P-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... 

Walls F, Padilla J, Joseph-Nathan P, Giral F, Escobar M, Romo J 1966 Studies in perezone derivatives, structures of the pipitzols and perezinone. Tetrahedron 22 2387-2399... 

Fraser-Reid s group have described further examples of the use of serial radical cyclization to prepare di- and triquinane natural products. The previously-reported intermediate (4) (Vol. 23, p. 264) has been ring-opened by a Vasella-Bernet reaction, and hence converted to the triquinane (5) with the silphinene skeleton (Scheme 2). Similar initial steps were involved in the conversion of (6) into silphiperfolene (7), and to transform (8) (Vol. 23, p. 265) into (-)-a-pipitzol (9) (mannose carbons numbered). ... 

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