Autoaggregation phenomena in quinine solutions

Abstract

The coexistence of monomer and dimers of quinine was establisbed in quinine solutions by investigating the temperature and concentration dependence of the NMR spectrsl parameters. Tbeir conformations were determined by a combination of accurate 2D NOESY and proton-selective relaxation rate measurements.

Rationalization of the multireceptorial character of chiral solvating agents based on quinine and its derivatives: Overview of selected NMR investigations

“…8 The same phenomenon occurred in quinine derivatives with hydrogen-bond donor groups at the C9 site, as in the case of the carbamate derivative 2a, the autoassociation constant of which once again was quite low (1.8 M À1 ). 3c The relevance of the structural features discussed above emerged clearly when we investigated the interaction mechanisms of underivatized quinine or its C9 acetylated or carbamoylated derivatives with selected enantiomeric substrates (Fig.…”

“…Autoassociation phenomena of quinine 8 were clearly evidenced by analysis of the dependence of the chemical shifts on the concentration (Table 1), as well as by protonselective relaxation-rate measurements, 9 allowing us to calculate the reorientational correlation times (t c ) for very diluted and concentrated solutions (Table 2) and revealing that its value in the highly concentrated solution was about twice of the corresponding value in the diluted one (Table 2).…”

“…6), in which the two quinoline rings faced each other and the hydroxyl function of one unity was directed toward the quinoline nitrogen of the other, likely causing a stabilizing hydrogen-bond interaction. 8 The two quinuclidine moieties were located anti to each other at the external part of the dimer (Fig. 6).…”

An Overview of Cinchona Alkaloids in Chemistry

“…Of course, the possibility that the variation in the enantioselectivity is caused by a concentration-dependent change in the conformational composition cannot be ruled out. However, according to the results obtained by Salvadori [25], at least in the case of the parent alkaloids, the conformation is similar in the dimer and monomer. Quite recently, Soós and coworkers proved by means of NMR NOESY experiments and computational studies that the quinine thiourea catalyst exists in a dimeric form under concentrated conditions due to H-bond and T-type p-p interactions (Figure 1.6) [28].…”

“…In 1992, the coexistence of the monomer and dimers of quinine in chloroform solution was established by Salvadori and coworkers by investigating the temperature and concentration dependence of the NMR spectral parameters (chemical shift, NOE effect, relaxation time, etc.). The mole fraction of dimers was about 40% in 0.6 M solution [25]. The structure of the dimer determined from NOESY and the relaxation rate data is given in Figure 1.5.…”