Heterocyclic Analogs of Thioflavones: Synthesis and NMR Spectroscopic Investigations †

The synthesis of several hitherto unknown heterocyclic ring systems derived from thioflavone is described. Coupling of various o-haloheteroarenecarbonyl chlorides with phenylacetylene gives 1-(o-haloheteroaryl)-3-phenylprop-2-yn-1-ones, which were treated with NaSH in refluxing ethanol to yield the corresponding bi- and tricyclic annelated 2-phenylthiopyran-4-ones. Detailed NMR spectroscopic investigations of the ring systems and their precursors are presented.


Introduction
The flavone system (2-phenyl-4H-chromen-4-one, shown in Figure 1, is the core of many biologically active compounds which play important roles in numerous biological processes. The relevance of flavone-based molecules has been thoroughly described in the literature [1][2][3][4][5]. Replacement of the ring oxygen atom in flavone by a sulfur atom results in thioflavone (4a, Figure 1), whose derivatives also exhibit interesting biological properties [6][7][8][9][10][11][12][13]. Moreover, thioflavones are valuable precursors for the synthesis of other condensed heterocyclic systems, such as benzothiazepines [14]. Analogs of type 4 thioflavone-systems -in which the condensed benzene ring of 4a is replaced by a heteroaromatic moiety (Figure 1) -seem to be of notable interest considering the OPEN ACCESS concept of bioisosterism [15][16][17]. Type 4 compounds with a pyridine, thiophene, benzo[b]thiophene or indole system annelated to the thiopyrane ring have been previously characterized. For instance, condensation of appropriate thiophenols A with ethyl benzoylacetate followed by subsequent cyclization of the obtained condensation products permits access to type 4 systems [18]. Another approach was employed by Becher in the synthesis of 2-phenyl-4H-thiopyrano [2,3-b]pyridin-4-one (4f) [19]. Ethyl 2-chloronicotinate (B) was transformed into the 2-tert-butyl congener C, which was condensed with acetophenone to yield D; the latter readily cyclized into 4f in an acidic medium. Moreover, the reaction between phenylpropiolates and thiophenols A and subsequent ring closure reaction of the formed substituted cinnamates E has also been used [20][21], for instance, in the synthesis of indole annelated 2-phenylthiopyran-4-ones [22]. Variants of 4 containing a thiophene or a benzo[b]thiophene moiety have been prepared via Friedel-Crafts acylation of the corresponding methylthio(benzo)thiophenes F with cinnamoyl chloride, to yield ketones G, and addition of bromine to the alkene double bond (compounds H) and subsequent cyclization with pyridine hydrochloride [23][24][25]. However, many of these methods require precursors that are neither commercially available nor readily synthesized. Moreover, the other above-mentioned approaches lack generality. For example, Friedel-Crafts-based methods are restricted to π-excessive heteroaromatic systems.
Here, we present a general approach for synthesizing heterocyclic analogs of type 4 based on a Sonogashira-type coupling of o-halo(hetero)aroyl chlorides 1 with phenylacetylene to yield alkynones 2 (Scheme 2). Reactions between 2 and NaSH in refluxing ethanol then yield the desired compounds 4 via an addition/cyclization step (Scheme 2). A related approach to thioflavone generation employing a microwave-assisted one-pot, three-component synthesis has been recently described by Müller [26] (based on an earlier report by Shvartsberg [27][28]). In the course of these investigations, the acetylenic component was varied to obtain different 2-substituted 4H-thiochromen-4-ones [26].

