Dimethyl acetylenedicarboxylate carbonyl ylide generation

In the dipole cascade reaction, a proton must be removed from the a-carbon atom in order to generate the azomethine ylide. When the a-position of the pyrrolidine ring was blocked by a benzyl group, formation of the azomethine ylide dipole could not occur. In fact, treatment of diazoketone 186 with rhodium(II) acetate in the presence of dimethyl acetylenedicarboxylate afforded only the carbonyl ylide-derived cycloadduct 187 in 95% yield [117]. 

One recent example of applying this process was reported by Johnson et al. for the preparation of furan derivatives (Scheme 16.12) [19]. This reaction involved the rhodium(II)-catalyzed intermolecular generation of carbonyl ylides A or B from diazosulfone (18) and aldehydes 19 or 21, respectively. The 1,3-dipolar cycloaddition of the resulting carbonyl ylides with an intramolecular tethered alkyne moiety or dimethyl acetylenedicarboxylate (DMAD, an intermolecular process), followed by elimination of phenylsulfinic acid, could access furan derivatives. 

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