Oxazoline, chiral remote asymmetric induction

In an example of remote asymmetric induction, chiral oxazoline (51) undergoes Grignard additions with moderate to good selectivity. The major product formed (52) results fiom coordination of the reagent to the oxazoline moiety, followed by nucleophilic attack fiom the bottom, as shown in Figure 12, The phenyl substituent of the oxazoline effectively prevents addition fiom the top face of the molecule. Systems such as (51) have been used to synthesize opticaUy active i thalides (54), as shown in Scheme 2. Typical optical purities of the isolated aromatic products range from 46-80%. Table 13 lists some of these results. 

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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