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The synthesis of austrodoral (1), a marine nor‐sesquiterpene that contains a unique bicyclic skeleton, has been achieved. The synthetic strategy is based on the ring contraction of a suitable optically active drimanic epoxy derivative, obtained from commercially available (+)‐sclareolide (4). Fluorosulfonic acid was found to promote the ring contraction efficiently. The nor‐sesquiterpene hydrocarbon 13, a key intermediate in the synthesis of sesquiterpene hydroquinones, has also been prepared in optically active form. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

The family of scalaranic sesterterpenoids has been known since the early 70s, but they have attracted the attention of synthetic organic chemists only in the last decade. These marine natural products are considered secondary metabolites of marine organisms and have shown promising biological properties. Synthetic approaches towards scalaranes are discussed in the present review. Scalaranes are quite complex compounds, with many chiral centers and their transformations are strongly directed by steric effects of different functional groups attached to the tetracyclic framework. Therefore, methods of assembling the scalaranic skeleton are examined first, followed by the synthesis of natural products or their analogues on the basis of the available tetracyclic template.

The paper reviews the known examples of cyclic terpenoids produced from open chain polyenic precursors by an "unusual" biosynthetic pathway, involving selective electrophilic attack on an internal double bond followed by cyclization. The resulting compounds possess cyclic backbones with pendant terminal prenyl groups. Synthetic approaches applied for the synthesis of such specifically functionalized compounds are also discussed, as well as biological activity of reported representatives.

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An efficient one step, retro-biomimetic procedure for the synthesis of natural products having the atisane structure is described (Scheme 2), natural products which are components of medicinal plants and possess relevant biological activity. Their structures were confirmed by chemical transformations and spectral data. The starting materials were the known ent-kaur-16-en-19-oic acid (1) and ent-trachyloban-19-oic acid (2), diterpenoids readily available from the waste of sunflower.Introduction. -Tetracyclic atisanes, beyeranes, kauranes, and pentacyclic trachylobanes represent an important group of biosynthetically closely related polycyclic diterpenes, many of which display a wide range of biological activities [1 -7]. According to the hypothesis of diterpene biogenesis [8], diterpenes belonging to the families of ent-beyerene (A), ent-kaurene (B), ent-trachylobane (C), and ent-atisene (D) might all arise from (À)-copalyl pyrophosphate E via nonclassical carbocations such as F as common intermediates (Scheme 1). This hypothesis [8] has been formulated on the basis of the known isoprene rule and, to the best of our knowledge, has not been turned down yet. Scheme 1 provides a general overview of this biogenetic scheme, and our primary intention was to perform the retro-biomimetic transformation of ent-kaurene (B) to ent-beyerene (A) or ent-atisene (D) (path B ! F ! A or D).The rearrangements of ent-kaurane-and ent-trachylobane-type diterpenes have been reported under the action of different reagents [9]. Most of the examples relate on the reactions involving the formation of the nonclassical carbocation of type F (Scheme 1). It is well-known from the work of Olah and co-workers [10] that superacids are very convenient generators of these species, and our own experience on the use of fluorosulfuric acid (¼ fluorosulfonic acid; FSO 3 H) as an efficient promoter of terpenoid cyclizations [11 -15] and rearrangements [16 -18] provided a motivation to investigate the behavior of ent-kaur-16-en-19-oic acid (1) [19] and ent-trachyloban-19-oic acid (2) under superacid treatment (Scheme 2).