**Investigating the Antiparasitic Potential of the Marine Sesquiterpene Avarone, Its Reduced Form Avarol, and the Novel Semisynthetic Thiazinoquinone Analogue Thiazoavarone**

**Concetta Imperatore 1,2,**†**, Roberto Gimmelli 3,**†**, Marco Persico 1,2,**†**, Marcello Casertano 1,2, Alessandra Guidi 3, Fulvio Saccoccia 3, Giovina Ruberti 3, Paolo Luciano 1, Anna Aiello 1,2, Silvia Parapini 4, Sibel Avunduk 5, Nicoletta Basilico 6, Caterina Fattorusso 1,2,\* and Marialuisa Menna 1,2,\***


Received: 9 January 2020; Accepted: 11 February 2020; Published: 14 February 2020

**Abstract:** The chemical analysis of the sponge *Dysidea avara* afforded the known sesquiterpene quinone avarone, along with its reduced form avarol. To further explore the role of the thiazinoquinone scaffold as an antiplasmodial, antileishmanial and antischistosomal agent, we converted the quinone avarone into the thiazinoquinone derivative thiazoavarone. The semisynthetic compound, as well as the natural metabolites avarone and avarol, were pharmacologically investigated in order to assess their antiparasitic properties against sexual and asexual stages of *Plasmodium falciparum*, larval and adult developmental stages of *Schistosoma mansoni* (eggs included), and also against promastigotes and amastigotes of *Leishmania infantum* and *Leishmania tropica*. Furthermore, in depth computational studies including density functional theory (DFT) calculations were performed. A toxic semiquinone radical species which can be produced starting both from quinone- and hydroquinone-based compounds could mediate the anti-parasitic effects of the tested compounds.

**Keywords:** *Dysidea avara*; avarone/avarol; redox-active compounds; quinones and hydroquinones; dioxothiazinoquinone; *Schistosoma mansoni*; *Plasmodium falciparum*; *Leishmania* spp.; 3D-SAR analysis; DFT studies
