YU Yun HE Jia-Xin HU Hui-Ling LIN Zhi-Lan GAO Yuan
(College of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen 518060, China)
Oxazine derivatives have been considered as one of the most important class of heterocycles, which attract the attention of synthetic chemists owing to their biological importance such as antitumor[1],antiviral[2-3], anti-thrombotic[4], progesterone receptor antagonists[5], inhibition of human leucocyte elastase[6], antibacterial and so on. Therefore, for the preparation of these complex molecules, large efforts have been directed towards the synthetic manipulation of 1,3-oxazine derivatives. As a result, a large number of reports have appeared in the literature.Soola et al.[7]synthesized 1,3-oxazines from the[4+2] cycloaddition of an alkene and an N-acylimine when the reaction was carried out in methylene chloride with BF3EtO2at 5 ℃. Markus et al.[8]reported the synthesis of 1,3-oxazines from the reaction of [2,3-c]pyridine-3-carboxylates with RNCO or p-nitrophenyl chloroformate, but in this reaction, two or three steps are needed. Recently,Kang et al.[9]have reported the synthesis of 1,3-oxazines from the reaction of intramolecular hydroamination of trichloroacetimidate using gold(1) as the catalyst. However, most of these reactions require harsh reaction conditions, expensive starting ma-terials or reagents and longer reaction time. Recently,we have found an unusual cascade reaction that involves a unique CˉC bond cleavage. This protocol affords an unusual and facile method for the preparation of 1,3-oxazin derivatives under mild conditions(Scheme 1) and the results of antibacterial activity studies of the newly synthesized compounds are presented in this paper.
Scheme 1
To the best of our knowledge, there are no reports about the crystal structures of such compounds and therefore, we reported the unexpected formations and crystal structures of 2-(4-(3,4-dimethoxyphenyl)-7,7-dimethyl-5-oxo-3,4,5,6,7,8-hexahydro-2H-be nzo[e][1,3]oxazin-2-ylidene)-3-oxobutanenitrile (3I)and 2-(4-(4-methoxy-phenyl)-7-methyl-5-oxo-3,4-dihydropyrano[3,4-e][1,3]oxazin-2(5H)-ylidene)-3-o xobutanenitrile (3II).
All chemicals and solvents were purchased from commercial sources and used without further purification unless stated otherwise. Two bacteria (E.coli 8099 and S. aureus ATCC6538) were presented by Guangdong institute of microbiology. The melting point was determined on an X-4 micromelting point apparatus and uncorrected. IR spectra were obtained as films on the KBr salt plates, using Pekin Elmer Spectrum One Version B spectrometer. Elemental analyses (C, H, N) were performed with a Vario EL elemental analysis instrument.1H NMR spectra were obtained on a Varian INOVA-400 MHz spectrometer in CDCl3and13C NMR spectra were recorded on a Bruker Advance DPX spectrometer in DMSO-d6at 305.8 K. The chemical shifts are reported in δ value (ppm) using tetramethylsilane as an internal reference. The reflection data were collected on a Riga-ku raxi Rapid diffractometer with an area detector at 273(2) K. The compounds,2-amimo-4-(3,4-dimethoxyphenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitril e 2a and 2-amino-4-(4-methoxyphenyl)-7-methyl-5-oxo-4,5-dihydropyrano[4,3-b]pyran-3-carbonitrile 2b, were prepared and purified according to the literature method[9-10].
In a 25 mL round-bottomed flask equipped with a magnetic stirrer, the obtained 2-amino-4-(3,4-dimethoxy-phenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4 H-chromene-3-carbonitrile (0.351 g, 1 mmol), acetic anhydride (1 mL) and a catalyst (pyridine, 2 drops)were mixed. The flask was stirred and heated in an 85 ℃ oil bath with reflux for 6.5 h, until TLC indicated the end of the reaction. The resulting solid was washed with water and filtered. The solid residue was recrystallized from 95% ethanol at room temperature to give colorless crystals. Yield: 72%, m.p.:209~211 ℃. Anal. Calcd. (%) for C22H24N2O5: C,66.65; H, 6.10; N, 7.07. Found (%): C, 66.51; H,5.66; N, 7.10. IR: (KBr) v: 2198 (C≡N), 1691(C=O)1664 (C=O) cm-1.1H NMR (400MHz, CDCl3) δ:1.08~1.15(s, 6H, 2CH3), 2.91(d, J = 3.58 Hz, 2H,CH2), 2.35(s, 1H, CH3), 2.61(m, 2H, CH2), 3.85~3.87(s, 6H, 2CH3), 5.31(s, 1H, CH), 6.76~6.83(m,3H, ArH), 11.78(s, NH).13C NMR (400Hz,DMSO-d6) δ: 193.05, 193.97, 161.84, 149.32,133.23, 119.63, 118.20, 112.91, 112.40, 111.53,71.24, 56.03, 55.99, 50.22, 48.58, 40.63, 40.42,40.21, 39.79, 39.58, 39.37, 39.26, 32.53, 28.68,28.20, 27.38.
