A variety of electrophilic alkenes will accept enol ate nucleophiles

chemoselective (Chapter 24) conjugate addition of the silyl ketene acetal on the enone is preferred to direct aldol reaction with the aldehyde. Then an aldol reaction of the intermediate silyl enol ether on the benzaldehyde follows. The stereoselectivity results, firstly, from attack of benzalde-hyde on the less hindered face of the intermediate silyl enol ether, which sets the two side chains trans on the cyclohexanone, and, secondly, from the intrinsic diastereoselectivity of the aldol reaction (this is treated in some detail in Chapter 34). This is a summary mechanism. 

A variety of electrophilic alkenes will accept enol(ate) nucleophiles 

The simplest and best Michael acceptors are those a, 3-unsaturated carbonyl compounds with exposed unsaturated (3 carbon atoms, such as exo-methylene ketones and lactones and vinyl ketones, and we shall see in the next section that these need to have their high reactivity modera ted in most applications. 

Thefactthatthia is an /V,/V ftnethyl -amide should remind you of the use of this kind of saturated amide id carbonyl substitution reactions with RU In Chapter 12. 

However, most a,[3-unsaturated ketones can be made to do conjugate addition by suitable choice of enol(ate) equivalent and conditions. Now we need to look at the best Michael acceptors, their reactions, and how to make them. 

The Mannich reaction provides stable equivalents of exo-methylene ketones 

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