Other Molecules with Pharmacological Activity
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7.1 2-Formylfuran Derivatives 2-Formylfuran (2FF) is a very important intermediate in organic synthesis. Some furan-based derivatives are muscarinic antagonists (Johansson et al. 1997) and show inhibitory activity towards cholesterol O-acyltransferase (Tanaka et al. 1998). A few 2-formylfuran derivatives (5-(4-fluor-phenyl)-2-formylfuran (5-(4FP)-FF) and 5-(4-brom-phenyl)-2-formylfuran (5-(4Br-P)-2FF) have been prepared and their bacteriostatic effects have been checked with good results. In order to know the action of potential drugs, such as the above-mentioned furan based derivatives, it is very important to see if the structure of the adsorbed species is the same as that of the free molecules. In these studies, a silver surface serves as an artificial biological interface (Dryhurst 1977). Prior to performing investigations on the abovementioned derivatives, the interest was focused on the 2FF molecule.
7.1.1 2-Formylfuran By the rotation of the COH group in the sample, two isomeric forms can be obtained. Several investigations have been performed on this molecular species up to now, and the conclusions drawn were quite different. Thus, the Raman and infrared data for the 2FF previously reported show intense doublets at 2800, 1670, 1420, and 1370 cm–1. Furthermore, it was found that for solutions of 2FF in organic solvents, the intensity distribution of the doublets at 1670 and 1470 cm–1 is a function of concentration. By analyzing the infrared and Raman spectra of 2FF recorded at different temperatures, Allen and Bernstein (Allen and Bernstein 1959) concluded the existence of an equilibrium between the cis- and trans-isomer forms. They found no temperature dependence for the gas phase (ΔH ≈ 0), but for the liquid phase there was a small dependence corresponding to an energy difference of 4.185 kJ mol–1. It was also showed by cryoscopy and infrared investiga-
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7 Other Molecules with Pharmacological Activity
tions that the carbonyl absorption around 1700 cm–1 is not due to the molecular association of 2FF but to the existence of rotational cis- and trans-isomers. Karabatsos and Vane’s nuclear magnetic resonance study (Karabatsos and Vane 1963) of coupling constants in a 2FF solution led to the result that the 2FF molecule exists completely in the cis-form. In contrast, Dahlqvist and Forsen (Dahlqvist and Forsen 1965), analyzing the nuclear magnetic resonance temperature dependence of 2FF dimethyl ether solution, found that the trans-form is more stable than the cis-form by ΔH = 4.39 kJ mol–1. In a microwave and far-infrared study of the 2FF molecule it was found that in the vapor phase the molecule is planar and both rotamers co-exist, the trans-form being the most stable. Miller et al. (Miller et al. 1982) also found from far-infrared studies that the trans-isomer is more stable than the cis-isomer by 8.36 kJ mol–1. The lack of a consensus between the experimental data on 2FF reported in the literature requires supplementary theoretical and experimental studies of this molecular species. Therefore, an in
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