## 1 Introduction  
Natural marine products have risen in prominence as a valuable source of rare secondary metabolites, drawing the attention of scientists and researchers worldwide. These compounds, often extracted from marine organisms such as corals, sponges, mollusks, and algae, are renowned for their remarkable chemical diversity. Research conducted in a plethora of marine ecosystems throughout the world has led to the emergence of a diverse spectrum of molecules possessing an extensive array of biological activities that hold great promise for strengthening medicine and advancing drug discovery [1; 2; 3]. In as much as many of these secondary metabolites have displayed potent antifungal, antibacterial, antiviral, and antimalarial properties [4; 5], their importance in fighting infectious diseases cannot be overstated. These unique compounds also offer promising pathways toward crafting innovative pharmaceuticals that address the growing challenges of drug-resistant pathogens [6]. Furthermore, the promise of marine natural products in tackling malaria has stirred significant excitement, given their capacity to impede the reproduction of malaria-causing parasites. This brings renewed hope in the battle against this highly destructive illness [7; 8; 9]. Marine sponges stand out as a remarkable reservoir of potential lead compounds in the continuous search for novel drugs. Their unique chemical compositions and the diversity of the bioactive molecules that can be obtained from them make these animals a valuable source of inspiration and innovation in the fields of medicinal chemistry and drug discovery [4; 5]. Among these interesting organisms, members of the genus _Plakortis_ have recently taken the spotlight given their well-known capacity to yield polyketide natural products with a remarkable structural diversity and widespread bioactivity [10; 11; 12]. Despite their potential, only a small number of marine polyketide-derived drugs are currently considered to be viable for medical therapeutic use due to their restricted accessibility [12]. Notwithstanding this, marine polyketides have become a focal point of intense research, and investigators throughout the world are diligently exploring the molecular mechanisms behind their biological properties while aiming to harness their therapeutic potential for the benefit of medical science and human health [13; 14; 15; 16]. In recent years, several polyketides possessing a [2(5H)-furanylidene]ethanoate (furanylidene) motif have been detected, extracted, and isolated from a wide variety of marine sponges, including species belonging to the genera _Agelas_ and _Plakortis_. More specifically, related compounds that were previously isolated from _Plakortis_ sponges have been shown to exhibit antimalarial and cytotoxic activities. This category of compounds includes the likes of spongogoritin A (**1**) [17], 9,10-dihydrospongosoritin A (**2**) [18], gracilioether B (**3**) [19], gracilioether C (**4**) [19], and gracilioether L (**5**) [20]. These compounds highlight the prevalence and significance of the furanylidene moiety in polyketides sourced from marine sponges and open new doors to further explore the biological functions and potential applications of these metabolites [21]. Figure 1 provides a visual representation of the chemical structures of compounds **1-5**. As part of our investigations, which aim to discover innovative bioactive compounds from Caribbean marine invertebrates, we recently conducted an extensive chemical analysis of the sponge _Plakortis halichondrioides_. Specimens of this species were carefully collected from waters surrounding Mona Island, which lies along the western coastline of Puerto Rico. We now wish to report the isolation and structure elucidation of three new _Plakortis_ polyketides, namely, graciloether M (**6**), 11,12-dihydrogracilioether M (**7**), and 9,10-dihydroplakortone G (**8**). Further research revealed that the latter compound was isolated together with known plakortone G (**9**) [22].