Aldol reactions antilsyn selectivity

Aldol Reactions of Ester Derivatives. The Titanium(IV) C/tlor/dc-catalyzed addition of aldehydes to 0-silyl ketene acetals derived from acetate and propionate esters proceeds with high stereoselectivity. Formation of the silyl ketene acetal was found to be essential for high diastereoselectivity. Treatment of the silyl ketene acetal, derived from deprotonation of the acetate ester with LICA in THF and silyl trapping, with a corresponding aldehyde in the presence of TiCU (1.1 equiv) afforded the addition products in 93 7 diastereoselectivity and moderate yield (51-67%). Similarly, the propionate ester provides the anti-aldol product in high antilsyn selectivity (14 1) and facial selectivity (eq 4). 

The blend SnC -ZnCli is an effective catalyst in the aldol reaction of silyl enol ethers with aldehydes (Eq. 87), acetals (Eq. 88), or ketones [122]. Product antilsyn ratios vary (32 69 to 89 11). The blend also catalyzes the Michael addition of silyl enol ethers with a,/3-unsaturated ketones (Eq. 89), yielding alkylation products (84-100 %) with anti selectivity antilsyn = 55 45 to 87 23). 

A common example for auxiliary-contfolled anti-aldol reactions is the Masamnne-Abiko process since the procedure covers the synthesis of both enantiomers depending on the nature of the auxiliary, which is readily available in both enantiomeric forms [15]. This anti-selective aldol reaction has been performed by using enantiomerically pure carboxylic esters derived from (-)- or (-i-)-norephedrine. The method is applicable to a wide range of aldehydes with high selectivity (both antilsyn and diastereoselectivity of anti-isomer) [16]. A typical example of this aldol approach is depicted in Scheme 2.113. It is proposed that the reaction proceeds via a six-membered Zimmerman-Traxler transition state. 

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