Other Remote Stereocenters

71 29 for the E- and 87 13 for the Z-silyl ketene acetals. The syn/anti diastereo-meric ratio was 1 1 for both axial rearrangement products. 

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Almost 50 years ago, Cram outlined a rule (Cram s rule), which proved to be fruitful in understanding, predicting, and controlling diastereoselectivity induced by a remote stereocenter [258,259], Numerous examples of 1,2 induction have confirmed over the time the predictive character of this rule [260], Afterwards, other important contributions of Felkin and coworkers and Anh... 

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. 

The C4 stereocenter dominates the stereochemical outcome of the Ireland-Claisen rearrangements in appropriately substituted allyl silyl ketene acetals. For substrates lacking a C4 stereocenter, other more remote stereocenters may play a significant role in the stereochemical outcome. There are comparatively few cases of remote stereocontrol, so most examples will be described in this section. The stereocenters will be discussed based on their location relative to the allyl silyl ketene acetal. Carbons a to the Cl carbon will be designated Cl, alpha to C4, C4 and so on (Fig. 4.1). 

Relationships between stereocenters vary between two extremes. On the one hand, stereocenters may interact strongly in a spatial sense if they are directly joined, proximate to one another, or part of a compact rigid-ring structure. On the other hand, two stereocenters which are remote from one another and/or flexibly connected may be so independent that one cannot be used to provide substrate spatial control for the other. Nonetheless, this latter type of stereorelationship may still be clearable if the target molecule can be disconnected to divide the two stereocenters between two precursors or if an appropriate enantioselective transform is available. 

The only other functional group is the conjugated unsaturated ester. This functionality is remote from the stereocenters and the ketone functionality, and does not play a key role in most of the reported syntheses. Most of the syntheses use cyclic starting materials. Those in Schemes 13.4 and 13.5 lead back to a para-substituted aromatic ether. The syntheses in Schemes 13.7 and 13.8 begin with an accessible terpene intermediate. The syntheses in Schemes 13.10 and 13.11 start with cyclohexenone. Scheme 13.3 presents a retrosynthetic analysis leading to the key intermediates used for the syntheses in... 

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