Unexpected ring-opening of 2,3-dihydropyridines

The reaction of 2,3-dihydropyridines with sulfonyl halides surprisingly yielded open chain dienes with sulfonylimine structure. The products were specific out of several possible isomers and, therefore, a separation of isomers was not necessary. All new compounds were characterized using FT-IR spectroscopy, HRMS, and NMR spectroscopy. A bicyclic by-product from the reaction of a 2,3-dihydropyridine with mesyl chloride was isolated and its structure elucidated using a single X-ray crystal analysis. Some biological activities, like antimicrobial and cytotoxic properties were investigated.

We already described some reactions of 2,3-dihydropyridines like benzylation in ring positions 1 and 3 [15][16][17] as well as the reaction with benzoyl halides to acyl derivatives [18] and investigated the antiprotozoal, antimicrobial, and anticancer potencies of these products [15][16][17][18]. It seems that the conjugated double bond system and a nitrogen in position 4 are important for those activities, since reduction of the double bonds to a piperidine-4-amine [16] or the hydrolysis to a keto group resulted in a complete loss of activity. To investigate how the electron density in the conjugated system influences the biological activities, we tried to connect the electron withdrawing sulfonyl group to the ring nitrogen by reaction of sulfonyl halides with 2,3-dihydropyridines. Surprisingly, the ring was cleaved and open chain sulfonylimines with diene structure were formed.

Results and discussion
Starting compounds were the bases 1a-1d of 6-unsubstituted tetrahydropyridin-4-ylidene ammonium salts (THPS) which were prepared from their 6-methylsulfanyl analogues via selective reduction with deactivated Raney nickel [19]. During the reaction of compounds 1a-1d with alkane-or arenesulfonyl chlorides a ring cleavage occurred. If an acid scavenger like triethylamine (TEA) was used, the sulfonylimino enamines 2a-5b were obtained, in the absence of an auxiliary base their hydrochlorides 6c-7c were isolated (Scheme 1).
As a mechanism of the ring cleavage, we assume a nucleophilic attack of the ring nitrogen at the sulfur of the sulfonyl halide. Subsequently one of the acidic protons in ring position 3 is removed by the auxiliary base or unreacted starting material. The formation of a new bond between ring atoms 2 and 3 and the cleavage between ring atom 2 and the ring nitrogen should occur simultaneously. Finally, the hydrochloride is given in acidic medium (Scheme 2).
The E-configuration at the double bond between C-2 and C-3 was proven by NMR spectroscopy: A cross-peak was found in a ROESY experiment between the NCH 2 groups of the piperidine ring of compound 4c and the protons in positions 2 and 4 indicating through space interactions between these protons (Fig. 1).
To investigate if lower reaction temperatures avoids the ring opening, we conducted the reaction of 1b with benzene sulfonyl chloride at − 70 °C (solid CO 2 /propan-2-ol). At this temperature the 4-chloro compound 8b was mainly formed (Scheme 3).
We investigated, therefore, the course of this reaction at different temperatures with the result, that by trend, the formation of 2b predominated at temperatures from − 21 to 20 °C, whereas its 4-chloro analogue 8b was formed as main product at very low temperatures like − 66 °C and − 70 °C ( Table 1).
The contrast of the yields determined using 1 H NMR spectroscopy to the isolated yields is a result of extensive cleaning procedures including repeated purification using CC as well as repeated crystallization. Only pure fractions were considered for the calculation of yields in the experimental part. Mixed fractions as well as mother liquors were not further separated.
During the attempts to form a hydrochloride of 2b, an isomerization of the double bond system to 9b was observed. Due to this positional change of the double bond we observed the following shifts of signals in 13 C NMR spectra of 9b compared to the hydrochlorides 6c, 7b, and 7c: the signals of C-3 and C-5 were shifted 3-4 ppm downfield, whereas, the resonance of C-1 shifted 17 ppm to lower frequencies. Furthermore, we observed a separation of the NCH 2 signals in 1 H NMR spectra due to the loss of rotatability caused by the formed double bond (Fig. 2).
The Z-configuration of the double bond in position 1 of compound 9b was confirmed by NOE-measurements. NOEs where observed between H-1 and H-2 as well as between H-2 and a proton of the NCH 2 group of the pyrrolidine ring. Furthermore H-4 and the protons of a methyl group and H-4 and a proton of the other NCH 2 group showed through space interactions (Fig. 2). Surprisingly, the bicyclic by-product 10c was isolated as by-product from the reaction of 1c with mesyl chloride. A single X-ray crystal analysis revealed 10c to be (1RS,4RS)-6,6-dimethyl-5-(methanesulfonyl)-7-(piperidin-1-yl)-2λ 6 -thia-5-azabicyclo[2.2.2]oct-7-en-2,2-dione. So far no compounds with a 2-thia-5-azabicyclo[2.2.2]octane ring system have been published (Fig. 3).
All atoms lie on general positions. The asymmetric unit consists of two molecules (s. Figs. 4,5) showing very similar geometric parameters.
In addition to the two molecules in 1R,4R configurations there exist two molecules in 1S,4S configurations in the unit cell related by inversion centers (Fig. 6).
Since, as already mentioned, some sulfonylimines showed antimicrobial and anticancer activities, we investigated some of them for their activities against Plasmodium falciparum as well as Trypanosoma brucei rhodesiense, which are the causative organisms of malaria tropica and sleeping sickness, respectively. Moreover, their cytotoxic properties were examined. All of the tested compounds are completely inactive against both parasites. The results are presented in Table 2.
In addition to that, we investigated the anticancer activity of compounds 2a, 2b, 3c, 4c, 7b, and 9b at 5 µM and 50 µM concentration against human leukemia cells (CCRF-CEM). The activities are shown in Fig. 7. The compounds clearly show more inhibitory activity at 50 µM concentration, but their inhibitory potential is low.
The investigation of the activities against some bacteria and yeast was done using drop plate methods. The results are presented in Table 3. Activity against the following  showing the atomic numbering scheme. The probability ellipsoids are drawn at the 50% probability level. The H atoms of the methyl groups and those of the piperidine ring were omitted for clarity, the other H atoms were drawn with arbitrary radii showing the atomic numbering scheme. The probability ellipsoids are drawn at the 50% probability level. The H atoms of the methyl groups and those of the piperidine ring were omitted for clarity, the other H atoms were drawn with arbitrary radii

