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Chiralne amidki litu : właściwości i wybrane zastosowania syntetyczne

Piwońska Magdalena

Wiadomości Chemiczne

2021

[Z] 75, 5-6

771--798

Reactions involving carbonyl groups are one of the most important transformations in organic chemistry. The nucleophilic properties of carbonyl compounds, when they are in the form of enolate ions, offer many possibilities for creating new carbon-carbon bonds in reactions with electrophiles. In the case of cyclic ketones, enolate formation can be stereocontrolled by deprotonation with the use of chiral lithium amides. Stereoselective formation of the chiral lithium enolate determines the stereochemistry of the product of the subsequent reaction with the electrophile. The induction of chirality in the reaction of enolate ions with electrophiles can also be achieved by using metals other than lithium, i.e. magnesium. When other alkali metals are used, an organometallic catalyst containing a chiral ligand must be present in the reaction. The presence of particular structural elements allow distinguishing the chiral lithium amides between eight major classes. Due to the high reactivity of lithium enolates, they are often converted into the silyl enol ether. This is done in two ways: internal quench (in situ reaction with TMSCl) or external quench. Due to aggregation of the chiral lithium amides and, thus, a decrease in asymmetric induction, the addition of LiCl is necessary for reactions run in external quench conditions. Although there are known examples of the use of chiral lithium amides in a catalytic amount in the deprotonation of epoxides, there is only one example of using less than stoichiometric amounts of chiral lithium amides in the deprotonation of ketones. There are many reports in the literature on the use of chiral lithium amides in total syntheses. The chiral lithium amides were used to form chiral enol silyl ether intermediates, e.g. in synthesis of chlortetaine or (+)-ibogamine. They are also used to form chiral lithium enolates which reacts directly with electrophiles, e.g. in synthesis of lasonolide A.