Synthesis of ( 2 S , 4 S )-2-Substituted-3-( 3-Sulfanylpropanoyl )-6-Oxohexahydropyrimidine-4-Carboxylic Acids as Potential Antihypertensive Drugs

Proceeding from natural amino acid L-asparagine and commercially available aldehydes a stereoselective synthesis was developed of (2S,4S)-2-alkyl(aryl)-3-(3-sulfanylpropanoyl)-6-oxohexahydropyrimidine-4-carboxylic acids, potential antihypertensive drugs, inhibitors of the angiotensin converting enzyme.


Introduction
Efficient antihypertensive drugs function as inhibitors of the angiotensin converting enzyme (ACE). This enzyme is involved in the regulator system rennin-angiotensin-aldosterone, and the distortion of its operation results in the majority of hypertensive human diseases [1]- [3]. The first of the drugs from this series was captopril, (S)-N-(3-sulfanyl-2-methyl-1-oxopropyl)-L-proline [4]. The later research on development of antihypertensive substances, ACE inhibitors, was directed to the replacement of the natural amino acid L-proline in the captopril molecule by synthetic cyclic amino acids of the heterocyclic series, among them derivatives of 1,3-oxazolidine [5], 1,3-thiazolidine [6], pipecoline [7], quinazoline [8], indole [9], and azepine [10]. In the series of the saturated pyrimidine derivatives the only example of the inhibitor activity with respect to ACE is known to be shown by 3-(3-sulfanylpropanoyl)-6-oxohexahydropyrimidine-4-carboxylic acid [11].
It was found by the analysis of published data that sodium (potassium) salts of 2-substituted 6-oxohexahydropyrimidine-4-carboxylic acids formed in the reaction of the natural amino acid L-asparagine with carbonyl compounds in alkaline medium. This reaction was investigated mainly by an example of derivatives of isobutyric, trimethylacetic, and benzoic aldehydes [12]- [16]. The process was found to occur with a high stereoselectivity giving prevailingly the spatial isomer with (2R,4S)-configuration of the substituents in the pyrimidine ring.

