Guaiane rearrangement

One interesting facet of the germacrane-type sesquiterpenoids is the conformation of the ten-membered ring. This aspect has previously been discussed in terms of transannular electronic effects (anomalous u.v. spectra) and transannular chemical reactions (Cope rearrangement and cyclisations to eudesmane and/or guaiane types). Recently, the power of two spectroscopic techniques has been brought to bear on this problem. The first of these is the use of the Nuclear Overhauser Effect (NOE) and the second is the A"-ray analysis of a suitable derivative. 

SCHEME 20 The Vanderwal guaiane synthesis with RCM and rearrangement. 

The africanane sesquiterpenes have an interesting rearranged guaiane-type carbon skeleton, which can be obtained from (/ )-pulegone, ring contraction, side chain elaboration, and RCM (Scheme 24) [59]. 

The same kind of rearrangement has been utilized in an intramolecular sense to transform the much more readily available decalin system into the bicyclo[5.3.0]decanes of the guaiane and pseudoguaiane sesquiterpenes. Thus, the Heathcock group synthesized confertin, as shown in Scheme 46 [82]. 

The same substrate (118) was treated in plant cell cultures of S. altissima (Asteraceae) for 10 days to give various hydroxylated products (121, 127, 125, 128-132) (Sakamoto et al., 1994). Guaiane (121) underwent further rearrangement C4-C5, cleavage, and C5-C10 transannular cyclization to the bicyclic hydroxyketone (128) and diketone (129) (Sakamoto et al., 1994) (Figure 20.43). 

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