Conclusion
The reaction of 2,3-dihydropyridines yielded unexpected sulfonylimines with diene structure. As a side product, 2]oct-7-en-2,2-dione was isolated whose structure was established with the aid of a single X-ray crystal analysis. The new sulfonylimines were investigated for some antimicrobial and cytotoxic activities. One compound showed distinct activity against Pseudomonas aeruginosa. Therefore, further investigations and optimizations of new sulfonylimines will be done to increase the antibacterial activity.

Experimental
Melting points were obtained on a digital melting point apparatus Electrothermal IA 9200. IR spectra: Bruker Alpha Platinum ATR FT-IR spectrometer (KBr discs). NMR spectra: Bruker Ascend 400, 5 mm tubes, spectra were acquired in CDCl 3 containing 0.03% TMS. Chemical shifts were recorded in parts per million (ppm), for 1 H spectra TMS (0.00 ppm) was used as internal standard and for 13 C spectra the central peak of the CDCl 3 peak was used as the internal reference (77.0 ppm). Some spectra were acquired in DMSO-d 6  The preparation of the hydroiodides of compounds 1a-1d was already reported by us [19]. The bases were set free by shaking with 2 M NaOH and subsequent extraction with CHCl 3 and used as starting materials without further purification.

Preparation of compounds 2a-5b
The bases 1a-1d were co-distilled twice with dry benzene and dissolved in dry dichloromethane. To this solution, dry triethylamine (TEA) and the corresponding arene-or alkanesulfonyl chloride was added. The reaction mixture was put under an Argon atmosphere and stirred at room temperature. Water was added and the mixture was stirred for 15 min and put into a separatory funnel. The organic layer was separated and the aqueous layer extracted five times with dichloromethane. The combined organic layers were dried over anhydrous sodium sulfate and filtered. The solvent was evaporated in vacuo and the residue was co-distilled twice with benzene and further purified.   activities of 2a, 2b, 3c, 4c, 5b, 7b, and 9b No clearance = inactive (-), clearance = active (+) and clear clearance = higher activity (++)

