Pathogenic microorganisms are causative agents for different types of serious and even lethal infectious diseases. Despite advancements in medication, bacterial and fungal infections continue to be a growing problem in health care. As more and more bacteria become resistant to antibiotics used in therapy and an increasing number of invasive fungal species become resistant to current antifungal medications, there is considerable interest in the development of new compounds with antimicrobial activity. The compounds containing a heterocyclic ring play an important role among organic compounds with biological activity used as drugs in human and veterinary medicine or as insecticides and pesticides in agriculture. Thiadiazoles belong to the classes of nitrogen–sulfur heterocycles with extensive application as structural units of biologically active molecules and as useful intermediates in medicinal chemistry. The potency of the thiadiazole nucleus is demonstrated by the drugs currently used. 1,3,4-Thiadiazoles and some of their derivatives are extensively studied because of their broad spectrum of pharmacological activities. The aim of this review was to highlight the main antimicrobial properties exhibited by derivatives possessing 2-amino-1,3,4-thiadiazole moiety. Many of the reported 2-amino-1,3,4-thiadiazole derivatives can be considered as lead compounds for drug synthesis, and several of them have demonstrated higher antimicrobial activity in comparison to standard drugs. Furthermore, taking into account the reactivity of the amine group in the derivatization process, 2-amino-1,3,4-thiadiazole moiety may be a good scaffold for future pharmacologically active 1,3,4-thiadiazole derivatives.

Neglected tropical diseases affect the lives of a billion people worldwide. Among them, the parasitic infections caused by protozoan parasites of the Trypanosomatidae family have a huge impact on human health. Leishmaniasis, caused by Leishmania spp., is an endemic parasitic disease in over 88 countries and is closely associated with poverty. Although significant advances have been made in the treatment of leishmaniasis over the last decade, currently available chemotherapy is far from satisfactory. The lack of an approved vaccine, effective medication and significant drug resistance worldwide had led to considerable interest in discovering new, inexpensive, efficient and safe antileishmanial agents. 1,3,4-Thiadiazole rings are found in biologically active natural products and medicinally important synthetic compounds. The thiadiazole ring exhibits several specific properties: it is a bioisostere of pyrimidine or benzene rings with prevalence in biologically active compounds; the sulfur atom increases lipophilicity and combined with the mesoionic character of thiadiazoles imparts good oral absorption and good cell permeability, resulting in good bioavailability. This review presents synthetic 2-amino-1,3,4-thiadiazole derivatives with antileishmanial activity. Many reported derivatives can be considered as lead compounds for the synthesis of future agents as an alternative to the treatment of leishmaniasis.

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An unexpected C-C bond cleavage was observed in trichloroacetamide-tethered ketones under amine treatment and exploited to develop a new synthesis of normophans from 4-amidocyclohexanones. The reaction involves an unprecedented intramolecular haloform-type reaction of trichloroacetamides promoted by enamines (generated in situ from ketones) as counter-reagents. The methodology was applied to the synthesis of compounds embodying the 6-azabicyclo[3.2.1]octane framework.

Viral infections have resulted in millions of victims in human history. Although great efforts have been made to find effective medication, there are still no drugs that truly cure viral infections. There are currently approximately 90 drugs approved for the treatment of human viral infections. As resistance toward available antiviral drugs has become a global threat to health, there is an intrinsic need to identify new scaffolds that are useful in discovering innovative, less toxic and highly active antiviral agents. 1,3,4-Thiadiazole derivatives have been extensively studied due to their pharmacological profile, physicochemical and pharmacokinetic properties. This review provides an overview of the various synthetic compounds containing the 2-amino-1,3,4-thiadiazole moiety that has been evaluated for antiviral activity against several viral strains and could be considered possible prototypes for the development of new antiviral drugs.Molecules 2020, 25, 942 2 of 22 due to the ability of viruses to undergo rapid mutations, the mechanisms involved in developing resistance to antiviral drugs are activated in most cases [1,3]. As resistance toward antiviral drugs is becoming a global health threat, there is an intrinsic need to identify new scaffolds that are useful in discovering innovative, less toxic and highly active antiviral agents [3,6,7]. Nitrogen-Containing Heterocycles and Thiadiazole Ring in Biology and Medicinal ChemistryNitrogen-containing heterocycles are widely distributed in nature and are essential in vegetal and animal metabolism. They are found in nucleic acids, vitamins, antibiotics, alkaloids, etc. [8][9][10][11][12][13][14]. In addition, nitrogen-containing heterocycles are important targets for medicinal chemistry, as they are found in more than half of the commercially available drugs and can also act as versatile intermediates in the synthesis of complex products that exhibit outstanding biological activities [15]. Most of the nitrogen-containing heterocyclic compounds exhibit better biological activity than non-nitrogen compounds [12,16,17]. Currently, there are approximately 90 drugs approved for use in the treatment of nine human viral infections caused by human immunodeficiency virus (HIV), hepatitis B virus (HBV), hepatitis C virus (HCV), herpes simplex virus (HSV), Influenza virus, human cytomegalovirus (HCMV), varicella-zoster virus (VZV), respiratory syncytial virus (RSV) and human papillomavirus [18]. Most of these drugs (e.g., acyclovir, cidofovir, idoxuridine, nevirapine, pleconaril, ribavirin, etc.) are nitrogen heterocycle molecules [3,19].Five-membered aromatic systems with three heteroatoms at symmetrical positions, such as the 1,3,4-thiadiazole ring, have been extensively studied due to their pharmacological profile and physicochemical and pharmacokinetic properties. 1,3,4-Thiadiazole derivatives are known as compounds having significant and diverse biological activities such as antibacterial, antifungal, antitubercular [20], analgesic and anti-inflammatory [21,22], antidepr...