Compound 3II
To a 25 mL round-bottomed flask equipped with a magnetic stirrer, the mixture of the obtained 2-amimo-4-(4-methoxyphenyl)-7-methyl-5-oxo-4,5-dihy dropyrano [4,3-b]-pyran-3-carbonitrile (0.310 g, 1 mmol), acetic anhydride (1 mL) and a catalyst (pyridine, 3 drops) was added. After that, it was stirred and heated in an 85 ℃ oil-bath with reflux for 5 h(monitored by TLC), allowed to cool to room temperature, then subsequently poured into water (200 mL). The precipitate was collected by filtration to afford the crude product which was purified by recrystallization from 95% ethanol at room temperature to furnish the compound as yellow block crystals. Yield: 77%, m.p.: 221~222 ℃. Anal.Calcd. (%) for C19H16N2O5: C, 64.77; H, 4.58; N,8.00. Found (%): C, 64.52; H, 4.42; N, 8.18. IR:(KBr) v: 3446(NH), 2205(C≡N), 1731(C=O) 1672(C=O) cm-1.1H NMR(400 MHz, DMSO-d6) δ:2.21~2.27(s, 6H, 2CH3), 3.74(s, 3H, CH3), 5.50(d,J = 2.79Hz, 1H, CH), 6.60(d, J = 0.80 Hz, 1H,=CH ), 6.91~6.93(m, 2H, ArH), 7.26~7.31(m, 2H,ArH), 11.54(d, J = 2.74 Hz, NH).13C NMR(400 Hz,DMSO) δ: 194.25, 165.11, 161.49, 160.08, 159.96,156.80, 132.20, 129.29, 117.78, 114.67, 100.50,98.10, 55.69, 49.13, 40.74, 40.53, 39.90, 28.22,19.93.
A colorless single crystal of compound 3I with dimensions of 0.45mm × 0.30mm × 0.27mm was selected and mounted on the top of a glass fiber. The data were collected by a Bruke P4 diffractometer equipped with a graphite-monochromatic MoKα (λ =0.71073 ?) radiation using an ω/2θ scan mode in the range of 3.00≤θ≤26.40° (ˉ20≤h≤20, ˉ9≤k≤9,ˉ2≤l≤21) at 173(2) K. A total of 13397 reflections were collected, of which 4153 were independent(Rint= 0.0528) and 3662 were observed with I >2σ(I). A yellow single crystal of compound 3II(0.72mm × 0.62mm × 0.22mm) was selected and mounted on the top of a glass fiber. The data were collected by a Rigaku raxis IP diffractometer equipped with a graphite-monochromatic CuKa radiation(λ = 1.54186 ?) using an ω/2θ scan mode in the range of 3.82≤θ≤68.20° (ˉ8≤h≤8, ˉ12≤k≤12,ˉ2≤l≤28) at 274(2) K. A total of 19142 reflections were collected, of which 5955 were independent(Rint= 0.0414) and 4920 were observed with I >2σ(I). The two structures were solved by direct methods using the program SHELXS-97[11](Sheldrick, 1997) and refined by full-matrix least-squares on F2. The non-hydrogen atoms were refined isotropically and all hydrogen atoms were positioned geometrically. For 3I, the final R = 0.0771, wR =0.1582 (w = 1/[σ2(Fo2) + (0.0466P)2+ 3.0000P],where P = (Fo2+ 2Fc2)/3). (Δ/σ)max= 0.028, S =1.030, (Δρ)max= 0.313 and (Δρ)min= ˉ0.215 e/?3.For 3II, the final R = 0.0515 and wR = 0.1241 (w =1/[σ2(Fo2) + (0.0592P)2+ 1.5000P], where P = (Fo2+ 2Fc2)/3). (Δ/σ)max= 0.003, S = 1.005, (Δρ)max=0.264 and (Δρ)min= ˉ0.366 e/?3.
Antibacterial activity was evaluated by film covering[12]. The concentration of tested compounds dissolved in DMF was 500 mg/L-1and 2 mL was covered at the surface of enamel (50mm × 50mm)with the excess DMF being removed by evaporation.200 uL of the growth bacteria which was diluted to the density of 10-4~10-5CFU/ml was plated on the surface containing the tested compound of enamel and the blank one, respectively, then covered by PE film (40mm × 40mm) and kept for 24 h at constant conditions of temperature ((36±1) ℃). The PE film and enamel were washed by saline (9.5 mL) to give the solution that was diluted to the density of 10-3,10-4and 10-5. The application of 0.2 mL was covered at the surface of PDA and then incubated at 37 ℃for 24 h.
