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Blocking one face: Benzylic carbocations, which are easily generated in situ from alcohols or acetates by Brønsted or Lewis acids, undergo selective facial discrimination in diastereoselective reactions (see scheme; FG=functional group). The A values are responsible for the facial selectivity. Catalytic amounts of various Lewis acids can be employed in discriminating one face of the carbocation.
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Cozzi et al. (2009) studied this question.
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