Simultaneous diastereo- and enantioseparation of Farnesoid X Receptor (FXR) agonists with a quinine carbamate-based chiral stationary phase

In the frame of a project aimed at finding non-steroidal farnesoid X receptor (FXR) agonists, we identified 4-(2,4-dimethoxyphenyl)-3,6-dimethyl-1-(2-tolyl)-4,8-dihydro-1H-pyrazole[3,4-e][1,4]thiazepin-7-one (1) as a hit endowed with FXR activity. Most of the compounds synthesized during the hit-to-lead optimization work were characterized by the presence of two chiral centres and were therefore obtained as mixtures of anti(±)- and syn(±)-diastereoisomers. A restricted sub-set of species harboured with a carboxylic acid group on the distal phenyl ring of the biphenyl [a(±)5 (A1) and s(±)5 (S1)] or the phenoxyphenyl [a(±)6 (A2) and s(±)6 (S2)] moiety at C-4 position of the pyrazole[3,4-e][1,4]thiazepin-7-one core, resulted in suitable diastereo- and enantioresolution with a quinine (QN) carbamate derived chiral stationary phase (CSP). Differently from the compounds usually analyzed with QN-based CSPs, the couples A1/S1 and A2/S2 were atypical selectands, in which the two chiral carbon atoms reside at a remote position with respect to the carboxylic function, the main “point-of-attack” to the CSP. We produced evidence that the scarcely employed normal-phase (NP) eluent systems, represent the elective choice for achieving the simultaneous diastereo- and enantioseparation of this class of compounds, over the usually preferred reversed-phase (RP) and polar-organic (PO) modes of elution. Indeed, after the optimization of the eluent composition, NP conditions allowed to obtain profitable enantioselectivity profiles, along with excellent diastereoselectivity levels [alpha(A1) = 1.07, RS(A1) = 1.15; alpha(S1) = 1.09, RS(S1) = 1.47; alpha(A2) = 1.08, RS(A2) = 1.31; alpha(S2) = 1.06, RS(S2) = 1.18]. The optimized NP methods are suitable for simultaneously providing information on the diastereo- and enantiopurity of the investigated compounds.