Pipitzol

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... 

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. 

FIGURE 1.10 The nucleoside analog AZT (27), the specific branched-chain compounds 29 and 30, and the sesquiterpene (-)-a-pipitzol (33, made from 31 and 32) available from sugar starting materials. 

On thermolysis, perezone (192) undergoes a [4+2] cycloaddition to produce equal amounts of the two pipitzols (193) and (194) i.e. there is no stereochemical induction by the chiral centre in (192). If, however, the reaction is carried at 0 °C in the presence of boron trifluoride etherate the cyclization becomes highly stereoselective producing largely a-pipitzol (193) and it has been shown that a stepwise mechanism (195)—K196)->(193) is operative in this case.108... 

The absolute stereochemistries of a- and /3-pipitzol have been unambiguously established as (177) and (178) respectively by the chemical transformation of a-pipitzol into (-)-a-cedrene (179) and by X-ray analysis of a-pipitzol benzoate. ... 

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