Sequential Synthesis, Olfactory Properties, and Biological Activity of Quinoxaline Derivatives.

A simple, practical, and rapid access to quinoxalin-2-ones 1, 1,2,3,4-tetrahydroquinoxalines 2, quinoxalines 3, and quinoxalin-2(1H)-ones 4 has been achieved, based on the copper-catalyzed quinoxalinone formation of 2-haloanilines and amino acids followed by their reduction and oxidation. The olfactory properties and lipid accumulation inhibitory activity in cultured hepatocytes of the quinoxaline derivatives were also evaluated.

Introduction

Quinoxaline and its derivatives are important structural units for the development of pharmaceuticals, agrochemicals, and functional materials (Figure ).[1] Some quinoxalines also have a pleasant smell and savory properties.[2] For instance, 5-methylquinoxaline is not only identified by its roasted coffee flavor but also displays a nutty-type odor with a high odor intensity and is utilized for a series of food products, such as coffee, nuts, meats, and cocoa.[2] Similarly, 2-methylquinoxaline has been identified in roasted coffee. However, reports on the odors of quinoxaline derivatives are still limited,[2c] and, therefore, the odors of many quinoxaline derivatives are unknown. Moreover, quinoxaline derivatives are known to display a diverse range of biological activities.[1] On the basis of recent reports,[3−5] we predicted that the quinoxaline derivatives may exert a liver-protective effect. Regarding the synthesis of the quinoxaline derivatives, a significant amount of literature dealing with the synthesis of quinoxaline derivatives has been published.[1a,6] We previously reported the synthesis of quinoxalin-2-ones 1 starting from 2-haloanilines and amino acids in the presence of a copper catalyst.[7] It was anticipated that a series of quinoxaline congeners would be derived from the quinoxalin-2-ones 1 by employing simple manipulations such as reductions and oxidations (Scheme ). In this report, we demonstrate the synthesis of a series of quinoxaline derivatives with different oxidation states, which include 1,2,3,4-tetrahydroquinoxalines 2, quinoxalines 3, and quinoxalin-2(1H)-ones 4 from quinoxalin-2-ones 1. Their olfactory properties and hepatic lipid accumulation inhibitory activity using lipid-overloaded HepG2 hepatocytes were also evaluated.

Figure 1

Examples of useful quinoxaline derivatives.

Scheme 1

Synthetic Approach for Quinoxalinones 1, 1,2,3,4-Tetrahydroquinoxalines 2, Quinoxalines 3, and Quinoxalin-2(1H)-ones 4

Results and Discussion

Synthesis

The syntheses of quinoxalinones were performed based on our previously reported procedure.[7] The reactions of 2-bromoanilines with α-amino acids in the presence of catalytic amounts of CuCl (5 mol %), DMEDA (40 mol %), and K3PO4 (2 equiv) afforded the desired quinoxalinones 1 in moderate to excellent yields (Table ). Quinoxalinones 1a, 1b, and 1k were synthesized for the first time in this study. When 2-bromo-4,6-dimethylaniline was utilized as a substrate, 1,2-dihydroquinoxaline 1k was produced instead of the corresponding desired quinoxalinone.

Table 1

Synthesis of Quinoxalinones and Their Olfactory Properties

The conversions of quinoxalinones 1 to 1,2,3,4-tetrahydroquinoxalines 2 were smoothly conducted using excess lithium aluminum hydride as the reducing agent (Table ).[8]

Table 2

Conversions of Quinoxalinones to 1,2,3,4-Tetrahydroquinoxalines and Their Olfactory Properties

The quinoxaline derivatives 3 were obtained by the oxidations of 1,2,3,4-tetrahydroquinoxalines 2 with vanadium (V) oxide.[9] When (1,2,3,4-tetrahydroquinoxalin-2-yl)methanol 2j was applied under these conditions, aldehyde 3j was obtained in 11% yield. The N-methyl substrate 2i did not afford 1-methyl-1,2-dihydroquinoxaline 3i.

The pathway to quinoxalin-2(1H)-one has been demonstrated using quinoxalinones 1a and 1c as model substrates (Scheme ). Oxidation of quinoxalinones 1a and 1c with V2O5 provided quinoxalin-2(1H)-one 4a and 4c in 67 and 68% yields, respectively. Unfortunately, the reductions of 4a and 4c to the corresponding 1,2-dihydroquinoxalines have not been successful.

Scheme 2

Synthesis of Quinoxalin-2(1H)-ones Starting from Quinoxalinones

Olfactory Properties

We next evaluated the olfactory properties of the quinoxaline derivatives 1–3 synthesized in this study, which were performed by five panelists (30–50-year-old men), including three flavorists, by direct smelling from a sample tube. The results are listed in Table –3 under the structures of the compounds. Quinoxalines 3a–c and 3e were discovered in nature[2] and their olfactory properties are already known; however, the other derivatives were evaluated for the first time. It was found that quinoxalinones 1 tended to have phenolic or medicine-like odors, although some of them have chocolate- (1e and 1j) or cacaolike (1f) smells. We assume that this would be the case because of the existence of a carbonyl group on the quinoxalinones 1. Some 1,2,3,4-tetrahydroquinoxalines 2 had odors of food such as nutty (2a) and roasty (2b and 2e). Others had phenolic (2c and 2g), medicine-like (2d, 2f, and 2m), and fishy (2k) odors. Their odor qualities depended on the number and position of the substituents and also their functional groups. Generally, many 1,2,3,4-tetrahydroquinoxalines have a pungent odor. Gratifyingly, the monomethyl-substituted quinoxalines have desirable odors, such as coffeelike (3c), cookielike (3b), and roast chestnutlike (3e). On the other hand, the dimethyl, ethyl, and formyl-substituted quinoxalines displayed a phenolic (3d, 3g, and 3j) and smoky odor (3f). Generally, quinoxalines 3 did not display a pungent odor and had mild and desirable incenses. In terms of each quinoxaline derivative, roasty (2a), peanutlike (2e), cookielike (3b), coffeelike (3c), and roast chestnutlike odors (3e) have been determined. These compounds have common structural units such as being monomethyl substituted, especially, the 5-methyl-substituted quinoxaline. The intensity levels of the odors of the quinoxaline derivatives synthesized in this study were found to be as follows: quinoxalines 3 > tetrahydroquinoxalines 2 > quinoxalinones 1.

Table 3

Synthesis of Quinoxalines and Their Olfactory Properties

Biological Activities

Recent research evidence has demonstrated that the cannabinoid type 2 (CB2) receptor has therapeutic value in terms of pain relief, inflammation, osteoporosis, treating cancers, and hepatoprotection. The activation of the CB2 receptors in Kupffer cells and hepatocytes has been reported to protect inflammatory liver injury and attenuate liver steatosis in vitro and in vivo.[3−5] In recent years, structurally diverse CB2-selective agonists exhibiting an analgesic activity in various pain models have been discovered, and Saari et al. have reported that the quinoxaline derivatives showed a low-potency partial CB2 receptor agonist activity.[3] Therefore, we predicted that the synthesized quinoxaline derivatives may exert a liver-protective effect. We screened the synthesized quinoxaline derivatives for lipid accumulation inhibitory activity using lipid-overloaded HepG2 hepatocytes.[5] The cells were exposed to 0.5 mM palmitate in the presence of each synthesized quinoxaline derivative for 24 h; then the intracellular neutral lipids were determined by Oil Red O staining. As shown in Figure A, we found that some quinoxalinones 1 effectively prevent palmitate-induced lipid accumulation in HepG2 hepatocytes. It is noteworthy that 8-methyl-3,4-dihydro-1H-quinoxalin-2-one (1a) and 3,6-dimethyl-3,4-dihydro-1H-quinoxalin-2-one (1f, 1m) showed a marked decrease in lipid accumulation by about 50% compared to that of the vehicle-treated control. In addition, tetrahydroquinoxalines 2a, 2e, 2i, and quinoxalines 3c and 3f slightly inhibited the hepatic lipid accumulation (Figure B,C). These results suggest that the quinoxalinone structure might play a potential role in lipid accumulation suppressing activity, and some flavors might act as biofunctional materials. In future, further studies are needed to examine the detailed molecular mechanism underlying their actions, including CB2 agonistic activity and assess their in vivo efficacy.

