Dicyclopropyl ketimines

The dicyclopropyl ketimine 198 prepared from the 0,N-cyclopropanone hemiacetal 194 (vide supra, Sect. 4.9, Eq. (62)), heated in xylene with ammonium chloride for 4 hr underwent ring expansion exclusively to the enamine 399 followed by isomerization to the cyclopropyl pyrroline 400. Although further ring expansion was not observed on prolonged heating, 400 was converted to the hydrobromide 401 with anhydrous HBr 2091 which upon heating to 140 °C for 10 min experienced a second cyclopropyl imine rearrangement to provide the pyrrolizidone 403 in 51 % yield, most probably via the HBr adduct 401 by cyclization to the pyrroline 402 followed by acid-induced hydrolysis, Eq. (95) 129). 

Recently, dicyclopropyl ketimines, which allow preparation of pyrrolizidinones, have been investigated (equation 31). The example depicted demonstrates that the cyclopropane ring with the piperidino group is considerably slower in the ring enlargement and therefore needs more drastic conditions (equimolar amounts of HBr) to occur. 

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