Phenols are widespread in nature, being the major components of several plants and essential oils. Natural phenols’ anti-microbial, anti-bacterial, anti-oxidant, pharmacological and nutritional properties are, nowadays, well established. Hence, given their peculiar biological role, numerous studies are currently ongoing to overcome their limitations, as well as to enhance their activity.
Natural phenols, mainly of vegetable origin, are receiving increasing attention, as insight into their biological activity increases.
In recent years, many reviews appeared about phenolic profiles of plants and/or essential oils, evidencing anti-microbial, anti-bacterial 
, antioxidant 
, as well as pharmacological 
and nutritional 
properties, together with a very informing book 
. In view of their importance, studies were aimed at breeding plants able to increase the content of bioactive phenols 
. The research in the field continues, and more and more plants are investigated for their phenolic content and related bioactivity 
. The antioxidant activity of natural phenols has been related to their scavenger ability towards free radicals 
. Particularly interesting is the possibility to encapsulate phenols—as well as other natural compounds—in chitosan biopolymers 
, or in β-cyclodextrin 
It must be noticed that the application of modern extractive techniques 
makes the determination of phenolic compounds in plant matrices more accessible and complete.
New applications of natural phenols in different fields are reported in fish aquaculture 
, sport performances 
, fish gelatin and gelatin from bovine skin modification by cross-linking with natural phenolic acids 
. Advanced extraction technologies allowed the use of phenolic extracts from some plants for food preservation 
. Moreover, technological applications are becoming available, such as anti-bacterial films based on cellulose/phenolic species 
, antimicrobials packaging films based on nano-encapsulation of bioactive oils through emulsion polymerization 
, fire-resistant phenolic foams 
and natural fiber-reinforced composites with lignin phenol binder 
Additionally, it is worth signaling the use of natural phenolic compounds as building blocks to obtain functional materials 
or as antioxidants for biodiesel 
With so much information collected and available, the next step was the effort to understand the structural factors responsible for bioactivity, examining the structure–activity relationship of antioxidant phenolic compounds 
From the chemical point of view, it may be interesting to look for chemical derivatization of natural phenols leading to eventually enhanced biological activity. As a matter of fact, treatment with diazomethane of phenolic extracts led to derivatives more suitable as antioxidants for lipophilic foods 
. Considering the importance for human health, representative methods to chemically modify the natural phenols were discussed 
, as well as reviews of enzymatic modification 
and of metabolic engineering for microbial biosynthesis of natural compounds, among which phenols, were reported 
2. Tailored Functionalization of Natural Phenols to Improve Biological Activity
Monophenol functionalization is attracting the interest of a growing number of researchers, since the synthesis of new biologically active derivatives starting from natural compounds is a proficient tool to improve their properties. In fact, tailored functionalization is a valuable strategy to overcome natural phenol weaknesses such as toxicity, low water solubility, as well as to mild their strong fragrances, that often limit their application 
Carvacrol, thymol and eugenol are amongst the most widespread phenols in nature, usually responsible for beneficial plant properties.
As an example, the antioxidant activity of tyrosol (2-(4-hydroxyphenyl)-ethanol), which is an abundant phenol in olive oil, responsible for oil beneficial properties 
, can be sensibly enhanced through esterification of the alcoholic hydroxyl group with different phenolic acids (Scheme 1
. Analogously, hydroarylation with cinnamic esters improves the antioxidant properties of tyrosol, especially in the presence of additional hydroxyl group in the aromatic ring of the acidic moiety (Scheme 1
Natural phenols and their derivatives with biological activities constitute a fast-growing research topic, in view of their many present and future applications. Their structural diversity offers many possibilities of chemical transformations, aimed at overcoming the drawbacks of natural phenols. However, apart from some guidelines that emerged from the huge number of publications, such as the need to meliorate stability and bioavailability of the bioactive compounds, the picture of structural requirements is not yet complete, in view of optimizing in vivo and in-field applications.