Oxazolines azomethine ylide generation

A possibility for azomethine ylide generation from 4-isoxazolines was first suggested by Baldwin, who demonstrated a thermal N—O bond cleavage of 4-isoxazoline systems (68JA5325). Cycloaddition of N-methylene(t-butyl)-amine V-oxide to dimethyl acetylenedicarboxylate takes place at 0°C, rapidly and quantitatively, to give 4-isoxazoline 146 (R = t-Bu, =R = COOMe), which isomerizes at 80°C into 4-oxazoline 148 (R = f-Bu, R = R = COOMe). The nitrone bearing an N-aryl substituent and the same acetylene directly afford 2-acylaziridine 147 (R = 2,4,6-trimethylphenyl, R = R = COOMe). Reaction of the N-t-butylnitrone with 3-methylbutyn-... 

Vedejs and Grissom" described the synthesis and reactions of azomethine ylides generated from 4-oxazolines. They found that alkylation of a 2,5-disubsti-tuted or a 2,4,5-trisubstituted oxazole 15 gave an A -alkyloxazolium salt, which was... 

A series of publications by Vedejs et al. (17-19) outlined the use of oxazolines in the generation of azomethine ylides. Reduction of the oxazolium salts 71 (R = Ph or Me) with PhaSiH/CsF led to formation of the unstable 4-oxazolines (72) and their valence bond azomethine ylide tautomers (17), which could be trapped in situ with DM AD to afford the bicyclic adducts 73, where R and R = Ph or Me, in 75% yield via [2 + 2] cycloaddition (Scheme 3.18). 

Generation of azomethine ylides by starting from oxazoline heterocycles consists of the following steps (1) valence isomerization of 4-isoxazolines, (2) valence isomerization of 4-oxazolines, and (3) cycloreversion of 5-oxazo-lidinones or oxazolidines. These three generation methods as variations of the oxazoline route are reviewed in this section. 

The last variation of the oxazoline route is a cycloreversion of 5-oxazoli-dinones or oxazolidines. Thermal decarboxylation of 5-oxazolidinones is a convenient method of generating nonstabilized azomethine ylides, since the heterocyclic precursors are accessible, and sometimes isolable, by simply heating a-amino acids and carbonyl compounds. This has been discussed in Section II,E. 

Generation of azomethine ylides by starting from oxazoline heterocycles consists of the following steps (1) valence isomerization of 4-isoxazolines,... 

Bis-oxazolines of much use in both the Diels-Alder and nitrone cycloadditions, have also been shown, by j0rgensen and coworkers, to be good ligands in the azomethine ylide cycloaddition. In particular, the zinc complex formed with Bu-BOX ligand (8.68) and zinc(II) triflate has been used to effect good to high ees in the reaction of ylides generated from iminoesters such as (8.202) with acrylates. 

A comparison of methods to generate azomethine ylides via 4-oxazohne ring opening vs. aziridine thermolysis has shown that the oxazoline method can produce the same azomethine dipole under kinetic control as the aziridine method under thermodynamic... 

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