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.
已报道了necine碱(-)-hastanecine(
吡咯烷
生物碱hastacine和punctanecine的亚基)的正式合成。关键中间体7由28.3%总产率的
L-苹果酸衍
生物二甲基酯11通过五个步骤合成。通过在二甲基酯11上进行Mitsunobu反应,获得了目标化合物C7的正确绝对立体
化学。其余立体中心(C1和C7a)所需的立体
化学来自硝酮9(通过完全位控获得)与
马来酸二甲酯的关键环加成反应中高度优选的外消旋方法。