Acoranes, Cedranes, and Zizaanes

Although the p-bisabolene (57) produced is optically active (31% optical purity), isomerization to y-bisabolene (215) prior to further cyclization results in the eventual formation of racemic a-cedrenes (214 and 216) 113). The absence of detectable amounts of spiro[4.5]de-canes indicates that the final cyclization is more rapid than the spiro cyclization step. Since both cedrene (214) and 2-epicedrene (216) are formed in comparable amounts, it would appear that the bisabolene precursor cyclizes to acorenyl carbonium ion intermediates with the secondary methyl and isopropyl groups both trans (as in 212) and cis, the latter affording to 2-epicedrene. 

Tomita and Hirose have reported that the reaction of P-acoradiene (226) with formic acid affords, after saponification, the newly discovered natural sesquiterpene allocedrol (227-OH) (297, 298), along with several other products. The structure of allocedrol is enantiomeric to that obtained by hydration of the carbonium ion intermediate (220) depicted in the zizaene biogenesis. This cyclization is presumably less efficient than the a-acoradiene- cedrene cyclization (213- 214) since ring 

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