Figure 2

Effects of quinoxaline derivatives on palmitate-induced lipid accumulation in HepG2 cells. The cells were incubated with each quinoxaline derivative (10 μM) in the presence of 0.5 mM palmitate for 24 h in serum-free Dulbecco’s modified Eagle’s medium (DMEM). The neutral lipid level normalized to the cellular protein content was determined by Oil Red O staining. (A) Quinoxalinones 1a–o. (B) Tetrahydroquinoxalines 2a–o. (C) Quinoxalines 3a–k. Compounds 1k, 4a, and 4c could not be dissolved in media, so they were not included in the data.

Conclusions

In conclusion, a protocol for synthesizing a series of quinoxaline derivatives, that is, the 3,4-dihydroquinoxalin-2(1H)-ones, quinoxalin-2(1H)-ones, 1,2,3,4-tetrahydroquinoxalines, and quinoxalines, varying in their four different oxidation states but with the same carbon skeleton, has been developed from readily available starting materials. These methods would serve the purposes of the fine tuning of odorants as well as the biological activities of pharmaceuticals and agrochemicals and also the functions of materials with the quinoxaline skeleton. Indeed, promising perfume and drug candidates among the synthetic quinoxaline derivatives obtained in this study have been observed in some cases. Studies for synthesizing other quinoxaline derivatives from 2-haloanilines and amino acids and a wide range of pharmaceutical and agrochemical activities for the synthetic quinoxalines are now under investigation and will be reported in due course.

Materials and Methods

Instruments

The 1H and 13C NMR spectra were determined in CDCl3 solution using 400 and 100 MHz spectrometers, respectively. The proton chemical shifts (δ) are relative to tetramethylsilane (TMS, δ = 0.0) as the internal standard and expressed in parts per million. Spin multiplicities are given as s (singlet), d (doublet), t (triplet), and m (multiplet) as well as brs (broad singlet). Coupling constants (J) are listed in hertz. The infrared spectra were recorded by an Fourier transform infrared spectrometer. Melting points were determined using a Büchi melting point B-540 apparatus and are uncorrected. The MS spectra were obtained using a mass spectrometer. The HRMS was determined using a JEOL JNM-AX 500 mass spectrometer.

Chemicals

All chemical reagents were commercially available. The solvents (chloroform (99%), dimethyl sulfoxide (DMSO) (99%), ethyl acetate (99%), hexane (99%), diethyl ether (99%), tetrahydrofuran (THF) (99%), and toluene (99%)) were purchased from Tokyo Chemical Industry Co., Ltd. and were directly used without further purification. Unless otherwise stated, the reactions were monitored by thin layer chromatography on silica gel plates (60 F254) and visualized with ultraviolet light or iodine spray. Column chromatography was performed on silica gel (60–120 mesh) using hexane and ethyl acetate.

General Procedure for the Preparation of 3,4-Dihydroquinoxalin-2(1H)-ones 1a–o

A suspension of 2-bromoaniline (1 equiv), amino acid (2–3 equiv), copper chloride (I) (5–15 mol %), potassium phosphate (2–3 equiv), and dimethylethylenediamine (20–60 mol %) in dry DMSO was degassed with N2 for 5 min and stirred at 110 °C under a nitrogen atmosphere for 24 h. The reaction mixture was quenched with water and extracted with AcOEt. The organic layer was washed with water and a saturated aqueous solution of ammonium chloride and then dried over Na2SO4. After filtration, the solvent was removed in vacuo. The residue was purified by silica gel column chromatography (eluent: hexanes/AcOEt) to afford the corresponding quinoxalin-2-one 1.

8-Methyl-3,4-dihydro-1H-quinoxalin-2-one (1a)

2-Bromo-6-methylaniline (1.11 g, 6.00 mmol), glycine (0.90 g, 12.0 mmol), copper chloride (I) (30 mg, 0.3 mmol), K3PO4 (2.55 g, 12.0 mmol), dimethylethylenediamine (0.21 g, 2.4 mmol), and dry DMSO (20 mL) were used, and the reaction mixture was extracted with CHCl3 instead of AcOEt: pale brown solid; 550 mg, 56% yield; R = 0.36 (hexanes/AcOEt = 1/1); mp 159.3–161.7 °C; 1H NMR (400 MHz, CDCl3) δ 8.07 (brs, 1H, NH), 6.81 (t, J = 7.8 Hz, 1H, Ar), 6.63 (d, J = 7.8 Hz, 1H, Ar), 6.56 (d, J = 7.8 Hz, 1H, Ar), 3.95 (d, J = 1.5 Hz, 2H), 3.87 (brs, 1H, NH), 2.24 (s, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 166.8, 133.7, 123.9, 123.5, 123.3, 121.4, 112.1, 47.1, 16.5; IR (KBr) ν 3223, 2974, 1689, 1601, 1486, 1388, 1299, 796, 732 cm–1; HRMS (FAB) m/z [M + H]+ calcd for C9H11ON2 163.0871, found 163.0876.

6-Methyl-3,4-dihydro-1H-quinoxalin-2-one (1b)

2-Bromo-4-methylaniline (1.49 g, 8.00 mmol), glycine (1.80 g, 24.0 mmol), copper chloride (I) (45 mg, 0.4 mmol), K3PO4 (5.00 g, 24.0 mmol), dimethylethylenediamine (0.28 g, 3.2 mmol), and dry DMSO (25 mL) were used: yellow solid; 1.10 g, 83% yield; R = 0.38 (hexanes/AcOEt = 1/1); mp 151.0–151.9 °C; 1H NMR (400 MHz, CDCl3) δ 9.20 (brs, 1H, NH), 6.66 (d, J = 8.0 Hz, 1H, Ar), 6.55 (d, J = 8.0 Hz, 1H, Ar), 6.49 (s, 1H, Ar), 3.96 (d, J = 1.2 Hz, 2H), 3.81 (brs, 1H, NH), 2.23 (s, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 167.2, 133.7, 133.5, 123.1, 120.1, 115.6, 114.6, 47.1, 20.9; IR (KBr) ν 3368, 3067, 1682, 1523, 1408, 1299, 804 cm–1; HRMS (FAB) m/z [M + H]+ calcd for C9H11ON2 163.0872, found 163.0858.

