Hydrogenation of pyrrolizin-3-ones; new routes to pyrrolizidines.

Pyrrolizin-3-ones (e.g. 1) can be easily hydrogenated to their hexahydro (pyrrolizidin-3-one) derivatives in the presence of heterogeneous catalysts. Good diastereoselectivity (up to >97:3, depending on catalysts and solvent) can be achieved if the pyrrolizin-3-one is substituted at the 1- (or 7-) position(s), but the selectivity is reduced if both positions are substituted. Subsequent deoxygenation of the pyrrolizidin-3-ones provides concise, diastereoselective routes to the necine bases (+/-)-heliotridane 5, (+/-)-isoretronecanol 6 and (+/-)retronecanol 7.