Five-membered Rings containing more than One Nitrogen

Five-membered Rings Containing More than One Nitrogen. - The A2-pyrazolines (169) are produced in excellent yields by SnC -catalysed ring expansion of the azocyclopropanes (168), which are readily available via oxidative cyclization of the 

B-stannylhydrazones (167).10 A useful aspect of this procedure is that the two-step conversion of the hydrazones (167) into the pyrazolines (169) is stereospecific the cyclopropanation proceeds with inversion and the ring expansion with retention of configuration at the 6-carbon. The first synthesis of 4-imidazolidinone (171) has been reported by Nitta et al., by a 

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