Chemistry
Synthesis of the target compounds 4 was accomplished via the sequence shown in Scheme 2. Precursors 1 are either commercially available or can be easily prepared by treatment of the corresponding carboxylic acids with thionyl chloride. Compounds 1 were transformed into 2 via Sonogashira-type coupling [29][30], a very important step in forming C-C bonds with terminal acetylenes [31]. A ligand-and copper-free Pd-catalyzed (Pd acetate, Et 3 N) version of this method for the coupling of carboxylic acid chlorides with different terminal acetylenes has been recently described by Nájera [32] and Srinivasan [33]. In preliminary tests in which the solvent, amount of triethylamine and the acid chloride/phenylacetylene ratio were varied, we adapted the reaction conditions for synthesis of the desired o-halo(hetero)arylynones 2a-h. The best results were obtained using two equivalents of acid chloride, a 3-14-fold molar excess of triethylamine, dichloromethane as the solvent and ambient temperature. Reasonably good yields of ynones 2a-h were obtained applying these reaction conditions. However, using of 1i and 1j as starting materials (both having the COCl group attached in the o-position to a pyridine-type nitrogen atom), we did not obtain the corresponding coupling products 2i or 2j. Instead, the respective N,N-diethylamides 5 and 6 were isolated as the main reaction products from the complex reaction mixtures (Scheme 3). A few reports in the literature have described N,N-diethylamide formation from acid chlorides and triethylamine [30,[34][35]. NaSH in refluxing ethanol (96%) was used for the conversion of ynones 2a-h into thiopyranones 4a-h (Scheme 2). In principle, it is possible to use Na 2 S as the SH donor [27][28]; however, poor results were obtained using this technique. Evidence of the proposed mechanism, a Michael addition of the hydrosulfide to the alkynone system [26], was obtained in the following experiment. When 2a1 was reacted with NaSH/ethanol at room temperature, we isolated a sulfur-containing product that contained a chlorine atom and an enone group. The high-resolution mass spectrum and the spectral data demonstrated that the product was not the expected intermediate 3a1, but rather the thioether 7 (Scheme 4), formed following reaction between two units of 3a1 by elimination of hydrogen sulfide.
The cis-position of alkene-H and the phenyl ring was confirmed by an NOE-difference experiment that employed irradiation of alkene-H resonance (Scheme 4). Initial attempts to synthesize 4a and 4f in a one-pot/two-step procedure (reactants: 1a or 1f, respectively, with phenylacetylene, by Pd(OAc) 2 /triethylamine catalysis in solvent; then addition of Na 2 S in DMF) gave low yields (<16%) of the desired thiopyranones.
The ynones 2 described here are valuable precursors in the synthesis of other condensed heterocyclic systems, such as annelated pyridin-4-ones 8 and pyran-4-ones 9 (Scheme 5). For example, 2a1 was reacted with methylamine to generate the corresponding Michael addition product 10 at a 73% yield (stereochemistry demonstrated by a NOESY experiment). Treatment of product 10 with K 2 CO 3 in dry DMF gave 1-methyl-2-phenyl-4(1H)-quinolinone (11) at a good yield. This type of compound was reported to exhibit interesting biological properties, such as anti-platelet [36], antimitotic [37], and anti-HIV-replication activity [38]. Furthermore, the well-known class of fluoroquinolinone type [39] antibacterial drugs, such as ciprofloxacin [40] or enoxacin [41], are structurally similar to 11. Investigations into the preparation of heterocyclic annelated pyridin-4-ones of type 8 are currently in progress and will be reported elsewhere.

Spectroscopic investigations
Alkynones 2 and thiopyranones 4 are predominantly novel structures. Representative structures (2a1, 2a2, 4a, 4d-f) have not been thoroughly investigated by spectroscopic methods, such as NMR. In regards to 13 C-NMR spectroscopy -in cases where such data are available (4a, 4f) -little [19] to no [27][28] signal assignments have been made. Hence, we present the results from an extensive NMR study ( 1 H, 13 C, 15 N) of compounds 2 and 4. Reliable and unambiguously assigned chemical shift data are important reference material for NMR prediction programs, such as CSEARCH [42]/NMRPRE-DICT [43] and ACD/C + H predictor [44]. Such programs have become very popular in the last few years, particularly for predicting 13 C-NMR chemical shifts. However, the quality of such predictions is highly dependent on the availability of authentic reference data from related structures. This criterion is frequently unfulfilled for rare condensed heteroaromatic systems, such as those described here.
Full and unambiguous assignment of all 1 H, 13 C and 15 N resonances was achieved by combining standard NMR techniques [45], such as fully 1 H-coupled 13 C-NMR spectra, APT, HMQC, gs-HSQC, gs-HMBC, COSY, TOCSY, NOESY and NOE-difference spectroscopy. Moreover, experiments with selective excitation (DANTE) of certain 1 H-resonances were performed, such as long-range INEPT [46] and 2D(δ,J) long-range INEPT [47]. The latter experiments were indispensable for the unambiguous mapping of long-range 13 C, 1 H coupling constants. Aside from the variable heteroaromatic system, the obtained data of the invariant parts of 2 and 4 show a high degree of consistency. Thus, the 3-phenyl-2-propyn-1-one substructure in 2a-h exhibits a carbonyl C-1 shift of 168.3-176.7 ppm, a C-2 shift of 87.2-88.5 ppm, and a C-3 shift of 91.9-96.2 ppm. Alkyne C-atoms, C-2 and C-3, can be easily distinguished by their coupling patterns: the C-2 signal appears as a singlet in the 1 H-coupled 13 C-NMR spectrum, whereas the C-3 signal is a triplet due to 3 J coupling with phenyl protons H-2/6 (which is confirmed by a correlation signal from the involved nuclei in the HMBC spectrum). The signals from the phenyl-C atoms are Ph C-1: 119.3-120.1 ppm, Ph C-2/6: 133.1-133.3 ppm, Ph C-3/5: 128.6-128.8 ppm and Ph C-4: 130.9-131.6 ppm. The thiopyranone system in compounds 4a-h is characterized by a chemical shift of 176.2-182.0 ppm for the carbonyl C-atom (split by a ~ 1 Hz 2 J coupling to the adjacent =C-H), 122.1-125.0 ppm for =C-H ( 1 J 163.3-164. 8 Hz) and 151.5-155.1 ppm for S-C-Ph ( 2 J to =CH ~ 3 Hz). The =C-H proton gives rise to a characteristic singlet signal between 6.99 and 7.32 ppm. As observed with compounds 2, systems 4 also exhibit small differences in Ph-C shifts within the series a-h (Ph C-1: 136.1-136.7 ppm, Ph C-2/6: 126.9-127.2 ppm, Ph C-3/5: 129.2-129.4 ppm, Ph C-4: 130.7-131.3 ppm). The descriptions of NMR spectra in the Experimental section were assigned based on systematic nomenclature and, hence, the numbering of atoms within the thiopyrane moiety is inconsistent.
The excellent utility of 2D (δ,J) long-range INEPT spectra with selective excitation for the definite mapping of 13 C, 1 H coupling constants is demonstrated by an example presented in Figure 2. In the 1 Hcoupled 13 C-NMR spectrum of 4d, the signal of C-7a is split by a 7.1, 5.9 and 4.7 Hz coupling, whereas coupling occurs with H-2, H-3 and H-6. Unequivocal assignment of these coupling constants based on literature data for the thiophene system is unreliable. However, after selective excitation of the H-2 resonance, the C-7a signal appears as a doublet of 5.  Furthermore, the NMR spectra of N,N-diethylcarboxamides 5 and 6, respectively, two different signals sets for the ethyl moieties (Et cis , Et trans , see Scheme 3) were found for each case. As expected, this indicates that there is restricted rotation around the amide bond under the recording conditions (CDCl 3 , ambient temperature).
In the IR spectra, the carbonyl C=O absorption from the ynones 2 appears in the range between 1,601 and 1,649 cm -1 , and those from the thiopyranones 4 between 1,606 and 1,635 cm -1 . Absorptions for the C≡C vibration in the IR spectra of compounds 2 appear from 2,196-2,202 cm -1 .
As reported for related structures [48], the mass spectra of 4 exhibit a fragmentation behavior that is characterized by the loss of CO, a pathway that is also observed for ynones 2.

