HETEROCYCLES

Paper | Regular issue | Vol. 83, No. 6, 2011, pp. 1303-1313
Received, 16th February, 2011, Accepted, 28th March, 2011, Published online, 13th April, 2011.
DOI: 10.3987/COM-11-12180

■ Synthesis of 3,1-Benzothiazines from 2-Alkenyl- and 2-Alkynylanilides and Lawesson Reagent

Kentaro Okuma,* Saori Ozaki, Noriyoshi Nagahora, and Kosei Shioji

Department of Chemistry, Faculty of Sciences, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan

Abstract

Reaction of 2-vinylacetanilide with P4S10 gave 2-vinylthioacetanilide, whereas reaction of 2-vinylacetanilide with Lawesson reagent (LR) afforded 2,4-dimethyl-4H-3,1-benzothiazine in 62% yield. Reaction of 2-alkynylanilides with LR gave 4-exomethylene-4H-3,1-benzothiazines in good yields.

INTRODUCTION
4H-3,1-Benzothiazines (1) are important compounds because of their biological and structural appeal. 4H-3,1-Benzothiazine derivatives with aromatic substituents at positions 2 and/or 4 exhibit many kinds of biological activities1 and are of interest for the production of recording materials and use in photographic and laser techniques.2 Moreover, they are valuable building blocks and can be used for the synthesis of indole derivatives in particular.3 Recent methods include the reaction of 2-alkenylanilides (2) having electron-withdrawing groups with LR,4 the reaction of 2-hydroxyalkylanilides with LR,5 the reaction of 2-alkynylthioformanilides with DBU,6 the reaction of 2-alkynylaniline with isothiocyanate,7 and the Friedel-Crafts reaction of isothiocyanate.8 However, there is no report on the direct synthesis of benzothiazines from 2-alkynylanilides (3). These results prompted us to investigate the reaction of 2-alkenylanilides 2 having electron-donating groups and 2-alkynylanilides 3 with LR, to find out whether the corresponding intramolecular cyclization would proceed or not. Herein, we describe a new approach to the synthesis of 4H-3,1-benzothiazines 1 and 4-exomethylene-4H-3,1-benzothiazines (4) based on a thionation process followed by a cyclization process from 2-alkenyl- and 2-alkynylanilides.

RESULTS AND DISCUSSION
2-Alkenylanilides 2 with electron-withdrawing groups (CO2Me and CN) underwent thionation and intramolecular cyclization to give 3,1-benzothiazines.4 As the intramolecular cyclization did not proceed by using 2-alkenylformanilide and P4S10 as the thionation reagent,6 we first attempted to react 2-alkenylanilide 2 with P4S10 or LR to investigate the possibility of tandem thionation and intramolecular cyclization.
Treatment of 2-vinylacetanilide (2a) with P4S10 in refluxing pyridine (6 h) resulted in the formation of 2-vinylthioacetanilide (5a) in 66% yield (Table 1, Entry 1). When the reaction was carried out by using LR as the thionation reagent, starting 2a was recovered almost quantitatively (Entry 2). The reaction of 2a with LR in refluxing chloroform also recovered starting 2a (Entry 3). However, when toluene was used as the solvent (refluxing for 12 h), 2,4-dimethyl-3,1-benzothiazine (1a) was obtained in 62% yield.

From these results, the optimum conditions for tandem thionation and intramolecular cyclization of 2 were judged to be LR (0.5 eq) in refluxing toluene. We then tried the reaction by using anilide (2b-2f) prepared from commercially available isopropenylaniline as substrates under these reaction conditions (Scheme 1). The reaction of 2-isopropenylacetanilide (2b) with LR gave 2-isopropenylthioacetanilide (5b) and 2,4,4-trimethyl-4H-3,1-benzothiazine (1b) in 19% and 68% yields, respectively (Table 2, Entry 1). When the reaction was carried out in refluxing toluene for 12 h, 1b was isolated in 89% yield (Entry 2). Other isopropenylanilides 2c-2f also afforded benzothiazines 1c-1f in good yields (Entries 3-6).

