Camphor silyl ketene acetals, derivatives

In the same way as enantiomerically pure a-halo ketones, a-halo esters have been prepared in high diastereomeric excesses by asymmetric halogenation of silyl ketene acetals generated from chiral esters So, a successfull procedure was first introduced in 1985, by W. Oppolzer (refs. 8,9), using derivatives of camphor-10-sulfonic acid as chiral auxiliaries (Fig. 4). 

Chiral silyl ketene acetals (Il)-(20) were recently introduced for diastereoselective aldol-type additions. Camphor derivatives (11)-(16) are conformationally rigid with one diastereotopic face of the enol silane sterically shielded. - A -Methylephedrine derivatives (17)-(20) are likely to bind to TiCU through the NMe2 group with consequent dramatic conformational constraint.As a result the Lewis acid mediated additions to C=X occur in a highly stereoselective way. The chiral auxiliaries can then be removed (and recycled) by reduction, saponification or displacement with various nucleophiles to give useful synthetic intermediates. 

Helmchen [67] and Oppolzer [68] investigated and documented the use of camphor-derived auxiliaries in Mukaiyama aldol reactions. Silyl ketene acetals 106 and 108 participate in diastereoselective additions to aldehydes in the presence of TiCl4, affording adducts with up to 99 1 diastereoselectivity (Equations 7 and 8). 

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