Cycloaddition of an enantiopure cyclic nitrone to maleate: Straightforward synthesis of the necine base (-)-hastanecine

A formal synthesis of the necine base (-)-hastanecine, subunit of the pyrrolizidine alkaloids hastacine and punctanecine, is reported. The key intermediate 7 is synthesized in five steps from dimesylate 11, derived from L-malic acid, in 28.3% overall yield. The correct absolute stereochemistry at C7 in the target compound is attained by means of a Mitsunobu reaction on dimesylate 11. The desired stereochemistry at the remaining stereogenic centres (C1 and C7a) derives from a highly preferential exo-anti approach in the key cycloaddition of nitrone 9 (obtained with complete regiocontrol) to dimethyl maleate.