Stereochemical Investigations of Diastereomeric N-[ 2-( Aryl )-5-methyl-4-oxo-1 , 3-thiazolidine-3-yl ]-pyridine-3-carboxamides by Nuclear Magnetic Resonance Spectroscopy ( 1 D and 2 D )

Some newN-[2-(aryl)-5-methyl-4-oxo-1,3-thiazolidine-3-yl]-pyridine-3-carboxamides were synthesized and their structures were investigated by IR,NMR (H, C, and 2D), andmass spectra.The presence of C-2 andC-5 stereogenic centers on the thiazolidinone ring resulted in diastereoisomeric pairs. The configurations of two stereogenic centers were assigned based upon H NMR analysis of coupling constants and 2D nuclear overhauser enhancement spectroscopy (NOESY) experiment. Resolution of the diastereoisomers was performed by high performance liquid chromatography (HPLC) using a chiral stationary phase.

Currently, nearly 50% of the drugs are in use as racemates.But stereochemical factors generally have important influence on biological activity of the drug molecules.The two enantiomers present in a racemic mixture can possess different biological activities; that is, one enantiomer has therapeutic value; the other enantiomer may be less effective, inactive, or highly toxic [21][22][23][24][25][26][27].Therefore, the identification and separation of stereoisomers are considered to be important.Chiral compounds bearing thiazolidin-4-one ring have also been studied for their stereochemistry.Several studies have been done on these compounds regarding enantiodifferentiation of stereoisomers in the presence of chiral auxiliary [28], separation of enantiomers by chiral HPLC [29,30], and determination of absolute conformations [31,32].
It is well known that combinations of two or more heterocyclic scaffolds in one molecule can provide a series of compounds with a broad spectrum of biological activity.Here, we combine thiazolidin-4-one and pyridine-3-carboxamide scaffolds together as part of an ongoing project directed towards the design and synthesis of biologically active nitrogen and sulfur containing heterocyclic compounds [33].Our research focused on stereochemical investigations on diastereomeric -[2-(aryl)-5-methyl-4-oxo-1,3-thiazolidine-3-yl]-pyridine-3-carboxamides (2a-f) (Figure 1) by one-and two-dimensional NMR techniques.In addition, the analytical chromatographic separation of some derivatives by chiral HPLC has been examined using a chiral column.operating at 499.7 MHz for 1 H and 124.9 MHz for 13 C, using tetramethylsilane (TMS) as an internal standard.Chemical shifts () were reported in parts per million (ppm).Spectral widths of 14 and 230 ppm were used in 1 H and 13 C NMR, respectively.The splitting patterns of
The structures of the compounds were determined by microanalysis, IR, 1 H-NMR, 13 C-NMR, HMBC, and ESI mass spectrometry.IR spectra of 2a-f showed common characteristic absorption bands at 3142-3176 cm −1 (NH), 1707-1732 cm −1 (thiazolidinone C=O), and 1670-1681 cm −1 (NH-C=O) which provided evidence for the ring closure reaction between 1a-f and 2-sulfanylpropanoic acid.Disappearance of the peak at 8 ppm corresponding to N=CH proton of 1af [37] and the observation of C-2 proton of 2a-f at 5.88-6.30ppm in the 1 H-NMR spectra were also taken as the proof of the formation of thiazolidin-4-one ring.
The structure of 2b was confirmed by the HMBC spectrum in which the correlations of C-8 ( C 164.  3).
For all diastereomeric compounds (Figure 4), it was observed that C-5 methine proton on the thiazolidinone moiety was coupled with C-6 methyl protons and appeared as two quartets (Table 1, Figure 5).Similarly the signal of C-6 methyl protons was coupled with C-5 methine and observed as two doublets for compounds 2b-2f.In all of the 1 H NMR spectra of compounds 2a-2f (except 2b) the higher frequency signals of C-5 methine appeared as a quartet of doublets due to the long-range coupling with the C-2 proton.The two diastereotopic C-2 hydrogens could be observed separately only for compounds 2c-2f.Aromatic protons of pyridyl and C-2 aryl rings gave signals between 6.9 and 9.0 ppm.In this region some of the aromatic peaks corresponding to two diastereomers could also be observed separately for all compounds (Figure 5).The N-H proton was observed at around 11 ppm as two singlets with unequal integral ratios for compounds 2b, 2c, and 2f and only one singlet for 2a, 2d, and 2e.
We have previously elucidated the stereostructures of some oxazolidine derivatives by NOESY experiment [38][39][40].The configurations of the major and minor stereoisomers of thiazolidin-4-one derivatives (2a-2f) were determined by means of 1 H NMR and NOESY spectra of compound 2f.The 1 H NMR spectrum of 2f showed that the major diastereomer had its C-5 methine signal (quartet) at a lower frequency (4.10 ppm,  H-5,CH 3 -6 = 7.0Hz) than the signal of the minor component (4.21 ppm, qd,  H-5,CH 3 -6 = 7.0 Hz  H-5,H-2 : 1.96 Hz).The signal of C-2 proton of compound 2f was observed as two separate signals (Δ: 0.04 ppm) corresponding to two diastereomers: a singlet at 6.26 ppm for the major diastereomer and a doublet at 6.22 ppm ( 4  H-2,H-5 = 1.96Hz) for the minor diastereomer.The observed longrange coupling constant ( 4 ) of the doublet, which is characteristic of trans protons [41], was consistent with that of the higher frequency quartet of minor diastereomer.Based on these results, the stereochemistry of the minor diastereomer was assigned as 2S, 5S or 2R, 5R, in which C-2 and C-5 methine protons are trans to each other (Figure 4).
NOESY spectrum for compound 2f was taken in order to further prove that the stereochemistry of the minor and the major diastereomers was 2S, 5S/2R, 5R and 2S, 5R/2R, 5S, respectively (Figure 7).Observation of the cross peaks at     1).Based on these results, the configurations of C-2 and C-5 centers of the major and minor diastereomers are given in Table 2.
The diastereomeric isomer ratios of compounds 2b and 2c obtained by the integration of the 1 H NMR signals have been found identical with those obtained by HPLC analysis.Therefore, with the knowledge of the configurations of the C-2 and C-5 centers of the major and minor diastereomers of 2b and 2c, the HPLC peaks (Figures 6(a) and 6(b)) marked by "♣" could be assigned to 2S, 5R or 2R, 5S (major) and the others to 2S, 5S or 2R, 5R (minor).
In order to determine the reason of the diastereoselectivity of the synthesis, samples of 2d and 2e were recrystallized once again from ethanol and the composition of crystals precipitated first was analyzed by NMR.We have found a different composition for 2d and 2e.Therefore, the different isomer ratios showed that the obvious diastereoselectivity upon recrystallization from ethanol was due to different solubilities of the diastereomeric isomers in ethanol which was observed previously [29,30,39,40] and not related to any remarkable favorable attack during ring closure.Nevertheless fractional crystallization of the product from ethanol allowed for an easy access to diastereomerically enriched 2b, 2d, and 2e (Table 2).
For 1 H NMR data of the other protons, see Section 2. b The signals corresponding to major diastereomer.c Coupling with C-2 methine was observed as a shoulder.

Table 2 :
The configurations of C-2 and C-5 centers of major and minor diastereomers.