Carbo-cyclic compounds

Carbo-cyclic Compounds.—Contrasted with the hetero-cyclic compounds, which we have just been discussing in a general way, we have other compounds whose constitution is also that of a closed ring or cycle, but this ring is composed of carbon groups only. To such we assign the names carbo-cyclic compounds or iso-cyclic compounds and they embrace not only hydrocarbons but also all of the different groups of derivatives which we have heretofore studied. 

Benzene.—By far the most important and most numerous of these carbo-cyclic compounds have as their mother substance the hydrocarbon benzene, and the names Benzene Series, Benzene Compounds, or Benzene Derivatives are commonly used as synonymous with carbo-cyclic compounds. This has led to the usual classification of organic... 

Part II Carbo-cyclic compounds including not only carbon ring compounds derived from benezene iso-cyclic, but also carbon ring compounds not derived from benzene, ali-cyclic in addition to these the hetero-cyclic compounds as a group. 

A. ALI-CYCLIC COMPOUNDS OR CARBO-CYCLIC COMPOUNDS NOT DERIVED FROM BENZENE. 

If now, by the reactions which we have discussed, we convert derivatives of the saturated open chain compounds into carbo-cyclic compounds, a strain is produced in the formation of the ring just as there is in the formation of ethylene. 

From these figures, which are the result of mathematical calculation, we see that the carbo-cyclic compounds which should be the most... 

While, however, penta-methylene is the most stable ali-cyclic compound of the poly-methylene group it is hexa-methylene which is of special interest and importance. This importance is due to the fact that it is the connecting link between the ali-cyclic compounds (carbo-cyclic compounds not derived from benzene, i.e. the poly methylenes) and benzene itself. Thus it becomes the connecting link between the aliphatic open-chain compounds and the very large and important division including benzene and its derivatives. 

B. CARBO-CYCLIC COMPOUNDS DERIVED FROM BENZENE, ISO-CYCLIC COMPOUNDS OR AROMATIC COMPOUNDS... 

Aromatic Compounds—The carbo-cyclic compounds which in number far exceed those of the aliphatic series were originally called aromatic compounds because many of them possess aromatic properties, e.g.j oil of wintergreen, oil of bitter almonds, etc. They were included with the paraffin compounds in the various groups of alcohols, aldehydes, acids, etc. Later it was found that they differed from the aliphatic compounds and finally it was shown that the hydrocarbon benzene is related to the aromatic compounds just as methane is to the aliphatic compounds, i.e,j as the mother substance. 

Benzene Series.—This gave rise to the use of the names benzene series and benzene compounds in place of the name aromatic compounds. As many of the compounds since discovered and belonging to this series are not aromatic the former names are better as all of them are related to benzene. Strictly speaking, however, the benzene series proper does not include all of the carbo-cyclic compounds related to benzene and which are included in the terms iso-cyclic or aromatic as distinct from aliphaticy e.g., naphthalene, etc. Generally speaking, however, the names are used synonymously. 

Cyclo-hexane.—=The resulting compound CeHi2 corresponds to the olefine unsaturated hydrocarbons, CnH2n, and is isomeric with hexene. Hexene, however, readily adds two atoms of hydrogen and yields hexane, whereas hexa-hydro benzene is with difliculty converted into hexane. The compound, therefore, is not unsaturated. More important still is the fact that it proves to be identical with hexa-methy-lene or cyclo-hexane which, as we have recently shown, is a carbo-cyclic compound represented as follows ... 

In the introduction to the benzene series or carbo-cyclic compounds (p. 458) the group of hetero-cyclic compounds was referred to. The representatives of this group which were mentioned at that time were all direct derivatives of the open chain compounds. They included the lactonesj the lactams, the di-basic acid anhydrides and the imides corresponding to the last, e.g. ... 

Carboxylic acids can be converted by ancxlic decarboxylation into radicals and/or carbocations. The reaction conditions are simple an undivided beaker-type cell as reaction vessel, controlled current supplied from an inexpensive d.c. power supply and meAanol as solvent are in most cases sufficient. A scale-up is fairly easy and the yields are in general good. By the radical pathway 1,/i-diesters, -diketones, -dienes and -dihalides, chiral intermediates for synthesis, pheromones and unusual fatty acids are accessible in just a few steps. The addition of the intermediate radicals to double bonds affords additive dimers, whereby four building units, two alkyl radicals from the carboxylates and two alkenes, can be coupled in one step. Five-membered hetero- or carbo-cyclic compounds can be prepared by intramolecular addition starting from unsaturated carboxylic acids. 

To determine the stability and reactivity of heterocyclic compounds, it is useful to compare them with their carbocyclic analogues. In principle, it is possible to derive every heterocycle from a carbo-cyclic compound by replacing appropriate CH2 or CH groups by heteroatoms. If one limits oneself to monocyclic systems, one can distinguish four types of heterocycles as follows ... 

Halogeno substituents at the 3- or 4-position of cinnoline are appreciably activated by N2 and Nl, respectively those at positions 5-8 or an extranuclear position have activities only marginally better than those in corresponding carbo-cyclic compounds. There seems to be little difference in reactivity of a fluoro, chloro, bromo, or iodo substituent at the same position. 

The addition of selenium reagents to double bonds has already been discussed above. However, a special case of such additions has received particular attention in the past five years this is in the cyclization of olefins. In this context phenyl-selenyl iodide is described as a new rea nt for the cyclization of dienes to carbo-cyclic compounds. The authors claim that phenylselenyl iodide is far more effective than the corresponding bromide or chloride for this transformation. The intramolecular attack of the carbon nucleophile in (287) onto the double bond in the cyclopentene ring is another example of a cyclization reaction and forms the basis for a synthesis of ( )-hirsutene, In this case, the tricycle (288)... 

---