Asymmetric synthesis reagent controlled, examples

Asymmetric synthesis introduces one or more new features of chirality in a molecule. Several approaches are possible. In general, preferential formation of an enantiomer or diastereoisomer is achieved as a result of the influence of a chiral element present in the substrate, a reagent, catalyst or the environment. Chirality control is also possible in the electronically excited state,584 as demonstrated in the following examples. 

The 1,4-addition of m anometallic reagents to a,3-unsaturated aldimines (11) 9 and aromatic aldimines (12) resembles additions to oxazolines (13) and (14), and provides a valuable method for generating a remote stereocenter (1,5-asyininetric induction). Other strategies have been employed for controlling azomethine diastereofacial selectivity. For example, organometallic treatment of chiral chromium complexes (15) of iV-arylaldimines leads to high stereoselectivi (presumably a result of steric control). Further investigations are required to assess the value of the chromium tricarbonyl complexes in asymmetric amine synthesis. 

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