Thus, the thionation and intramolecular cyclization of anilides 2 with LR afforded 3,1-benzothiazines 1 in moderate to good yields.
Previously, we have reported the synthesis of 3,1-benzoxazines by active halogen mediated intramolecular cyclization of 2-alkenylanilides.9 As thioanilides are more nucleophilic than the corresponding anilides, the formation of 3,1-benzothiazines by intramolecular cyclization would proceed without any activating reagents. Although the two-step synthesis of 4-alkenyl-3,1-benzothiazine was accomplished by the reaction of 2-alkynylformanilides with P4S10 followed by the addition of DBU,6 the sequential thionation and cyclization of 2-alkynylanilides 3 was not reported. The only reported example was the reaction of anilides 3 with phenylisothiocyanates, which afforded 2-imino-3,1- benzothiazines, and 2-imino substituents were required.7 Thus, we then tried to react anilide 3 with LR to find out whether tandem thionation and cyclization would proceed. Starting 2-alkynylanilides 3 were synthesized by the acylation of 2-alkynylanilines, which were easily synthesized by the Sonogashira coupling reaction.10 Treatment of [2-(2-phenylethynyl)phenyl]benzanilide (3a) with LR (0.5 eq) at rt for 36 h resulted in the almost recovery of 3a (Scheme 2, Table 3, Entry 1). When 1 eq of LR was used, small amount of 2-(phenylethynyl)thiobenzanilide (6a) was formed (Entry 2). When the reaction was carried out in refluxing toluene for 5 h, (Z)-4-benzylidene-2-phenyl-4H-3,1-benzothiazine (4a) were obtained in 82% yield (Entry 3). When P4S10 was used as the thionation reagent, thioanilide 6a was obtained in 42% yield and further cyclization did not proceed (Entry 4). It is noteworthy that isolated 6a was allowed to react with 0.1 eq of LR in refluxing toluene to afford 4a in 85% yield.

As the optimum conditions for tandem thionation and intramolecular cyclization would be LR (0.5 eq) in refluxing toluene, we then applied these conditions to a variety of 2-alkynylanilides (3b-3i) (Scheme 3). The results are shown in Table 4. The reaction of 3b with LR in refluxing toluene for 12 h gave 3,1-benzothiazine 4b in 57% yield (Entry 2).

When a sterically hindered anilide, such as 2-(3,3-dimethyl-1-butynyl)phenylacetanilide 3c, was used as substrate, prolonged reaction time is required (48 h) (Entry 3). Electron-donating groups, such as a methoxy or a methyl group, at para position gave relatively high yields of benzothiazines 4f and 4g (Entries 6 and 7). Electron-withdrawing groups at para position 3h-3i gave moderate yields of products 4h and 4i (Entries 8 and 9).
The reaction would proceed as follows: LR reacted with anilide 3a to afford to give the corresponding thioanilide 6a. Thiocarbonyl sulfur of 6a intramolecularly attacked triple bond to afford cyclized prodict, which finally produced 4a (Scheme 4).

The stereochemistry of 4 was determined from proton NMR spectra. The exo-alkene signal of 4c appeared at 6.14 ppm. The reported example of (Z)-benzylideneisochromene showed a signal at 6.32 ppm assignable to the exo-alkene proton, which is similar to 4c.11 NOESY spectrum of 4c shows the correlation between exomethylene proton and 5-H proton (Figure 1). Fortunately, as single crystals of 4c were obtained, the X-ray crystallographic analysis of 4c could be performed. As shown in Figure 2, stereochemistry of 4c has Z-form.

In summary, 2-vinylacetanilide 2a reacted with P4S10 to afford thioacetanilide 5a, whereas the reaction of anilides 2 with LR gave 3,1-benzothiazines 1 in good yields. The reaction of 2-alkynylanilides 3 with LR in refluxing toluene gave 4-methylene-3,1-benzothiazines 4 in good yields. We have successfully synthesized 3,1-benzothiazines from 2-alkenylanilides and 2-alkynylanilides by thionation and intramolecular cyclization in one-pot operation.

