## 1 Introduction  
In the last decades, with the aid of various investigation techniques, the study of the chemical composition of wood and in particular of its extractives has gained noticeable importance. Wood represents a natural resource, largely used for energy purposes, in the construction, furniture, pulp and paper industries, and for the creation of chemical products and compounds of great industrial use, widely used in the cosmetics, biomedical, pharmaceutical and food sectors, and even in biodiesel production [1, 2]. The field of wood chemistry and wood-derived products covers various aspects of chemical technology applied to the wood sector: from the amounts of lignin, cellulose and hemicelluloses, analysis of extractives, up to the characterization of adhesives and studies on the interphase between wood and adhesive formulations or finishing products. Many research groups have implemented the wood modification processes to make the wood material more suitable for any type of application [3]. Thermal modification is a technique, based on the principle according to which wood, subjected to high temperatures, modifies its chemical-physical structure, increasing its durability, dimensional stability and altering its colour; this technique is very effective in improving some wood properties without resorting to the use of chemical additives [4]. However, it can reduce its mechanical properties [5]. The use of different heating techniques over a wide range of time and temperatures allows a more detailed analysis of the wood concerning its heterogeneous thermal response. Although numerous studies have been conducted on improving the overall properties of wood, there has recently been a growing interest in characterizing the thermal degradation of modified wood products [6; 7], which could be related to both the increase in demand for wood and the new concept of environmental management policies [8; 9]. Recently, the European research program has been placing a lot of importance on biorefinery and green chemistry; one of the main purposes is to guarantee and produce innovative and renewable products using local bioresources such as wood and residual biomass. Although several studies have reported the profile of secondary metabolites and bioactivity of extractives obtained from the bark or other parts of the tree [10], the profiles of natural compounds have not yet been well studied and defined, obtained from heat-treated wood, even that is of growing interest. Some authors have shown the influence of heat treatment on extractives and related antioxidant properties of poplar [11], beech and spruce wood [12; 13], highlighting potential alternative applications of biomass beyond energy or industrial uses. Other researchers have also studied the change in yield of extractives following heat treatment of beech and found that it was particularly relevant at 160 \({}^{\circ}\)C, possibly due to hemicellulose decomposition [14]. On the contrary, it has been shown that the heat treatment of beech (_Fagus sylvatica_ [PERSON], 1753) and spruce (_Picea abies_ [PERSON], 1881) at 220 \({}^{\circ}\)C produces structural alterations both of thermally labile carbohydrates and of some lignin fractions [15]. The thermal decomposition process of wood and wood products is based on a series of complex reactions influenced by many factors, such as heating rate, temperature, exposure time, pressure, humidity, biomass composition and size of the particles. Most of the changes in the properties of wood are related to changes that occur at a chemical level, i.e., the microstructure of wood is altered due to the action of heat. During the heat treatment process, volatile extractives are formed by the alteration and degradation of polysaccharides and lignin [16] and are usually removed first from the wood, followed by new products and compounds resulting from various chemical reactions. The chemical changes occur in the degradation of its structural polymers.