Reversibility stereoselection

When P"Bu3 is replaced by the diphosphine ligand Cy2PCH2PCy2 in the presence of water as additive, the addition takes place with the same regioselectivity but with the reverse stereoselectivity and the (Z)-enamides are obtained preferentially [54]. 

A reversed stereoselectivity was also obtained during the protonolysis of the corresponding aluminum ate complex54. 

It is of particular interest that the borane complex of (S-68) produces the (R)-alcohol, while the reversed stereoselectivity with the same degree of asymmetric induction... 

Reversing stereoselectivity of a Diels-AIder reaction by preorgamzation of reactants within a porphyrinic host... 

Sten T, Qvisen S, Uutela P, et al. Prominent but reverse stereoselectivity in propranolol glucuronidation by human UDP-glucuronosyltransferases 1A9 and 1A10. Drug Metab Dispos 2006 34(9) 1488-1494. 

When [Ru(R(or S)-PhEtbpy)3]2+ was used as a photosensitizer, however, the enantiomer excess was negligibly small, while the yield was similar to that of the reaction with [Ru(( — )-menbpy)3]2+ [39]. This complex provides the reverse stereoselectivity to that of the photoreduction of Co(acac)3 by [Ru(( — )-men-bpy)3]2+ (see above). These reasons are ambiguous. 

The dissolving metal reduction (Ca/NHa) occurred with reversed stereoselectivity. 

Acetonitrile is frequently used as the solvent of choice to enhance the formation of 1,2-trans-glycosidic linkages, by the intermediacy of a kinetically formed a-glycdsyl nitrilium ion [25], Accordingly, coupling of la with 4 in CHjCN gave a reversed stereoselectivity (a/p, 1 3) compared with toe reaction carried out in Et O or CHjClj (Scheme 6). 

A notably stereoselective reaction of an arylcerium reagent has been repOTted by Terashiina et al. As shown in Scheme 18, the cerium reagent provides adducts (11) and (12) in a ratio of 16 1 in 95% combined yield. In contrast, the organolithium reagent gives a lower and reversed stereoselectivity. 

The stereoselectivity of the reduction of bicyclic 1,3-dioxolanes 2 JO has been studied [IM3, KU2, KU3]. DIBAH in excess leads predominantly to rrans-substi-tuted isomers, while Et3SiH-TiCl4 or Zn(BH4)2-TiCl4 in CHjClj gives the reverse stereoselectivity (Figure 2.25). Other reagents such as AlHBrj or AlHClj are less stereoselective [KU4]. 

The Z silyl enol ether 35 from the ketone 6 also shows reversed stereoselectivity from the lithium enolate, giving the anti aldol 7 in high yield.12 Further study1 revealed that these reactions, which take place via an open (rather than a chelated) transition state 37, are thermodynamically... 

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