Spectroscopic analysis of imidazolidines Part V: 1 H and 13 C NMR spectroscopy and conformational analysis of N -acylimidazolidines

The conformational features of a series of 1-acyl-3-arylimidazolidines 1-14 were correlated with their 1 H and 13 C-NMR spectra. Due to the hindered rotation around the N-CO bond, these compounds exist in solution as a mixture of non-isolable Z/E diastereomers, the Z-isomer being the most abundant in all cases. The analysis of NOESY spectra confirmed the complete assignment of the 1 H-NMR resonances of each rotamer. Two-dimensional heteronuclear HMQC and HMBC spectra of selected compounds permitted the assignment of 13 C-NMR signals. We analyzed the influence of the imidazolidine ring substituents on the equilibrium abundance of Z/E isomers. We also studied the spectroscopic and conformational properties of a 1-acyl-2,3-diarylimidazolidine 15 , which revealed the effect of the C-2 substitution.


Introduction
Tetrahydroimidazoles (imidazolidines) are cyclic aminals of pharmacological interest.Some of these compounds have biological activity, which is strongly dependent on their substitution patterns. 1In order to elucidate structure-activity correlations, the study of their conformational features, specially the substituent orientation in molecules of biological interest, has been the object of several investigations.
3][4] Our previous studies showed that the nitrogen atoms of 1,3diaryl substituted imidazolidines undergo fast inversion, independent of the presence or absence of C-2 substitution, whereas in the case of 1-alkyl-3-arylimidazolidines fast nitrogen inversion occurs only in the absence of C-2 substitution.In the presence of a C-2 substituent, inversion of the N-alkyl nitrogen is slow and the imidazolidine shows a trans-like distribution of its N-alkyl and C-2 substituents. 2,3][7][8][9] In particular, the E/Z equilibrium in heterocyclic tertiary amides and related compounds has been reviewed. 10MR spectroscopic methods have been successfully used for obtaining information about the stereochemistry of amides. 5,6In line with our previous studies related to spectral and biological properties of imidazolidines, [1][2][3][4] we prepared a series of 1-acyl-3-arylimidazolidines 11 (heterocyclic tertiary amides).Because the complete knowledge of structure, including stereochemistry, is essential for lead optimization in drugs discovery, we report herein our studies concerning the structural features of several of those compounds (1-15) (Table 1) as studied by NMR spectroscopy.We were particularly interested in determining the influence of the steric and/or electronic effects of the acyl and aryl groups on the conformational features of such compounds as well as the influence of C-2 substitution.

Results and Discussion
N-Acylimidazolidines can undergo three dynamic processes that result from their azacycloalkane character, namely ring reversal, amide rotation and nitrogen inversion.
The room temperature 1 H-NMR spectra of the C-2 unsubstituted imidazolidines 1-14 display in all cases two sets of signals that correspond to two different species (Figure 2).For both forms, the 1 H-NMR spectra show isochronicity of C-2, C-4 and C-5 methylene hydrogens of the heterocyclic ring that appear as singlets in the 4.61-5.16ppm range for H-2 or triplets in the 3.42-3.73ppm and 3.78-4.18ppm ranges for H-4 and H-5, respectively (Table 2).
[7][8][9]    The assignment of the 1 H-NMR signals was performed by analysis of their one-dimensional spectra and confirmed by the correlations observed in the NOESY spectra of selected compounds, 1, 8 and 12, as models of N-formyl, N-alkanoyl and N-aroyl derivatives respectively (Figure 4).The remaining compounds of the series were assigned by analogy.In all cases, the abundances of each rotamer were calculated from the intensity of the C-2 methylene signals (Table 2).In all cases, the most abundant isomer corresponds to the Z diastereomer that has the R 1 group trans to the C-2 methylene.
The Z/E abundance ratio increases with the volume of the R 1 substituent, being smallest in the formyl derivatives 1, 2 and largest for the isobutyryl compounds 9-11.Intermediate abundance ratios are found for the N-benzoyl derivatives 13 and 14.The Z/E equilibrium is also sensitive to the electronic properties of the substituents in the aryl moiety, R 3 , with increased preference for the Z diasteromer in the case of electron-withdrawing groups.As an example we can compare the ratio %Z / %E for the N-phenyl derivative 5 (55.2/44.8)and the N-4chlorophenyl derivative 6 (59/41).
Several conclusions follow the analysis of the 1 H-NMR data.The chemical shift of H-2 and H-5 reflects the increased electronegativity of the acyl-substituted nitrogen as compared to with alkyl-or aryl-substituted nitrogens.It has been reported that the H-2 and H-4/H-5 of 1,3dibenzylimidazolidines resonate around 3.50 and 2.35 ppm, respectively, 3 whereas those of 1,3-diaryl substituted compounds move downfield, resonating at around 4.60 and 3.55 ppm, respectively, 2 due to electron delocalization through the aniline moiety.In the case of 1-acyl-3arylimidazolidines 1-14, the chemical shift of H-2 and H-5 signals around 4.70-5.16ppm and 3.78-4.18ppm, respectively, indicates further deshielding, with their value depending on R 1 and the Z/E configuration of the compound.

