Perezone-pipitzol transformation

Yet another [4 + 2] cycloaddition, rather rare, is that between a pentadienyl cation and an alkene. The best known example is the perezone-pipitzol transformation 6.9 —> 6.11, where it is heavily disguised. It can be understood as beginning with an intramolecular proton transfer to give the intermediate 6.10, which can then undergo an intramolecular [4 + 2] cycloaddition with the pentadienyl cation, emphasised in bold, acting as the 4-electron component and the pendant alkene, also bold, as the 2-electron component. 

While the chirality of perezone (552) has been known for some time that of the a- (555) and p-pipitzols (554) which are derivable from 552 by thermolysis was rigorously proven only recently by chemical transformation to cedrene and x-ray diffraction The cycliration of 552 has been shown to involve a ncerted [4+2] cycloaddition which lacks stereochemical induction by the chiral center already present, since 555 and 554 are obtained in equimolar amounts However, a stepwise mechanism having higher stereoselectivity is followed by 552 in the presence... 

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See also in sourсe #XX -- [ ]

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