Pseudopterosin A

Pseudopterosins have anti-inflammatory and analgesic activity, with a mechanism of action different from the common non-steroidal anti-inflammatory drugs, NSAIDs.^[4] Commercially, pseudopterosins are found in skin creams as topical anti-inflammatory agents.^[5]

Elisabethatriene (2) has been identified as a key intermediate for the synthesis of the class of pseudopterosins and seco-pseudopterosins. A proposed mechanistic pathway for the synthesis of elisabethatriene from geranylgeranyl pyrophosphate (GGPP, 1), is described below. Elisabethatriene synthase, a diterpene cyclase enzyme, catalyzes the transformation of the diterpene GGPP to a 10-membered carbon skeleton followed by hydride migration towards the bicyclic ring system.^[1][6] This cyclase enzyme has been identified as a key enzyme in forming the carbon skeleton of pseudopterosins in one step. An alternative mechanism has been proposed in which a six-membered ring is formed first, then a second ring closing for the bicyclic system.^[6]

The biosynthesis of the pseudopterosins continues with an aromatization to erogorgiaene (3), two oxidations to a dihyroxyerogorgiaene (4, then 5), and another oxidation to an ortho-hydroxyquinone (6). Ring closure (7), re-aromatization to (8) and glycosylation yield Pseudopterosin A (9). This is a plausible biosynthetic pathway, and intermediates 2, 3, 6, 7, and 8 have been identified using radio labeling studies.^[1] An alternative mechanism has been proposed with no hydroxyquinone intermediate (6). Rather, molecule 3 undergoes two subsequent oxidations at C-6 and C-7 to a structure resembling 8, then glycosylation to pseudopterosin.^[6]

The branching point for the biosynthesis of the tricyclic pseudopterosins versus the bicyclic seco-pseudopterosins occurs at compound 11, the aromatized bicycle erogorgiaene. 11 is oxidized once then hydroxylated followed by glycosylation to make the bicyclic seco-pseudopterosins.

The proposed synthesis of artificial anti-inflammatory metabolites is modeled after pseudopterosins and is based on the bicyclic seco-pseudopterosin structure 6.^[1]