**2.10. Biosynthesis of Pogostol**

Biosynthesis of pogostol **34** by the endophytic fungus *Geniculosporium* was investigated by Dickschat and co-workers [22]. In this study, six 13C labeled isotopomers of mevalonolactone were synthesized and used in feeding experiments with the endophytic fungus *Geniarlosperium*. Feeding experiments with **35a** and **35b** gave insights into the stereochemical course of the terpene cyclization. The methyl group of the mevalonolactone that is labeled in these two isotopomers is converted into terminal (z)-methyl group of FPP (C-13). Both feeding experiments showed that the deprotonation step leading to germacrene A **36** proceeds with stereospecific deprotonation of C-13 and not C-12 of FPP (**Figure 5**).

**Scheme 13.** Mechanism of pogostol **34** formation from FPP.

**Figure 6.** Absolute stereochemistry of (−)-pogostol **37**—correlation of (−)-pogostol **37** and (+)-bulnesol **38**.

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**Scheme 14.** Mechanism proposed for cyclization and rearrangement of FPP to patchoulol **39**.

**Figure 5.** Biosynthesis of Pogostol **34** using isotopomers of mevalonolactone.

The volatile fraction was extracted by closed loop stripping apparatus followed by direct <sup>13</sup>CNMR analysis (CLSA-NMR) newly developed by the same group. The biosynthesis of pogostol **34** proceeds through initial formation of germacrene-A **36**. Protonation of 4,5 double bond initiates a second cyclization to cation which gets neutralized with water to give pogostol **34** (**Scheme 13**).

In view of correlation of (−)-pogostol **37** with (+)-bulnesol **38** with known absolute stereochemistry, (−)-pogostol be represented by the stereostructure **37** [23–25]. The stereostructure **34** thus represents (+)-pogostol (**Figure 6**).
