Germacrane systems

The trans-cis isomerization process observed in the eudesmane/germacrane ring system has been utilized for the synthesis of the c/s-fused sesquiterpene occidentalol, 48a, and its 7-epi isomer, 48b (Scheme ll)22. Photolysis of the traws-fused diene 45 at —78°C afforded triene 46, which upon warming underwent disrotatory electrocyclization to give 47a and 47b as a 1 2 mixture of diastereomers. Apparently, the carboalkoxy group imparts... 

In order to explain the stereospecificity of the thermally induced cyclization of preisocalamendiol (315) to give dehydroisocalamendiol (316), Terada and Yamamura have carried out molecular mechanics calculations on the ground-state conformers and probable transition states of (315). The computed values of the steric energies of the possible transition states are in good agreement with the observed experimental facts. Other studies related to the conformational aspects of germacrane sesquiterpenoids include an examination of the NOE effects in the isofuranodiene (317), which shows the 1,5-diene system to be in a crossed... 

A quite detailed review of transannular cyclizations was published wherein their important role in biomimetic syntheses of sesquiterpenes as well as explanation of the biogenetic formation of the polycyclic natural compounds from their monocyclic precursors is discussed. The great significance of these transformations for the synthesis of natural products is also emphasized in a series of reviews which describe the cyclizations to fonn terpene derivatives, e.g., of the germacrane and humulene systems -zoe... 

Recent studies on the Cope rearrangement of certain germacrane sesquiterpenoids have disclosed some interesting results on the reversibility and conformational control of stereochemistry in this reaction. Jain et al have demonstrated reversibility in three similar systems associated with costunolide and its derivatives [242, R = CH, a-Me, and CH2N(Me)2]. On the other hand, Takeda et 6,180 Qjjjy observed the reversibility of the Cope rearrangement but... 

The most chemically obvious cyclisation is that which forms the germacrane skeleton. However, this system also contains a strained... 

This reductive alkylation was used in a synthesis of 8, which contains the 10-membered ring system of germacrane sesquiterpenes. Thus the tetralonc 5 was converted by way of 6 into the tosylate 7. On treatment with LDA in THF at 24°, 7 underwent a Grob-type fragmentation to give 8. 

Germacranes often are portrayed as being derived from a 2- -6- -intermediate. These sesquiterpenes possess a 4,10-dimethyl-7-isopropyl cyclodecane system and often have double bonds in positions 1 (in germacrane A epoxide, 20)... 

Germacranes with trans,trans 1,5-diene system are thermally labile and undergo Cope rearrangement to give products with elemane skeleton. Thus, costunolide (195) on heating gives saussurea lactone (205), albeit, in low yield. 

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