X-ray single-crystal structural analysis revealed that both compounds have [1,3]-oxazin skeleton (Fig. 1 for compound 3I, Fig. 2 for compound 3II). In compound 3I, the C(6)ˉN(1) (1.311(3) ?) bond is remarkably shorter than the normal CˉN bond (1.47 ?)[11], but close to the C=N double bond (1.34 ?);the C(6)=C(11) (1.400(3) ?) is remarkably longer than the normal C=C bond (1.32 ?)[13], indicating delocalization of the electrons. Owing to the p-π conjugate effect, the O(2)ˉC(6) (1.358(3) ?) and O(2)ˉC(5) (1.395(3) ?) bonds are shorter than the normal CˉO bond (1.43 ?)[11]. The torsion angles of C(22)ˉO(5)ˉC(18)ˉC(19) and C(21)ˉO(4)ˉC(17)ˉ C(19) are 2.9(3) and ˉ3.0(4)°, respectively, which demonstrate that two methoxyl groups are nearly planar in relation to the benzene ring. In compound 3II, atoms C(1), N(1), C(2), O(1), C(3) and C(4)form a six-membered ring adopting half-chair conformation in the molecule, with the inter-atomic distance of 1.315(3) ? for N(1)ˉC(2), displaying a partial double-bond character, which is mainly due to delocalization of the lone pair of N(1) into the C(2)=C(16) antibonding orbital, resulting in an elongation of the formally double C(2)=C(16) and a shortening of the formally single N(1)ˉC(2), which increase a value of the N(1)ˉC(2)ˉ(16) (124.6(2)°)bond angle relative to the expected ideal value of 120°. The triple bond length of C(17)≡N(2) (1.149(3)?) is in accordance with the value of 1.146(3) ? found in a similar push-pull family of compounds[14].The planar substituent at C(2) atom forms torsion angles N(1)ˉC(2)ˉC(16)ˉC(18) = ?179.5° and O(5)ˉC(18)ˉC(16)ˉC(2) = ?5.0° with the oxazine ring. As shown, the molecules of the compounds adopt E isomer about the C=C double bonds. In the crystal structure of compound 3I, there exist interand intramolecular hydrogen bonds. The intramolecular N(1)ˉH(1A)··O(3) hydrogen bond forms a six-membered ring, which further enhances the structural rigidity of the N(1)ˉC(6)ˉC(11)ˉC(13)(C(12)ˉN(2)) moiety. In the intermolecular hydrogen bonding, the atom N(1) of oxazine ring in the molecule acts as a hydrogen-bond donor, via atom H(1A), to atom N(2) of the cyano-group, leading to the formation of a nearly linear N(1)ˉH(1A)··N(2).This intermolecular hydrogen bond links the molecules into infinite chains which are stacked along the b axis (Table 1, Fig. 3). In the crystal structure of compound 3II, the molecule is not coplanar but is stabilized by intra- and intermolecular hydrogen bonds (Table 1). The intramolecular N(1)ˉH(1B)··O(5) hydrogen bond is long (2.641(2)?) for the H-atom transfer along it, of which the O serves as an acceptor. Two molecules are stacked by O(5)ˉH(1)··O(8) intermolecular hydrogen bonds to form dimers which are stacked along the a axis (Fig. 4),and those dimmers interact other dimers to generate a network.
Table 1. Hydrogen Bond Lengths (?) and Bond Angles (°) for Compounds 3I and 3II
Fig. 1. Crystal structure of compound 3I
Fig. 2. Crystal structure of compound 3II
Fig. 3. Crystal packing of compound 3I along the b axis
Fig. 4. Crystal packing of compound 3II along the a axis
As shown in Scheme 2, our strategies of synthesizing these compounds were through the reaction of compound 1 or 2 with acetic anhydride in the presence of a substoichiometric amount of pyridine.Based on the success in preparing compounds 3I and 3II, we deemed that under basic conditions the reaction of compounds having the 2-amino-4-aryl-4H-pyran-3-carbonitrile skeleton with acetic anhydride afforded [1,3]-oxazines and this is a rearrangement reaction rather than a simple reaction of amino acylation.
Scheme 2. Synthetic procedure for compounds 3I and 3II
Although the detailed mechanism of the above reaction remains to be unclear, a possible mechanism is proposed in Scheme 3. Initially, nucleophilic attack of a carbonyl of the acetic anhydride by compound A gives intermediate B, eliminating 1 equiv of acetic acid to form the unstable intermediate C. Due to the strong electron-withdrawing groups at the ?-positions of the compounds C, an addition-elimination reaction occurs when pyridine attacks a vinylic carbon atom of C; this opens the chain E-enamine intermediate E which undergoes an intramolecular nucleophilic addition to afford the final products 3I or 3II.
The inhibitory activities of 2 (2a and 2b) and 3(3I and 3II) are shown in Table 2. 3 exhibits significant effects higher than 2. The probable reasons are that compound 3I or 3II with nitrogenoxygen heterocycle favored to combination of drug and role of the targets for enhancing the antibacterial activities of compounds.
Table 2. Fungicidal Activities of 2 and 3
Scheme 3. Possible reaction mechanism of the reaction
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