EXPERIMENTAL
General
All chemicals were obtained from commercial suppliers and were used without further purification. NMR spectra (1H at 400 MHz; 13C at 100 MHz) were recorded in CDCl3, and chemical shifts are expressed in ppm relative to internal TMS for 1H- and 13C-NMR. Melting points were uncorrected.

Marterial
All reagents (2-isopropenylaniline, LR, and P4S10) were purchased from TCI or Aldrich. 2-Vinylaniline was synthesized by reduction of 2-nitrostyrene12 with Sn/HCl.
2-Alkynylanilines were synthesized by Sonogashira coupling reaction.10 2-Vinylanilides were synthesized by the method reported in the literature.9,13 2-Alkynylanilides were synthesized by the reaction of 2-alkynylanilines with acyl chlorides.14 Followings are new compounds. Compound 3c: colorless needles; 1H NMR (CDCl3) δ = 1.38 (s, 9H, t-Bu), 7.03 (dd, 1H, J = 7.6 Hz and 7.6 Hz, Ar), 7.32 (dd, 1H, J = 7.6 Hz and 7.6 Hz, Ar), 7.40 (d, 1H, J = 7.6 Hz, Ar), 7.48-7.57 (m, 3H, Ar), 7.94 (d, 2H, J = 6.8 Hz, Ar), 8.58 (d, 1H, J = 8.4 Hz, Ar), 8.86 (br, 1H, NH). MS (GC): Calcd for C19H19NO: 277.15. Found; 277.87 (M+). Compound 3e: colorless needles. 1H NMR (CDCl3) δ = 1.28 (dd, 3H, J = 7.4 Hz and 7.6 Hz, CH3), 2.46 (dt, 2H, J = 7.6 Hz and 7.6 Hz, CH2), 7.05 (dd, 1H, J = 7.6 Hz and 7.6 Hz, Ar), 7.34-7.40 (m, 4H, Ar), 7.49-7.55 (m, 3H, Ar), 8.04 (br, 1H, NH), 8.44 (d, 1H, J = 8.0 Hz, Ar). MS (GC): Calcd for C17H15NO: 249.12. Found; 248.97 (M+). Compound 3f: colorless needles. 1H NMR (CDCl3) δ = 2.43 (s, 3H, CH3), 7.09 (dd, 1H, J = 7.6 Hz and 7.6 Hz, Ar), 7.27 (d, 2H, J = 8.0 Hz, Ar), 7.40-7.44 (m, 3H, Ar), 7.85 (d, 2H, J = 8.0 Hz, Ar), 8.62 (d, 1H, J = 8.8 Hz, Ar), 8.93 (br, 1H, NH). MS (GC): Calcd for C22H17NO: 311.13. Found; 310.97 (M+). Compound 3g: colorless needles. 1H NMR (CDCl3) δ = 3.88 (s, 3H, OCH3), 6.96 (d, 2H, J = 8.8 Hz, Ar), 7.09 (dd, 1H, J = 7.4 Hz and 7.6 Hz, Ar), 7.39-7.41 (m, 3H, Ar), 7.53-7.57 (m, 3H, Ar), 7.92 (d, 2H, J = 8.8 Hz, Ar), 8.09 (d, 1H, J = 8.8 Hz, Ar), 8.60 (d, 1H, J = 8.4 Hz, Ar), 8.88 (br, 1H, NH). MS (ESI): Calcd for C22H17NO2: 327.13. Found; 352.64 (M +2+ Na+). Compound 3h: colorless needles. 1H NMR (CDCl3) δ = 7.12 (dd, 1H, J = 7.6 Hz and 7.6 Hz, Ar), 7.41-7.43 (m, 3H, Ar), 7.45 (d, 2H, J = 8.4 Hz, Ar), 7.52-7.56 (m, 3H, Ar), 7.74 (d, 1H, J = 8.8 Hz, Ar), 7.89 (d, 1H, J = 8.4 Hz, Ar), 8.58 (d, 1H, J = 8.4 Hz, Ar), 8.88 (br, 1H, NH). MS (ESI): Calcd for C21H14ClNO: 331.08. Found; 356.53 (M +2+ Na+). Compound 3i: colorless needles. 1H NMR (CDCl3) δ = 7.14 (dd, 1H, J = 7.6 Hz and 7.6 Hz, Ar), 7.41-7.46 (m, 4H, Ar), 7.52-7.58 (m, 3H, Ar), 7.74 (d, 2H, J = 8.0 Hz, Ar), 8.06 (d, 2H, J = 8.0 Hz, Ar), 8.60 (d, 1H, J = 8.4 Hz, Ar), 8.93 (br, 1H, NH). MS (GC): Calcd for C22H14F3NO; 365.10. Found; 364.85(M+).
Reaction of 2-vinylacetanilide 2a with P4S10 in refluxing pyridine