Crystal structure determination of 10c
All the measurements were performed using monochromatized Mo K α radiation at 100 K: C 14  The structure was solved by direct methods (SHELXS-97) [22] and refined by full-matrix least-squares techniques against F 2 (SHELXL-2014/6) [23]. The non-hydrogen atoms were refined with anisotropic displacement parameters without any constraints. The H atoms of the tertiary C-H groups were refined with individual isotropic displacement parameter and all X-C-H angles equal at a C-H distance of 1.00 Å. The H atoms of the CH 2 groups were refined with common isotropic displacement parameters for the H atoms of the same group and idealized geometry with approximately tetrahedral angles and C-H distances of 0.99 Å. The H atoms H18 and H28 were put at the external bisectors of the C-C-C angle at a C-H distance of 0.95 Å but the individual isotropic displacement parameters were free to refine. The H atoms of the methyl groups were refined with common isotropic displacement parameters for the H atoms of the same group and idealized geometries with tetrahedral angles, enabling rotations around the C-C bonds, and C-H distances of 0.98 Å. For 427 parameters final R indices of R1 = 0.0300 and wR 2 = 0.0870 (GOF = 1.050) were obtained. The largest peak in a difference Fourier map was 0.715 e Å −3 . The final atomic parameters, as well as bond lengths and angles are deposited at the Cambridge Crystallographic Data Centre (CCDC 2,065,356).  ( 2 E ) -4 -C h l o r o -N -[ 5 -m e t hy l -3 -( p i p e r i d i n -1 -y l ) hexa-2,4-dien-1-ylidene]benzene-1-sulfonamide (4c, C 18 H 23 ClN 2 O 2 S) Reaction of 1 g of 1c (5.20 mmol) in 51 cm 3 of CH 2 Cl 2 with 1.152 g of 4-chlorobenzene-1-sulfonyl chloride (5.46 mmol) in the presence of 5.262 g of TEA (52 mmol) yielded after 2 d a residue which was purified by CC using (CH 2 Cl 2 :MeOH = 30:1) as eluent. Fractions containing 4c were combined and evaporated. The residue was recrystallized twice from ethyl acetate/cyclohexane and once from ethanol. Yield: 220 mg (12%) of 4c as off-white needles. 30 cm 3 of CH 2 Cl 2 with 579 mg of benzenesulfonyl chloride (3.28 mmol) yielded after 5 d a residue which was purified by CC using (CH 2 Cl 2 :MeOH = 20:1) as eluent. Fractions containing 6c were combined and evaporated and the residue subjected to CC with (CH 2 Cl 2 :MeOH = 9:1) as eluent. Fractions containing only 6c were combined and evaporated and the residue was recrystallized from ethanol/ethyl acetate giving 31 mg of 6c. Impure fractions containing 6c were combined, evaporated and the residue purified using CC with (CH 2 Cl 2 :MeOH = 9:1) as eluent giving a yellow resin which was recrystallized from ethanol/ethyl acetate and subsequently from ethanol giving additional 35 mg of 6c.  Ethyl acetate was added with stirring and the solid was sucked off, washed with ethyl acetate, and purified using CC with (CH 2 Cl 2 :MeOH = 9:1) as eluent giving a yellow solid. Yield: 50 mg (4%) of 7b. For analytical purposes it was dissolved in CHCl 3 , filtered, the solvent evaporated, and the residue recrystallized from ethanol giving fine-particle yellow needles. R f = 0.87 (CH 2 Cl 2 :MeOH = 9:1); m.   (14.42 mmol) in 120 cm 3 of CH 2 Cl 2 with 2.548 g of benzenesulfonyl chloride (14.43 mmol) in the presence of 1.46 g of TEA (14.41 mmol) was started at − 70 °C (solid CO 2 /2-propanol) and the reaction batch was allowed to come up to room temperature. It was stirred for 2 d. After workup according to the synthesis of 2b a residue was yielded which was purified by treatment with charcoal and subsequent by CC using (CH 2 Cl 2 :MeOH = 39:1) as eluent giving an orange resin. The slightly impure fractions were combined, evaporated, and the residue recrystallized repeatedly yielding additional product as off-white needles. Yield: 317 mg (6%) of 8b. R f = 0.12 (CH 2 Cl 2 :MeOH = 60:1); m.p.: 127 °C; 1