(S)-3-Methyl-3,4-dihydro-1H-quinoxalin-2-one (1c)[10]

2-Bromoaniline (688 mg, 4.00 mmol), l-alanine (713 mg, 8.00 mmol), copper chloride (I) (20 mg, 0.2 mmol), K3PO4 (1.75 g, 8.00 mmol), dimethylethylenediamine (140 mg, 1.6 mmol), and dry DMSO (10 mL) were used: white solid; 562 mg, 87% yield; R = 0.39 (hexanes/AcOEt = 1/1); mp 116.2–118.5 °C (lit. 112.4 °C);[10] [α]D22 = +54.20 (c 0.20, CHCl3) (lit. [α]D22 = +8.30 (c 0.60, CHCl3));[10]1H NMR (400 MHz, CDCl3) δ 8.59 (brs, 1H, NH), 6.89 (dt, J = 8.0, 2.8 Hz, 1H, Ar), 6.80–6.73 (m, 2H, Ar), 6.68 (d, J = 8.0 Hz, 1H, Ar), 4.02 (q, J = 6.6 Hz, 1H), 3.85 (brs, 1H, NH), 1.46 (d, J = 6.6 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 169.8, 133.4, 125.6, 123.7, 119.5, 115.5, 114.0, 51.8, 17.9.

dl-3-Ethyl-3,4-dihydro-1H-quinoxalin-2-one (1d)[11]

2-Bromoaniline (1.37 g, 8.00 mmol), a-aminobutynic acid (2.36 g, 24.0 mmol), copper chloride (I) (40 mg, 0.4 mmol), K3PO4 (5.02 g, 24.0 mmol), dimethylethylenediamine (280 mg, 3.2 mmol), and dry DMSO (25 mL) were used: yellow solid; 1.38 g, 98% yield; R = 0.24 (hexanes/AcOEt = 2/1); mp 85.8–87.4 °C (lit. 77.4–78.5 °C);[11]1H NMR (400 MHz, CDCl3) δ 9.30 (brs, 1H, NH), 6.88 (dt, J = 7.6, 1.6 Hz, 1H, Ar), 6.79–6.71 (m, 2H, Ar), 6.67 (d, J = 7.6 Hz, 1H, Ar), 3.99 (brs, 1H, NH), 3.87 (ddd, J = 8.4, 5.2, 1.2 Hz, 1H), 1.93–1.74 (m, 2H, CH2), 1.03 (t, J = 7.6 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 169.1, 133.0, 125.2, 123.8, 119.2, 115.3, 114.0, 57.5, 25.1, 9.6.

(S)-3,8-Dimethyl-3,4-dihydro-1H-quinoxalin-2-one (1e)[12]

2-Bromo-6-methylaniline (186 mg, 1.00 mmol), l-alanine (180 mg, 2.00 mmol), copper chloride (I) (5 mg, 0.05 mmol), K3PO4 (405 mg, 2.00 mmol), dimethylethylenediamine (35 mg, 0.4 mmol), and dry DMSO (5 mL) were used: pale brown solid; 125 mg, 71% yield; R = 0.38 (hexanes/AcOEt = 3/2); mp 104.1–105.4 °C; [α]D23 = +61.37 (c 0.18, CHCl3); 1H NMR (400 MHz, CDCl3) δ 8.12 (brs, 1H, NH), 6.80 (dd, J = 7.8, 7.6 Hz, 1H, Ar), 6.63 (d, J = 7.6 Hz, 1H, Ar), 6.56 (d, J = 7.8 Hz, 1H, Ar), 3.97 (q, J = 6.6 Hz, 1H), 3.86 (brs, 1H, NH), 2.24 (s, 3H, Me), 1.45 (d, J = 6.6 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 169.3, 133.5, 124.0, 123.4, 123.2, 121.4, 112.2, 51.6, 17.4, 16.5.

(S)-3,6-Dimethyl-3,4-dihydro-1H-quinoxalin-2-one (1f)[13]

2-Bromo-4-methylaniline (1.48 g, 8.00 mmol), l-alanine (1.42 g, 16.0 mmol), copper chloride (I) (40 mg, 0.4 mmol), K3PO4 (3.41 g, 16.0 mmol), dimethylethylenediamine (0.28 g, 3.2 mmol), and dry DMSO (20 mL) were used: pale yellow solid; 1.13 g, 81% yield; R = 0.55 (hexanes/AcOEt = 1/1); mp 128.5–131.7 °C; [α]D22 = +48.76 (c 0.18, CHCl3); 1H NMR (400 MHz, CDCl3) δ 8.92 (brs, 1H, NH), 6.66 (d, J = 7.8 Hz, 1H, Ar), 6.56 (d, J = 7.8 Hz, 1H, Ar), 6.50 (s, 1H, Ar), 3.99 (q, J = 6.6 Hz, 1H), 3.79 (brs, 1H, NH), 2.24 (s, 3H, Me), 1.44 (d, J = 6.6 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 169.5, 133.5, 133.2, 123.2, 120.1, 115.3, 114.7, 51.9, 20.9, 17.9.

(S)-3,7-Dimethyl-3,4-dihydro-1H-quinoxalin-2-one (1g)

2-Bromo-5-methylaniline (558 mg, 3.00 mmol), l-alanine (534 mg, 6.00 mmol), copper chloride (I) (15 mg, 0.15 mmol), K3PO4 (1.27 g, 6.00 mmol), dimethylethylenediamine (106 mg, 1.20 mmol), and dry DMSO (10 mL) were used: pale yellow solid; 408 mg, 77% yield; R = 0.38 (hexanes/AcOEt = 1/1); mp 161.7–164.0 °C; [α]D22 = +48.69 (c 0.16, CHCl3); 1H NMR (400 MHz, CDCl3) δ 8.93 (brs, 1H, NH), 6.70 (d, J = 8.0 Hz, 1H, Ar), 6.59 (d, J = 8.0 Hz, 2H, Ar), 3.99 (q, J = 6.6 Hz, 1H), 3.75 (brs, 1H, NH), 2.25 (s, 3H, Me), 1.45 (d, J = 6.8 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 170.0, 131.0, 129.3, 125.7, 124.2, 116.1, 114.1, 52.1, 20.6, 17.7; IR (KBr) ν 3678, 2958, 1660, 1621, 1562, 1397, 1193, 886, 822 cm–1; HRMS (FAB) m/z [M + H]+ calcd for C10H13ON2 177.1028, found 177.1031.

(S)-3,5-Dimethyl-3,4-dihydro-1H-quinoxalin-2-one (1h)[12]

2-Bromo-3-methylaniline (97 mg, 0.5 mmol), l-alanine (90 mg, 1.0 mmol), copper chloride (I) (7.8 mg, 0.07 mmol), K3PO4 (212 mg, 1.00 mmol), dimethylethylenediamine (26 mg, 0.3 mmol), and dry DMSO (2.5 mL) were used, and the reaction mixture was stirred for 48 h: dark yellow solid; 56 mg, 61% yield; R = 0.34 (hexanes/AcOEt = 3/2); mp 124.5–125.3 °C; [α]D23 = +5.55 (c 0.18, CHCl3); 1H NMR (400 MHz, CDCl3) δ 8.80 (brs, 1H, NH), 6.78 (dq, J = 6.1, 1.0 Hz, 1H, Ar), 6.69 (dd, J = 7.8, 1.7 Hz, 1H, Ar), 6.65 (dd, J = 7.8, 1.7 Hz, 1H, Ar), 4.05 (dq, J = 6.5, 2.0 Hz, 1H), 3.72 (brs, 1H, NH), 2.18 (s, 3H, Me), 1.48 (d, J = 6.8 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 169.4, 131.5, 125.2, 125.1, 121.9, 118.9, 113.5, 51.8, 18.1, 16.6.

4-Methyl-3,4-dihydro-1H-quinoxalin-2-one (1i)[14]

2-Bromoaniline (1.03 g, 6.0 mmol), sarcosine (1.07 g, 12.0 mmol), copper chloride (I) (59 mg, 0.6 mmol), K3PO4 (2.55 g, 12 mmol), dimethylethylenediamine (212 mg, 2.4 mmol), and dry DMSO (14 mL) were used: gray solid; 644 mg, 66% yield; R = 0.35 (hexanes/AcOEt = 3/2); mp 144.0–146.7 °C (lit. 140–143 °C);[14]1H NMR (400 MHz, CDCl3) δ 9.41 (brs, 1H, NH), 6.99 (dt, J = 8.1, 2.0 Hz, 1H, Ar), 6.83–6.70 (m, 2H, Ar), 6.67 (d, J = 8.1 Hz, 1H, Ar), 3.78 (s, 2H), 2.85 (s, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 167.6, 136.1, 126.3, 124.2, 119.1, 115.4, 111.5, 54.7, 37.3.

