Product Placement

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Science  04 Nov 2011:
Vol. 334, Issue 6056, pp. 571
DOI: 10.1126/science.334.6056.571-c

Asymmetric catalysis relies on a chiral agent to bias the stereochemical outcome of a reaction. If the desired product has just one chiral center, it is usually straightforward to prepare either enantiomer by using the appropriate enantiomer of catalyst (though in cases where the catalyst is a scantly modifi ed a natural product, it may be moderately more expensive or time-consuming to prepare its unnatural isomer). Reactions that generate two chiral centers in one product pose more of a conundrum though. There is no obvious means of modifying a catalyst to modulate the relative sense of those two centers; in some cases, it may be necessary to pursue entirely different approaches to bond construction, let alone catalyst design, in order to obtain each distinct diastereomer. Tian et al. present a rare instance in which fairly simple modifications to the reaction conditions switch the diastereoselectivity of a single catalyst. Depending on the solvent and acid cocatalyst, their quinuclidine-derived organocatalyst directs thiols to add in either syn or anti disposition with respect to the α-alkyl substituent in α,β-unsaturated ketones. Both pathways proceed with high enantioselectivity.

J. Am. Chem. Soc. 133, 10.1021/ja207847p (2011).

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