To a solution of 2-vinylacetanilide 2a (0.081 g, 0.50 mmol) in pyridine (5 mL) was added P4S10 (0.056 g, 0.25 mmol) in one portion. After refluxing for 6 h, the reaction mixture was washed with water and extracted with EtOAc (7 mL x 3). The combined extract was washed with water, dried over sodium sulfate, filtered, and evaporated to give pale brown oil, which was chromatographed over alumina by elution with hexane:EtOAc (1:1) to afford pale yellow oil of 2-vinylthioacetanilide 5a (0.056 g, 0.32 mmol, 64%). 5a: yellow oil; (lit.,15 yellow oil) (rotational isomeric mixture) 1H NMR (CDCl3) δ = 2.39 (s, 3H, CH3), 2.76 (s, 3H), 5.39 (d, 1H, Jcis =11.2 Hz, =CHH) 5.44 (d, 1H, Jcis = 11.2 Hz, =CHH), 5.74 (d, 1H, Jtrans = 17.6 Hz, =CHH), 5.80 (d, Jtrans =17.6 Hz, =CHH), 6.74 (dd, 1H, J = 11.2 and 17.6 Hz, =CH), 6.82 (dd, 1H, J = 11.2 and 17.6 Hz, =CH), 7.16-7.62 (m, 4H, Ar). 8.52 (br, 1H, NH), 9.22 (br, 1H, NH). 13C NMR (CDCl3) 5a’: δ = 30.33 (CH3), 118.32 (=CH2), 126.92, 127.47, 128.66, 129.26 (Ar), 131.47 (=CH), 134.77, 135.75, (Ar), 205.85 (C=S). 5a”: δ = 34.83 (CH3), 117.47 (=CH2), 128.56, 127.86, 128.50, 129.08, 132.06 (=CH), 134.31, 136.42 (Ar), 202.61 (C=S).
Reaction of 2a with LR in toluene
To a solution of 2-vinylacetanilide 2a (0.081 g, 0.50 mmol) in toluene (5 mL) was added LR (0.10 g, 0.25 mmol) in one portion. After refluxing for 12 h, the reaction mixture was filtered through short silica gel column, and evaporated to give brown oil, which was chromatographed over silica gel by elution with hexane:EtOAc (1:1) to afford pale yellow oil of 2,4-dimethyl-4H-3,1-benzothiazine 1a (0.053 g, 0.30 mmol, 60%). 1a: yellow oil, 1H NMR (CDCl3) δ = 1.44(d, 3H, J = 7.2 Hz, CH3), 2.42 (s, 3H, CH3), 4.04 (dt, 1H, J = 7.0 Hz and 7.2 Hz, CH), 7.10 (d, 1H, J = 7.2 Hz, Ar), 7.22-7.26 (m, 1H, Ar), 7.31-7.33 (m, 2H, Ar). 13C NMR (CDCl3) δ = 24.46 (CH3), 29.07 (CH), 37.50 (CH3), 124.82, 126.13, 127.29, 127.82, 128.42, 142.67 (Ar), 160.26 (C=N). HRMS: Calcd for C10H11NS; 177.0612. Found; 177.0605 (M+).
Other reactions were carried out in a similar manner.
2-Isopropenylthioacetanilide (5b): colorless needles: mp 84-86 °C (rotational isomeric mixture); 1H NMR (CDCl3) δ = 2.04 (s, 3H, =CCH3), 2.06 (s, 3H, =CCH3), 2.47 (s, 3H, CH3), 2.66 (s, 3H, CH3), 5.00 (s, 1H, =CH), 5.02 (s, 1H, =CH), 5.32 (s, 1H, =CH), 5.33 (s, 1H, =CH), 7.10-7.37 (m, 5H + 4H, Ar), 8.21 (d, 1H, J = 8.0 Hz, Ar), 8.68 (br, 1H, NH), 8.80 (br, 1H, NH). 13C NMR (CDCl3) 5b’: δ = 23.58 (CH3), 30.08(CH3), 116.99 (=CH2), 126.91, 127.55, 128.54, 129.65, 135.24 (=CH), 140.02, 142.68 (Ar), 200.65 (C=S). 5b”: δ = 24.09 (CH3), 35.92 (CH3), 118.08 (=CH2), 126.01, 127.22, 128.40, 128.57 (Ar), 135.07 (=CH), 138.25, 142.46, (Ar), 204.69 (C=S). Anal. Calcd for C11H13NS: C, 69.07; H, 6.85; N, 7.32. Found: C, 68.99; H, 6.77; N, 7.70.