(S)-3-Hydroxymethyl-3,4-dihydro-1H-quinoxalin-2-one (1j)[10]

2-Bromoaniline (4.3 g, 25 mmol), l-serin (7.9 g, 75 mmol), copper chloride (I) (248 mg, 2.5 mmol), K3PO4 (16 g, 75 mmol), dimethylethylenediamine (1.7 g, 20 mmol), and dry DMSO (60 mL) were used, and the reaction mixture was stirred for 70 h: pale brown solid; 2.16 g, 48% yield; R = 0.33 (hexanes/AcOEt = 1/4); mp 128.7–134.0 °C (lit. 80.3 °C);[7] [α]D22 = +17.44 (c 0.18, CHCl3) (lit. [α]D22 = +19.6 (c 0.3, CHCl3));[10]1H NMR (400 MHz, CDCl3) δ 8.13 (brs, 1H, NH), 6.92 (dt, J = 7.6, 1.2 Hz, 1H, Ar), 6.77 (dt, J = 7.6, 1.2 Hz, 1H, Ar), 6.72 (dd, J = 7.6, 1.2 Hz, 2H, Ar), 4.16 (brs, 1H, NH), 4.11 (t, J = 6.0 Hz, 1H), 4.06 (dd, J = 10.8, 6.0 Hz, 1H), 3.90 (dd, J = 10.8, 6.0 Hz, 1H), 2.63 (s, 1H, OH); 13C NMR (100 MHz, CDCl3) δ 162.1, 133.0, 124.7, 124.2, 119.7, 115.4, 114.3, 63.0, 57.0.

6,8-Dimethyl-1H-quinoxalin-2-one (1k)

2-Bromo-4,6-dimethylaniline (1.6 g, 8.0 mmol), glycine (1.2 g, 16.0 mmol), copper chloride (I) (40 mg, 0.4 mmol), K3PO4 (3.4 g, 16 mmol), dimethylethylenediamine (0.28 g, 3.2 mmol), and dry DMSO (20 mL) were used: yellow solid; 883 mg, 63% yield; R = 0.36 (hexanes/AcOEt = 1/1); mp 215.2–217.9 °C; 1H NMR (400 MHz, CDCl3) δ 11.42 (brs, 1H, NH), 8.31 (s, 1H), 7.51 (s, 1H, Ar), 7.23 (s, 1H, Ar), 2.55 (s, 3H, Me), 2.42 (s, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 156.5, 150.1, 133.7, 133.6, 133.1, 127.5, 127.1, 124.0, 20.8, 16.5; IR (KBr) ν 3389, 3158, 3032, 1669, 1560, 1473, 861 cm–1; HRMS (FAB) m/z [M + H]+ calcd for C10H11ON2 175.0871, found 175.0862.

(R)-3-Methyl-3,4-dihydro-1H-quinoxalin-2-one (1l)[10]

2-Bromoaniline (688 mg, 4.00 mmol), d-alanine (715 mg, 8.00 mmol), copper chloride (I) (20 mg, 0.2 mmol), K3PO4 (1.75 g, 8.00 mmol), dimethylethylenediamine (70 mg, 0.8 mmol), and dry DMSO (15 mL) were used: pale yellow solid; 564 mg, 87% yield; R = 0.31 (hexanes/AcOEt = 1/1); mp 116.4–117.3 °C (lit. 112.4 °C);[10] [α]D23 = −62.19 (c 0.19, CHCl3); 1H NMR (400 MHz, CDCl3) δ 9.23 (brs, 1H, NH), 6.88 (dt, J = 7.8, 1.8 Hz, 1H, Ar), 6.82–6.71 (m, 2H, Ar), 6.68 (d, J = 7.8 Hz, 1H), 4.02 (q, J = 6.8 Hz, 1H), 3.89 (brs, 1H, NH), 1.46 (d, J = 6.8 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 169.8, 133.4, 125.6, 123.7, 119.5, 115.5, 114.0, 51.8, 17.9.

(R)-3,6-Dimethyl-3,4-dihydro-1H-quinoxalin-2-one (1m)[13]

2-Bromo-4-methylaniline (744 mg, 4.00 mmol), d-alanine (713 mg, 8.00 mmol), copper chloride (I) (20 mg, 0.2 mmol), K3PO4 (1.70 g, 8.00 mmol), dimethylethylenediamine (0.14 g, 1.6 mmol), and dry DMSO (15 mL) were used: pale orange solid; 604 mg, 86% yield; R = 0.41 (hexanes/AcOEt = 1/1); mp 125.7–127.5 °C; [α]D22 = −64.70 (c 0.18, CHCl3); 1H NMR (400 MHz, CDCl3) δ 9.12 (brs, 1H, NH), 6.67 (d, J = 7.6 Hz, 1H, Ar), 6.56 (d, J = 7.6 Hz, 1H, Ar), 6.49 (s, 1H, Ar), 4.00 (q, J = 6.4 Hz, 1H), 3.80 (brs, 1H, NH), 2.24 (s, 3H, Me), 1.44 (d, J = 6.4 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 169.6, 133.5, 133.2, 123.2, 120.1, 115.3, 114.7, 51.9, 20.9, 17.9.

(R)-3,8-Dimethyl-3,4-dihydro-1H-quinoxalin-2-one (1n)[12]

2-Bromo-6-methylaniline (746 mg, 4.00 mmol), d-alanine (713 mg, 8.00 mmol), copper chloride (I) (20 mg, 0.2 mmol), K3PO4 (1.7 g, 8.0 mmol), dimethylethylenediamine (141 mg, 1.60 mmol), and dry DMSO (15 mL) were used: pale orange solid; 493 mg, 70% yield; R = 0.42 (hexanes/AcOEt = 1/1); mp 93.7–95.2 °C; [α]D21 = −65.39 (c 0.16, CHCl3); 1H NMR (400 MHz, CDCl3) δ 8.07 (brs, 1H, NH), 6.81 (t, J = 7.8 Hz, 1H, Ar), 6.64 (d, J = 7.8 Hz, 1H, Ar), 6.56 (d, J = 7.8 Hz, 1H, Ar), 3.97 (q, J = 6.1 Hz, 1H), 3.85 (brs, 1H, NH), 2.24 (s, 3H, Me), 1.45 (d, J = 6.6 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 169.2, 133.5, 124.1, 123.2, 1232, 121.4, 112.2, 51.7, 17.5, 16.5.

(R)-3,7-Dimethyl-3,4-dihydro-1H-quinoxalin-2-one (1o)

2-Bromo-5-methylaniline (372 mg, 2.00 mmol), d-alanine (358 mg, 4.00 mmol), copper chloride (I) (11 mg, 0.1 mmol), K3PO4 (849 mg, 4.00 mmol), dimethylethylenediamine (70 mg, 0.8 mmol), and dry DMSO (7 mL) were used: pale brown solid; 253 mg, 72% yield; R = 0.31 (hexanes/AcOEt = 1/1); mp 156.0–159.1 °C; [α]D22 = −47.03 (c 0.15, CHCl3); 1H NMR (400 MHz, CDCl3) δ 8.98 (brs, 1H, NH), 6.69 (d, J = 7.8 Hz, 1H, Ar), 6.59 (d, J = 7.8 Hz, 2H, Ar), 3.98 (q, J = 6.8 Hz, 1H), 3.75 (brs, 1H, NH), 2.24 (s, 3H, Me), 1.45 (d, J = 6.6 Hz, Me); 13C NMR (100 MHz, CDCl3) δ 170.0, 130.9, 129.3, 125.7, 124.1, 116.0, 114.1, 52.1, 20.6, 17.7; IR (KBr) ν 3737, 2951, 1660, 1620, 1561, 1396, 1193, 886, 822 cm–1; HRMS (FAB) m/z [M + H]+ calcd for C10H13ON2 177.1028, found 177.1019.

