Azomethine ylides generation from aziridines

The azomethine ylides, generated from aziridines 1, xmderwent 1,3-dipolar cycloaddition wiffi dimethyl acetyl-enedicarboxylate in supercritical CO2 (Scheme 1) [17]. Under photolysis conditions, dihydropyrrole 2 was formed with 43% yield. It was foxmd that using small quantity of acetonitrile as a cosolvent improved the yield to 63%. The photolysis of similar aziridine (1, = Bn, = Ph) was not efficient and the thermolysis was used to generate a reac-... 

The similar [3 + 2] cycloaddition of benzyl allenoates with azomethine ylides, generated from aziridines 1, was accompanied by a retro-aldol-type fragmentation with elimination of benzaldehyde and therefore leading to ffie... 

P.J.S. Gomes, C.M. Nimes, A.A.C.C. Pais, T.M.V.D. Pinho e Melo, L.G. Arnaut, 1,3-EHpolar cycloaddition of azomethine ylides generated from aziridines in supercritical carbon dioxide. Tetrahedron Lett. 47 (2006) 5475-5479. 

Garner et al. (90,320) used aziridines substituted with Oppolzer s sultam as azomethine ylide precursors. The azomethine ylide generated from 206 added to various electron-dehcient alkenes, such as dimethyl maleate, A-phenylmalei-mide, and methyl acrylate, giving the 1,3-dipolar cycloaddition product in good yields and up to 82% de (for A-phenylmaleimide). They also used familiar azomethine ylides formed by imine tautomerization (320). Aziridines such as 207 have also been used as precursors for the chiral azomethine ylides, but in reactions with vinylene carbonates, relatively low de values were obtained (Scheme 12.59) (92). 

Although trifluoropropene reacts poorly with the stabilized azomethine ylide generated by thermal ring opening of an aziridine, trifluoromcthyl-substituted alkenes are excellent dipolarophiles with azomethine ylides generated from organosilylated compounds, or with melalloazomethine ylides. 

Azomethine ylides (Section 4.03.6.1.1) have been generated from a wide variety of aziridines using both thermal and photochemical methods. With carbon-carbon unsaturated dipolarophiles, pyrrolines or pyrrolidines are obtained. With hetero double bonds, however, ring systems of interest to this discussion result. 

The chiral dipolarophiles of Garners and Dogan, which were derived from Oppolzer s sultam, have been previously discussed in Section 3.2.1 and, in an extension to these results, the sultam moiety was used as the stereodirecting unit in enantiopure azomethine ylides (56). The ylides were generated either by thermo-lytic opening of N-substituted aziridines or by the condensation of the amine functionality with benzaldehyde followed by tautomerism. These precursors were derived from the known (+)-A-propenoylbornane-2,10-sultam. Subsequent trapping of the ylides with A-phenylmaleimide furnished the cycloaddition products shown in Schemes 3.60 and 3.61. 

The thermolytic preparation by De Shong et al. (74) of azomethine ylides from aziridines and their intermolecular reactions are the first examples of singly stabilized ylides of this type. However, the protocol has been further extended to include intramolecular processes. Aziridines tethered to both activated and unactivated alkenes were subjected to flash vacuum thermolysis generating cycloadducts in moderate-to-excellent yields. While previously singly activated alkenes had furnished low material yields via an intermolecular process, the intramolecular analogue represents a major improvement. Typically, treatment of 222 under standard conditions led to the formation of 223 in 80% yield as a single cis isomer. Similarly, the cis precursor furnished adduct 224 in 52% yield, although as a 1 1 diastereomeric mixture (Scheme 3.77). 

Azomethine ylides can be generated from oxazolidines in the liquid phase by thermolysis and in the gas phase by FVP (90TL6017 92T8947). Depending on the other substituents and the FVP conditions, from 2-isopropyloxazolidines (41) either aziridines or enamines, or a mixture of them, are obtained by ring closure or hydrogen shift of the azomethine ylide. 

When reacted with 1-aryl-3,3,3-trifluoropropynes, azomethine ylide 14, generated from the aziridine, provides pyrrolines in high yield, with no rcgioseleclivily. These cycloadducts can be converted into the corresponding pyrroles or pyrrolidines. ... 

Stereochemical studies on the ring-opened azomethine ylide 1,3-dipoles thermally or photolytically generated from sterically defined aziridines are important in order to learn the geometry of transient ylide species. Though the aziridine route has a limitation in that an appropriate ylide-stabilizing... 

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