2,4,4-Trimethyl-4H-3,1-benzothiazine (1b). yellow oil: 1H NMR (CDCl3) δ = 1.59 (s, 6H, CH3), 2.41 (s, 3H, CH3), 7.26-7.37 (m, 4H, Ar). 13C NMR (CDCl3) δ = 28.37 (CH3), 29.46 (2CH3), 43.44 (C(CH3)2), 121.16, 127.44, 127.91, 128.00, 130.13, 143.13 (Ar), 161.18 (C=N). HRMS: Calcd for C11H13NS; 191.0769. Found; (M+) 191.0772.
4,4-Dimethyl-2-phenyl-4H-3,1-benzothiazine (1c) (0.11 g, 0.44 mmol, 89%).
1c: yellow oil, 1H NMR (CDCl3) δ = 1.68 (s, 6H, 2CH3), 7.34-7.41 (m, 3H, Ar), 7.48-7.51 (m, 3H, Ar), 7.55 (d, 1H, J = 8.0 Hz, Ar), 8.15 (d, 2H, J = 7.6 Hz, Ar). 13C NMR (CDCl3) δ = 29.11 (2CH3), 43.54 (C(CH3)2), 121.49, 127.89, 128.13, 128.28, 128.33, 128.70, 131.15, 131.59, 138.61, 143.93 (Ar), 161.09 (C=N). HRMS: Calcd for C16H15NS; 253.0925. Found; 253.0934 (M+).
4,4-Dimethyl-2-(p-tolyl)-4H-3,1-benzothiazine (1d). yellow oil, 1H NMR (CDCl3) δ = 1.66 (s, 6H, CH3), 2.41 (s, 3H, CH3), 7.25-7.39 (m, 5H, Ar), 7.52 (d, 1H, J = 7.8 Hz, Ar), 8.04 (d, 2H, J = 6.4 Hz, Ar). 13C NMR (CDCl3) δ = 21.75 (CH3), 29.09 (2CH3), 43.49 (C(CH3)2), 121.47, 127.85, 128.03, 128.06, 128.32, 129.44, 131.24, 135.99, 142.07, 144.05 (Ar), 161.00(C=N). HRMS: Calcd for C17H17NS; 267.1082. Found; (M+) 267.1083.
4,4-Dimethyl-2-(p-anisyl)-4H-3,1-benzothiazine (1e). yellow oil, 1H NMR (CDCl3) δ = 1.67 (s, 6H, CH3), 3.88 (s, 3H, OCH3), 6.98 (d, 2H, J = 8.8 Hz, Ar), 7.17 (d, 1H, J = 7.6 Hz, Ar), 7.26-7.40 (m, 2H, Ar), 7.52 (d, 1H, J = 7.6 Hz, Ar), 8.12 (d, 2H, J = 8.8 Hz, Ar). 13C NMR (CDCl3) δ = 27.75 (2CH3), 42.38 (C(CH3)2), 54.40 (OCH3), 112.83, 120.21, 126.41, 126.63, 126.68, 128.90, 129.64, 129.86, 142.41, 159.80 (Ar), 161.55(C=N). HRMS: Calcd for C17H17NOS; 283.1031. Found; 283.1027 (M+).
4,4-Dimethyl-2-(p-chlorophenyl)-4H-3,1-benzothiazine (1f). yellow oil, 1H NMR (CDCl3) δ = 1.66 (s, 6H, CH3), 7.37-7.40 (m, 3H, Ar), 7.44 (d, 2H, J = 8.4 Hz, Ar), 7.52 (d, 1H, J = 7.6 Hz, Ar), 8.09 (d, 2H, J = 8.4 Hz, Ar). 13C NMR (CDCl3) δ = 27.80 (2CH3), 42.40 (C(CH3)2), 120.23, 126.69, 126.81, 127.24, 127.60, 128.23, 129.65, 135.61, 136.44, 142.36 (Ar), 158.47 (C=N). HRMS: Calcd for C16H14ClNS; 287.0535. Found; 287.0530 (M+).
Reaction of N-[2-(2-phenylethynyl)phenyl]benzanilide 3a with P4S10
To a solution of benzanilide 3a (0.15 g, 0.50 mmol) in toluene (5 mL) was added P4S10 (0.055 g, 0.25 mmol) in one portion. After refluxing for 15 h, the reaction mixture was washed with water and extracted with EtOAc (7 mL x 3). The combined extract was washed with water, dried over sodium sulfate, filtered, and evaporated to give pal brown oil, which was chromatographed over alumina gel by elution with hexane:EtOAc (10:1) to afford yellow crystals of N-[2-(2-phenylethynyl)phenyl]- thiobenzanilide 6a (0.12 g, 0.38 mmol, 76%). 