Conjugated unsaturated systems electrophilic attack

In the field of stereochemical induction, efficient new asymmetric syntheses have resulted from conjugate addition of Grignard reagents to the readily prepared chiral unsaturated systems (45) and (46). Further details and developments have appeared concerning the stereoselective a-addition of the thioacrolein dianion (47) to ketones and enones, in contrast to the random nature of the Li reagent. In the same vein, the Mg salt derived from the sterically protected urethane (48) undergoes exclusive y-attack with electrophiles [cf. species (19b) above]. ... 

There are many examples of nucleophilic reagents that add to a, 3-unsaturated aldehydes and ketones in a manner in which the addition is formally 1,4. This result is called conjugate addition. Under basic conditions, these transformations involve initial attack by the nucleophile to the p-carbon atom, followed by electrophilic addition (normally of a proton) on the carbonyl oxygen the nucleophile and electrophile add at the 1 and 4 positions relative to one another. The enolate formed in the early stages of the reaction is generally quickly protonated to give an enol. The enol wiU subsequently tautomer-ize to the ketone. A general mechanistic scheme is shown below for the 1,4 addition of water to an a,p-unsaturated carbonyl system. 

We begin by introducing the chemistry of enolates and enols. Especially important is a reaction between enolate ions and carbonyl compounds, called the aldol condensation. This process is widely used to form carbon-carbon bonds both in the laboratory and in nature. Among the possible products of aldol condensation are a,/3-unsaturated aldehydes and ketones, which contain conjugated carbon-carbon and carbon-oxygen tt bonds. As expected, electrophilic additions may take place at either bond. However, more significantly, a, -unsaturated carbonyl compounds are also subject to nucleophilic attack, a reaction that may involve the entire conjugated system. 

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