Influence of Anomeric Effect on Conformational Reactivities

4-elimination of 193, given in the Eqs. 52 and 53, represents two pathways leading to two different iminium ions 194 and 195, whose relative distribution will depend on whether the reaction is performed under thermodynamic or kinetic control. The species 194 is a cyclooctene derivative and the species 195 a cyclohexane derivative. The cyclohexane derivative 195 must, of course, predominate in a thermodynamically controlled reaction. Like the species 193, the species 196 also represents two pathways for 1,4-elimination as shown in Eqs. 54 and 55. Fortunately, both the pathways yield the same product 197. The reactions shown in Eqs. 56 and 57 are examples of elimination resulting from the 1,3-diol mono-tosylate system. An electron pair orbital on the hydroxylic oxygen that is antiperiplanar to the cleaving central ac c bond provides the necessary electronic push in which the former makes the latter weak, and therefore labile for cleavage. The reaction shown in Eq. 57 was used by Corey and coworkers in a synthesis of 

In the Eqs. 58 and 59, an oxy anion provides the necessary electronic push for the 1,4-elimination to take place smoothly. In Eq. 58, this oxy anion was generated in situ by hydride reduction of the carbonyl group [27]. The in situ oxy anion generation in Eq. 59 was achieved by the cleavage of the ester group on reaction with an alkoxide ion in a protocol that is typical of transesterification [28]. 

As shown in Eq. 63, the bicyclic species 216 is quantitatively transformed into the ra-cyanocarboxylic acid 218 [32]. The intermediate species 217 formed from the reaction with hydroxide ion has two electron pairs, one on each of the two oxygen atoms located on Cl, antiperiplanar to the CTci-c2 bond. The antiperiplanar relationship between Jci-c2 and cjC3 C4 and also between ctc3-c4 and ctn-ots arc the other requirements for the overall fragmentation to be successful. Note that the nitrile function emerges from a process of anti-elimination. 

Methoxide ion-catalyzed transformation of the tricyclic enedione 229 into the isomeric mixture of the ester 232, Eq. 67, is yet another interesting example. The (7c i c2 bond is weakened by the two antiperiplanar electron pair orbitals, one on each the two oxygen atoms on Cl. Moreover, ctC c.2 bond is nearly parallel to the p orbitals of the nc=c bond that allows their electron densities to mix well enough to facilitate formation of the dienolate as in 231 [34]. Protonation on either face of the dienolate is responsible for formation of the isomeric mixture of the end product 232. 

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