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Ketone, /-butyl ethyl stereoselectivity

A similar preference for formation of the syn aldol is found for other Z-enolates derived from ketones in which one of the carbonyl substituents is bulky. Ketone enolates with less bulky substituents show a decreasing stereoselectivity in the order r-butyl > i-propyl > ethyl.2c This trend parallels a decreasing preference for stereoselective formation of the Z-enolate. [Pg.69]

E -Enolates often react with lower stereoselectivity than those of the corresponding Z-enolates. A classic example to illustrate this point is a study carried out by Heathcock et al.6 (Scheme 2.IV). When the carbonyl compounds 1 were deprotonated with lithium diisopropylamide (LDA) and the resulting enolates were subsequently treated with benzaldehyde at -72° C, the aldol products desired (2) were obtained in 83 to 99% yield. The Z-enolates derived from t -butyl and 1-adamantyl ethyl ketones afforded syn -products in excellent levels of diastereoselectivity. The fact that the syn/anti ratios directly reflect the isomeric purity of the reacting enolates hints that the Z-enolates in these cases undergo aldol reaction through a chairlike six-membered transition state (Scheme 2.III,... [Pg.49]

Ethyl fluoroacetate gives a 1 1 mixture of the ( )- and (Z)-enolate, which reacts with aldehydes and ketones to give mixtures of syn and anti aldols (equation 83). ° Stereoselectivity is generally low, the syn.anti ratio ranging from 50 50 for 2-butanone to 4 1 for ethyl r-butyl ketone. However, since a mixture of enolates was employed in this study, there remains the possibility that one of the enolates is highly stereoselective, whereas the other one is stereorandom. [Pg.209]

Stereoselective aldol condensation. Aldol condensation has been shown to be subject to kinetic stereoselection, with (Z)-enolates giving mainly the erythro-aldol and (E)-enolates giving mainly the (Areo-aldol. This observation has been extended to preformed (Z)- and (E)-lithium enolates generated under kinetic control with LDA in THF or ether at — 72°. When one of the alkyl groups is sterically demanding, complete kinetic stereospecificity can be obtained. As the bulk of the alkyl group decreases, stereoselection also decreases. Thus the kinetic enolate of ethyl /-butyl ketone (1) is the (Z)-isomer a, and it reacts with benzalde-hyde to form the erythro-aldol (2) with no detectable amounts of the threo-aldol. [Pg.152]

The treatment of an ester (or lactone) with a base and a silyl halide or trillate gives rise to a particular type of sUyl enol ether normally referred to as a silyl ketene acetal. The extent of O- versus C-silylation depends on the structure of the ester and the reaction conditions. The less-bulky methyl or ethyl (or 5-tert-butyl) esters are normally good substrates for O-silylation using LDA as the base. Acyclic esters can give rise to two geometrical isomers of the silyl ketene acetal. Good control of the ratio of these isomers is often possible by careful choice of the conditions. The f-isomer is favoured with LDA in THF, whereas the Z-isomer is formed exclusively by using THF/HMPA (1.24). Methods to effect stereoselective silyl enol ether formation from acyclic ketones are less well documented. ... [Pg.14]

An exception is the silyl enol ether of ethyl t-butyl ketone, which gave predominantly the anti stereoisomer. This is opposite to the stereoselectivity observed with the lithium enolate. The results are considered to be consistent with a noncyclic transition state for the reactions. [Pg.69]


See other pages where Ketone, /-butyl ethyl stereoselectivity is mentioned: [Pg.193]    [Pg.193]    [Pg.193]    [Pg.377]    [Pg.167]    [Pg.377]    [Pg.1739]    [Pg.89]    [Pg.117]    [Pg.286]    [Pg.192]    [Pg.233]    [Pg.634]    [Pg.192]    [Pg.233]    [Pg.634]    [Pg.58]    [Pg.80]    [Pg.301]    [Pg.30]    [Pg.1739]    [Pg.192]    [Pg.233]    [Pg.634]    [Pg.24]   
See also in sourсe #XX -- [ Pg.193 , Pg.195 ]

See also in sourсe #XX -- [ Pg.193 , Pg.195 ]




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