Results and Discussion
The goal of this study is the search for new and synthetically accessible potential antihypertensive substances, ACE inhibitors, compounds where the cyclic amino acid fragment is a derivative of pyrimidine-4-carboxylic acid.
Compounds 2а-i were obtained in 70% -90% yields after maintaining L-asparagine, an appropriate aliphatic or aromatic aldehyde, and equivalent quantity of sodium hydroxide in methanol solution for 10 -12 h at 25˚С (see Scheme 1 and Table 1).
The first objects of our investigation were the condensation products of L-asparagine with a series of aliphatic aldehydes, compounds 2а-f. The cyclic pyrimidine structure of these compounds is unquestionable, as shows the appearance in the 1 Н NMR spectra of the typical АВХ system due to the diastereotopic character of Н-5 and Н-4 protons, of the signals of Н-2 atom in the region 4.2 -4.5 ppm, and in the 13 С NMR spectra, of the signal of sp 3hybridized С-2 atom in the region 65 -70 ppm. In the spectra of all obtained compounds 2а-f two sets of resonance signals from (2R,4S)-and (2S,4S)-stereoisomeric forms of the pyrimidine ring are observed. The assignment of the stereoisomeric forms of compounds 2а-f is based on the formerly established criteria and rules ob-  A i tained at the use of 1 Н and 13 С NMR spectroscopy [13]- [15], and also on the data of X-ray diffraction analysis [13] [16]. In keeping with the comparison of the 1 Н and 13 С NMR spectra of compounds 2а-f to the analogous spectral characteristics published in [13]- [16] the (2R,4S)-configuration was assigned to the main isomer. In the 1 Н NMR spectrum of this stereoisomer the protons Н-5 and Н-4 are characterized by larger spin-spin coupling constant (J AВ = 17 Hz, J AX + J BX = 16 Hz), and the signals of Н-5 and Н-4 protons are considerably shifted upfield than the analogous signals of the stereoisomer with (2S,4S)-configuration. The corresponding coupling constant in the latter isomer is smaller (J AВ = 15 Hz, J AX + J BX = 13 Hz). The position of the configurational equilibrium of compounds 2а-f is shifted to a large extent to (2R,4S)-stereoisomer, therefore it is impossible to obtain a clear correlation between the logarithms of the constant of the configurational equilibrium and the Taft steric constants of alkyl substituents. The largest fraction of the minor (2S,4S)-isomer (13%) was observed in the D 2 O solution of compound 2e, isovaleric aldehyde derivative.
In the 1 Н and 13 С NMR spectra in D 2 O of the L-asparagine condensation products with aromatic aldehydes (compounds 2g-i) alongside the signals of two configuration isomers of the pyrimidine form signals appear of a linear form А. Not entering into details of the previously found by us [17] spectral distinctions between the cyclic and linear forms of compounds 2g-i we only mention that the typical signs of the latter in the 1 Н NMR spectrum is a significant downfield shift of the protons of СН 2 СН group as compared with analogous signals Н-5 and Н-4 of cyclic forms, the appearance of the proton signal of the azomethine group at 8.25 -8.35 ppm, and in the 13 С NMR spectrum, of the downfield signal at 165 ppm (C=N).
By an example of compound 2g, the condensation product of L-asparagine with benzaldehyde, we studied the dependence of the position of the tautomeric equilibrium on the nature of the applied solvent (see Table 2). In the crystalline state compound 2g exists in the cyclic pyrimidine (2R,4S)-form as confirmed by the solid state 13 С NMR spectrum [17]. In all solvents the coexistence of tautomeric forms was observed, and in going to aprotic polar solvents (DMSO-D 6 and DMF-D 7 ) the fraction of the linear form A significantly increased.
Hence the products of L-asparagine condensation with acetic, propionic, valeric, isobutyric, isovaleric, and hydrocinnamic aldehydes in alkaline medium have the cyclic pyrimidine structure, and in D 2 O solutions they are present as two cyclic spatial stereoisomers with a significant prevalence of the (2R,4S)-form. In neither case the appearance in the solutions of the linear imine form А was observed. In this respect compounds 2а-f fundamentally differ from the condensation products of L-asparagine with a series of aromatic aldehydes 2g-i where the occurrence of the ring-chain tautomerism has been found and alongside two pyrimidine stereoisomers a linear form exists in solutions.
The acylation products 3а-i possess the (2S,4S)-configuration of the substituents of the pyrimidine ring, in their solutions in DMSO-D 6 , in keeping with the data of 1 Н and 13 С NMR spectra, the presence of two cis, transconformers has been found in the ratio ~3:2. It was confirmed by the coalescence of doubled signals at the re-  gistering 1 Н NMR spectra in DMSO-D 6 at higher temperature (~80˚С -85˚С). Proceeding from the comparison of the 1 Н and 13 С NMR spectra of compounds 3а-i with the published [13]- [16] spectral characteristics of 3-alkanoyl-2-substituted 6-oxohexahydropyrimidine-4-carboxylic acids the trans-configuration was ascribed to the main isomer. This conformer is characterized by the downfield shift of Н-4 and Н-2 signals in the 1 Н NMR spectrum compared the analogous signals of the minor cis-isomer. The position of the conformational equilibrium is governed by the nature of the applied solvent, and in going from СDCl 3 to polar solvents, as show the data for compounds 3а-i, the stability of the more polar cis-conformer increases. The removal of S-acetyl protection in compounds 3а-i occurs in the ammonia solution within several hours at room temperature and leads to the formation of (2S,4S)-2-alkyl(aryl)-3-sulfanylpropanoyl-6-oxohexahydropyrimidine-4-carboxylic acids 4а-i in 50% -65% yields (see Scheme 3 and Table 4). In the 1 Н NMR spectra in DMSO-D 6 of compounds 4а-i a triplet signal is observed from the proton of the SH group in the region 1.45 -2.10 ppm. Like in compounds 3а-i, in the solutions of products 4а-i cis-and trans-conformers are present due to the effect of the hindered amide rotation.

Experimental
1 Н and 13 С NMR spectra were registered on a spectrometer Bruker AV-400 at operating frequencies 400 and 100 MHz respectively (internal reference HMDS). The stereoisomeric composition of obtained compounds was estimated by the integration of the appropriate signals in the 1 Н NMR spectra. The specific optical rotation was measured on a polarimeter P-161M at the wavelength of the plane-polarized light 589 nm. Elemental analysis of newly obtained compounds was carried out on a CHN Analyzer Hewlett Packard 185B. The purity of prepared compounds was checked by TLC on Silufol UV-254 plates, eluent benzene-acetone, 1:1.