Rearranged Labdanes

Labdane isomerizes to halimane by migration of the methyl group (C-20) from C-10 to C-9. An additional shift of the methyl group (C-19) from C-4 to C-5 leads from halimane to elerodane. 

---

J. de Pascual Teresa, J.G. Urones, I.S. Marcos, F. Bermejo, P. Basabe, A rearranged labdane salmantic acid from Cistus laurifolius, Phytochemistry, 22, 2783 2785 (1983). 

The Amherstiae copals have been a rich source of labdane and rearranged labdane diterpenoids. The full description of oliveric acid (7) isolated from one of these, Daniellia oliveri, has appeared. ... 

Chettaphanin-I (411), first isolated from roots of Andenochlaena siamensis, represents another class (Chettaphanane) of rearranged labdanes, in which one hydride shift and one methyl shift have occurred sequentially (340). A more... 

Chatterjee A, Banerjee A, Bohlmann F 1977 Crotocaudin a rearranged labdane type nor-diterpene from Croton daudatus Geisel. Tetrahedron 33 2407-2414... 

Rearranged labdane skeletons arise via a series of associated hydride and methyl shifts, usually involving concerted tra/ts-migrations of the participating substituents. 

Four bicyclic type diterpenoids have been reported from Salvia species labdanes, neo-clerodanes and clerodanes, seco-clerodanes, rearranged clerodanes. 

Diterpenes of this series are formed by rearrangement of the labdane skeleton during cyclization (Hanson, 1991 West, 1981) (Fig. 22.11). Many highly oxygenated diterpenes of this series are found in the Lamiaceae and Verbena-ceae. There are over 400 known clerodanes many of these are from Teucrium species (Lamiaceae) (Hanson, 1991). 

Scheme 11.65 Njardarson s synthesis of labdane diterpenoids involving a stereoselective Cu-catalyzed vinyloxirane-dihydrofuran rearrangement.

Recendy, in 2013, the Njardarson group utilized the stereoselective vinyloxirane-dihydrofuran rearrangement to s)mthesize labdane diterpenoids.The decalin core structure 112 obtained in enantiopure form from inexpensive juniper berries was subjected to Shi s epoxidation to form the vinyloxirane 113 in a stereoselective fashion. The vinyloxirane was tiien treated with Cu(hfacac)(COD) catalyst to afford dihydrofur an 114, which was rapidly converted to labdane natural products, 115 and 116 tScheme tl.65T... 

Among the normal-labdanes, the simplest rearrangement product, the relhaniane skeleton (Figure 3, No. 28), results from the concerted migration of C-9a-H to... 

Steps proposed for the biogenesis of rearranged ewMabdanes appear to correspond to the mechanisms indicated for the normal-labdanes, differing only in the opposite orientivity of the migrating groups. 

Figure 8. Diterpene skeleton products of the rearrangement of normal-labdane precursors alternative routes for the biogenesis of diterpenes with the relhaniane skeleton, scheme 1 for compounds reported from Relhania acerosa 90) and scheme 2 for compounds from Koanophyllon conglobatum 42),...

Vlad, P.F., N.D. Ungur, and M.N. Koltsa Cyclization and rearrangements of diterpenoids. I. Synthesis of tetracyclic diterpenoids with a new carbon skeleton from labdanes. Tetrahedron 39, 3947 (1983). 

---