Synthesis and E/Z Configuration Determination of Novel Derivatives of 3-Aryl-2-(benzothiazol-2'-ylthio) Acrylonitrile, 3-(Benzothiazol-2'-ylthio)-4-(furan-2''-yl)-3-buten-2-one and 2-(1-(Furan-2''-yl)-3'-oxobut-1''-en-2-ylthio)-3-phenylquinazolin-4(3H)-one

Knoevenagel condensation of 2-(benzothiazol-2-ylthio) acetonitrile (2) with either furan-2-carbaldehyde or thiophene-2-carbaldehydes leads to E-isomers 4a–b exclusively, while the condensation of the compound 2 with benzaldehyde or para-substituted benzaldehydes with an electron-donating group afforded E/Z mixtures 4c–e with preferentially formation of the E-isomer. Condensation of furan-2-carbaldehyde (3a) with either 1-(benzothiazol-2'-ylthio) propan-2-one (5) or 2-(2'-oxo propylthio)-3-phenyl-quinazolin-4(3H)-one (9) leads exclusively to the Z-isomers of 6 and 10, respectively. The structures of the newly synthesized compounds were elucidated by elemental analyses, 1H-NMR and 13C-NMR spectra, COSY, HSQC, HMBC, NOE, MS and X-ray crystallographic investigations.

Based on the abovementioned effects of benzothiazoles, 3-arylacrylonitriles and quinazolin-4-(3H)ones a series of novel 3-arylacrylonitriles with 2-mercaptobenzothiazoles moiety in the position 2-of the acrylonitrile were synthesized to obtain new potential biologically active agents. On other hand, a series of 4-aryl-3-buten-2-ones with either 2-mercaptobenzothiazole or quinazolin-4-(3H)-one rings in the 3-position of 4-aryl-3-buten-2-one were synthesized. The structures of the newly synthesized compounds have been established by x-ray diffraction studies and on the basis of their spectral data.

Result and Discussion
In continuation to our recent research programme dealing with the synthesis of heterocyclic systems, particularly those containing the 2-mercaptobenzothiazole moiety [15,16], 1-(benzothiazol-2yl-thio)acetonitrile (2) was readily prepared in an excellent yield by treatment of 2-mercaptobenzothiazole (1) with α-chloroacetonitrile in refluxing acetone containing anhydrous potassium carbonate (cf. Scheme 1). The structure of 2 was unambiguously confirmed by X-ray crystallography [17] as well as on the basis of its spectral data (cf. Figure 1 and Tables 1-3).    Knoevenagel condensation of the ethanolic solution of 1-(benzothiazol-2-yl-thio) acetonitrile (2) with either hetero-2-carbaldehydes 3a-b or benzaldehyde or para-substituted benzaldehydes 3c-e with an electron-donating group at the para-position, as depicted in Scheme 2. The reactions were carried out in the presence of a catalytic amount of piperidine at the reflux temperature leading to novel 3-aryl-2-(benzothiazol-2'-ylthio)acrylonitriles 4a-e (cf. Scheme 2). The new 3-aryl-2-(benzothiazol-2'-ylthio)acrylonitrile derivatives 4a-e which were formed are highly conjugated systems containing heteroaromatic or phenyl rings, or para-substituted phenyl rings with an electron-donating group in the para-position such as -OCH 3 or -OH. One the other hand, all systems are linked to the benzothiazole nuclei through an S linkage at the α-vinylic carbon. The formation of a carbon-carbon double bond usually lead to the creation of acrylonitriles with either E or Z configuration or a mixture of E and Z isomers (cf. Scheme 2). The yields are based on isolated products and the E/Z ratio was affected by the kind of substitution of the aryl aldehydes 3a-e. The E/Z ratio was determined on the basis of the 1 H-NMR spectra of the products. The structures of isolated S-alkylated acrylonitrile products 4a-e were confirmed on the basis of elemental analysis and spectral data. The mass spectrum of 4a revealed a molecular ion peak (M + ) with m/z 284. The chemical shifts of protons for 4a were assigned using the COSY (correlation spectroscopy) measurement which provided the proton-proton couplings. The 1 H-NMR revealed in addition to an aromatic multiplet, a singlet signal for a vinylic proton (H-3), that appears at δ H 8.06 ppm. Moreover, the chemical shifts of carbons for compound 4a were assigned using HSQC (Heteronuclear Single Quantum Coherence) and HMBC (Heteronuclear Multiple Bond Coherence) measurements. The 13 C-NMR spectrum for 4a is characterized by two signals at δ C 142.2 and δ C 91.9 ppm for the carbon-carbon double bond group. The 13 C signal of the β carbon at the higher frequency coupled with a proton, while the 13 C signal of the α-carbon appears at lower frequency (cf. Figure 2).   