General Procedure for Synthesis of 1,2,3,4-Tetrahydroquinoxalines

Lithium aluminum hydride (6.5–8.8 equiv) was dissolved in dry THF under nitrogen at 0 °C. To the stirred solution, over a period of 5 min, was added a solution of quinoxalin-2-one in dry THF at 0 °C, and the mixture was refluxed for 18 h. The resulting reaction mixture was quenched with water and AcOEt and filtered through a Celite plug; then the solution was dried over Na2SO4. After filtration, the solvent was removed in vacuo. The residue was purified by silica gel column chromatography (eluent: hexanes/AcOEt) to afford the corresponding 1,2,3,4-tetrahydroquinoxaline 2.

5-Methyl-1,2,3,4-tetrahydroquinoxaline (2a)[15]

8-Methyl-3,4-dihydro-1H-quinoxalin-2-one 1a (100 mg, 0.60 mmol), lithium aluminum hydride (152 mg, 4.00 mmol), and dry THF (10 mL) were used: brown liquid; 69 mg, 76% yield; R = 0.28 (hexanes/AcOEt = 1/1); 1H NMR (400 MHz, CDCl3) δ 6.51 (dd, J = 12.4, 6.5 Hz, 2H, Ar), 6.40 (dd, J = 6.5, 2.4 Hz, 1H, Ar), 3.48 (dd, J = 6.5, 2.4 Hz, 2H), 3.40 (dd, J = 6.5, 2.4 Hz, 2H), 2.08 (s, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 133.1, 131.6, 122.1, 120.3, 117.9, 112.8, 41.7, 41.1, 16.9.

6-Methyl-1,2,3,4-tetrahydroquinoxaline (2b)[16]

6-Methyl-3,4-dihydro-1H-quinoxalin-2-one 1b (100 mg, 0.60 mmol), lithium aluminum hydride (200 mg, 5.30 mmol), and dry THF/dry diethyl ether = 1:2 (15 mL) were used: brown solid; 87 mg, 93% yield; R = 0.37 (hexanes/AcOEt = 1/1); mp 99.1–102.2 °C (lit. 100–102 °C);[16]1H NMR (400 MHz, CDCl3) δ 6.41 (d, J = 8.0 Hz, 2H, Ar), 6.32 (s, 1H, Ar), 3.38 (s, 3H), 3.38 (brs, 2H, NH), 2.17 (s, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 133.7, 131.1, 128.3, 119.0, 115.4, 114.9, 41.5, 41.5, 20.7.

(S)-2-Methyl-1,2,3,4-tetrahydroquinoxaline (2c)[11,17]

(S)-3-Methyl-3,4-dihydro-1H-quinoxalin-2-one 1c (403 mg, 2.50 mmol), lithium aluminum hydride (840 mg, 22.0 mmol), and dry THF (80 mL) were used: brown solid; 360 mg, 98% yield; R = 0.28 (hexanes/AcOEt = 4/1); mp 62.0–65.2 °C (lit. 88.1–89.5 °C);[8] [α]D19 = −23.8 (c 0.22, EtOH) (lit. [α]D25 = −33.8 (c 1.06, EtOH));[17]1H NMR (400 MHz, CDCl3) δ 6.61–6.55 (m, 2H, Ar), 6.53–6.45 (m, 2H, Ar), 3.57 (brs, 1H, NH), 3.50 (t, J = 6.1 Hz, 1H), 3.30 (dd, J = 10.8, 2.2 Hz, 1H), 3.03 (t, J = 9.4 Hz, 1H), 1.18 (d, J = 6.1 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 133.5, 133.1, 118.6 (2C), 114.4, 114.4, 48.2, 45.6, 19.8.

dl-2-Ethyl-1,2,3,4-tetrahydroquinoxaline (2d)[11]

3-Ethyl-3,4-dihydro-1H-quinoxalin-2-one 1d (484 mg, 2.70 mmol), lithium aluminum hydride (910 mg, 24.2 mmol), and dry THF (45 mL) were used: orange solid; 434 mg, 97% yield; R = 0.33 (hexanes/AcOEt = 4/1); mp 67.0–68.0 °C (lit. 67.9–69.2 °C);[8]1H NMR (400 MHz, CDCl3) δ 6.60–6.54 (m, 2H, Ar), 6.51–6.46 (m, 2H, Ar), 3.64 (brs, 1H, NH), 3.37 (dd, J = 10.5, 2.9 Hz, 1H), 3.28 (dq, J = 7.2, 2.9 Hz, 1H), 3.06 (dd, J = 10.5, 7.2 Hz, 1H), 1.52 (quint, J = 7.2 Hz, 2H, CH2), 1.00 (t, J = 7.6 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 133.5, 133.4, 118.7, 118.5, 114.4, 114.4, 51.7, 46.3, 27.1, 10.0.

(S)-2,5-Dimethyl-1,2,3,4-tetrahydroquinoxaline (2e)

(S)-3,8-Dimethyl-3,4-dihydro-1H-quinoxalin-2-one 1e (400 mg, 2.30 mmol), lithium aluminum hydride (603 mg, 16.0 mmol), and dry THF (40 mL) were used: orange liquid; 335 mg, 91% yield; R = 0.38 (hexanes/AcOEt = 2/1); [α]D22 = +6.52 (c 0.29, CHCl3); 1H NMR (400 MHz, CDCl3) δ 6.53 (d, J = 6.8 Hz, 1H, Ar), 6.49 (d, J = 6.8 Hz, 1H, Ar), 6.40 (dd, J = 6.8, 2.4 Hz, 1H, Ar), 3.53–3.42 (m, 1H), 3.47 (brs, 1H, NH), 3.38 (dd, J = 10.8, 2.8 Hz, 1H), 3.06 (dd, J = 10.8, 8.3 Hz, 1H), 2.08 (s, 3H, Me), 1.17 (d, J = 6.3 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 133.1, 131.1, 121.9, 120.2, 117.9, 112.6, 48.5, 45.4, 19.8, 17.0; IR (NaCl) ν 3373, 2962, 2854, 1597, 1493, 1299, 1171, 966, 757 cm–1; HRMS (FAB) m/z [M + H]+ calcd for C10H15N2 163.1236, found 163.1209.

(S)-2,7-Dimethyl-1,2,3,4-tetrahydroquinoxaline (2f)

(S)-3,6-Dimethyl-3,4-dihydro-1H-quinoxalin-2-one 1f (795 mg, 2.50 mmol), lithium aluminum hydride (1.31 g, 36.0 mmol), and dry THF (80 mL) were used: brown solid; 677 mg, 93% yield; R = 0.33 (hexanes/AcOEt = 2/1); mp 50.6–52.4 °C; [α]D22 = −17.44 (c 0.22, CHCl3); 1H NMR (400 MHz, CDCl3) δ 6.43 (d, J = 7.8 Hz, 1H, Ar), 6.39 (d, J = 7.8 Hz, 1H, Ar), 6.33 (s, 1H, Ar), 3.45 (brs, 1H, NH), 3.29 (d, J = 9.8 Hz, 2H, CH2), 3.00 (t, J = 9.8 Hz, 1H, NH), 2.17 (s, 3H, Me), 1.17 (d, J = 6.1 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 133.6, 130.6, 128.3, 119.0, 115.2, 114.7, 48.4, 45.9, 20.6, 19.9; IR (KBr) ν 3350, 3305, 2953, 2852, 1593, 1523, 1354, 1301, 848, 800 cm–1; HRMS (FAB) m/z [M + H]+ calcd for C10H15N2 163.1235, found 163.1261.

