Bridged-ring system

In bridged ring compounds, rings share more than two atoms, and the bridge can consist of one or more atoms. We have already met an example in bornane (see Section 3.3.2), which we used as an illustration of how a cyclohexane ring can be forced into a boat conformation to achieve the necessary bonding. 

We should compare this system with a 1,4-disubstituted cyclohexane such as 4-methylcyclo-hexanecarboxylic acid (see Section 3.4.4). There is a plane of symmetry in this molecule, so there are no chiral centres but geometric isomers exist, allowing cis and trans stereoisomers. The restrictions imposed by bridging have now destroyed any possibility of geometric isomerism. 

When we move on to camphor, a ketone derivative of bornane, we find this can exist in two enantiomeric forms because the plane of symmetry has been destroyed. Nevertheless, there are only two configurational isomers despite the presence of two chiral centres bridging does not allow the other two variants to exist. 

P-Pinene is representative of a bicyclo[3,l,l]heptane system. This natural product has two chiral centres, but can exist only in the (-1-)- and (—)-enantiomeric forms shown. 

The tropane alkaloids (—)-hyoscyamine and (—)-hyoscine are found in the toxic plants deadly nightshade (Atropa belladonna) and thornapple (Datura stramonium) and are widely used in medicine. Hyoscyamine, usually in the form of its racemate atropine, is used to dilate the pupil of the eye, and hyoscine is employed to control motion sickness. Both alkaloids are esters of (—)-tropic acid. 

Many bridged ring systems are named by the von Baeyer system. Von Baeyer names are used mostly for bridged ring systems and occasionally for nonbridged ring systems. Examples of von Baeyer names are bicyclo[3.2.1]octane and tricyclo[7.4.1.0 ]tetradecane. 

Bicyclo denotes two rings and octane denotes a total of eight skeletal atoms in the ring system. [3.2.1] gives the sizes of the three bridges connecting two bridgehead atoms. 

Fused ring systems that have other bridges are usually named by prefixing the name of the bridge to the name of the fused ring system. The names of hydrocarbon bridges are derived from the names of the parent hydrocarbons by replacing the final -ane, -ene, etc., by -ano, -eno, etc. Thus, -CH2- is methano and -CH=CH- is etheno. 

Heterocyclic systems are named by replacement nomenclature. Unsaturation is denoted by -ene and -yne suffixes. For more information, see Eckroth, D. R., J. Org. Chem., 32, 3312,1967. 

Some common cage structures are usually named trivially as parents. CAS names all of these systematically. 

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The individual bonds of a bridged ring system which are eligible for inclusion in the set of strategic bond disconnections are those which meet the following criteria. 

Cis-Trans Isomerism of Fused and Bridged Ring Systems... 

Moreover, compounds of type 7-127, which were obtained from 7-125 by reaction with Pb(OAc)4, can undergo a further domino process when treated with potassium carbonate in a mixture of water and methanol [57]. This includes saponification of the acetate moieties in 7-127 to provide 7-129 via the unstable cyclic hemiacetal 7-128 (Scheme 7.35). Retro-Claisen reaction and ring closure with the proposed intermediates 7-130 and 7-131 led to the bridged ring-system 7-132 as a mixture of di-astereomers with preference of the (3-isomer. 

Cineole provides an example of a bridged ring system. The essence of this problem is to spot that we cannot bridge the opposite carbons of a cyclohexane chair conformation it is necessary to have the cyclohexane in a boat form. Then the answer is readily obtained. 

Cyclization reactions in systems with restricted mobility (most notably ring systems) to form either fused or bridged ring systems, in order to determine the relative disposition of substituents, has played an important role in the structural elucidation of many natural products 109. The method is based on the precondition that only one relative configuration will allow ring closure for steric reasons. Two recent examples may serve to illustrate the procedure. 

Some unusual nucleophile functional groups that have been utilized in cyclofunctionalizations to form bridged ring systems include cyclic hemiacetals (equation 18)73 and epoxides (equation 19).87 The cyclizations in equation (20) involve a cyclic enol ether as the reactive ir-system and have been used in syn-... 

The reaction of 92 with sodium carbonate in MeOH affords the ester 93, while the reduction of 92 with LAH leads to ring cleavage, affording the bridged ring system 94 (Scheme 19). Then 94 was converted to the quinolizidines 97 and 98 via 95 and 96, respectively, in a two-step reaction in 62% and 58% yields <2002SL85>. 

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