Nuclear Overhauser Effect (NOE) experiments were run to establish the stereo-orientation for acrylonitrile derivatives 4 as either E or Z isomers. NOE experiments for 4a showed an enhanced triplet signal (H-4'') at δ H 6.84 ppm upon irradiating the doublet signal (H-5'') at δ H 8.15 ppm. On irradiating the doublet signal (H-3") at δ H 7.30 ppm both the triplet signal (H-4") and the singlet signal (H-3) at δ H 6.84 and 8.06 ppm, respectively, were enhanced, while the benzothiazole protons showed no effect, confirming that the benzothiazol-2-ylthio moiety and 2'-furyl groups are on opposite sides of the double bond as required by the E-form. The structure of 4a was unambiguously confirmed by X-ray crystallography [18] (cf. Figure 3 and Tables 4-6).   The condensation of 2 with thiophene-2-carbaldehyde (3b) afforded a mixture of (E)-and (Z)-2-(benzothiazol-2'-ylthio)-3-(2''-thienyl) acrylonitriles in a ratio of 6:0.5 based on the 1 H-NMR spectrum, which revealed, in addition to the expected aromatic signals, two downfield signals at δ H 8.52 and 8.70 ppm assignable to the E and Z-vinylic protons. Upon recrystallization of the crude product from a mixture of ethanol and diethyl ether in a ratio of 3:2 one isomer 4b was obtained. The 1 H-NMR spectrum of the crystallized product showed the vinylic proton resonance at δ H 8.52 ppm. In order to determine the configuration of the obtained isomer, different NMR experiments have been carried out such as COSY, HSQC and NOE experiments. The NOE experiments showed a strong NOE coupling between H-3 and (vinylic-H) and the thiophene protons H-3'' and H-4'' and no coupling between the benzothiazole and the thienyl ring protons, indicating that the benzothiazole and the thienyl rings are on opposite sides of the double bond, confirming that compound 4b exists in the E-configuration.

Scheme 2. Syntheses of 4.
On the other hand, condensation of 2 with either benzaldehyde or para-substituted benzaldehydes 3c-e in refluxing ethanol containing a catalytic amount of piperidine, afforded a mixture of E and Z isomers of 3-aryl-2-(benzothiazol-2'-ylthio) acrylonitriles 3c-e in a ratio of 7:2 based on the 1 H-NMR spectra of the crude products. The chemical shifts of the 1 H-and 13 C-spectra were assigned on the basis of the proton-proton and the carbon-proton coupling patterns observed in the COSY and HSQC spectra. The 1 H-NMR spectra display vinylic proton singlets in the δ H 8.12-8.29 ppm region. The chemical shifts for the carbon-carbon double bond are observed at δ C 91-97 and at δ C 157-158 ppm for the αand β carbon, respectively. Moreover, Nuclear Overhauser Effect (NOE) experiments were run to establish the stereo-orientation for either the E or Z isomers of 2-(benzothiazol-2'-ylthio)-3-(4''methoxyphenyl)acylonitrile 4d. On irradiating the singlet of the vinylic proton at δ H 8.17 ppm the doublet of H-2'' of phenyl ring at δ H 8.00 ppm was enhanced. On irradiating the doublet signal of the phenyl ring H-3'' at δ H 7.15 ppm both the doublet of the phenyl ring H-2'' and the para-methoxy group protons at δ H 8.00 and 3.87 ppm, respectively, were enhanced, while the benzothiazole protons showed no effect, indicating that the benzothiazole and the 4-methoxyphenyl rings are on opposite sides of the double bond, as required by an E-form.
