# Trepenoids in Marine Heterobranch Molluscs
## 4 Diterpenoids
### Bicyclic Diterpenoids  
Nudibranchs, pleurobranchoids, sea hares, and pulmonates possess bicyclic diterpenoids. In sea slugs, _Phyllidiaella pustulosa_ from China represents the first finding of isocyanide diterpenoids in the family, some of them previously identified from _Acanthella_ sponges, thus confirming the prey-predator relationship [126; 127; 128]. These diterpene isonitriles are kalihinene, amphilectene (48), kalihinol-A (49), and kalihinol-E (Figure 7). _P. pustulosa_ from Vietnam also contains these compounds and some sterols [129]. Three isonitriles, pustulosaisonitriles-1 (50), -2, and -3 (Figure 7) are also found in _P. pustulosa_ from Australia, with pustulosaisonitrile-1 exhibiting moderate levels of antimalarial activity _in vitro_[188]. The aeolid _Spurilla_ sp. from Argentina presents spurillin B together with other compounds [183].  
In pleurobranchoids, _Pleurobranchaea meckeli_ from the Mediterranean possesses two labdane aldehyde diterpenes in the mantle [189], which are similar to those of the pulmonate _Trimusculus reticulatus_ (see below).  
Regarding sea hares, _Aplysia punctata_ presents three brominated diterpenes, glandulaurecianols A-C (51) (Figure 7), from the red algae _Laurencia glandulifera_[190], as well as punctad (52) (Figure 7) [191]. All these compounds possess a laurencianol skeleton, a known antibacterial diterpene from the red algae _L. obtusa_[192]. Similarly, the Atlantic and Mediterranean _A. depilans_ present the guanidine diterpenes dicyclol-A and B (53) (Figure 7), from the brown algae _Dictyota coriacea_ which they feed upon [193], while the Atlantic sea hares also present steroids and peroxy sterols [194; 195]. _A. kurodai_ presents many halogenated and brominated mono- and diterpenes distributed in different body parts. These include aplysin-20 (54), isoaplysin-20, aplysiadiol (55), epi-aplysin-20, and _ent_-isoconcinndiol (Figure 7) [196; 197; 198], derived from isococinndiol of the red algae _L. snyderae_[199], as well as some monotereponoids (see above). _A. kurodai_ may also present aplykurodin A (56) and B [200] (Figure 7), as well as several alkaloids and other compounds [3]. Dolabellanes [81; 82] from brown Dictyaceae algae are found in _Dolabella auricularia_ from the Gulf of California, while in the Indian Ocean it presentsthe similar dolatriol, together with the monoterenoid (\(-\))-loliolide [83; 84]. _Dolabella auricularia_ is also the source of the bromo-chloro-ditrepenoid, dolabello A (57) (Figure 7) [201].  
The pulmonate genus _Trimusculus_ does not possess the typical propionate's common in this group, instead it possesses a single type of labane diterpenoids, some of them similar to those of the pleurobranchoid _Pleurobranchaea neckelii_[189]. The South African _T. costatus_ possesses the labdanes 68,7a-diacetoxylab-8,13-dien-15-ol and 2a,68,7a-triacetoxylabda-8,13-dien-15-ol, which are toxic to _Artenia salina_ and inhibit feeding of the predatory fish _Pomadasys commercii_[202]. The New Zealand _T. reticulatus_ secrets several diterpenes from its mantle and foot that are deterrents against sea stars [203]. Similarly, related species from Chile and South Africa, such as _T. costatus_ and _T. peruvianus_, present cytotoxic and anti-feedant activities [204; 205; 206; 207].  
Figure 7: Structures of selected diterpenoids from Heterobranch molluscs: (**48**) amphilectene; (**49**) kalihinol A; (**50**) pustulosaisonitrile-1; (**51**) glandulaurencianol A; (**52**) punctatol; (**53**) dictyl B; (**54**) aplysin-20; (**55**) aplysindiol; (**56**) aplykurodin A; (**57**) dolabello A.