133733-27-4Relevant academic research and scientific papers
Alkyl substituent effects on pipecolyl amide isomer equilibrium: Efficient methodology for synthesizing enantiopure 6-alkylpipecolic acids and conformational analysis of their N-acetyl N'-methylamides
Swarbrick, Martin E.,Gosselin, Francis,Lubell, William D.
, p. 1993 - 2002 (1999)
Enantiopure 6-alkylpipecolic acid hydrochlorides 1a-e were synthesized in five steps and 15-59% overall yields from α-tert-butyl β-methyl N- (PhF)aspartate (3) via an approach featuring selective hydride reduction to the corresponding aspartate β-aldehyde 2, aldol condensations with the enolates of various methyl alkyl ketones, and diastereoselective intramolecular reductive aminations. The influence of the 6-position substituent on the equilibrium and the energy barrier for isomerization of the amide N-terminal to pipecolate was then explored via the synthesis of N- acetyl N'-methylpipecolinamide (16) and its (2S,6R)-6-tert- butylpipecolinamide counterpart 17, and their conformational 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 of the bulky 6-alkyl substituent on the acetamide geometry and isomerization barrier were less pronounced in the pipecolate series relative to the respective proline amides.
