*3.1.1.2. 13C-NMR date*

**3. Diterpenoid classification**

classification of clerodane diterpenes is given in **Figure 2**.

**Figure 2.** Clerodane skeleton, cis and trans structures of clerodane.

*3.1.1.1. Clerodane diterpenes by biotransformation from endophytic fungi*

Clerodane diterpenes are the natural group of secondary metabolites holding an utmost pharmacological significance. These are bicyclic structures consisting of a fused ring (decalin moiety from C-1 to C-10) with a six-carbon side chain (C-11 to C-16) attached at C-9. The rest of the carbons (C-17 to C-20) are bonded at C-8, C-4, C-5, and C-9, correspondingly [25]. Only 25% of the clerodane diterpenes showed 5:10 *cis* ring junction, while the rest possess 5:10 *trans* ring fusion as presented here in the form of columbin and clerodin, respectively. Columbin exhibited dose-dependent anti-inflammatory activity as well as chemopreventive activity against colorectal cancer [26–28]. During the last 25 years, over 1300 diterpenoids and nor-diterpenoids with the clerodane carbon skeleton have been isolated [29, 30]. The detailed

Three strains of endophytic fungi *L. gonubiensis*, *N. ribis*, and *P. stromaticum* produced one known and five unknown compounds (B1–B4) through a process of biotransformation, while compounds Q1–Q2 are derived as chemical derivatization of compound 2 [30]. These compounds were actually isolated for the first time from *Croton argyrophylloides* (Euphorbiaceae) and further biotransformed by *Cunninghamella echinulata* and *Rhizopus stolonifer* fungi and produced a new diterpene, as previously described by Monte et al. [31] and Mafezoli et al.

**3.1. Bicyclic diterpenoids**

114 Terpenes and Terpenoids

*3.1.1. Clerodane diterpenes*

[32] (**Figure 3**).

13C-NMR spectra of substrate 1 and B2 suggested the C-7 hydroxylation making the signal at δ 70.1 (CH) in B2. Compound B2 is identified as new metabolite (4S,5S,7R,8R,9S,10S)- 4,7-dihydroxy-15,16-epoxy-3,12-dioxocleroda-13(16),14-diene, and its molecular formula C20H28O5 is sorted by HRMS. Compound B3 was unique for cultures of *N. ribis*. The 13C-NMR spectrum of B3 showed the presence of one at δ 72.2 in the spectrum confirmed that compound 1 was regioselectively bioreduced at C-3. The new compound B3 was named (3*R*,4*S*,5*S*,7*R*,8*S*,9*R*,10*S*)-3,4,7-trihydroxy-15,16-epoxy-12-oxocleroda-13(16),14-diene, which is in agreement with the molecular formula C20H30O5 . The biotransformation product B4 was obtained only in the *P. stromaticum* culture. The 13C-NMR spectrum of B4 showed no reduction of carbonyl group at δ 213.5 (C-3) and the appearance of carbinol methane group at δ 71.8 (C-6). And it is named as (4S,5R,6R,8S,9R,10S)-4,6-dihydroxy-15,16-epoxy-3,12-dioxocleroda-13(16),14-diene, which is in agreement with the molecular formula C20H28O5 . The new compound Q1 was named (4S,5S,7R,8R,9S,10S)-7-propionyloxy-4-hydroxy-15,16-epoxy-3,12 dioxocleroda-13(16),14-diene. The new derivative Q2 was named (4*S*,5*S*,7*R*,8*R*,9*S*,10*S*)-7 benzyloxy-4-hydroxy-15,16-epoxy-3,12-dioxocleroda-13(16),14-diene [30].
