2-(3,4-Dichlorophenyl)-N-methyl-N-[2-(1-pyrrolidinyl)-1-substituted- ethyl]-acetamides: the use of conformational analysis in the development of a novel series of potent opioid kappa agonists

This paper describes the synthesis of a series of N-[2-(1-pyrrolidinyl)ethyl]acetamides (1), methylated at C1 and/or C2 of the ethyl linking group, and their biological evaluation as opioid kappa agonists. Conformational analysis of corresponding desaryl analogues 2 suggested that only those compounds capable of occupying an energy minimum close to that of the known kappa agonist N-[2-(1-pyrrolidinyl)cyclohexyl] acetamide U-50488 might possess kappa agonist properties. Starting from chiral Amino acids, other alkyl and aryl substituents were introduced at C1 of the ethyl-linking moiety, giving compounds capable of adopting the same conformation as U-50488. The most potent of these, 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-phenyl-2-(1-pyrrolidinyl)ethyl] acetamide (8), was 146-fold more active than U-50488 in vitro in the mouse vas deferens model and exhibited potent naloxone-reversible analgesic effects (ED50 = 0.004 mg/kg sc) in an abdominal constriction model.