Crown ethers lithium amides

Thermodynamic control. Note that it is also possible for the aldolate adduct to revert to aldehyde and enolate, and equilibration to the thermodynamic product may afford a different diastereomer (the anti aldolate is often the more stable). The tendency for aldolates to undergo the retro aldol addition increases with the acidity of the enolate amides < esters < ketones (the more stable enolates are more likely to fragment), and with the steric bulk of the substituents (bulky substituents tend to destabilize the aldolate and promote fragmentation). On the other hand, a highly chelating metal stabilizes the aldolate and retards fragmentation. The slowest equilibration is with boron aldolates, and increases in the series lithium < sodium < potassium, and (with alkali metal enolates) also increases in the presence of crown ethers. ... 

The catalytic polyaddition reaction of divinylbenzene and disecondary amines in the presence of a lithium amide yields polyamines with a p-vinylbenzyl (VB) substituent as in equation (21) using iV,iV -diethylethylenediamine. Macromonomers of poly(crown ethers) were also prepared by this method. Macromonomers of polypeptides were synthesized by the ring-opening polymerization of y-benzyl-L-glutamate-N-carboxyanhydride initiated by N-methyl-N-(4-vinylphenethyl) ethylenediamine, and of DL-phenylalanine-N-carboxyanhydride initiated by m, p-vinylbenzyl-... 

Lithium aluminohydride (LiAlH4, LAH) is soluble in ethers. In diethylether and dioxane it forms tight ion pairs, but in THF and in DME it forms loose ion pairs [ADI, WSl]. LAH is used either in solution, as a suspension, or in a solid-liquid phase transfer medium (benzene, 15-crown-5) [DCl, GL4]. It is also used adsorbed onto silica gel [KH2, KH3] however, its reducing power is so diminished under the latter conditions that it can selectively reduce ketoesters to hydroxyesters or amide esters into amide alcohols [ks5]. 

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