(S)-2,6-Dimethyl-1,2,3,4-tetrahydroquinoxaline (2g)

(S)-3,7-Dimethyl-3,4-dihydro-1H-quinoxalin-2-one 1g (400 mg, 2.20 mmol), lithium aluminum hydride (603 mg, 16.0 mmol), and dry THF (40 mL) were used: yellow solid; 256 mg, 69% yield; R = 0.35 (hexanes/AcOEt = 2/1); mp 85.2–87.2 °C; [α]D22 = −7.75 (c 0.22, CHCl3); 1H NMR (400 MHz, CDCl3) δ 6.43–6.36 (m, 2H, Ar), 6.33 (s, 1H, Ar), 3.46 (t, J = 6.1 Hz, 1H), 3.29 (dd, J = 10.8, 2.4 Hz, 2H), 3.02 (t, J = 8.5 Hz, 1H), 2.17 (s, 3H, Me), 1.17 (d, J = 6.3 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 133.2, 131.0, 128.2, 119.0, 115.1, 114.6, 48.3, 45.8, 20.6, 19.8; IR (KBr) ν 3364, 3329, 2960, 2912, 2855, 1602, 1522, 1299, 1171, 867, 800 cm–1; HRMS (FAB) m/z [M + H]+ calcd for C10H15N2 163.1235, found 163.1214.

(S)-2,8-Dimethyl-1,2,3,4-tetrahydroquinoxaline (2h)

(S)-3,5-Dimethyl-3,4-dihydro-1H-quinoxalin-2-one 1h (100 mg, 0.5 mmol), lithium aluminum hydride (133 mg, 3.5 mmol), and dry THF (10 mL) were used: yellow liquid; 65 mg, 85% yield; R = 0.39 (hexanes/AcOEt = 3/1); [α]D22 = −1.07 (c 0.34, CHCl3); 1H NMR (400 MHz, CDCl3) δ 6.55–6.43 (m, 2H, Ar), 6.41 (dd, J = 6.5, 2.7 Hz, 1H, Ar), 3.60–3.50 (m, 1H), 3.30 (dd, J = 10.4, 2.9 Hz, 1H), 3.02 (dd, J = 10.4, 8.0 Hz, 1H), 2.09 (s, 3H, Me), 1.22 (d, J = 6.3 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 132.6, 131.5, 121.7, 120.3, 117.9, 112.6, 48.0, 46.0, 20.1, 16.9; IR (NaCl) ν 3379, 3965, 2854, 1598, 1491, 1298, 1174, 762 cm–1; HRMS (FAB) m/z [M + H]+ calcd for C10H15N2 163.1236, found 163.1210.

1-Methyl-1,2,3,4-tetrahydroquinoxaline (2i)[18]

4-Methyl-3,4-dihydro-1H-quinoxalin-2-one 1i (530 mg, 3.2 mmol), lithium aluminum hydride (869 mg, 23.0 mmol), and dry THF (50 mL) were used: orange liquid; 396 mg, 82% yield; R = 0.34 (hexanes/AcOEt = 4/1); 1H NMR (400 MHz, CDCl3) δ 6.67 (dt, J = 7.6, 1.5 Hz, 1H, Ar), 6.61–6.54 (m, 2H, Ar), 6.47 (dd, J = 7.6, 1.5 Hz, 1H, Ar), 3.50–3.45 (m, 2H), 3.29–3.24 (m, 2H), 2.86 (s, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 136.2, 134.2, 118.7, 118.2, 113.6, 111.7, 49.9, 41.3, 39.1.

(S)-1,2,3,4-Tetrahydro-2-(hydroxymethyl)quinoxaline (2j)[19]

(S)-3-Hydroxymethyl-3,4-dihydro-1H-quinoxalin-2-one 1j (0.8 g, 4.5 mmol), lithium aluminum hydride (1.2 g, 31 mmol), and dry THF (80 mL) were used: dark orange liquid; 587 mg, 79% yield; R = 0.32 (hexanes/AcOEt = 1/4); [α]D22 = −35.75 (c 0.31, CHCl3) (lit. [α]D20 = −12.6 (c 0.40, CHCl3);[19]1H NMR (400 MHz, CDCl3) δ 6.64–6.56 (m, 2H, Ar), 6.55–6.48 (m, 2H, Ar), 3.66 (dd, J = 10.8, 4.8 Hz, 1H), 3.56 (dd, J =10.8, 6.8 Hz, 1H), 3.53–3.46 (m, 1H), 3.50 (brs, 2H, NH), 3.28 (dd, J = 10.8, 3.2 Hz, 1H), 3.16 (dd, J = 10.8, 6.0 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 133.1, 132.9, 119.2, 118.7, 114.9, 114.7, 64.6, 51.5, 42.6.

5,7-Dimethyl-1,2,3,4-tetrahydroquinoxaline (2k)

6,8-Dimethyl-1H-quinoxalin-2-one 1k (471 mg, 2.70 mmol), lithium aluminum hydride (720 mg, 19.0 mmol), and dry THF (47 mL) were used: brown solid; 360 mg, 82% yield; R = 0.31 (hexanes/AcOEt = 1/1); mp 81.3–82.9 °C; 1H NMR (400 MHz, CDCl3) δ 6.33 (s, 1H, Ar), 6.24 (s, 1H, Ar), 3.49–3.42 (m, 3H), 3.42–3.32 (m, 3H), 2.15 (s, 3H, Me), 2.06 (s, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 133.3, 129.0, 127.6, 122.5, 121.0, 113.5, 41.8, 41.4, 20.5, 16.8; IR (KBr) ν 3247, 2923, 1592, 1488, 1347, 1243, 1130, 830, 684 cm–1; HRMS (FAB) m/z [M + H]+ calcd for C10H15N2 163.1236, found 163.1239.

(R)-2-Methyl-1,2,3,4-tetrahydroquinoxaline (2l)[20]

(R)-3-Methyl-3,4-dihydro-1H-quinoxalin-2-one 1l (200 mg, 1.20 mmol), lithium aluminum hydride (328 mg, 8.60 mmol), and dry THF (20 mL) were used: dark brown solid; 142 mg, 78% yield; R = 0.35 (hexanes/AcOEt = 2/1); mp 66.3–69.1 °C (lit. mp 71–72 °C);[17] [α]D19 = +4.09 (c 0.22, CH2Cl2) (lit. [α]D20 = +3.70 (c 0.50, CH2Cl2));[20]1H NMR (400 MHz, CDCl3) δ 6.61–6.55 (m, 2H, Ar), 6.53–6.47 (m, 2H, Ar), 3.58 (brs, 1H, NH), 3.55–3.42 (m, 1H), 3.30 (dd, J = 10.7, 2.9 Hz, 1H), 3.02 (dd, J = 10.7, 8.2 Hz, 1H), 1.18 (d, J = 6.3 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 133.5, 133.1, 118.6 (2C), 114.4, 114.4, 48.2, 45.7, 19.8.

