The stereochemistry of base-catalyzed H/D exchange on 13
![](/images/gifchars/beta2.gif)
-substituted ethyl butanoates in ethanol-
dhas been studied in order to analyze the steric and electronic factorswhich control the diastereoselectivity ofelectrophilic attack on enolate anions. Electrophilic deuterationof the enolate anion also determines the stereoselectivityof 1,4-conjugate addition of ethanol-
d to
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,
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-unsaturated esters. Experimental conditions were selectedwhichrigorously exclude the effects of ion pairing and aggregation. Theresearch showed that stereoelectronic factorsgenerally produce higher stereoselection than steric effects do.Electronegative heteroatom substituents at C-3produceda 10:1 ratio of the2
R*,3
R*/2
R*,3
S*2-deuteriobutanoates. In the most stable transition states forelectrophilic attack,these electronegative substituents occupy an antiperiplanar position tothe forming C-D bond. Only with a
![](/images/gifchars/beta2.gif)
-
tert-butyl substituent did steric effects produce high stereoselection, andit fell off rapidly with a decrease in carbonbranching. Protonation of acyclic
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-ethoxy aldehyde and ketoneenolates follows the same diastereoselectivity patternas the
![](/images/gifchars/beta2.gif)
-ethoxy ester enolate, but protonation of the cyanocarbanionfrom a
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-ethoxy nitrile gives much lowerstereoselection.