Pentacyclic hydrocarbons, structure

The stereochemistry of a thermally induced 10e electrocyclization (predicted to be disrotatory) has not been firmly established and the main synthetic application is found in the formation of azulenes and ring-fused azulenes as in the transformation (452) to (453). Thermolysis of (454) with spontaneous elimination of dimethylamine from intermediate (455) afforded the fused azulene structure (456). The chemistry of even higher order (12e to 20e") pericyclic processes has been recently reviewed. An example of an unusual sequence of pericyclic processes is the transformation of heptahendecafulvadiene (457) to the pentacyclic hydrocarbons (462) and (463) in a 2 1 ratio. The pathway for this transformation can be viewed as an initial conrotatory 20e electrocyclization followed by a cascade of 10e and 6e pericyclic processes. ... 

Of the fifteen pentacyclic hydrocarbons, five are known sarcomatogens and two are borderline cases again, several derivatives are active. Biological screening results are less complete for the hexacyclic aromatic hydrocarbons. Only the well-known carcinogens are listed in Tables 4.4 and 4.5. Table 4.4 lists the active alternant, parent polycyclic aromatic hydrocarbons under the names currently accepted Table 4.5 gives the activities of tested methylene derivatives which possess structures similar to those in the homologous series of alternant parents. 

Taraxa sterol, a-lactucerol, a-anthesterol a simple, unsaturated, pentacyclic triterpene alcohol, M, 426.73, m.p. 227 °C [a]D-r50°, structurally a derivative of the parent hydrocarbon, 5a-taraxastane. It occurs as the acetate in the latex of the dandelion (Taraxacum officinale) and other members of the Compo-sitae. 

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