Enantioselective Syntheses of 2-Alkyl-, 2,6-Dialkylpiperidines and Indolizidine Alkaloids Through Diastereoselective Mannich-Michael Reactions
作者:Markus Weymann、Waldemar Pfrengle、Dieter Schollmeyer、Horst Kunz
DOI:10.1055/s-1997-3185
日期:1997.10
Aldimines of 2,3,4,6-tetra-0-pivaloyl-β-D-galactosylamine react with 1-methoxy-3-trimethylsilyloxybuta-1,3-diene in a Mannich-Michael condensation reaction sequence to give 2-substituted N-galactosyl-5,6-dehydropiperidin-4-ones 3 with high diastereoselectivity. The X-ray analysis of the 2-propyl derivative 3a proved (R)-configuration of the major diastereomer and led to the correction of our earlier assignment of configuration for (-)-coniine hydrochloride 9a obtained from this intermediate. Despite their low reactivity, these enaminones 3 can be converted into chiral 2,6-cis-disubstituted piperidinones 12 with high stereoselectivity by reaction with organocuprates in combination with hard electrophiles. Enantiomerically pure alkaloids such as (-)-dihydropinidine and gephyrotoxine 167B have been synthesized according to this methodology.
2,3,4,6-四-O-特戊酰基-β-D-半乳糖胺的烯胺与1-甲氧基-3-三甲基硅氧基丁-1,3-二烯在曼尼希-迈克尔缩合反应序列中反应,以高非对映选择性得到2-取代的N-半乳糖基-5,6-脱氢哌啶-4-酮3。2-丙基衍生物3a的X射线分析证实了主要非对映异构体的(R)构型,并纠正了我们之前对该中间体得到的(-)-毒芹碱盐酸盐9a的构型分配。尽管这些烯胺酮3的反应性较低,但通过与有机铜酸盐和硬亲电试剂的反应,可以高立体选择性地将其转化为手性的2,6-顺式-二取代哌啶酮12。根据这一方法,已经合成了高对映纯度的生物碱,如(-)-二氢矮牵牛素和gephyrotoxine 167B。