Pyrrolines cyclopropylimines

An experimental and computational investigation into the photorearrangement of N-cyclopropylimines to yield pyrrolines (Scheme 122) has shown that the, regiochem-istry and stereochemistry can be understood in terms of a mechanism involving barrierless evolution in three different (S2, Si, and So) singlet states and sequential decay through two different (S2/S1 and S1/S0) conical intersection funnels.183... 

Campos, P.J., Soldevilla, A., Sampedro, D., and Rodriguez, M.A. (2001) N-Cyclopropylimine-1 -pyrroline rearrangement. A novel photochemical reaction. Organic Letters, 3, 4087 -089. 

Several related carbene complexes 28 with M = Cr, Mo, or W, have been shown to undergo photocycloadditions with alkenes, in particular acrylates, to furnish a series of 1 -pyrrolines 29 in moderate yields. The mechanism is believed to involve an initial cyclopropanation of the alkene, followed by a light-induced [l,3]-sigmatropic rearrangement of an intermediate N-cyclopropylimine <020M4076>. 

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). 

Af-Cyclopropylimines undergo rearrangement to cyclic imines (pyrrolines) under... 

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. 

Two total syntheses of sceletium alkaloid A4 (42) have been reported. The first, which involves a rearrangement of a cyclopropylimine to a pyrroline, is condensed in Scheme 12. The general pattern of the second (Scheme 13) is similar. ... 

Scheme 11.42 Diradical mechanism for iV-cyclopropylimine-pyrroline rearrangement.

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