In a similar manner, the 1-(benzothiazol-2'-ylthio)propan-2-one (5) used in our experiments has been recently prepared in an excellent yield by treatment of 2-mercaptobenzothiazole (1) with α-chloroacetone in refluxing acetone containing anhydrous potassium carbonate [15]. The structure of 5 was unambiguously confirmed by X-ray crystallography [19] (cf. Scheme 3, Figure 4 and Tables 7-9). Condensation of 5 with the furan-2-carbaldehyde in refluxing ethanol containing a catalytic amount of piperidine afforded 3-(benzothiazol-2'-ylthio)-4-(furan-2''yl)-3-buten-2-one in excellent yield (cf. Scheme 3). The structure of the isolated product was confirmed on the basis of its elemental analysis and spectral data (see Experimental section).    The mass spectrum of 6 revealed a molecular ion peak [M + ]. with m/z 301 The effect of the conjugatation of the carbonyl group with the carbon-carbon double bond reduces the frequency of the absorption at υ max 1662cm −1 . It is considered lower than the isolated carbonyl group in compound 5, which revealed a carbonyl stretching band at υ max 1720 cm −1 [15]. The chemical shift of protons for 6 were assigned using COSY (correlation spectroscopy) measurements which provided the protonproton coupling. The 1 H-NMR spectrum showed a resonance at δ H 8.25 ppm corresponding to H-4 of the vinylic proton. Moreover, the 13 C-NMR chemical shift assignments were straightforward using HSQC (Heteronuclear Quantum Coherence) measurements (cf. Figure 5). The 13 C-NMR spectrum of the reaction product revealed a low field signal at δ C 194.8 that corresponds to the carbonyl carbon. Also were revealed two low field signals at ca. δ C 148 and 136 ppm corresponding to a carbon coupled with a proton. As in the furan ring system, the carbon resonating at ca. δ C 148 ppm corresponds to C-5'', while the carbon resonating at ca. δ C 136 ppm corresponds to the vinylic carbon C-4. The complete assignment of H 1 and 13 C chemical shifts for 6 are presented in Figure 6.
Moreover, the configuration of the product 6 was assigned as the Z-isomer based on Nuclear Overhauser Effect (NOE) experiments; on irradiating the methyl proton at δ H 2.54 ppm only the signal at δ H 8.25 ppm for the vinylic proton was enhanced and there is no effect on the benzothiazole protons or furyl protons, which indicate that acetyl group and the vinylic proton are on the same side of the carbon 3,4 double bond as numbered, therefore it has the Z configuration. On irradiating the vinylic H-4 at δ H 8.25 ppm both the singlet and doublet signals at δ H 2.54 and 7.47 ppm, respectively, were enhanced. The signal at δ H 2.54 ppm corresponds to the acetyl group, while the doublet signal at δ H 7.47 ppm corresponds to the furyl system H-3". On other hand, on irradiating the doublet signal of the H-4' proton at δ H 7.92 ppm only the triplet signal at δ H 7.32 ppm was enhanced. These two signals correspond to the benzothiazole H-4' and H-5' protons. On irradiating the triplet signal H-4'' at δ H 6.74 ppm both the H-5'' and H-3'' signals at δ H 8.05 and 7.47 ppm, respectively, were enhanced. The NOE experiments show that the Z-isomer of compound 6 was preferred over the E-isomer indicating that the furyl and 2-benzothiazol-2-thio nuclei are on the same sides of the double bond, but away from each other in space.

Thus it can be concluded that Knoevenagel condensation of 2-(benzothiazol-2-ylthio) acetonitrile with furan-2-carbaldehyde leads preferentially to (E)-2-(benzothiazol-2'-ylthio)-3-(furan-2''-yl)
acrylonitrile (4a). In this conformation the lone pair electrons of the sulfur atom can conjugated with the π orbital of the double bond, and thereby stabilize the molecule. That is why this conformation is the most stable conformer. On the other hand, the Knoevenagel condensation of 3-(benzothiazl-2'ylthio) propan-2-one with furan-2-carbaldehyde lead preferentially to (Z)-3-(benzothiazol-2'-ylthio)-4-(furan-2''-yl)-3-buten-2-one. By changing the size and the geometry of the group attached to the α-carbon from a cyano group to an acetyl group, the conformation changes from E to Z. One may therefore conclude that under these circumstances the most E-isomer is unlikely to be the most stable conformer, because of the steric strain between the acetyl group and furan ring. The steric strain makes this conformation energetically highly unfavorable in the E-isomer so the conformer changes from E to Z. On the other hand, 2-(2'-oxopropylthio)-3-phenylquinazolin-4(3H)-one (9) was readily prepared in good yield by the treatment of 2-mercapto-3-phenylquinazolin-4(3H)-one (8) [20] with α-chloroacetone in refluxing acetone containing potassium carbonate. The structure was established on the basis of its spectral data. The mass spectrum of the reaction product showed a molecular ion peak at m/z 310. The 1 H-NMR spectrum revealed, in addition to aromatic signals, two upfield singlets at δ H 2.33 and 4.06 ppm, assignable to methyl and methylene protons, respectively. Moreover, the 13 C-NMR spectrum showed two downfield signals at δ C 202.5 and 161.1 ppm. The signal at δ C 202.5 ppm corresponds to the ketone carbonyl carbon, while that at δ C 161.1 ppm corresponds to the cyclic amide carbon .The data are therefore consistent with structure 9 (cf. Scheme 4). Condensation of 2-(2'-oxopropylthio)-3-phenylquinazolin-4(3H)-one (9) with furan-2-carbaldehyde (3a) afforded a product which could have been either E or Z-isomer of α,β-unsaturated ketone 10 (cf. Scheme 4). The structure of α,β-unsaturated ketone 10 was deduced from its elementanal anaylsis and spectra data. The mass spectrum revealed a molecular ion peak (M + ) with m/z 388. The chemical shifts of the protons for 10 were assigned using COSY (correlation spectroscopy) measurements which provided the proton-proton coupling. The 1 H-NMR revealed in addition to an aromatic multiplet, a singlet for a vinylic proton (H-1') which appears at low field at ca. δ H 7.88 ppm. Moreover, the 13 C-NMR chemical shift assignments were straightforward using HSQC (Heteronuclear Single Quantum Coherence) measurements (cf. Figure 7). The 13 C-NMR spectrum for 10 is characterized by two signals at δ C 132.0 and δ C 119.7 ppm for the α,β-unsaturated ketone group. The vinylic carbons at the higher frequency is the one coupled with a proton, while the other one is a disubstituted sp 2 carbon. The complete assignment of H 1 and 13 C chemical shift for 10 are presented in Figure 8.
