Tetracyclic hydrocarbons, structure

Of the six purely-aromatic tetracyclic hydrocarbons, only chrysene, benz[a] anthracene and benzo[ft]phenanthrene possess even marginal activity but, as shown (Tables 4.1,4.2, and 4.3), many benz[a] anthracene derivatives are active, as are some benzo[c]phenanthrene derivatives. Substitution of one, two, or three methyl groups at 6, 7, 8, or 12 in benz[a] anthracene markedly increases activity [41]. Studies of N-mustard derivatives of anthracenes and benz[a] anthracenes [42] emphasise the parallelism between carcinogenic and carcino-static properties of poly-nuclear derivatives [43]. The marked influence of a small change in chemical structure is illustrated by the sequence of anti-tumour activities lO-ethyl-9-anthryl > 9-anthryl lO-methyl-9-anthryl for the change of meso substituent from Et to H to Me [42]. 

Steroids are important lipids whose structures are based on a tetracyclic system. Most steroids function as hormone chemical messengers, and thus these molecules have been discussed in detail in chapter 5. Structurally, steroids are heavily modified triterpenes that are biosynthesized starting from the acyclic hydrocarbon squalene and progressing through cholesterol to the final steroid product Bloch and Cornforth, who were awarded Nobel Prizes in medicine (1964), contributed greatly to the elucidation of this remarkable biosynthetic transformation. 

Granting that a tricyclic ring system takes precedence over a tetracyclic ring in simplicity, the other structural modifications, namely introduction of oxygen functions and iinsaturation, are high order deviations from the simple hydrocarbon. However, we are concerned only with compounds containing the steroid skeleton. 

Rao et al. isolated ishwarone (391) as one of the chief constituents of the roots of A. indica [347], Subsequent investigations by Ganguly et al. and Govindachari et al. led to the first structural assignment of ishwarone (391) as a novel tetracyclic sesquiterpene ketone based on eremophilane skeleton, the first of its kind to occur in the nature [348,349]. Ishwarane (388), reported from the roots of A. indica by Govindachari et al. is the first tetracyclic sesquiterpene hydrocarbon based on the eremophilane skeleton [334], A tetracyclic sesquiterpene alcohol, ishwarol (389) is also reported from the roots of A. indica [350]. Pakrashi et al. isolated (125)-... 

Polycyclic compounds are those that contain more than one ring. The lUPAC classifies polycyclic structures according to the minimum number of bond cleavages required to generate a noncyclic structure. The structure is bicyclic if two bond disconnections yield an open-chain structure, tricyclic if three, tetracyclic if four, and so on. Adamantane, a naturally occurring hydrocarbon found in petroleum, for example, is tricyclic because three bond cleavages are needed before an open-chain structure results. 

Cueurbitacins tetracyclic triterpenes found as their glycosides in the Cucurbitaceae and Cruciferae. These toxic, bitter compounds are structurally related to the parent hydrocarbon, cucurbitane [19(10-9P)-abeo-5p-lanostane], which differs from lanostane (see Lanosterol) in the formal shift of the 10-methyl group to the 9p-position. Cucurbitacin E was formerly known as elaterin. C. have a laxative action some serve as insect attractants, and a few have antineo-plastic and antigibberellin activity. 

Lanoatarol, kryptosterol 5a-lanosta-8(9),24-dien-3P-ol, a tetracyclic triterpene alcohol. M, 426.7, m.p. 140 °C, [o]i3 + 60° (CHCI3). L. is also a zoosterol (see Sterols) present in large amounts in the wool fat of sheep. Structurally, it is based on the hydrocarbon 5a-lanostane ring system. It is biosynthesized from squalene, via 2,3-epoxysqualene, and is an important intermediate in the synthesis of all further tetracyclic triterpenes of the lanostane type and of the steroids. 

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