Enantioselective synthesis of indolizine derivatives by rearrangement-cyclization of isoxazoline-5-spirocyclopropanes
作者:Ernesto G. Occhiato、Antonio Guarna、Laura Michela Spinetti
DOI:10.1016/s0040-4020(01)81553-2
日期:1993.1
(3S,7S,8aS)-3-Methyl-7-hydroxyoctahydroindolizine (13) was prepared with an enantiomeric excess of 74%. in five steps starting from enantiomerically pure (S)-(+)-5-nitro-2-pentanol (5). (3S,5S,7S,8aS)-(-)-3-Methyl-5-phenyl-7-hydroxyoctahydroindolizine (15) and (3S,5R,7R,8aR)-(+)-3-methyl-5-phenyl-7-hydroxyoctahydroindolizine (16) were also obtained starting from (5) with an ee of 96%. The synthetic strategy required the enantioselective enzymatic reduction of 5-nitro-2-pentanone (4) to 5 (> 99% ee) and its conversion to (R)-(-)-2-chloro-5-nitropentane (7) (> 90% ee). Cycloadditions of the corresponding nitrile oxide prepared in situ from 7 with methylenecyclopropane (8) or 1-methylene-2-phenylcyclopropane (9) produced chiral isoxazolines 1 and 2, which were converted by thermolysis to 2,3,5,6-tetrahydro-3-methylindolizin-7(1H)-one (10), and 2,3,5,6-tetrahydro-3-methyl-5-phenylindolizin-7(1H)-ones (11) and (12) respectively. The enantioselectivity of the thermal rearrangement is dependent on the experimental conditions and on the structures of the chiral isoxazolines. Catalytic hydrogenation of the indolizinones 10, 11 and 12, afforded the substituted hydroxyoctahydroindolizines 13, 15 and 16 with high stereoselective control of all stereogenic centers.
(3S,7S,8aS)-3-甲基-7-羟基八氢吲哚嗪(13)以(3S,7S,8aS)-3-甲基-7-羟基八氢吲哚嗪的形式制备,其对映体过量为74%,从旋光纯的(S)-(+)-5-硝基-2-戊醇(5)开始,通过五步反应合成。同样地,从(5)开始,还获得了(3S,5S,7S,8aS)-(-)-3-甲基-5-苯基-7-羟基八氢吲哚嗪(15)和(3S,5R,7R,8aR)-(+)-3-甲基-5-苯基-7-羟基八氢吲哚嗪(16),其ee为96%。合成策略需要5-硝基-2-戊酮(4)通过酶促不对称还原生成(5)(> 99% ee),随后将其转化为(R)-(-)-2-氯-5-硝基戊烷(7)(> 90% ee)。将7原位制备的腈亚胺与甲基环丙烷(8)或1-甲基-2-苯基环丙烷(9)进行环加成,产生手性异恶唑烷1和2,随后通过热裂解分别转化为2,3,5,6-四氢-3-甲基吲哚嗪-7(1H)-酮(10)、2,3,5,6-四氢-3-甲基-5-苯基吲哚嗪-7(1H)-酮(11)和(12)。热重排的对映选择性取决于实验条件和手性异恶唑烷的结构。催化氢化吲哚嗪酮10、11和12,得到取代的羟基八氢吲哚嗪13、15和16,对所有手性中心均具有高度的立体选择性控制。