# Novel Aromatic Estolide Esters from Biobased Resources by a Green Synthetic Approach
## 1 Introduction
However, the utilization of an aromatic alcohol or acid in these reactions is difficult to accomplish. In fact, only one report was published in this topic until now, addressing the synthesis of cinnamate estolides from milkweed seed oil. Heterogeneous chemical catalysts have been used for the epoxidation of unsaturated groups of this oil, followed by epoxy ring opening to yield polyhydroxy triacylglycerglycerols, which reacted with ferulic acid [22]. The antioxidant properties of these compounds were also demonstrated, as they prevented lipid peroxidation in phospholipid vesicles [23]. Copolymers with alternating RCA and 4-hydroxycinnamic acid derivatives were obtained by pre-coupling the monomer molecules, followed by self-condensation of the resulting hetero-dimers using various chemical catalysts [24]. Lipases are extremely versatile enzymes, being able to catalyze a wide range of reactions including the synthesis of oligomers (estolides) [25] or co-oligomers [26] of HFAs, particularly ricinoleic acid. RCA is a biobased hydroxy acid with great potential for sustainable manufacturing of green chemicals, although in some countries its source, castor bean (_Ricinus communis_), is considered an invasive plant [27]. [PERSON] et al. studied the estolide size profile and the evolution of the polymerization degree of RCA over a 24 h reaction time [28]. [PERSON] et al. obtained RCA trimers and tetramers directly from castor oil by transesterification with lipase A from _Candida antarctica_[17]. The estolide esters were also obtained with lipases, in both native or immobilized form. [PERSON] et al. used lipase B from _Candida antarctica_ immobilized by adsorption to yield polyglycerol polyricinoleate, an important food emulsifier [29]. Another naturally occurring hydroxy acid, lesquerollic acid from lesquerella oil, was also investigated for the lipase-catalyzed synthesis of estolide esters by endcapping with oleic, stearic or 2-ethylhexanoic acids [30]. The synthesis of aromatic esters catalyzed by lipases is more difficult compared to aliphatic esters, due to the lower substrate specificity towards aromatic substrates, as demonstrated by our earlier works [31; 32]. The selection of an appropriate enzyme is essential since this substrate specificity is strongly influenced by the structure of the binding site of the enzyme [33]. The objective of the present work was the valorization of RCA, a biomass-based unsaturated hydroxy fatty acid, for the green synthesis of new high added-value oligomers, using a cascade approach involving hydrolysis of castor oil, enzymatic synthesis of oligo-ricinoleates and endcapping with an aromatic acid or alcohol. Alongside RCA, the aromatic acids and aromatic alcohols can also be obtained from renewable resources. 4-Hydroxycinnamic acids can be recovered from agricultural by-products and also synthesized by engineered microorganisms [34]. 4-Hydroxycinnamyl alcohols have been produced from inexpensive phenylpropanoic acids by a three-step biocatalytic cascade reaction using genetically engineered _Escherichia coli_ strains [35]. Enzymatic synthesis of such aromatic estolide derivatives has not yet been reported. Since the estolides still hold a hydroxy and a carboxy group available for functionalization, the selection of either an aromatic acid or an aromatic alcohol is possible for the synthesis of the end capped oligomeric esters, using the suitable biocatalyst. The aim of this first study was to demonstrate the possibility of synthesizing estolides of ricinoleic acid endcapped by an aromatic functional group at either the carboxy or hydroxy end. The obtained estolide esters are biobased compounds, synthesized from a vegetable oil-derived hydroxy acid and aromatic acids or alcohols also selected based on their availability from renewable raw materials by enzymatic or chemoenzymatic pathways. Advanced techniques were employed for the characterization of the obtained aromatic oligoesters, highlighting the rheological properties as being essential for their possible utilizations.