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

摘要

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-pyrazole3,4-e1,4thiazepin-7-one (1) as a hit endowed with FXR activity. Most of the compounds synthesised during the hit-to-lead optimisation work were characterised 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 pyrazole3,4-e1,4thiazepin- 7-one core, resulted in suitable diastereo- and enantioresolution with a quinine (QN) carbamate-derived chiral stationary phase (CSP). Differently from the compounds usually analysed 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 optimisation of the eluent composition, NP conditions allowed to obtain profitable enantioselectivity profiles, along with excellent diastereoselectivity levels (α(A1)∈=∈1.07, R_S(A1)∈=∈1.15; α(S1)∈=∈1.09, R_S(S1)∈=∈1.47; α(A2)∈=∈1.08, R_S(A2)∈=∈1.31; and α(S2)∈=∈1.06, R_S(S2)∈=∈1.18). The optimised NP methods are suitable for simultaneously providing information on the diastereo- and enantiopurity of the investigated compounds.