468095-29-6

Proline-like 2,4-dialkyl-5-phosphonylpyrrolidines 2 were obtained stereoselectively by reduction of the corresponding β-iminophosphonates 1 with NaBH4. The detailed characterization of compounds 2 was accomplished by 1H and 13C NMR as well as by crystal structure analysis. When 1 was reduced with LiAlH4 the reaction gave dephosphonylated pyrrolidines 3. The latter method is suitable for providing substituted pyrrolidines regioselectively. A possible mechanism for the dephosphonylation is proposed.

In this work, we show that the tin hydride-mediated reaction of allene-tethered dithiosemicarbazides 4 is a convenient method for the preparation of five-membered unsaturated nitrogen heterocycles. The sulfur-directed intermolecular attack of the tin radical at the semicarbazide moiety leads to an allene-tethered iminyl radical, which then undergoes a 5-exo-dig cyclization leading to both the 3H-pyrroles 5 and the alkylidene pyrrolines 6; thermal isomerization of 5 to 6 occurs in some cases.