Metalloenamines reactivity

A similar asymmetric addition occurs in the case of chiral a,P-unsaturated oxazolines, and yields chiral dialkylpropanoic acids after hydrolysis (Scheme 108).382-384 A different type of reaction of chiral oxazolines leads to both chiral dialkylpropanoic acids and chiral dialkylacetic acids. In this case the chelated lithium oxazoline derivative is alkylated stereospecifically, as a consequence of the metalloenamine reactivity and the chelate geometry. 

Highly reactive non-stabilized 2-azaallyl anions (48) have been found to react with alkenes by efficient 3 + 2-cycloaddition to form 1-pyrrolines (50) after loss of LiX from intermediate (49)7 The 1-pyrrolines are deprotonated under the reaction conditions to give 1-metalloenamines which can be trapped by electrophiles. 

The control of mixed aldol condensations between aldehydes and ketones which present several possible sites for enolization is a challenging problem. Such reactions are normally carried out by converting the carbonyl compound which is to serve as the nucleophile to an enolate, silyl enol ether, or metalloenamine. The reactive nucleophile is then allowed to react with the second reaction component. As long as the addition step is faster than proton transfer, or other mechanisms of interconversion of the nucleophilic and electrophilic components, the adduct will have the desired structure. The term directed aldol condensations is given to such reactions. Directed aldol condensations must be carried out under conditions designed to ensure that the product structure is that which is desired. Scheme 2.3 illustrates some of the procedures which have been developed to achieve this goal. 

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