*2.2. Formylpyrroles*

In addition to the acyl-, carboxy-, and carboxamido-pyrroles (**1**–**3**, **23**–**25** and **26**–**34**) shown in the previous Section (cf. Section 2.1), the formylpyrroles constitute another distinct family of the marine pyrrole alkaloids [109].

In the course of an investigation of the South China Sea sponge *Mycale lissochela* in 2017, two new formylpyrroles **96** and **97** bearing an aliphatic side chain with a terminal nitrile group were isolated (Figure 12) [110]. Both mycalenitrile-15 (**96**) and mycalenitrile-16 (**97**) showed excellent and good inhibition effects against PTP1B (protein-tyrosine phosphatase 1B, a recognized target for diabetes and obesity) with IC50 values of 8.6 μmol/L and 3.1 μmol/L, respectively, resulting from the unsaturated side chain [110].

**Figure 12.** Mycalenitrile **96** and **97** as well as the pyrrole-terpenoid **98**.

An additional formylpyrrole, cinerol I (**98**), was isolated from the sponge *Dysidea cinerea* and belongs to the meroterpenoid family (Figure 12) [111]. Cinerol I (**98**), which lacks the unsaturated side chain present in compounds **96** and **97**, showed no inhibitory activity against PTP1B, ATP-citrate lyase (ACL), or SH2 domain-containing phosphatase-1 (SHP-1) [111].

Five new formylpyrroles **99**–**103** were isolated from the marine cyanobacterium *Moorea producens* in 2017 (Figure 13) [112]. Biosynthetically, they are suggested to originate from the amino acid tryptophan, the indole moiety of which is partly reduced to forge the annellated tetramethylenepyrrole framework. Further annellated pyrroles are depicted in Section 2.4. All pyrroles described herein feature a 3-formyl group, and compound **103** additionally carries a purine unit. The five isolated pyrroles **99**–**103** showed no noteworthy cytotoxicity or antibacterial properties [112].

**Figure 13.** Representation of five tetrahydroindoles **99**–**103** isolated from *Moorea producens*.
