Cyclohexane with fused rings

Polycyclic compounds are common in nature, and many valuable substances have fused-ring structures. For example, steroids, such as the male hormone testosterone, have 3 six-membered rings and 1 five-membered ring fused together. Although steroids look complicated compared with cyclohexane or decalin, the same principles that apply to the conformational analysis of simple cyclohexane lings apply equally well (and often better) to steroids. 

Some examples of different types of hydrocarbons are given in Figure 9.1. Nonaromatic compounds without ring structure are termed aliphatic, whereas those with a ring structure (e.g., cyclohexane) are termed alicyclic. Aromatic hydrocarbons often consist of several fused rings, as in the case of benzo[a]pyrene. 

In principle, a number of different types of acetal or ketal might be produced. In this section, we want to exemplify a small number of useful reactions in which two of the hydroxyl groups on the sugar are bound up by forming a cyclic acetal or ketal with a snitable aldehyde or ketone reagent. Aldehydes or ketones react with 1,2- or 1,3-diols under acidic conditions to form cyclic acetals or ketals. If the diol is itself cyclic, then the two hydroxyl groups need to be cA-oriented to allow the thermodynamically favourable fused-ring system to form (see Section 3.5.2). Thus, dx-cyclohexan-1,2-diol reacts with acetone to form a cyclic ketal, a 1,2-O-isopropylidene derivative usually termed, for convenience, an acetonide. 

Bornane monoterpenes are exemplified by camphene (2,2-dimethyl-3-methylene-bicyclo[2,2,1]heptane), a structure in which two fused cyclopentane rings share three Cs. We can simply represent the camphene skeleton as a cyclohexane with a methylene (—CH2—) cross-link (G6(-CH2—)). The keto derivative camphor (camphor smell), the ether eucalyptol (eucalyptus smell) and the simple bornene a-pinene (pine smell) are familiar examples. 

The mechanism of dehydrohalogenation under basic conditions of trons-fused bicyclo[4,n,0]alkane halohydrins (563)—(565) has been studied. Three reaction types are noted (i) epoxide formation, (ii) ketone formation, and (iii) ring contraction. trans-Diaxial chlorohydrins corresponding to (563)—(565) gave epoxides (566)—(568) with relative rates (derived from bimolecular rate constants) of 1 3 17. This rate sequence was rationalized in terms of deformation of the cyclohexane ring brought about by the nature of the fused ring. In particular, deformation is probably towards the half-chair conformation favoured by the cyclohexane epoxide which is formed in the slow step. trans-Diequatorial chlorohydrins represented by (563)—(565)... 

This chapter follows much the same pattern as has been established in previous volumes. Material of theoretical and specific structural significance on five- and six-membered rings and related simple fused-ring systems is included to the exclusion of a wide range of such information found inter alia under general synthesis and identification. Also included is a selection of the more important aspects of reactivity associated with simply functionalized cyclohexanes, cyclopentanes, and their fused-ring systems. 

The same methodology applies to heteroarylpentenes which lead to heterocyclic compounds fused with a cyclohexane (cyclopentane fused-heterocycles are not available since the five-membered ring closure onto the aromatic is too slow to compete with the oxidative elimination) (equation (64)) [128]. Even though the reactions are usually performed under thermal conditions in aqueous acetic acid at 90 °C, it has been shown to proceed faster under sonication, although in slightly lower yields. 

When two cyclohexane rings are fused—fused rings share two adjacent carbons—one ring can be considered to be a pair of substituents bonded to the other ring. As with any disub-stituted cyclohexane, the two substituents can be either cis or trans. The trans isomer (in which one substituent bond points upward and the other downward) has both substituents in the equatorial position. The cis isomer has one substituent in the equatorial position and one in the axial position. Trans-fused rings, therefore, are more stable than cis-fused rings. 

Nematogenic, fused-cyclohexane systems with rra -ring junctions have been made based on decalins [38-40], perhydrophe-nanthrenes [40 -42], perhydrochrysenes [43], and perhydroanthracenes, perhydro-naphthacenes and perhydrobenza[a]anthra-cenes [44]. 

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