(R)-2,7-Dimethyl-1,2,3,4-tetrahydroquinoxaline (2m)

(R)-3,6-Dimethyl-3,4-dihydro-1H-quinoxalin-2-one 1m (200 mg, 1.10 mmol), lithium aluminum hydride (302 mg, 7.90 mmol), and dry THF (20 mL) were used: brown solid; 122 mg, 66% yield; R = 0.36 (hexanes/AcOEt = 2/1); mp 44.2–45.4 °C; [α]D22 = +8.17 (c 0.22, CHCl3); 1H NMR (400 MHz, CDCl3) δ 6.43 (d, J = 8.0 Hz, 1H, Ar), 6.38 (d, J = 8.0 Hz, 1H, Ar), 6.32 (s, 1H, Ar), 3.56–3.44 (m, 1H), 2.78 (dd, J = 10.4, 2.6 Hz, 1H), 2.99 (dd, J = 10.4, 8.4 Hz, 1H), 2.17 (s, 3H, Me), 1.17 (d, J = 6.4 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 133.6, 130.6, 128.3, 119.0, 115.1, 114.7, 48.4, 45.9, 20.6, 19.9; IR (KBr) ν 3350, 3304, 2955, 2913, 2852, 1598, 1522, 1363, 1303, 871, 802 cm–1; HRMS (FAB) m/z [M + H]+ calcd for C10H15N2 163.1235, found 163.1220.

(R)-2,5-Dimethyl-1,2,3,4-tetrahydroquinoxaline (2n)

(R)-3,8-Dimethyl-3,4-dihydro-1H-quinoxalin-2-one 1n (200 mg, 1.10 mmol), lithium aluminum hydride (302 mg, 7.90 mmol), and dry THF (20 mL) were used: yellow liquid; 335 mg, 95% yield; R = 0.38 (hexanes/AcOEt = 2/1); [α]D21 = −9.83 (c 0.28, CHCl3); 1H NMR (400 MHz, CDCl3) δ 6.53 (d, J = 6.8 Hz, 1H, Ar), 6.49 (d, J = 6.8 Hz, 1H, Ar), 6.40 (dd, J = 6.8, 2.2 Hz, 1H, Ar), 3.53–3.43 (m, 1H), 3.48 (brs, 1H, NH), 3.39 (dd, J = 10.7, 2.8 Hz, 1H), 3.06 (dd, J = 10.7, 8.3 Hz, 1H), 2.08 (s, 3H, Me), 1.17 (d, J = 6.3 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 133.1, 131.1, 121.9, 120.3, 117.9, 112.6, 48.5, 45.4, 19.8, 17.0; IR (NaCl) ν 3366, 2965, 2853, 1596, 1490, 1294, 1171, 966, 762 cm–1; HRMS (FAB) m/z [M + H]+ calcd for C10H15N2 163.1235, found 163.1226.

(R)-2,6-Dimethyl-1,2,3,4-tetrahydroquinoxaline (2o)

(R)-3,7-Dimethyl-3,4-dihydro-1H-quinoxalin-2-one 1o (350 mg, 2.00 mmol), lithium aluminum hydride (528 mg, 14.0 mmol), and dry THF (35 mL) were used: yellow solid; 241 mg, 74% yield; R = 0.38 (hexanes/AcOEt = 2/1); mp 101.9–106.6 °C; [α]D23 = +7.36 (c 0.27, CHCl3); 1H NMR (400 MHz, CDCl3) δ 6.44–6.36 (m, 2H, Ar), 6.33 (s, 1H, Ar), 3.51–3.40 (m, 1H), 3.46 (brs, 1H, NH), 3.28 (dd, J = 10.6, 2.9 Hz, 1H), 3.01 (dd, J = 10.6, 8.3 Hz, 1H), 2.17 (s, 3H, Me), 1.17 (d, J = 6.3 Hz, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 133.2, 131.0, 128.2, 119.0, 115.1, 114.7, 48.3, 45.8, 20.6, 19.8; IR (KBr) ν 3367, 3326, 2961, 2911, 2853, 1603, 1522, 1298, 1170, 867, 799 cm–1; HRMS (FAB) m/z [M + H]+ calcd for C10H15N2 163.1235, found 163.1226.

General Procedure for Synthesis of the Quinoxalines

A suspension of 1,2,3,4-tetrahydroquinoxaline, V2O5 (2.5–3.0 equiv), and silica gel in toluene was stirred at reflux temperature under Ar for 24 h. The reaction mixture was cooled and filtered through a Celite plug using CHCl3. The solvents were removed in vacuo, and the residue was purified by silica gel column chromatography (eluent: hexanes/AcOEt) to afford the corresponding quinoxaline 3.

5-Methylquinoxaline (3a)[21]

5-Methyl-1,2,3,4-tetrahydroquinoxaline 2a (98 mg, 0.66 mmol), V2O5 (300 mg, 1.60 mmol), silica gel (500 mg), and toluene (5 mL) were used: yellow liquid; 62 mg, 65% yield; R = 0.34 (hexanes/AcOEt = 9/1); 1H NMR (400 MHz, CDCl3) δ 8.85 (dd, J = 6.2, 1.8 Hz, 2H), 7.95 (dd, J = 8.2, 0.7 Hz, 1H, Ar), 7.67 (t, J = 8.2 Hz, 1H, Ar), 7.62 (d, J = 6.2 Hz, 1H, Ar), 2.81 (s, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 144.6, 143.7, 143.2, 142.2, 137.7, 130.0, 129.8, 127.4, 17.3.

6-Methylquinoxaline (3b)[21]

6-Methyl-1,2,3,4-tetrahydroquinoxaline 2b (296 mg, 2.00 mmol), V2O5 (900 mg, 5.00 mmol), silica gel (1.6 g), and toluene (12 mL) were used: yellow liquid; 184 mg, 64% yield; R = 0.28 (hexanes/AcOEt = 4/1); 1H NMR (400 MHz, CDCl3) δ 8.69 (d, J = 8.0 Hz, 2H), 7.90 (d, J = 8.6 Hz, 1H, Ar), 7.79 (s, 1H, Ar), 7.51 (d, J = 8.6 Hz, 1H, Ar), 2.51 (s, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 144.8, 144.0, 143.0, 141.4, 140.6, 132.3, 128.9, 128.2, 21.7.

2-Methylquinoxaline (3c)[21]

2-Methyl-1,2,3,4-tetrahydroquinoxaline 2c (222 mg, 1.50 mmol), V2O5 (552 mg, 3.00 mmol), silica gel (1.2 g), and toluene (9 mL) were used: dark orange liquid; 120 mg, 55% yield; R = 0.30 (hexanes/AcOEt = 9/1); 1H NMR (400 MHz, CDCl3) δ 8.75 (s, 1H), 8.07 (dd, J = 7.8, 1.7 Hz, 1H, Ar), 8.02 (dd, J = 7.8, 1.7 Hz, 1H, Ar), 7.73 (dquint, J = 7.8, 1.7 Hz, 1H), 2.78 (s, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 153.7, 146.0, 142.0, 140.9, 130.0, 129.1, 128.9, 128.6, 22.5.

2-Ethylquinoxaline (3d)[22]

2-Ethyl-1,2,3,4-tetrahydroquinoxaline 2d (162 mg, 1.00 mmol), V2O5 (460 mg, 2.50 mmol), silica gel (0.8 g), and toluene (6 mL) were used: pale orange liquid; 104 mg, 69% yield; R = 0.40 (hexanes/AcOEt = 4/1); 1H NMR (400 MHz, CDCl3) δ 8.77 (s, 1H), 8.06 (dq, J = 8.0, 2.0 Hz, 2H, Ar), 7.73 (dquint, J = 8.0, 1.6 Hz, 2H, Ar), 3.06 (q, J = 7.6 Hz, 2H), 1.45 (t, J = 7.6 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 158.5, 145.5, 142.1, 141.2, 129.9, 129.1, 128.9, 128.8, 29.6, 13.4.

