Double stereodifferentiation substrate control

Conversely, the addition of enantiomerically pure chiral dialkylboranes to enantiomerically pure chiral alkenes can also take place in such a way that substrate control and reagent control of diastereoselectivity act in the same direction. Then we have a matched pair. It reacts faster than the corresponding mismatched pair and with especially high diastereoselectivity. This approach to stereoselective synthesis is also referred to as double stereodifferentiation. 

Keywords Stereogenic reactions. Mechanism control. Substrate and reagent control. Stereoselectivity, Simple diastereoselectivity, exo-endo-Diastereoselectivity, Double stereodifferentiation, Chiral catalysts, Chiraphor and catalaphor. Hybrid catalysts. Chiral auxiliary... 

Scheme 47 shows a case 2 example for double stereodifferentiation, the problem being to reduce enone 47-1 preferentially to alcohol 47-2 or 47-3 [110]. The substrate control (DIBAH or L-selectride) is essentially zero, so that the chiral hydride donor must do the job. It can be seen that BINAL-H [ 111 ] is ineffective whereas diborane plus the CBS catalyst [112] shows a very pronounced reagent control so that either one of 47-2 and 47-3 may be generated selectively for the formation of 47-3 the reagent control is much higher than for 47-2, which is surprising in view of the low substrate control of the process. 

With chiral /J-oxo esters double stereodifferentiation via combined catalyst and substrate control is obtained with (+ )-3-(methoxycarbonyl)camphor (3) and a palladium catalyst modified with (+)- and ( —)-Diopn. 

For practical reasons, the stereochemical results of acyclic and cyclic compounds are treated separately. In both cases, the reaction can be enantio- or diastereodifferentiating. In the first case, the stereochemical differentiation is controlled by the reagent and in the second case the stereochemical outcome is either determined by the chiral substrate or by both the chiral substrate and the chiral reagent (double stereodifferentiating reactions)81-83. 

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