Cyclopropylimines synthesis

For synthesis of certain alkaloids, the requisite cyclopropylimines were prepared by bisalkylation of the appropriately substituted benzyl cyanide. Cyanides could be selectively reduced in high yield to the corresponding aldehydes by employing diisobutylaluminium hydride. Treatment of these aldehydes with a primary amine completes the synthesis of the imines. Rearrangement191,432-435 with acids afforded the 2-pyrrolines in high yield (equation 26). 

The cyclopropylimine ynoline rearrangement (equation 68) has been exploited by Stevens in alkaloid syntheses. - Wenkert s cyclopropylcarbinyl rearrangement (equation 69) served extremely well in the design of 1,4-dicarbonyl synthons or 3,-y-unsaturated carbonyl compounds which then were expressed in numerous syntheses of terpenoid and alkaloid natural products. Donor-acceptor concepts continue to be express in the applicability of these rearrangements to organic synthesis. 

Although prolonged heating of the pyrroline (192) does not lead to further ring expansion, a second cyclopropylimine rearrangement takes place on heating in the presence of anhydrous hydrobromic acid at 140°C for 10 min, giving after hydrolysis the pyrrolizidinone (193) (equation 128). This sequence opens a route to pyrrolizidines that are of obvious interest in natural product synthesis. 

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