Abstract
A series of caffeic acid derivatives were synthesized via a modified Wittig reaction which is a very important tool in organic chemistry for the construction of unsaturated carbon–carbon bonds. All reactions were performed in water medium at 90 °C. The aqueous Wittig reaction worked best when one unprotected hydroxyl group was present in the phenyl ring. The olefinations in the aqueous conditions were also conducted with good yields in the presence of two unprotected hydroxyl groups. When the number of the hydroxyl groups was increased to three, the reaction yields were worse, and the derivatives 12, 13, and 18 were obtained with 74%, 37%, and 70% yields, respectively. Nevertheless, the Wittig reaction using water as the essential medium is an elegant one-pot synthesis and a greener method, which can be a safe alternative for implementation in organic chemistry. The obtained compounds were tested for their antioxidant activity, and 12, 13, and 18 showed the highest activities. Moreover, all synthesized compounds displayed no cytotoxicity, and can therefore be used in the pharmaceutical or cosmetic industry.
Highlights
Caffeic acid (CA) is a natural phenolic acid which is synthesized by plants as a secondary metabolite
The latest studies have disclosed that the presence of free phenolic hydroxyl groups and their number and position in the phenyl ring are essential for the strength of the antioxidant activity, while their protection renders them inactive [14]
The reaction of aromatic aldehydes with two unprotected hydroxyl groups in the aromatic ring resulted in favorable yields
Summary
Caffeic acid (CA) is a natural phenolic acid which is synthesized by plants as a secondary metabolite. CA and its natural and synthetic derivatives show potent antioxidant activity, even in low concentrations. It has been proved in many biological investigations that caffeic acid and its analogues display anti-inflammatory, antibacterial, antiviral, and antitumor activities [1,2,3,4,5]. The latest studies have disclosed that the presence of free phenolic hydroxyl groups and their number and position in the phenyl ring are essential for the strength of the antioxidant activity, while their protection renders them inactive [14]. The type of spacer between the carboxylic group and the aromatic ring of the phenolic acid markedly influences their antioxidant profile—derivatives with methylenic, ethylenic, and unsaturated chains display the highest activity [15,16]
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