Allylic esters, chelated rearrangement

Enolates of allyl esters of a-amino acids are also subject to chelation-controlled Claisen rearrangement.249... 

The use of a heteroatom a to the ester carbonyl group allows for the formation of a chelate with the metal counterion hence, the geometry of the ester enolate can be assured.336-338358359 This approach was used in the rearrangement of the glycine allylic esters 13 to y,8-unsaturated amino acids in good yields and excellent diastereoselectivity (Scheme 26.13).358 The enantioselectivity could be reversed by using quinidine instead of quinine. 

Chelate enolate Claisen rearrangements of a-amido substituted allylic esters... 

Kazmaier, U. Synthesis of y,5-unsaturated amino acids via ester enolate Claisen rearrangement of chelated allylic esters. Amino Acids... 

Claisen rearrangement. The enolate Claisen rearrangement of allyl esters of dipeptides is under chelation control by the addition of MnClj. 

Claisen rearrangement. Allyl esters of A-acylglycine form chelates on treatment with LDA and NiCl2. Such chelates undergo stereoselective rearrangement. ... 

Tab. 5.2.1 Chelate Claisen rearrangement of a-hydroxy allylic esters 1.

Chelate Claisen Rearrangements of Metal Enolates of a-Alkoxy Allyl Esters... 

Chelate Enolate Claisen Rearrangement 261 Tab. 5.2.11 Rearrangement offluorinated allylic ester 115. 

Regarding the yield, the Ireland-Qaisen rearrangement of the silylated allylic ester 117a under standard conditions was superior to the chelate Claisen rearrangement discussed in the next chapter. 

Chelate Enolate Claisen Rearrangement 263 Irdand-Claisen Rearrangement of Cyclic Allylic Esters... 

Deprotonation of N-protected amino acid allylic esters such as 123 with LDA at -78 °C, and subsequent addition of a metal salt (MX ), presumably results in the formation of a chelated metal enolate 123, which undergoes Qaisen rearrangement upon warming to room temperature, giving rise to unsaturated amino acid 124 (Scheme 5.2.36) [66]. 

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. 

The chelated ester enolate Claisen rearrangement of allylic glycinates 9 is carried out with zinc(II) chloride, which is added to the enolate at — 78 C (Table 19). The rearrangement occurs as the reaction mixture is allowed to warm to room temperature over 1 hour. The 2-amino-3,3-difluoro-4-[(2-methoxyethoxy)methoxy] alk-4-enoic acids 10 arc converted directly into the corresponding methyl esters, which can be hydrolyzed to the methyl 2-amino-3,3-difluoro-4-oxoal-kanoates. 

Table 19. Synthesis of 2-Amino-3,3-difluoro-4-[(2-methoxyethoxy)meth-oxy]alk-4-cnoic Acids 10 by the Chelated Ester Enolate Claisen Rearrangement of Allylic Glycinates 943...

The presence of a benzyloxymethyl substituent at the allylic position of ester (202) leads to a predominance of the E)-syn rearranged product with Hf and Ti enolates (Table 18, entries 10 and 11). In these cases favorable chelation between the ester enolate and the benzyloxy oxygen stabilizes transition state (X Scheme 15). When the benzyloxy substituent is more remote (Table 18, entries 12-14) such chelation is less favorable and the reaction proceeds through transition state (U). 

The synthesis of Cbz-protected D-valine methyl ester (296) (Scheme 40) begins with addition of an organometallic reagent to the ester function of 282. The resulting phosphonate 290 undergoes a Wittig reaction with isobutyraldehyde to afford 291. Chelation-controlled reduction of the ketone with zinc borohydride furnishes the a /-alcohol 292 (98% de). A [3,3] rearrangement of trifluoroacetimidate 293 produces allylic amine 294. Elaboration of the olefin to an ester furnishes the D-valine derivative 296 with 85% ee [101]. 

Alternatively, the allyl sulphinyl anion (8) gave the threo adduct (9), which underwent sulphoxide - sulphinate ester rearrangement, leading to the hex-2-enitol derivative (10)(Scheme 4) the threo isomer predominated by non-chelation control, which has previously been difficult to achieve. ... 

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