**2.2. Acid catalyzed rearrangement of moreliane based triketone. Characterization of keto lactone, a 1-11 seco-moreliane**

An interesting molecular rearrangement has been reported by Morales and co-workers [5]. They observed that triketone **5** on treatment with *p*-TSA in benzene resulted in the formation of a keto lactone **6**, a 1–11 seco-moreliane derivative and also the first representative of this group (**Figure 1**).

The rearrangement depicted in **Scheme 3** involves initial cyclobutane ring expansion of the protonated triketone, generation of carbocationic intermediate **7** which rearranges *via* transition state in to protonated seco-moreliane **8**. These steps are supported by DFT calculations.

**Figure 1.** Skeletons of longipinane, moreliane and 1–11 seco-moreliane.

yield. Exploitation of *exo*-isomer with *m*-CPBA followed by acid catalyzed opening afforded (+)-cymbodiacetal **12** (**Scheme 5**). The uncertainty in absolute stereochemistry was independently established by X-ray crystallography. These studies also clarified discrepancies in the

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**Scheme 4.** Molecular rearrangement of (−)-modhephene **9** to (−)-isocomene **10** and (−)-triquinane **11**.

previously published work [8, 9].

**Scheme 5.** Total synthesis of (+)-cymbodiacetal **12**.

**Scheme 3.** Acid catalyzed rearrangement of triketone **5** to 1–11 seco-moreliane derivative **6**.