2,5-Dimethylquinoxaline (3e)[23]

2,5-Dimethyl-1,2,3,4-tetrahydroquinoxaline 2e (162 mg, 1.00 mmol), V2O5 (455 mg, 2.50 mmol), silica gel (0.8 g), and toluene (6 mL) were used: orange liquid; 141 mg, 89% yield; R = 0.37 (hexanes/AcOEt = 4/1); 1H NMR (400 MHz, CDCl3) δ 8.75 (s, 1H), 7.85 (d, J = 8.3 Hz, 1H, Ar), 7.63 (t, J = 8.3 Hz, 1H, Ar), 7.54 (d, J = 6.8 Hz, 1H, Ar), 2.79 (s, 3H, Me), 2.78 (s, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 153.2, 144.6, 142.1, 140.1, 137.3, 129.7, 129.0, 126.5, 22.4, 17.3.

2,7-Dimethylquinoxaline (3f)[23]

2,7-Dimethyl-1,2,3,4-tetrahydroquinoxaline 2f (252 mg, 1.56 mmol), V2O5 (682 mg, 3.75 mmol), silica gel (1.2 g), and toluene (9 mL) were used: pale orange solid; 194 mg, 79% yield; R = 0.40 (hexanes/AcOEt = 2/1); mp 76.2–76.5 °C (lit. 79–80 °C);[20]1H NMR (400 MHz, CDCl3) δ 8.67 (s, 1H), 7.95 (d, J = 8.5 Hz, 1H, Ar), 7.78 (s, 1H, Ar), 7.53 (dd, J = 8.5, 1.7 Hz, 1H, Ar), 2.76 (s, 3H, Me), 2.59 (s, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 153.6, 145.1, 142.1, 140.5, 139.4, 131.2, 128.6, 127.5, 22.6, 21.8.

2,6-Dimethylquinoxaline (3g)[23]

2,6-Dimethyl-1,2,3,4-tetrahydroquinoxaline 2g (163 mg, 1.00 mmol), V2O5 (545 mg, 3.00 mmol), silica gel (0.8 g), and toluene (6 mL) were used: red solid; 122 mg, 77% yield; R = 0.40 (hexanes/AcOEt = 2/1); mp 73.0–74.1 °C (lit. 74–75 °C);[23]1H NMR (400 MHz, CDCl3) δ 8.69 (s, 1H), 7.89 (d, J = 8.8 Hz, 1H, Ar), 7.83 (s, 1H, Ar), 7.56 (dd, J = 8.8, 2.0 Hz, 1H, Ar), 2.75 (s, 3H, Me), 2.58 (s, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 152.8, 145.9, 141.0, 140.5, 139.3, 132.2, 128.1, 128.0, 22.4, 21.7.

2,8-Dimethylquinoxaline (3h)[24]

2,8-Dimethyl-1,2,3,4-tetrahydroquinoxaline 2h (81 mg, 0.5 mmol), V2O5 (227 mg, 1.25 mmol), silica gel (0.4 g), and toluene (5 mL) were used: pale yellow liquid; 61 mg, 77% yield; R = 0.33 (hexanes/AcOEt = 19/1); 1H NMR (400 MHz, CDCl3) δ 8.71 (s, 1H), 7.92–7.87 (m, 1H, Ar), 7.60–7.55 (m, 2H, Ar), 2.78 (s, 3H, Me), 2.77 (s, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 152.6, 145.5, 141.3, 141.0, 137.0, 130.0, 128.5, 127.0, 22.7, 17.2.

Quinoxaline-2-carbaldehyde (3j)[25]

1,2,3,4-Tetrahydro-2-(hydroxymethyl)quinoxaline 2j (116 mg, 0.70 mmol), V2O5 (322 mg, 1.80 mmol), silica gel (560 mg), and toluene (5 mL) were used: pale yellow solid; 12 mg, 11% yield; R = 0.39 (hexanes/AcOEt = 4/1); mp 98.1–99.6 °C (lit. 98–100 °C);[25]1H NMR (400 MHz, CDCl3) δ 10.30 (s, 1H, CHO), 9.44 (s, 1H), 8.31–819 (m, 2H, Ar), 8.00–7.87 (m, 2H, Ar); 13C NMR (100 MHz, CDCl3) δ 192.7, 145.9, 144.4, 142.5, 141.9, 132.9, 131.2, 130.5, 129.6.

5,7-Dimethylquinoxaline (3k)

5,7-Dimethyl-1,2,3,4-tetrahydroquinoxaline 2k (168 mg, 1.0 mmol), V2O5 (455 mg, 2.50 mmol), silica gel (0.8 g), and toluene (6 mL) were used: orange liquid; 130 mg, 80% yield; R = 0.34 (hexanes/AcOEt = 4/1); 1H NMR (400 MHz, CDCl3) δ 8.78 (d, J = 2.0 Hz, 2H), 7.71 (s, 1H, Ar), 7.28 (s, 1H, Ar), 2.77 (s, 3H, Me), 2.56 (s, 3H, Me); 13C NMR (100 MHz, CDCl3) δ 144.5, 143.3, 142.8, 140.7, 140.2, 137.0, 132.4, 126.1, 21.8, 17.2; IR (NaCl) ν 3450, 2920, 1619, 1492, 1355, 1082, 1032, 876 cm–1; HRMS (FAB) m/z [M + H]+ calcd for C10H11N2 159.0923, found 159.0932.

Olfactory Evaluation

The olfactory properties of a series of quinoxalinones 1, 1,2,3,4-tetrahydroquinoxalines 2, and quinoxalines 3 were evaluated three times every 1 or 2 days. The evaluation was carried out between 11:00 and 12:00 a.m. before lunch to obtain the most sensitive periods. The five male panelists (35–50 years old) included a licensed smell examiner, one of the national licensees in Japan, an analyst of volatile components, and three flavorists. They had been trained in the objective evaluation and expression of odors for both business and academic research for over 15 years. They individually sniffed each compound kept in a tube (0.8 cm × 5 cm). The amounts of the compounds were about 20 mg each. Each panelist used as many odor-descriptive terms as possible. Common terms used by more than 2 persons were selected as the odorant attributes.

Biological Activity

Cell Culture

The human hepatocyte cell line HepG2 (ATCC, Manassas, VA) was cultured in DMEM with 5% FBS, 100 units/mL streptomycin, and 100 μg/mL penicillin, at 37 °C in a humidified 5% CO2 atmosphere. Prior to the experiments, the cells were incubated overnight with serum-free DMEM. After treatment, the cells were washed twice with ice-cold phosphate-buffered saline (PBS) and lysed with radio-immunoprecipitation assay buffer containing 1% Triton X-100, the phosphatase inhibitor cocktail, and the protease inhibitor cocktail. The protein concentration of each lysate was determined using the BCA Protein Assay Reagent Kit (Nacalai Tesque, Kyoto, Japan).

Palmitate Treatment and Oil Red O Staining

To induce lipid overloading, HepG2 cells at the subconfluence were exposed to palmitate-containing media. Culture media containing palmitate were prepared, as previously described.[5] Briefly, sodium palmitate was dissolved in 50% v/v ethanol, diluted in serum-free DMEM containing 1% w/v fatty acid free BSA, and incubated at 37 °C for 2 h with shaking. The control medium containing ethanol and BSA was similarly prepared. To evaluate the intracellular neutral lipid accumulation, Oil Red O staining was performed according to the published procedure with some modifications.[5] Briefly, the cells were fixed in 10% paraformaldehyde in PBS for 10 min, stained with 1.8 mg/mL Oil Red O (Sigma-Aldrich, St. Louis, MO) in a 60% aqueous isopropanol solution for 15 min at room temperature, and rinsed twice with 60% aqueous isopropanol and PBS. To quantify the cellular lipid content, the Oil Red O was extracted with 100% isopropanol, and then its absorbance at 490 nm was measured using a microplate reader. The cellular neutral lipid content was normalized to the protein concentration.