Azomethine ylides, cycloaddition with substituted aziridines

Zhang and coworkers reported a Y(OTf)3-catalyzed formal [3 + 2]-cycloaddition of alkenes with azomethine ylide, providing highly substituted pyrrolidines with moderate to excellent di-astereoselectivity. The azomethine ylide was generated from a selective carbon-carbon bond cleavage of the corresponding A-tosyl aziridine under the mild reaction conditions (eq 13). ... 

The intermolecular reaction of imines with acceptor-substituted carbene complexes generally leads to the formation of azomethine ylides. These can undergo several types of transformation, such as ring closure to aziridines [1242-1245], 1,3-dipolar cycloadditions [1133,1243,1246-1248], or different types of rearrangement (Figure 4.9). 

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. 

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). 

With trifluoromethyl-substituted azomethine ylides, generated in situ by the ring opening of CAV-2-bcnzoyl-l -methyl-3-(trifluoromcthyl)aziridine (2)124 or by protonation of trifluoro-methylated thioamidinium salts, 25 1,3-dipolar cycloadditions with both electron-deficient and electron-rich alkenes can be achieved rcgiosclcctively. However, stereoselectivity depends on both the ylide configuration and electronic interactions. 

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