Synthesis and Antimicrobial Activity of New Schiff Base Compounds Containing 2-Hydroxy-4-pentadecylbenzaldehyde Moiety

Various novel Schiff base compounds have been synthesized by reaction of 2-hydroxy-4-pentadecylbenzaldehyde with substituted benzothiophene-2-carboxylic acid hydrazide and different substituted aromatic or heterocyclic amines in the presence of acetic acid in ethanol. The structures of all these compounds were confirmed by elemental analysis, IR, 1H-NMR, 13C-NMR, and mass spectral data and have been screened for antibacterial and antifungal activity. 1. Introduction Compounds with the structure of –C=N–(azomethine group) are known as Schiff bases, which are usually synthesized by condensation of primary amines and active carbonyl groups. Schiff bases are an important class of compounds in the medicinal and the pharmaceutical field, including antibacterial [1, 2], antifungal [3, 4], and antitumor activity [5, 6]. Heterocyclic-containing Schiff bases can show dramatically increased biological activities. As evident from literature [7], it was noted that a lot of research has been carried out on Schiff bases, but no work has been done on this particular type of Schiff base. Aromatic Schiff bases possessing long alkyl chains have received overwhelming attention due to their possibility to show mesomorphic properties such as smectic and nematic phases [8–11]. In this paper, we report the synthesis of a novel Schiff base from substituted benzothiophene-2-carboxylic acid hydrazide, various aromatic and heterocyclic amines with 2-hydroxy-4-pentadecylbenzaldehyde as a moiety. We also report the results of biological screening for possible antibacterial and antifungal activity of the resulting derivatives and we discuss the relationship of molecular structure and the bioactivity (Table 1). Table 1: Antibacterial and antifungal activity of the tested compounds. 2. Results and Discussion Cashew nut shell liquid (CNSL) is obtained as a by-product from mechanical processing for edible use of cashew kernel (Anacardium occidentale L.) and is a mixture of anacardic acid, cardanol, and smaller amounts of cardol and 2-methyl cardol. Due to the easy thermal decarboxylation of anacardic acid, the main component of distilled CNSL is cardanol (yield up to 70–80% and purity up to 90%) as a mixture of saturated (3-n-pentadecylphenol), monoolefinic [3-(n-pentadeca-8-enyl)phenol], diolefinic [3-(n-pentadeca-8,11-dienyl)phenol], and triolefinic [3-(n-pentadeca-8,11,14-trienyl)phenol] long-chain phenols, with an average value of two double bonds per molecule. Cardol and methyl cardol are present in smaller percentages (Figure 1). Figure 1: Chemical structure of components in