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Syn-anti dichotomy

Syn elimination and the syn-anti dichotomy have also been found in open-chain systems, though to a lesser extent than in medium-ring compounds. For example, in the conversion of 3-hexyl-4-d-trimethylammonium ion to 3-hexene with potassium ec-butoxide, 67% of the reaction followed the syn-anti dichotomy. In general syn elimination in open-chain systems is only important in cases where certain types of steric effect are present. One such type is compounds in which substituents are found on both the P and the y carbons (the unprimed letter refers to the branch in which the elimination takes place). The factors that cause these results are not completely understood, but the following conformational effects have been proposed as a partial explanation. The two anti- and two syn-periplanar conformations are, for a quaternary ammonium salt ... [Pg.1305]

In order for an E2 mechanism to take place a base must approach the proton marked. In C this proton is shielded on both sides by R and R. In D the shielding is on only one side. Therefore, when anti elimination does take place in such systems, it should give more cis product than trans. Also, when the normal anti elimination pathway is hindered sufficiently to allow the syn pathway to compete, the anti — trans route should be diminished more than the anti — cis route. When syn elimination begins to appear, it seems clear that E, which is less eclipsed than F, should be the favored pathway and syn elimination should generally give the trans isomer. In general, deviations from the syn-anti dichotomy are greater on the trans side than on the cis. Thus, trans olefins are formed partly or mainly by syn elimination, but cis olefins are formed entirely by anti elimination. Predominant syn... [Pg.988]

Bailey Saunders Chem. Commun. 1968, 1598. J. Am. Chem. Soc. 1970, 92. 6904. For other examples of syn elimination and the syn-anti dichotomy in open-chain systems, see PAnkovA Sicher ZAvada Chem. Commun. 1967, 394 PAnkovA Vitek ValiikovA fteficha ZAvada Collect. Czech. Chem. Commun. 1972, 37. 3456 Schlosscr An Helv. Chim. Acta 1979, 62. 1194 Sugita Nakagawa Nishimoto Kasai Ichikawa Bull. Chem. Soc. Jpn. 1979, 52. 871 PAnkovA KociAn KrupiCka ZAvada Colled. Czech. Chem. Commun. 1983, 48. 2944. [Pg.988]

This new use of DMSO has not only yielded information relating to the recently exposed syn-anti dichotomy (Sicher, 1972), but has also revealed new facets of the century-old Hoffmann-Saytzeff orientation controversy. [Pg.182]

Syn-anti dichotomy An unusual result that occurs when alkenes are formed by an elimination reaction from large rings, in which the cis isomer is formed by //// -elimination, but the trans isomer is formed by vf/z-elimination. [Pg.391]

Stereochemistry of the Palladium-Catalyzed Allylic Substitution— The Syn-Anti Dichotomy in the Eormation of (-Tr-Allyljpalladium Complexes and Their Equilibration. [Pg.1479]

There is a dichotomy in the sense of syn-anti diastereofacial preference, dictated by the bulkiness of the migrating group [94]. The sterically demanding silyl group results in syn diastereofacial preference but the less demanding proton leads to anti preference (Sch. 35). The anti diastereoselectivity in carbonyl-ene reactions can be explained by the Felkin-Anh-like cyclic transition-state model (Ti) (Sch. 36). In the aldol reaction, by contrast, the now inside-crowded transition state (Ti ) is less favorable than Tg, because of steric repulsion between the trimethylsilyl group and the inside methyl group of aldehyde (Ti ). The syn-diastereofacial selectivity is, therefore, visualized in terms of the anti-Felkin-like cyclic transition-state model (T2 )-... [Pg.821]

Pankova, M., Sicher, J., Zavada, J. Syn-anti elimination dichotomy a common feature in Hofmann elimination. Chem. Commun. 1967, 394-396. [Pg.601]

Even when the retroaldol reaction is fairly facile, stereoisomer equilibration can be slow. This phenomenon is illustrated in Scheme 16. A solution of the lithium aldolate (243) and benzaldehyde equilibrates to (244) and p-anisaldehyde with a half-life of 15 min at 0 °C. However, the syn lithium aldolate (244) equilibrates with its anti diastereomer (246) with a half-life of approximately 8 h at room temperature. The reason for this apparent dichotomy is that enolate (245) is so stereoselective in its reactions with aldehydes. Since the kinetic syn.anti ratio is 98.7 1.3, the syn aldolate must dissociate approximately 75 times in order for one syn aldolate molecule to be converted into one anti aldolate molecule. Of course, for less stereoselective enolates, such as the cyclohexanone enolate referred to above, stereochemical isomerization will more nearly parallel the rate of actual aldol reversal. [Pg.235]

There is a dichotomy in the sense of syn- vs antz-diastereofacial preference, dictated by the bulkiness of the migrating group [80]. The sterically demanding silyl group shows syn-diastereofacial preference but the less demanding proton leads to anti-preference (Scheme 33). The anfi-diastereoselectivity in carbonyl-... [Pg.1101]


See other pages where Syn-anti dichotomy is mentioned: [Pg.1304]    [Pg.1305]    [Pg.1357]    [Pg.987]    [Pg.988]    [Pg.372]    [Pg.373]    [Pg.183]    [Pg.1484]    [Pg.1484]    [Pg.1485]    [Pg.1304]    [Pg.1305]    [Pg.1357]    [Pg.987]    [Pg.988]    [Pg.372]    [Pg.373]    [Pg.183]    [Pg.1484]    [Pg.1484]    [Pg.1485]    [Pg.918]    [Pg.931]    [Pg.918]    [Pg.931]    [Pg.918]    [Pg.931]    [Pg.88]    [Pg.12]    [Pg.922]    [Pg.922]    [Pg.38]    [Pg.922]    [Pg.208]   
See also in sourсe #XX -- [ Pg.987 , Pg.988 ]

See also in sourсe #XX -- [ Pg.372 ]

See also in sourсe #XX -- [ Pg.391 ]




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Dichotomies

Syn-anti

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