**1. Introduction**

Anthracyclines were primarily recognized as antibiotics due to their antibacterial properties in 1939 [1]. However, the chemical characterization of the anthracyclines which includes a rigid planar aromatic ring that remains bound to an amino-sugar by a glycosidic bond (**Figures 1** and **2**). Quinone and hydroquinone groups of these molecules on adjacent rings allow gain and loss of electrons in the conversion of quinone to the semiquinone radical [2, 3]. This semiquinone free radical converts back to quinine under aerobic conditions resulting in the formation of superoxide anion and hydrogen peroxide. The excessive formation of these free radical consequences in lipid peroxidation within cell membranes, DNA damage and finally cell death. This makes them the potent non-selective anti-cancer drugs i.e., they are used in the treatment of a wide range of cancer like small cell lung cancer, breast cancer, lymphoblastic and myeloblastic leukemia, etc. [1, 4–6]. There is a high probability that a cancer patient will be administered with anthracycline at some stage of their chemotherapy session. Daunomycin and doxorubicin were the earliest anthracyclines isolated from *Streptomyces peucetius* and were effective against a wide range of human cancers. Owing to significant antitumor potential, the World Health Organization (WHO) has included daunomycin and doxorubicin

**Figure 1.**

*Chemical structure of doxorubicin and daunorubicin. Dotted circle and dotted line arrow represents probable substitution position in anthracyclines. Doxorubicin and daunorubicin differ at C14 position encircled with 2. Green dotted line is used to depict aglycone and sugar moieties of anthracycline drugs. Chemical structures were rendered using ChemDeaw software.*

#### **Figure 2.**

*Chemical structure of epirubicin and idarubicin. Epirubicin and doxorubicin (Figure 1) differs at position no. 3 (C4′ of sugar moiety, stereoisomer). Idarubicin differs from daunorubicin (Figure 1) at position 1 by absence of methoxy group. Chemical structures were rendered using ChemDraw software.*

in the model list of medicines [7]. But it has been discovered that repeated administration of these drugs can impart chemotherapy-resistance to the tumors and cardiotoxicity [4]. To reduce or subside these side effects, major efforts are being done to find better alternatives. Therefore, the study of more than 2000 analogs has been done so far [8].

Regarding the chemical structure of daunomycin C27H29NO10 and doxorubicin C27H29NO11, they share the same carbon skeleton (**Figure 1**). The difference in their chemical structure comes at the side chain at C-14 position; daunomycin has a hydrogen atom whereas doxorubicin has an alcohol group (**Figure 1**) [5, 6].