6a: yellow needles, mp 90-92 oC. 1H NMR (CDCl3) δ = 7.22-7.55 (m, 11H, Ar), 7.62 (d, 1H, J = 7.6 Hz, Ar), 7.96 (br, 2H, Ar), 10.03 (br, 1H, NH). 13C NMR (CDCl3) δ = 84.48 (q-C), 98.24 (q-C), 115.75 (q-C), 119.35 (q-C), 121.18 q-C), 122.23 (q-C), 126.06, 127.02, 128.95, 129.35, 129.37, 131.54, 131.65, 132.12, 140.25, 144.05 (Ar), 196.75 (C=S). Anal. Calcd for C21H15NS: C, 80.48; H, 4.82; N, 4.47. Found: C, 81.10; H, 4.87; N, 5.10. Analytical data of 7a was little bit off, however, spectral data of which clearly support the structure.
Reaction of N-[2-(2-phenylethynyl)phenyl]propionamide 3e with LR
To a solution of propionamide 3e (0.12 g, 0.50 mmol) in toluene (5 mL) was added LR (0.10 g, 0.25 mmol). After refluxing for 18 h, the reaction mixture was filtered through short silica gel column, and evaporated to give a pale brown oil, which was chromatographed over silica gel by elution with hexane:EtOAc (5:1) to give yellow oil of (Z)-4-benzylidene-2-ethyl-4H-3,1-benzothiazine (4e) (0.10 g, 0.38 mmol). 4e: yellow oil, 1H NMR (CDCl3) δ = 1.28 (t, 3H, J = 7.6 Hz, CH3), 2.57 (q, 2H, J = 7.6 Hz, CH2), 7.03 (s, 1H, =CH), 7.30-7.35 (m, 2H, Ar), 7.40-7.47 (m, 6H, Ar), 7.61 (d, 1H, J = 7.6 Hz, Ar). 13C NMR (CDCl3) δ = 12.10 (CH3), 34.77 (CH2), 121.83 (=CH), 124.38, 124.87, 126.64, 127.66, 128.45, 128.52, 129.09, 129.41, 129.91, 136.06, 142.43 (Ar), 164.02 (C=N). HRMS: Calcd for C17H15NS; 265.0925. Found; 265.0924 (M+).
Other reactions were carried out in a similar manner.
(Z)-4-Benzylidene-2-phenyl-4H-3,1-benzothiazine (4a): yellow oil, 1H NMR (CDCl3) δ = 7.13 (s, 1H, =CH), 7.32-7.57 (m, 11H, Ar), 7.66 (d, 1H, J = 8.0 Hz, Ar), 8.05 (d, 2H, J = 8.2 Hz, Ar). 13C NMR (CDCl3) δ = 122.45 (CH), 124.54, 126.11, 126.22, 127.77, 127.81, 128.60, 128.72, 128.83, 129.64, 129.68, 129.97, 131.77, 136.03, 137.69, 143.13 (Ar), 158.10 (C=N). HRMS: Calcd for C21H15NS; 313.0925. Found; 313.0921 (M+).
(Z)-4-Pentylidene-2-phenyl-4H-3,1-benzothiazine (4b): yellow oil, 1H NMR (CDCl3) δ = 0.94 (t, 3H, J = 7.2 Hz, CH3), 1.40-1.54 (m, 4H, 2CH2), 2.30 (dd, 2H, J = 7.2 Hz and 7.6 Hz, CH2), 6.09 (dd, 1H, J = 7.2 Hz and 7.2 Hz, =CH), 7.27 (dd, 1H, J = 7.6 Hz and 8.2 Hz, Ar), 7.36 (dd, 1H, J = 7.6 Hz and 8.2 Hz, Ar), 7.45-7.51 (m, 5H, Ar), 8.08 (d, 2H, J = 8.2 Hz, Ar). 13C NMR (CDCl3) δ = 12.95 (CH3), 21.38, 27.55, 30.06 (CH2), 120.99 (=CH), 122.48, 123.30, 125.99, 126.33, 127.23, 127.48, 127.99, 128.25, 130.32, 136.73, 141.77 (Ar), 157.57(C=N). HRMS: Calcd for C19H19NS; 293.1238. Found; 293.0236 (M+).

