Steric screening: In alkylations of metalated nitriles containing vicinal methyl groups and a trisubstituted CC bond, the butene moiety screens electrophilic attack from one diastereotopic face as the quaternary stereocenter is installed (see scheme; LDA=LiNiPr2). Excellent stereoselectivity is maintained with a wide range of electrophiles to yield acyclic nitriles with contiguous tertiary and quaternary stereocenters.
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Fleming et al. (2007) studied this question.
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