Conclusions
We have presented a widely applicable method for the preparation of heterocyclic annelated 2phenylthiopyran-4-ones via a cross-coupling addition-cyclization approach starting from o-haloheteroaroyl chlorides and phenylacetylene. Detailed NMR spectroscopic studies of the title compounds and their precursors were provided.

General
Melting points were determined on a Reichert-Kofler hot-stage microscope and are uncorrected. Hz/data point in the 13 C-NMR spectra. Systematic names were generated with ACD/Name [49] according to the IUPAC recommendations and were checked manually [50]. For chromatographic separations, Kieselgel 60 (70-230 mesh, Merck) was used. Light petroleum had a boiling point of 40-65 °C.

General procedure for the synthesis of o-halo(hetero)aroyl chlorides 1b-h
A suspension of the appropriate acid (2 mmol) in toluene (20 mL), DMF (5 drops) and SOCl 2 (20 mmol, 2.38 g) was refluxed for 3 h or overnight. The solvent and excess SOCl 2 were removed under reduced pressure. Additional toluene (4  5 mL) was added, and the solvent was removed under reduced pressure. The remaining acid chloride was immediately used in subsequent reaction steps (with no further purification).

General procedure for the synthesis of ynones 2a-h
The appropriate acid chloride 1 (2 mmol) was dissolved in dry CH 2 Cl 2 (3 mL). Triethylamine (2 mL), phenylacetylene (1 mmol, 102 mg) and Pd(II) acetate (10 µmol, 2 mg) were added to the solution, which was stirred at room temperature under a N 2 atmosphere (N 2 balloon) for the indicated time. The solvent was removed under reduced pressure and water (20 mL) was added to the residue. The resulting solution was acidified with 5% HCl and extracted with CH 2 Cl 2 or EtOAc (2  15 mL). The combined organic layers were washed with a saturated NaHCO 3 solution and a saturated NaCl solution and were then dried over anhydrous Na 2 SO 4 . After removal of the solvent, the residue was purified by column chromatography through a silica gel column (eluent given below). An analytically pure sample was obtained by recrystallization from the appropriate solvent, which is indicated below.

General procedure for the synthesis of thiopyranones 4a-h
A suspension of NaSH hydrate (~60%, 1.2 mmol, 112 mg) was refluxed in EtOH (20 mL) until a cloudy solution was formed. Ynone 2 (0.4 mmol) in EtOH (2 mL) was then added to the solution, which was refluxed for the indicated time. The solvent was removed under reduced pressure and water (20 mL) was added to the residue. The resulting solution was acidified with 5% HCl and extracted with CH 2 Cl 2 (2  15 mL). The combined organic layers were washed with a saturated NaHCO 3 solution and a saturated NaCl solution. They were then dried over anhydrous Na 2 SO 4 . After removal of the solvent, the residue was purified by column chromatography through a silica gel (eluent as indicated below). An analytically pure sample was obtained by recrystallization from an appropriate solvent.