Azomethine ester derivatives

New Liquid Crystalline Poly(azomethine esters) Derived from PET Waste Bottles... 

Several variations and extensions of this HHT method have recently been reported. The mildness of this reaction was exemplified through the synthesis of glyphosate thiol ester derivatives 35. The requisite thioglycinate HHT 34 was prepared in high yield by a novel, methylene-transfer reaction between r-butyl azomethine and the ethyl thioglycinate... 

Dipolar addition to nitroalkenes provides a useful strategy for synthesis of various heterocycles. The [3+2] reaction of azomethine ylides and alkenes is one of the most useful methods for the preparation of pyrolines. Stereocontrolled synthesis of highly substituted proline esters via [3+2] cycloaddition between IV-methylated azomethine ylides and nitroalkenes has been reported.147 The stereochemistry of 1,3-dipolar cycloaddition of azomethine ylides derived from aromatic aldehydes and L-proline alkyl esters with various nitroalkenes has been reported. Cyclic and acyclic nitroalkenes add to the anti form of the ylide in a highly regioselective manner to give pyrrolizidine derivatives.148... 

Similar products could be generated via azomethine yhdes derived by a formal 1,2-H shift from the precusor imine, rather than by the decarboxylative approach outlined above (82,83). For example (83), condensation of aldehyde 272 with the requisite amino ester 273 led to the intermediate ylide, which dehvered adducts 274 and 275 in a 1.2 1 ratio in 75% yield. Grigg has once again applied this protocol to the synthesis of a wide range of complex molecular frameworks (276... 

In a similar study to that outlined by Grigg, Kanemasa et al. (68) has demonstrated the intramolecular cycloaddition of azomethine ylides derived from either amino acids or esters. Treatment of the amino methyl ester 302 with... 

The azomethine yhde derived from 79 has also been used in reactions with chiral ( )-y-alkoxy-a,p-unsaturated esters 80 (Scheme 12.27). The corresponding tetra-substituted pyrrolidines 81 were obtained with complete regiocontrol in fair to excellent de (125). 

For the examples reported so lar, usually in the presence of die alkene the azomethine ylide is generated in situ by photochemical disrotatory ring-opening of the aziridine (254 nm) in dioxane yielding pyrrolidines. (Obviously, thermal conrotatoiy ring-opening of aziridines demands conditions, which oppose an efficient stereocontrol of the subsequent cycloaddition with a,P-unsaturated carboxylic ester derivatives.)... 

The unconjugated azomethine chromophore in an aliphatic environment (RjR2C=NR3) shows a weak uv maximum at about 235 nm (a = ca. 100). In the case of the simple a-amino acid ester derivatives, a broad end absorption is observed with an apparent shoulder in the 220-230-nm region due to the two overlapping chromophores (—CH=N— and —COOH). The ORD spectra of a series of N-neopentylidene derivatives (49), obtained from a reaction of pivaldehyde (McaC—CHO) with amino acid esters, were measured... 

Scheme 1.10 Cydoaddition of azomethine ylides derived from imino esters with N-arylmaleimides.

In the same context, azomethine ylides derived from imino esters were reacted by Arai and Awata with methyleneindolinones in the presence of an in situ generated chiral nickel catalyst derived from their original chiral imid-azoline-aminophenol ligand 11. Remarkably, the process afforded the corresponding chiral spiro[pyrrolidine-3,3 -oxindoles] in excellent yields and exo selectivities, as well as enantioselectivities in almost all cases of substrates studied, as summarised in Scheme 1.11. Since the spiro-oxindole skeleton is... 

The 3 + 2-cycloaddition reaction of azomethine ylides with c-deficient alkenes produced polysubstituted l- and D-unnatural prolines. Also, phosphoramidite-(7u(OTf)2 complexes catalyse the 1,3-dipolar cycloaddition reactions of azomethine ylides with nitroalkenes to yield exo-tetrasubstituted proline esters." The 1,3-dipolar cycloaddition of non-stabilized azomethine ylides, from iV-alkyl-a-amino acids and aldehydes, with 3-substituted coumarins provides l-benzopyrano[3,4-c]pyrrolidines in good yields and high regio- and stereo-selectivity." The organocatalytic 1,3-dipolar cycloaddition of azomethine ylides, derived from azlactones, with methyleneindolinones produced spirooxindoles with high yields (up to 95%) and high diastereo- (93 7 dr) and enantioselectivity (98% ee). ... 

Coldham and coworkers reported a tandem condensation/cyclization/intramolec-ular cycloaddition cascade process to form fused tricychc amines, using azomethine ylides (derived from a-amino-acids or esters). This chemistry was apphed for the constmction of the pyrrolo[l,2-a]azepine ring system of the Stemona alkaloids (Scheme 4.20) [39]. Condensation of the aldehyde 96 bearing a dipolarophile (an... 

Dipolar [3 + 2] cycloadditions are one of the most important reactions for the formation of five-membered rings [68]. The 1,3-dipolar cycloaddition reaction is frequently utihzed to obtain highly substituted pyrroHdines starting from imines and alkenes. Imines 98, obtained from a-amino esters and nitroalkenes 99, are mixed together in an open vessel microwave reactor to undergo 1,3-dipolar cycloaddition to produce highly substituted nitroprolines esters 101 (Scheme 35) [69]. Imines derived from a-aminoesters are thermally isomerized by microwave irradiation to azomethine yhdes 100,... 

Azomethine ylides of pyrrolo[l,2- ]pyrazine <1996JOC4655> and 3,4-dihydro pyrrolo[l,2-tf]pyrazine <1997T9341> undergo 1,3-dipolar cycloadditions with a number of dipolarophiles. For example, the ylide 178 reacts with propargylic ester 179 to give the tricyclic derivative 180 (Equation 43). 

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