Sesquiterpenoid products intermediates

Collado and co-workers made detailed studies of the chemistry of the sesquiter-penoid caryophyllene and its hydroxylated products including rearrangements in-duced by superacids. They have recently reported novel rearrangements of the sesquiterpenoid panasinsane derivatives 213 to provide three products and interpreted the transformations by the involvement of the common carbocationic intermediate 214 [Eq. (5.304)]. 

The cytotoxic sesquiterpenoid (-)-quadrone, isolated from the fungus Aspergillus terreus, possesses the constitution and absolute stereochemistry shown in (218). The tricyclic carbon skeleton of this interesting natural product is the same as that found in compound (198), which, as described above (Scheme 28), is readily prepared by thermolysis of the tricyclic diene (197). Thus, it appeared that the Cope rearrangement of a suitably substituted and functionalized derivative of (197) might serve effectively as a key intermediate in a total synthesis of ( )-quadione (218) that is, successful Cope rearrangement of a substrate, such as (219), would provide, stereoselectively, the tricyclic substance (220). Presumably, the intermediate (220) could then be converted into the keto aldehyde (221), which had already been transformed into ( )-quadrone (218). ... 

In summary, these important investigations have rationalized the stereochemistry of the 1,3-hydride shift (which transfers a positive centre from C-10 or C-11 at the distal end of FPP to C-1 and so allows initiation of further isomerization or a second cyclization of the Cio or Ci i ring) on the basis of the ring size of the products. This presumably results from fixation of specific conformations of the intermediates on a hypothetical enzyme surface. The larger diversity of structure for sesquiterpenoids than for di- or tri-terpenoids could then reflect the larger conformational flexibility of a Cio or Cii ring (which are often involved in the formation of the former) than of the Ce rings that are usually implicated in the latter. 

In this review, the results of our investigations in the field of the synthesis of a number of optically-active, drimanic compounds from norlabdanic derivatives, the cleavage products of many available labdanoids, and also from a mixture of neoabienols (7) are summarized. The respective literature data are also discussed in the present article. A number of the prepared drimanes are naturally occurring biologically active substances. Others have been used as intermediates in the syntheses of important natural bioactive polyfunctional drimane sesquiterpenoids. Below the syntheses of the representatives of the drimane (1) group are described. 

---