Lactams. XXIII. Thermal cis-trans isomerization of the 5-ethyl-2-oxo-4-piperidineacetic acid system: Effect of an N-substituent.
作者:TOZO FUJII、MASASHI OHBA、SHIGEYUKI YOSHIFUJI、SHIGEAKI AKIYAMA
DOI:10.1248/cpb.33.1062
日期:——
In order to investigate the effect of an N-substituent on thermal cis-trans isomerization of the 5-ethyl-2-oxo-4-piperidineacetic acid system, the N-alkyl analogs (±)-1b, c and (±)-2b, c and the N-(2-arylethyl) analogs (-)-1f, g, i and (+)-2f, g, i were separately heated neat at 180°C, and the propress of their cis-trans isomerization reactions was followed by determining the isomer ratios in the reaction mixtures according to the previously reported C-13 nuclear magnetic resonance spectroscopic method. It has been found that in all cases the reaction comes to equilibrium within 28-130 min, when the cis and trans isomers exist in a ration of 1 : 2. These results together with those obtained previously with the analogs (±)-1a, d, e, (-)-1h, (±)-2a, d, e, and (+)-2h indicate that a higher and/or bulkier N-substituent tends to decrease the rate of cis-trans isomerization of the 5-ethyl-2-oxo-4-piperidineacetic acid system, which is a useful synthon for the synthesis of benzo-[a] quinolizidine-type Alangium alkaloids. Among the substrates used in the present equilibration study, (±)-1b, c and (±)-2b, c were prepared from the corresponding N-unsubstituted lactam esters (±)-3 and (±)-7 by the"lactim ether method."
为了研究 N-取代基对 5-乙基-2-氧代-4-哌啶乙酸体系热顺反异构化的影响,将 N-烷基类似物 (±)-1b, c 和 (±)-2b, c 以及 N-(2-芳基乙基)类似物 (-)-1f, g, i 和 (+)-2f, g. i 分别在 180°C 下纯净加热,并根据之前报道的 C-13 核磁共振法测定反应混合物中的异构体比率,跟踪它们的顺反异构化反应的进展、i 分别在 180°C 温度下加热,并根据之前报告的 C-13 核磁共振光谱法确定反应混合物中的异构体比率,从而跟踪它们顺反异构化反应的进展情况。结果发现,在所有情况下,当顺式和反式异构体的比例为 1 : 2 时,反应在 28 至 130 分钟内达到平衡。这些结果以及之前在类似物 (±)-1a、d、e、(-)-1h、(±)-2a、d、e 和 (+)-2h 中获得的结果表明,较高和/或较宽松的 N 取代基往往会降低 5-乙基-2-氧代-4-哌啶乙酸体系的顺反异构化速率,而该体系是合成苯并-[a] 喹嗪类阿兰生物碱的有效合成物。在本平衡研究中使用的底物中,(±)-1b, c 和 (±)-2b, c 是通过 "乳醚法 "从相应的 N-未取代内酰胺酯 (±)-3 和 (±)-7 制备的。