Hopanes, Fernanes, Adinanes, and Filicanes

Another noteworthy characteristic of this triterpene family is the frequent absence of an oxygen function at C3. Whereas greater than 98% of all other known C30 triterpenes possess a C3 oxygen grouping, about half of the fifty-four members of this particular class lack such functionality (7). The non-incorporation of squalene-2-3-oxide into A -fernene (together with a positive incorporation of squalene) supports the expectation that these non-oxygenated triterpenes are biosynthesized by a proton-initiated cyclization of squalene itself (496) The incorporation of six tritium atoms into A -fernene from is consistent 

The three fernene isomers (409, 413, and 414), all of which are natural triterpenes, and various functionalized variants thereof, suffer rearrangement in the retro-biogenetic direction to -neohopene (412) (501, 504—506). Evidently A -fernene (413) is relatively resistant to the acidic conditions since mixtures of (412) and (413) are often obtained. Both A -and A -fernene (414 and 409) are converted to the A -isomer under relatively mild conditions. The apparent ease of migration of the double bond into the A position in this series contrasts with the rapid rearrangement of multiflorenyl acetate (372-OAc) to P-amyrin acetate (352, R = OAc) mentioned in the preceding section. 

Advantage may be taken of the conjugative stabilization afforded by additional unsaturation (A double bond or 11-ketone) in order to effect 

Scott Handbook of Naturally Occurring Compounds, Vol. II Terpenes. New York Academic Press. 1972. 

Ruzicka, L., a. Eschenmoser und H. Heusser The Isoprene Rule and the Biogenesis of Terpenic Compounds. Experientia 9, 357 (1953). 

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