Conjugate addition reactions stereoselectivity

Given the above possible reaction mechanism, it is then intriguing to speculate that another approach to the same stereoselective reduction of a vinyl sulphone could be achieved by the use of a suitably sterically hindered organosilane, as outlined in equation (64). Such a reaction would provide an interesting test for the stereoelectronics of a conjugate addition reaction by a second-row heteroatom to a vinyl sulphone. 

As is the case for aldol addition, chiral auxiliaries and catalysts can be used to control stereoselectivity in conjugate addition reactions. Oxazolidinone chiral auxiliaries have been used in both the nucleophilic and electrophilic components under Lewis acid-catalyzed conditions. (V-Acyloxazolidinones can be converted to nucleophilic titanium enolates with TiCl3(0-/-Pr).320... 

Selective labelling of the two diastereotopic methyl groups of i-leucine (144) has enabled their fates during secondary metabolic reactions to be elucidated [66]. Moreover, in the context of protein interactions, differentiation of the leucine pro-R and pro-S methyl groups in protein NMR spectra allows molecular recognition phenomena to be studied [67]. Recently, efficient routes to both forms of Relabeled leucine, based on application of an auxiliary-controlled stereoselective conjugate addition reaction (Scheme 6.27) have been described [68]. Thus, starting... 

Some examples of the synthetic usefulness of zincate reagents in organic synthesis are stereoselective alkylation reactions of cyclic ketones, conjugate-addition reactions... 

Stereoselective intramolecular conjugate addition reactions (Scheme 4) of dithiane anions tethered to an a,/ -unsaturated nitrile have been developed to advantage for the synthesis of axially substituted indolizidines and quinolizidines.81 The control of axial nitrile orientation by a peg-in-a-pocket template effect has been discussed. 

Aluminum salen complexes have been identified as effective catalysts for asymmetric conjugate addition reactions of indoles [113-115]. The chiral Al(salen)Cl complex 128, which is commercially available, in the presence of additives such as aniline, pyridine and 2,6-lutidine, effectively catalyzed the enantioselective Michael-type addition of indoles to ( )-arylcrolyl ketones [115]. Interestingly, this catalyst system was used for the stereoselective Michael addition of indoles to aromatic nitroolefins in moderate enantiose-lectivity (Scheme 36). The Michael addition product 130 was easily reduced to the optically active tryptamine 131 with lithium aluminum hydride and without racemization during the process. This process provides a valuable protocol for the production of potential biologically active, enantiomerically enriched tryptamine precursors [116]. 

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