(Z)-4-t-Butylmethylene-2-phenyl-4H-3,1-benzothiazine (4c): yellow needles: mp:84-85 °C, 1H NMR (CDCl3) δ = 1.35 (s, 9H, 3CH3), 6.14 (s, 1H, =CH), 7.30 (dd, 1H, J = 6.4 Hz and 7.6 Hz, Ar), 7.36 (dd, 1H, J = 6.4 Hz and 7.6 Hz, Ar), 7.45-7.52 (m, 5H, Ar), 8.08 (d, 2H, J = 7.6 Hz, Ar). 13C NMR (CDCl3) δ = 30.15 (3CH3), 33.75 (C(CH3)3), 122.84, 123.48 (Ar), 124.32 (=CH), 127.91, 128.58, 128.84, 129.05, 129.18, 131.70, 138.01, 139.45, 143.25 (Ar), 158.81(C=N). EI-MS: Calcd for C19H19NS; 293. Found; (M+) 293. Anal. Calcd for C19H19NS: C, 77.77; H, 6.53; N, 4.77. Found: C, 77.59; H, 6.53; N, 4.94.
X-Ray crystallographic data for 4c: Mr = 293.41, a = 11.512(8) Å, b = 12.053(8) Å, c = 24.029(16) Å, V = 3218(4) Å3, T = 293 K, monoclinic, space group P21/a, Z = 8, 6342 dependent reflections, R1 = 0.0575, wR2 = 0.1278. Crystallographic data have been deposited with Cambridge Crystallographic Data Centre. Deposition number CCDC-815982.
(Z)-4-Benzylidene-2-methyl-4H-3,1-benzothiazine (4d): yellow oil, 1H NMR (CDCl3) δ = 2.36 (s, 3H, CH3), 7.02 (s, 1H, =CH), 7.30-7.46 (m, 8H, Ar), 7.61 (d, 1H, J = 7.8 Hz, Ar). 13C NMR (CDCl3) δ = 27.90 (CH3), 121.51 (=CH), 124.39, 124.88, 126.56, 127.74, 128.52, 128.78, 128.91, 129.40, 129.96, 135.95, 142.40 (Ar), 158.91(C=N). HRMS: Calcd for C16H13NS; 251.0769. Found; 251.0776 (M+).
(Z)-4-Benzylidene-2-(p-tolyl)-4H-3,1-benzothiazine (4f): yellow needles: mp146-148 oC, 1H NMR (CDCl3) δ = 2.41 (s, 3H, CH3), 7.12 (s, 1H, =CH), 7.24-7.26 (m, 2H, Ar), 7.31-7.39 (m, 2H, Ar), 7.43-7.47 (m, 3H, Ar), 7.53-7.57 (m, 3H, Ar), 7.64 (d, 1H, J= 8.0 Hz, Ar), 7.94 (d, 2H, J= 8.0 Hz, Ar). 13C NMR (CDCl3) δ = 21.83 (CH3), 122.52 (=CH), 124.53, 126.06, 126.29, 127.74, 128.56, 128.57, 128.58, 129.53, 129.56, 129.65, 129.91, 135.00, 136.08, 142.28, 143.23 (Ar), 158.01 (C=N). EI-MS: Calcd for C22H17NS; 327. Found; (M+) 327. Anal. Calcd for C22H17NS: C, 80.47; H, 5.42; N, 4.21. Found: C, 80.70; H, 5.23; N, 4.28.
