Abstract: The invention relates to chiral cyclic ?-amino acids of Formula (I) and their salts formed with pharmaceutically acceptable acids or bases, wherein the main meanings of the substituents are as follows: R stands for C1-4 Alk; X stands for —COOH, —CONH2, —CONH(C1-4 Alk), —CON(C1-4 Alk)2, —COO(C1-4 Alk), —COPhe-O—(C1-4 Alk) or —CH2OH; Y stands for —NH2, —NHBoc, —NHFmoc, —NH(C1-4 Alk), —N(C1-4 Alk)2, —NHCH2Ph, or Ar—NH—C(?X0)—N(R0)— wherein Ar stands for a phenyl group substituted by C1-4 alkoxy or halogen, X0 stands for O or S, and R0 stands for hydrogen or benzyl; and X+Y stands for —CONH— vagy —CON(Boc)-; with the proviso that when X stands for —COOH, then Y may be only different from —NH2. The invention also relates to pharmaceutical compositions having multidrug-resistance reversing effect that contain one or more compound(s) of Formula (I) or a salt thereof and inert pharmaceutical carriers and/or auxiliary agents. The invention also relates to carboxylic acids of Formula (XX) and their salts.

Abstract: The invention relates to chiral cyclic ?-amino acids of Formula (I) and their salts formed with pharmaceutically acceptable acids or bases, wherein the main meanings of the substituents are as follows: R stands for C1-4 Alk; X stands for —COOH, —CONH2, —CONH(C1-4 Alk), —CON(C1-4 Alk)2, —COO(C1-4 Alk), —COPhe-O-(C1-4 Alk) or —CH2OH; Y stands for —NH2, —NHBoc, —NHFmoc, —NH(C1-4 Alk), —N(C1-4 Alk)2, —NHCH2Ph, or Ar—NH—C(=X0)—N(R0)— wherein Ar stands for a phenyl group substituted by C1-4 alkoxy or halogen, X0 stands for O or S, and R0 stands for hydrogen or benzyl; and X+Y stands for —CONH— vagy —CON(Boc)-; with the proviso that when X stands for —COOH, then Y may be only different from —NH2. The invention also relates to pharmaceutical compositions having multidrug-resistance reversing effect that contain one or more compound(s) of Formula (I) or a salt thereof and inert pharmaceutical carriers and/or auxiliary agents. The invention also relates to carboxylic acids of Formula (XX) and their salts.