Nonconjugated isomers

We present here examples of this condensation with an aromatic aldehyde and a cyclic ketone. Both of these examples are useful because, although other methods are available for their preparation, problems often attend these syntheses. In the synthesis of cyclohexy11deneaceton1tr11e, for example, the standard method results exclusively In the g.y-lsomer and none of the a,g-Isomer. In Part A of this procedure, cyclohexanone Is condensed with acetonitrile to give predominantly the conjugated Isomer (80-83%) whicfi is then separated from the nonconjugated isomer by selective bromination. 

A number of important families of heterocycles belong to this class and many of the important systems are oxo derivatives. 2-Hydroxyfuran 64 exists almost exclusively as the equilibrating 3H- and 5//-furan-2-one tautomers 66 and 67. Tautomerism favoring the nonconjugated isomers also occurs in hydroxypyrroles and hydroxythiophenes. Similar behaviour is observed with simple amino derivatives 2-aminofuran 65 is too unstable to be detected <2006AHC (92)1 >. The 3-oxo tautomer 68 is the preferred form of 3-hydroxyfuran. 

The nonconjugated isomer 18 of 15, which has lost the capability of undergoing a 1,5-hydrogen shift because of the interrupting methylene group, opens to 19 when heated to 130"C. In this case no reverse reaction is observed. ... 

Yields of pyrylium salts are often higher in diacylations of alkenes than in the acylation of an equilibrium mixture of unsaturated ketones. This may be attributed to the preferential formation of the nonconjugated isomer in the first acylation, whereas isomerization of the (usually) thermodynamically more stable isomer is required before an a,p-unsaturated ketone may acylate. 

In addition to the trans monoene isomers, there are a substantial number of trans diene isomers identified in intestinal contents of ruminants. These include a multitude of conjugated and nonconjugated isomers. The conjugated diene isomers can be divided into double bond systems with a cis-trans, a trans-cis, a cis-cis, or a trans-trans geometric configuration. Total production of CLA isomers across the rumen averaged 9.2 and 4.7 g/d for the cis-trans and trans-trans CLA classes, respectively, when averaged across three cattle studies (14, 20, 23). 

In addition to the acyclic compounds just discussed, it is common to see cyclic molecules with conjugated 7i-bonds. An important cyclic diene is cyclopen-tadiene (5). The numbers are omitted from the name because there is only one possible structure. This is not the case for cyclohexadiene derivatives because there is the conjugated 1,3-cyclohexadiene (6) and the nonconjugated 1,4-cy-clohexadiene, 7. With all dienes that have larger rings, there is one conjugated isomer but there may be several possible nonconjugated isomers. 

Problem 6.11. Based upon Equation 2.2 and the observation that 13-cyclohexadi-ene has a rt re at 263 nm while that for its nonconjugated isomer is about 190nm, calculate the difference in energies of the excited states. 

Scheme 6.56. A path for the acid-catalyzed isomerization of a nonconjugated alkadiene (1,4-cyclohexadiene) to its conjugated isomer (1,3-cyclohexadiene). Note that although the reaction is reversible, that is, the protonated intermediate could have lost the same proton originally acquired to return to the nonconjugated isomer, it is shown as occurring in one direction (to the most stable isomer).

First, in common with other conjugated rr-electron systems that we have seen, a,p-unsaturated aldehydes and ketones are more stable than their nonconjugated isomers. Under conditions chosen to bring about their interconversion, for example, the equilibrium between a p,y-unsaturated carbonyl and its a,p-unsaturated analog favors the conjugated isomer. 

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