Moreover, the configuration of the product 10 was assigned as the Z-isomer based on Nuclear Overhauser Effect (NOE) experiments; on irradiating the methyl proton at δ H 2.48 ppm the vinylic proton signal at δ H 7.88 ppm was enhanced. There is no effect on the 3-phenylquinazolin-4(3H)-one or furan protons, which indicates that the acetyl group and the vinylic proton are on the same side of the C1',2' double bond as numbered, which therefore has Z configuration. On irradiating the vinylic H'-1 at δ H 7.88 ppm the methyl signal at δ H 2.48 ppm was enhanced.
On the other hand irradiation of the H-8 proton at δ H 8.08 ppm has no effect, confirming that the 3-phenylquinazolin-4(3H)-one moiety and 2'-furyl groups are on same sides of the double bondas required by a Z-form. The structure of 10 was also confirmed by X-ray crystallography [21] (cf. Figure 9 and Tables 10-12).   In particular, we were interested in the reasons for the predominance of the s-cis conformation of the Z configuration and the E/Z determination of the prepared novel derivatives of 2-(benzothiazol-2'ylthio)-3-arylacrylonitrile, 3-(benzothiazol-2'-ylthio)-4-(furan-2''-yl)-3-buten-2-one and 2-(1-(furan-2''-yl)-3-oxobut-1-en-2-ylthio)3-phenylquinazolin-4(3H)-one in accordance with expectations, it was ascertained that s-trans conformations were more stable than s-cis conformations for both the E and Z molecular configurations. Thus the disparity displayed here where s-cis conformations in case of compound 6 dominated over the s-trans conformation was of interest and one supposition was that electronic interactions could be responsible, e.g., by delocalization of a sulfur lone electron pair with the unsaturated segments residing in the newly formed heterocyclic ring and attendant side-chains or through hyperconjugation. The s-cis conformational preference over s-trans in a structurally similar system has been reported previously [22] and an electronic cause was also postulated, though that system differed significantly in the distribution of unsaturation. It is obvious to note that compounds 4c-e are predominantly in the E/Z form and this is due to the large range of mesomeric effects due to large conjugations in the phenyl group present in such compounds. However, the existence of compounds 4a and 4b in the E form is due to the existence of the heterocyclic, furan and thiophene rings, respectively, where less conjugation occurs in such compounds.

General
Melting points are reported uncorrected and were determined on a Gallenkamp apparatus. The Infrared spectra were recorded on a Jasco FT/IR-6300 FT-IR using KBr disks. 1 H-NMR and 13 C-NMR spectra were measured on a Bruker DPX 400 MHz and Bruker AVANCE ΙΙ 600 MHz spectrometers, with DMSO-d 6 or CDCl 3 as solvent using TMS as an internal standard. The methods used for the purpose of NMR assignment were COSY, HSQC and HMBC. The chemical shifts are expressed as δ unit in parts per million (ppm) and TMS = 0.00 ppm. The following abbreviation are used: s = singlet, d = doublet, t = triplet; q = quartet; m = multiple; br. = broad. Mass spectra were measured on GC/MS DFS, THERMO instrument. Microanalyses were performed on a CHNS-Vario Micro Cube analyzer, Single crystal X-ray crystallography was perfomed using a Rigaku Rapid ΙΙ located at the Chemistry Department of Kuwait University. Compound 5 was prepared according to our recent reference [15] and its X-ray data was reported in reference [19]. (2) phenylquinazolin-4(3H)-one afforded compounds 6 and 10 respectively. The condensation products 6 and 10 were characterized by spectroscopic measurements and were shown to be the Z-isomers.