The reverse order of irradiation, check details consisting in saturation of H-5, led to the enhancement of H-3 resonance in (3 S ,5 R )-3a (4.0 %), but not in (3 S ,5 S )-3a. Fig. 2 Selected nOe correlations in (3 S ,5 S )-3a and (3 S ,5 R )-3a The difference in the chemical shifts of the protons H-3 and H-5 adjacent to the stereogenic carbon atoms was another 1H NMR spectroscopic
feature useful for distinguishing between the respective diastereoisomers of 3. With the exception of 3e, the signals corresponding to H-5 in (3 S ,5 S )-3 were shifted downfield (Δδ = 0.26–0.38 ppm [parts per million]) compared to those of (3 S ,5 R )-3. This observation was in agreement with the pseudoequatorial arrangement of these protons with respect to the 2,6-DKP ring in (3 S ,5 S )-3, and their
pseudoaxial position in (3 S ,5 R )-3. On the contrary, resonances of the H-3 protons in all (3 S ,5 S )-3 isomers were shifted upfield (Δδ = 0.14–0.32 ppm). Although these protons in both respective diastereoisomers occupy the same pseudoaxial positions, the slightly stronger shielding in (3 S ,5 R )-3 could be attributed to the anisotropic effect see more of the phenyl ring present in the spatial vicinity. The racemic 2,6-DKPs 3f, g were synthesized from the corresponding N-substituted glycines in a similar manner, according to the reaction sequences depicted in Scheme 2. Notably, the chemical yields of racemic U-5C-4CR products 1f and g (18 and 24 %, respectively) were significantly lower than those observed for N-unsubstituted adducts 1a–e. Scheme 2 Synthesis of racemic 2,6-DKP derivatives 3f, g Anticonvulsant screening Compounds 3a–f were evaluated in the in vivo animal models of epilepsy within the Anticonvulsant Screening Program (ASP) of The National Institute of Neurological Disorders and Stroke (NINDS), at The National Institutes of Health according to well-established Nintedanib (BIBF 1120) protocols described in the “Experimental” section of this article. The compounds were screened using maximal electroshock seizure
(MES) and subcutaneous metrazole (scMET) tests (White et al., 2002). The first of these tests uses an electrical stimulus to induce generalized tonic–clonic seizures and is capable of identifying compounds that prevent the spread of seizure. The latter employs chemically induced seizures to recognize agents that raise the seizure threshold. The most promising compounds were subjected to an evaluation of anticonvulsant activity using a minimal clonic seizure (6 Hz) test (Brown et al., 1953; Barton et al., 2001; Kaminski et al., 2004), which is p38 MAPK apoptosis regarded as a preliminary model of pharmacoresistant limbic seizures. Additionally, a standardized rotorod test for neurological toxicity (TOX; Dunham and Miya, 1957) was performed for each compound. The results are summarized in Tables 1 and 2.