)O\({}_{2}\), _tert_-butyl hydroperoxide (TBHP), or even O\({}_{2}\) as oxidants. Since the 1960s, numerous complexes of molybdenum have been used as catalysts for efficient epoxidation of olefins together with hydroperoxides [17]. More recently, scientists have further developed molybdenum complexes for this purpose [18]. Some complexes have attracted our interest, i.e., [MoO\({}_{2}\)L] complexes (L being a tridentate Schiff base ligand) since their synthesis is quite easy [19; 20]. These complexes have a long shelf life and have been shown to exhibit interesting reactivity under green conditions. For example, epoxidations of cycloocene and cyclohexene [19; 20; 21; 22] and the natural substrate limonene has been demonstrated [23].