Alkyl Substituent Effects on Pipecolyl Amide Isomer Equilibrium: Efficient Methodology for Synthesizing Enantiopure 6-Alkylpipecolic Acids and Conformational Analysis of Their <i>N</i>-Acetyl <i>N</i>‘-Methylamides
作者:Martin E. Swarbrick、Francis Gosselin、William D. Lubell
DOI:10.1021/jo982181h
日期:1999.3.1
analysis by proton NMR spectroscopy and coalescence experiments. The presence of the tert-butyl substituent augmented the population of the amide cis-isomer and lowered the barrier for pipecolyl amide isomerization in water. Compared with the results from our previous examination of N-acetyl-5-tert-butylproline N'-methylamides (Beausoleil, E.; Lubell, W. D. J. Am. Chem. Soc. 1996, 118, 12902), the consequences
通过选择性地将氢化物还原为相应的天冬氨酸的方法,以五个步骤合成了对映体纯的6-烷基胡椒酸盐酸盐1a-e,总产率为15-59%,由α-叔丁基β-甲基N-(PhF)天门冬氨酸(3)合成β-醛2,与各种甲基烷基酮的烯醇式缩醛醇缩合,以及非对映选择性分子内还原胺化。然后,通过合成N-乙酰基N'-甲基哌嗪酰胺(16)及其(2S,6R)-,探索了6位取代基对酰胺N端异构化为哌酸酯的平衡和能垒的影响。 6-叔丁基哌嗪酰胺对应物17,并通过质子NMR光谱和聚结实验对它们进行构象分析。叔丁基取代基的存在增加了酰胺顺式异构体的数量,并降低了在水中胡椒基酰胺异构化的障碍。与我们之前对N-乙酰基-5-叔丁基脯氨酸N'-甲基酰胺的检测结果相比(Beausoleil,E.; Lubell,WDJ Am。Chem。Soc。1996,118,12902),笨重的结果6相对于相应的脯氨酸酰胺,在胡椒酸酯系列中,乙酰胺几何形