(Z)-4-Benzylidene-2-(p-methoxyphenyl)-4H-3,1-benzothiazine (4g): yellow needles: mp124-126 oC, 1H NMR (CDCl3) δ = 3.86 (s, 3H, OCH3), 6.94 (d, 2H, J = 6.8 Hz, Ar), 7.12 (s, 1H, =CH), 7.34-7.38 (m, 2H, Ar), 7.43 (dd, 3H, J = 7.6 Hz and 8.0 Hz, Ar), 7.52 (dd, 3H, J = 7.6 Hz and 8.0 Hz, Ar), 7.63 (d, 1H, J = 7.6 Hz, Ar), 8.02 (d, 2H, J = 6.8 Hz, Ar). 13C NMR (CDCl3) δ = 55.68 (OCH3), 114.14 (Ar), 122.48 (=CH), 124.55, 126.05, 126.37, 127.74, 128.56, 128.35, 128.56, 129.39, 129.51, 129.68, 129.90, 130.32, 136.13, 143.36, 157.38 (Ar), 162.71 (C=N). EI-MS: Calcd for C22H17NOS; 343. Found; 343 (M+). Anal. Calcd for C22H17NOS: C, 76.94; H, 4.99; N, 4.08. Found: C, 76.85; H, 5.05; N, 4.28.
(Z)-4-Benzylidene-2-(p-chlorophenyl)-4H-3,1-benzothiazine (4h): yellow needles, mp 153-155 oC, 1H NMR (CDCl3) δ = 7.13 (s, 1H, =CH), 7.33-7.48 (m, 7H, Ar), 7.51-7.53 (m, 3H, Ar), 7.65 (d, 1H, J = 7.6 Hz, Ar), 7.99 (d, 2H, J = 8.4 Hz, Ar). 13C NMR (CDCl3) δ = 121.14 (=CH), 123.29, 124.34, 125.31, 126.61, 127.31, 127.38, 127.70, 127.73, 128.14, 128.31, 128.72, 134.57, 134.71, 136.60, 141.64 (Ar), 155.45 (C=N). EI-MS: Calcd for C21H14ClNS; 347. Found; 347 (M+). Anal. Calcd for C21H14ClNS: C, 72.51; H, 4.06; N, 4.03. Found: C, 72.53; H, 4.10; N, 4.23.
(Z)-4-Benzylidene-2-(p-trifluoromethylphenyl)-4H-3,1-benzothiazine (4i): yellow oi1, 1H NMR (CDCl3) δ = 7.14 (s, 1H, =CH), 7.34-7.57 (m, 8H, Ar), 7.66-7.71 (m, 3H, Ar), 8.16 (d, 2H, J = 8.4 Hz, Ar). 13C NMR (CDCl3) δ = 122.38 (=CH), 124.55, 125.31 (Ar), 125.64 (q, J = 15.0 Hz, CF3), 126.74, 127.95, 127.97, 128.61, 129.37, 129.55, 129.85, 130.07, 132.96, 133.28, 135.77, 140.69, 142.74 (Ar), 156.49 (C=N). HRMS: Calcd for C22H14F3NS; 381.0799. Found; 381.0791 (M+).

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