Effects of C2-alkylation, N-alkylation, and N,N'-dialkylation on the stability and estrogen receptor interaction of (4R,5S)/(4S,5R)-4,5-bis(4-hydroxyphenyl)-2-imidazolines.

(4R,5S)/(4S,5R)-4,5-Bis(4-hydroxyphenyl)-2-imidazolines bearing 2,2'-H (3a), 2,2'-Cl (3b), 2,2',6-Cl (3c), and 2,2'-F (3d) substituents in the aromatic rings were C2-alkylated (5a-i), N-alkylated (7, 7a-c), and N,N'-dialkylated (9a-c). The synthesis started from the diastereomerically pure (1R,2S)/(1S,2R)-1,2-diamino-1,2-bis(4-methoxyphenyl)ethanes 1a-d, which were cyclized to the imidazolines 2a-d and 4a-i with triethylorthoesters or iminoethers. Ether cleavage with BBr(3) yielded the (4R,5S)/(4S,5R)-4,5-bis(4-hydroxyphenyl)-2-imidazolines 3a-d and 5a-i. The N-alkylation and N,N'-dialkylation of 2b, employed for obtaining 7a-c and 9a-c, were performed prior to the ether cleavage with alkyl iodine in dry THF. By use of HPLC, the influence of the substitution patterns in the aromatic rings and alkyl chains at the C2- or N-atoms on the hydrolysis rate of the imidazolines was studied under in vitro conditions. It appeared that only imidazolines with C2- or N-alkyl substituents show sufficient stability to interact as heterocycles with the estrogen receptor (ER). The resulting gene activation was monitored in a luciferase assay using ERalpha-positive MCF-7-2a breast cancer cells stably transfected with the plasmid ERE(wtc)luc. It is interesting to note that C2-alkylation led to a strong reduction or even a complete loss of activity whereas N-alkylation improved the estrogenic profile. The (4R,5S)/(4S,5R)-N-ethyl-4,5-bis(2-chloro-4-hydroxyphenyl)-2-imidazoline 7b has proven to be the most active compound in this structure-activity relationship study (EC(50) = 0.015 microM).