12-Z (major)
12-E (minor) The chemical shift of the H-2 and H-5 signals in formamides 1 and 2 reflect shielding of hydrogens that are cis with respect to the oxygen.Hence, the H-2 chemical shift in the Z diastereomer is smaller than in the E isomer, whereas the inverse situation occurs at H-5.On the other hand, the phenyl of the benzoyl groups exhibits a larger shielding effect on the adjacent cis methylenes than the carbonyl group.Thus, the H-2 signals of the E isomers of 12 and 13 resonate around 4.65 ppm, about 0.3 ppm upfield from the values observed for the Z diastereomer (around 4.93 ppm).The same effect is observed for the H-5 signals of the same compounds, which resonate approximately 0.2 ppm upfield in the Z isomer as compared to the E isomer.This correlation is absent for imidazolidines with R 1 = H or alkyl groups, compounds 1 to 11, which show chemical shift differences smaller than 0.07 ppm for the H-2 and H-5 signals of the diastereomers.
The H-4 signals of compounds 1-14 appear in the 3.42-3.73ppm range, values that are similar to those previously observed in 1,3-diarylimidazolidines. 2 Chemical shift differences between the diastereomeric forms are small, being 0.03 ppm for the formyl derivatives 1-2, 0.09-0.11ppm for the alkyl-substituted compounds 3-11 and 0.06 for the N-benzoylimidazolidines 12-

14.
In all the series, the aromatic proton signals of the N-aryl group of both diastereomers appear in general with the expected multiplicity.On the other hand, the benzoyl aromatic hydrogens of both diastereomeric forms of 12 and 13 appear as broad singlets, indicating a low barrier for their interconversion, a fact previously observed for other benzamides. 7The most likely explanation for this behavior is the competive stabilization between the amide moiety and the phenyl substituent (cross conjugation) which lowers the partial N-CO double bond character (Figure 5).compound are in line with the experimental data measured by integration of 1 H NMR signals.
Data are reported in Table 4.The 1,2,3-trisubstituted imidazolidine 15 shows a single set of signals in the 1 H-and 13 C-NMR spectra, indicating the presence of only one diastereomer.The analysis of HMQC and HMBC spectra yielded the complete assignment of 13 C-NMR signals that is listed in Table 3.The H-4, adjacent to the N-aryl group, appears as two differentiated multiplets at around 3.30-3.50ppm and at 3.75-3.85ppm.In contrast, the H-5 signals are a complex multiplet around 3.90-4.00ppm.Due to the presence of the phenyl substituent, the H-2 is more deshielded than in compound 13 and appears as a singlet at 6.59 ppm, partially overlapping with the ortho hydrogen of the N-4-methoxyphenyl group.The N-aryl hydrogens of 15 resonate as two resolved doublets at 6.55 and 6.77 ppm, whereas the N-benzoyl ortho hydrogens are a broad signal at 7.60-7.70ppm.The other aromatic hydrogens appear as a complex multiplet resonating on the 7.20-7.60ppm range.
The NOESY spectrum of 15 displays a clear interaction between the N-aryl hydrogens (H-7, 6.55 ppm) and both hydrogens of the adjacent methylene group (H-4), indicating that, under our experimental conditions, inversion of the N-aryl substituent is fast even in the presence of the C-2 phenyl group.On the other hand, observation of an NOE cross-peak between H-2 (6.69 ppm) and the ortho hydrogens of the N-benzoyl group (H-11, 7.71 ppm) indicates a short distance among them, suggesting the E configuration of the N-CO amide bond, as shown in Figure 7.

