Chelation peptide enolates

Chelated peptide ester enolates [35] and, more recently, peptide amide enolates [36] have also been shown to be suitable nucleophiles in Pd-catalyzed allylations. The absolute configuration of the newly formed stereogenic center is directed... 

Zinc enolates like 41 served as nucleophiles in a study aimed at a rhodium-catalyzed reaction with enantiomerically enriched allylic phosphates. Nonracemic products were obtained under efficient chirality transfer from the substrate and with remarkable diastereoselectivity [23]. Kazmaier enolates were used to manipulate the backbone of small peptides [27] by palladium-catalyzed allylations of a terminal glycine ester [28]. In the course of these studies, central glycine was also allylated in a diastereoselective manner through assumed chelated zinc enolates of glycine in a peptidic bond to proline or JV-alkyl amino acids [29]. Although this is, of course, not an asymmetric synthesis, the protocol is remarkable inasmuch as, in contrast to the palladium-mediated conversion, the noncatalyzed reaction of the enolate with the corresponding allylic bromide occurs in a more or less stereorandom manner. 

It has been shown that the Claisen rearrangement of lithium enolates of amino acid enynol esters allows the synthesis of very sensitive y, 5-unsaturated amino acids with conjugated enyne side chains.The chelate-enolate Claisen rearrangement has also been applied to the synthesis of unsaturated polyhydroxylated amino acids, polyhydroxylated piperidines, and unsaturated peptides. ... 

Maguesium aminoester enolates are of much iuterest for the syuthesis of complex amiuoacids aud peptides. These chelate euolates have beeu used as uucleophiles for a wide rauge of stereoselective transformatious, as diastereoselective Michael additious aud Claiseu rearrangement . Two typical examples are presented in equation 95. 

Using the manganese enolates, the chiraUty is transfered from the chiral ester moiety towards the peptide backbone, but not inside the peptide backbone. Therefore, the influence of the chelating metal salt in the rearrangement of several dipeptide crotyl esters was investigated. 

Allylic esters of peptides have been shown to undergo Claisen rearrangement after deprotonation in the presence of tin chloride to afford allylated peptides. Subsequent iV-allylation and ring closure provided the corresponding cyclic peptides. The same authors have made attempts to use the chiral backbone of a given peptide to control the stereochemical outcome of its modification using a chelate-enolate Claisen rearrangement. 

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