Chelation allylic esters

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

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

A wide range of carbon, nitrogen, and oxygen nucleophiles react with allylic esters in the presence of iridium catalysts to form branched allylic substitution products. The bulk of the recent literature on iridium-catalyzed allylic substitution has focused on catalysts derived from [Ir(COD)Cl]2 and phosphoramidite ligands. These complexes catalyze the formation of enantiomerically enriched allylic amines, allylic ethers, and (3-branched y-8 unsaturated carbonyl compounds. The latest generation and most commonly used of these catalysts (Scheme 1) consists of a cyclometalated iridium-phosphoramidite core chelated by 1,5-cyclooctadiene. A fifth coordination site is occupied in catalyst precursors by an additional -phosphoramidite or ethylene. The phosphoramidite that is used to generate the metalacyclic core typically contains one BlNOLate and one bis-arylethylamino group on phosphorus. 

Some chelating ligands provide a high to excellent enantioselectivity in the alkylation of cyclic allylic esters. In 1995, Minami and Helmchen independently found some /3-phosphinocarboxylic acids 121 and 122 as effective chiral ligands for the allylic alkylation (Equations (24) d08a (25) ). Helmchen and Kudis also discovered... 

Allyl chloride is used to make intermediates for downstream derivatives such as resins and polymers. Approximately 90% of allyl chloride production is used to synthesize epichlorohydrin, which is used as a basic building block for epoxy resins and in glycerol synthesis. Allyl chloride is also a starting material for allyl ethers of phenols, bisphenol A and phenolic resins, and for some allyl esters. Other compounds made from allyl chloride are quaternary amines used in chelating agents and quaternary ammonium salts, which are used in water clarification and sewage sludge flocculation (Kneupper Saathoff, 1993). 

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. 

Besides allylic alkylation reactions chelated zinc ester enolates 231 also give good results in various types of standard enolate reactions, including alkylations, aldol reactions" and Michael additions" . 

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

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

Waldmann and coworkers [104, 264, 289] and Cativiela and coworkers [286] proposed the use of acrylamides of a-aminoesters as dienophiles. Phenylalanine, valine and isoleucine derivatives lead to mediocre selectivities, but cy-doadditions of various dienes with a,P-unsaturated amides of (S)-proline benzyl or allyl esters 9.66 at 0°C are stereoselective. Obtaining different isomers depends on whether the Lewis acid is EtAlCh or TiCLj. These results have been interpreted by the approach of the diene to the least hindered face of either a monodentate complex (EtAlC ) or a chelate (TiC ). In both cases, the unsaturated amide is under... 

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

In 1984 Fujisawa et al. observed a directing effect due to a CT oxygen stereocenter (Scheme 4.29) [32]. Tandem deprotonation and silylation of the j8-hydroxy allylic ester presumably gave the intermediate sUyloxy silyl ketene acetal. The Z-config-uration of the silyl ketene acetal was a consequence of chelation of the intermediate dianion by the Id cation. The authors postulated that the allylic alkene prefer-... 

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... 

Like a-aUcoxy substituted allylic esters, the a-amido substituted analogues are also able to form chelated ester enolates such as 122 (Fig. 5.2.3). 

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]. 

In contrast to the well-investigated chelation controlled ester enolate Claisen rear-rangments, the corresponding rearrangement of N-allylated amides is much less appUed. Tsunoda et al. evaluated the synthetical potential of N-crotyl glycolamides and glycinamides 215 [125] and the results obtained are listed in Table 5.2.23. 

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