1.3- diploar cycloaddition

Oxidation of oxime 422 with aqueous sodium hypochlorite has been used to synthesize the central piperidine ring of the tricyclic system 423 in moderate yield, which presumably proceeds via an intramolecular 1,3-diploar cycloaddition of the intermediate nitrile oxide (Equation 114) <2000EJ0645>. 

Fukushima and Ibata reported direct evidence for the formation of an acyl nitrile ylide intermediate during Rh2(OAc)4-catalyzed reactions of a-diazoaceto-phenones 133 with nitriles in the presence of dimethylacetylene dicarboxylate (DMAD). The authors isolated mixtures of both 5-aryl-2-phenyloxazoles 137 and tetrasubstituted pyrroles 138 (Scheme 1.37). They proposed that 137 arose from the acyl nitrile yhde 136 via 1,5-cyclization, whereas 1,3-diploar-cycloaddition of 136 with DMAD followed by a 1,5-hydrogen shift produced 138. The authors showed that 138 did not arise via a Diels-Alder reaction of 137 with DMAD. 

The first synthesis of pentasubstituted pyrroles was achieved by using PylC (Ik) in the U-4CR on a solid phase [50], According to Mjalli and coworkers, pyridine was necessary as a cosolvent in the U-4CR as it helped buffer the reaction mixture and stabilize the isocyanide. The acyclic Ugi product 108 can be directly converted to the miinchnone 109 by heating (100°C) in acetic anhydride and trapped in situ with alkynes bearing electron-withdrawing groups via 1,3-diploar cycloaddition to afford pyrrole 110 (Scheme 7.29). 

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