Reactions of Azomethine Ylides

For the reactions of other 1,3-dipoles, the catalyst-induced control of the enantio-selectivity is achieved by other principles. Both for the metal-catalyzed reactions of azomethine ylides, carbonyl ylides and nitrile oxides the catalyst is crucial for the in situ formation of the 1,3-dipole from a precursor. After formation the 1,3-di-pole is coordinated to the catalyst because of a favored chelation and/or stabiliza-... 

Cobalt, Manganese, and Silver Catalysts for Reactions of Azomethine Ylides... 

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... 

Dipolar cycloaddition reaction of azomethine ylides to alkynes or alkenes followed by oxidation is one of the standard methods for the preparation of pyrroles.54 Recently, this strategy has been used for the preparation of pyrroles with CF3 or Me3Si groups at the (3-positions.55 Addition of azomethine ylides to nitroalkenes followed by elimination of HN02 with base gives pyrroles in 96% yield (Eq. 10.48).56... 

Tominaga and coworkers have reported the formation of indolizine by the reaction of azomethine ylide with l-nitro-2-phenylthioethylene (Eq. 10.86).146... 

Diastereoselective reactions of azomethine ylides with chiral vinyl sulfoxides have also been conducted (Scheme 12.35) (162-164). The 1,3-dipolar cycloaddition of (R)s-p-tolyl vinyl sulfoxide (106) with l-methyl-3-oxidopyridinum (105) gave three of the four possible diastereomers, and one of these isomers 107 was used for the enantioselective synthesis of the (75)-(—)-2a-tropanol 108 (162). 

For intramolecular 1,3-dipolar cycloadditions, the application of nitrones and nitrile oxides is by far most common. However, in increasing frequency, cases intramolecular reactions of azomethine ylides (76,77,242-246) and azides (247-259) are being reported. The previously described intermolecular approach developed by Harwood and co-workers (76,77) has been extended to also include intramolecular reactions. The reaction of the chiral template 147 with the alkenyl aldehyde 148 led to the formation of the azomethine ylide 149, which underwent an intramolecular 1,3-dipolar cycloaddition to furnish 150 (Scheme 12.49). The reaction was found to proceed with high diastereoselectivity, as only one diaster-eomer of 150 was formed. By a reduction of 150, the proline derivative 151 was obtained. 

Ammonium ylides generated in this way are useful as a route to alkenes. Of more interest, nonstabilizcd immonium ylides can be prepared in situ from imines and trapped with a dipolarophile. This sequence provides a route to pyrrolines by reaction of azomethine ylides with acetylenedicarboxylic acid esters. 

A one-pot, double intramolecular 1,3-dipolar cycloaddition reaction of azomethine ylides was developed by reaction of 4 equiv of an O-allyl salicyl-adehyde with a fluorous amino ester under microwave heating to generate a novel hexacyclic ring system 13 that contains seven stererocenters (Scheme 15) [45]. 

Several organofullerene donor-acceptor molecular material hybrid systems have been synthesized via 1,3-dipolar cycloaddition reactions of azomethine ylides, via Bingel cyclopropanation and methanofullerene formation intermediates as well as via cycloaddition reactions, that have already been discussed in previous sections. The majority of such hybrid systems possess always as acceptor unit the fullerene core and as donor moieties porphyrins, tetrathiafulvalenes, ferrocenes, quinones, or electron-rich aromatic compounds that absorb visible light [190-193]. The most active research topic in this particularly technological field relies (i) on the arrangement of several redox-active building blocks in... 

Coldham, I. and Hufton, R. (2005) Intramolecular dipolar cydoaddition reactions of azomethine ylides. Chemical Reviews, 105, 2765-2810. 

In a subsequent study a number of fulleroproline-containing dipeptides and other derivatives, such as 201 and 202 (Scheme 1.15), were prepared and isolated as optically pure diastereoisomers or resolved into the enantiomers.241 The most distinctive feature in the CD spectra of these adducts is a sharp extremum at 428 nm, the sign of which is characteristic of the absolute configuration of the newly generated stereogenic center at the a-C-atom of the proline unit. To establish this relationship, the absolute configuration of the isolated diastereoisomers 201 and 202 (Scheme 1.15), synthesized by reaction of azomethine ylides generated from enantiomerically pure 203 and... 

We have too little information to understand the reactivity of azomethine ylides toward nonactivated acetylenes since acetylenes bearing no electron-withdrawing substituent(s) have rarely been utilized in cycloaddition trapping of azomethine ylides. To the best of our knowledge, the only example is the reaction of azomethine ylide 7, stabilized by two ester moieties, with acetylene itself in acetone at 125°C, producing pyrrole 210 after dehydrogenation with chloranil (66TL397). 

Table VIII summarizes the cycloaddition reactions of azomethine ylides with maleimides and maleic anhydrides. The upper part of the table involves endo-selective cycloadditions and the lower part nonselective reactions. Endo selectivity is closely related to the substitution pattern of the azomethine ylides used. The azomethine ylides that show high endo selectivity to these cyclic olefins bear an ylide-stabilizing substituent of the carbonyl or cyano type (EWG) at one carbon and an aryl, heteroaryl, acyl, or 1-alkenyl substituent (R ) at the other. These key substituents must be present for high endo selectivity, although the substituent (R) on the ylide nitrogen is equally important.

The reaction of azomethine ylides with cyclopropenones afforded pyridin-4(l//)-ones 3. ... 

Scheme 3.28 1,3 Dipolar cycloaddition reaction of azomethine ylides with maleate.

Chiral induction in cycloaddition reactions of azomethine ylide has been achieved when the a-amino acid (233) reacts with isatin or benzthiophen derivatives (234 X = NH, NBu, S) and menthyl acrylate. The course of the reactions involves decarboxylative azomethine ylide formation followed by cycloaddition through an endo transition state ultimately affording the cycloadduct (61) (Table 9) in 67-82% diastereomeric excess <91TL5417>. 

Dipolar cycloaddition reactions of azomethine ylides are probably the most widely used reactions for functionalization of [60]fullerene. This reaction has also been used to obtain SWNT derivatives and, occasionally, microwave irradiation has been used [89]. Pyrrolidino-SWNT 76 was synthesized by reaction of pristine... 

Dipolar cycloaddition reactions are interesting processes for the preparation of several heterocyclic systems due to the generally mild reaction conditions and the simultaneous formation of several bonds in a single operation. The preparation of pyrrolidines via cycloaddition reactions of azomethine ylides is a well known process and has been extensively studied. A strategy based on the reaction of azomethine ylides with a,P-unsaturated ketones was selected by Hollinshead... 

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... 

The 1,3-dipolar cycloaddition reaction of azomethine ylides with thioketones has been used to prepare 1,3-thiazoIidines. The metallated azomethine ylides 67 were generated in situ by treating a-amino acid ester imines 66 with lithium bromide and DBU. The ylides were then treated with highly reactive thioketones such as thiobenzophenone or fluorene-9-thione, to afford 1,3-thiazolidine derivatives 68 (main isomer) and 69 (minor isomer) in good yield and in diastereoisomeric ratios of between 2 1 and 4 1 <01H(55)691>. 

Intramolecular dipolar cycloaddition reactions of azomethine ylides 05CRV2765. 

Reactions of Azomethine Ylides Derived from Aldimines... 

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