Conclusions
We studied the spectroscopic and conformational characteristics of a series of 1-acyl-3arylimidazolidines at room temperature.Analysis of the NMR data shows that C-2 unsubstituted imidazolidines 1-14 exist as an equilibrium mixture of E/Z diastereomers, more abundant on their Z isomer.In the case of N-alkanoyl derivatives 1-11, preference for the Z configuration increases with the size of the acyl group, reflecting the importance of steric hindrance in the E/Z equilibrium.
The NMR data also show shielding of the methylene hydrogens adjacent to N-1, specifically H-2 and H-5, by the amide carbonyl and the phenyl of the benzoyl group.At room temperature, the 1,2,3-trisubstituted compound 15 exists as a single diastereomer that, based on NOESY evidence, was later ascribed to the E isomer of the N-CO bond.

Experimental Section
General.Reagents, solvents and starting materials were purchased from commercial sources and purified according to standard procedures.Melting points are uncorrected values taken on a Büchi capillary apparatus.One-and two-dimensional 1 H-and 13 C-NMR spectra were recorded on a Bruker Avance II 500 spectrometer at room temperature (297.5°K).Standard sample concentrations were 10 mg/mL for 1 H-and 25 mg/mL for 13 C-NMR, using deuterochloroform as solvent.Chemical shifts (δ) are given in ppm relative to internal TMS and coupling constants are reported in Hz.Splitting multiplicities are reported as singlet (s), broad singlet (bs), doublet (d), triplet (t), quartet (q), heptet (h) and multiplet (m).HMQC, HMBC and NOESY (mixing time, 0.9 seconds) spectra were recorded on a Bruker Avance II 500 spectrometer.Electron ionization mass spectra were recorded with a GC-MS Shimadzu QP-1000 spectrometer operating at 20 eV.TLC analyses were carried out on Silica gel 60 F 254 using chloroform-methanol (9:1) as solvent.
1-Acyl-3-arylimidazolidines (1-15).These were prepared from the reaction of N-acyl-N´arylethylenediamine and the corresponding aldehyde, following literature procedure. 11The preparation of compounds 3-6, 8, 10 and 12 has been previously described. 11Melting points, yields and elemental analysis of the new compounds are given below. 1 H-and 13 C-NMR data are presented in Tables 2 and 3. Computational Studies.Four model compounds, 2, 8, 10 and 12, were investigated computationally by GRID SEARCH, using the conformational analysis module of the SYBYL 8.1 program, 12 running on a PC-Linux workstation.The optimized conformation of each molecule in the data set was energy minimized employing MMFF94 force field and the Powell conjugate gradient algorithm with a convergence criterion of 0.05 kcal/mol•Å and Gasteiger-Hückel charges. 13,14The percentage of rotamers, was determined taking 10% of the total generated conformers by the program.

Figure 5 .
Figure 5. Competitive resonance stabilization between the amide moiety and the benzoyl group.

Figure 7 .
Figure 7. Correlations observed in the NOESY spectrum of compound 15

Table 4 .
Abundances of Z diastereomer