4-Amino-2-( p -tolyl)-7 H -chromeno[5,6-d]oxazol-7-one

: The new 4-amino-2-( p -tolyl)-7 H -chromeno[5,6-d]oxazol-7-one was successfully prepared through the Au/TiO 2 -catalyzed NaBH 4 activation and chemoselective reduction of the new 4-nitro-2-( p -tolyl)-7 H -chromeno[5,6-d]oxazol-7-one. The latter was synthesized by the one-pot tandem reactions of 6-hydroxy-5,7-dinitrocoumarin with p -tolylmethanol under Au/TiO 2 catalysis. The dinitrocoumarin was obtained by the nitration of 6-hydroxycoumarin with cerium ammonium nitrate (CAN). The structure of the synthesized compounds was conﬁrmed by FT-IR, HR-MS, 1 H-NMR and 13 C-NMR analysis. Preliminary biological tests show low anti-lipid peroxidation activity for the title compound.

Aminocoumarins are valuable building blocks for the synthesis of fused pyridocoumarins presenting significant biological activities such as antibacterial [22], antifungal [23], antimalarial [24], antioxidant [25] and wound-healing [26]. Pyridocoumarins are prepared from aminocoumarins through the one-pot Povarov reactions with aromatic aldehydes and cyclic enol ethers [27], the reactions with vinyl ketones [28], or under Vilsmeier conditions [29] or with phenylacetylene and benzaldehydes under catalysis by I 2 [30] or by other Lewis acids [25,31]. The cycloisomerization of propargylaminocoumarins, prepared from aminocoumarins, followed by oxidation, led also to pyridocoumarins under catalysis by AgSbF 6 [32] or BF 3 .Et 2 O [33] or Au/nanoparticles [34]. The need for the synthesis of new compounds, to probe novel biological activity containing a heterocyclic ring fused to the pyridocoumarin moiety, led us to the synthesis of amino-substituted fused oxazolocoumarins. In continuation of our interest on fused oxazolocoumarin [8,22] and pyridocoumarin [25,33,34] derivatives, we would like to report here the synthesis of novel amine 7, through a selective reduction procedure, and the biological evaluation of the products. The reactions studied and the synthesized products are depicted in Scheme 1.

Synthesis
The starting material for this procedure was the 6-hydroxy-5,7-dinitrocoumarin (4), which was synthesized in 62% yield along with 6-hydroxy-5-nitrocoumarin (2) (22% yield) and 6-hydroxy-7-nitrocoumarin (3) (14% yield) by nitration of 6-hydroxycoumarin (1) with cerium ammonium nitrate (CAN) in CH3CN at r.t., according to the literature [35]. In this paper, the authors obtained 3 in 50% yield using 1 equiv. of CAN, while by using 2 equiv. of CAN they isolated compound 3 in 74% yield along with compound 2 (12%). No evidence for the presence of the dinitro-derivative 4 was noticed. When we performed the above reaction with 0.5 equiv. of CAN, only compound 2 [36] (10 %) was isolated along with 85% of the starting compound 1. The spectral data of compound 4 resemble well with that given in the literature [37], where the preparation was achieved by using nitric/acetic acids. The reaction of 4 with p-tolylmethanol (5) in a sealed tube in toluene in the presence of Au/TiO2 (4 mol%) at 150 ᵒC led to 4-nitro-2-(p-tolyl)-7H-chromeno[5,6-d]oxazol-7-one (6) (45% yield) accompanied by 4-amino-2-(p-tolyl)-7H-chromeno[5,6-d]oxazol-7-one (7) (13%). This reaction was performed in analogy to our recent work on the synthesis of fused oxazolocoumarins by the treatment of o-hydroxynitrocoumarins with benzyl alcohol catalyzed by Au/TiO2 or Ag/TiO2 or FeCl3 [21]. During this process, a simultaneous reduction of nitro-to amine-group and oxidation of benzyl alcohol to benzaldehyde occurred, followed by imine formation from the amine and benzaldehyde, cyclization by addition of hydroxy-group to imine and oxidation of the intermediate oxazoline to oxazole. The selective reduction of the 5-nitro group of coumarin in comparison to the 7-nitro group by the intermediate gold-hydride [21] could be attributed to a possible complexation of gold to the 3,4-double bond of coumarin. In the 1 H-NMR spectrum of 6, there are

Synthesis
The starting material for this procedure was the 6-hydroxy-5,7-dinitrocoumarin (4), which was synthesized in 62% yield along with 6-hydroxy-5-nitrocoumarin (2) (22% yield) and 6-hydroxy-7-nitrocoumarin (3) (14% yield) by nitration of 6-hydroxycoumarin (1) with cerium ammonium nitrate (CAN) in CH 3 CN at r.t., according to the literature [35]. In this paper, the authors obtained 3 in 50% yield using 1 equiv. of CAN, while by using 2 equiv. of CAN they isolated compound 3 in 74% yield along with compound 2 (12%). No evidence for the presence of the dinitro-derivative 4 was noticed. When we performed the above reaction with 0.5 equiv. of CAN, only compound 2 [36] (10 %) was isolated along with 85% of the starting compound 1. The spectral data of compound 4 resemble well with that given in the literature [37], where the preparation was achieved by using nitric/acetic acids.
The reaction of 4 with p-tolylmethanol (5) in a sealed tube in toluene in the presence of Au/TiO 2 (4 mol%) at 150 • C led to 4-nitro-2-(p-tolyl)-7H-chromeno[5,6-d]oxazol-7-one (6) (45% yield) accompanied by 4-amino-2-(p-tolyl)-7H-chromeno[5,6-d]oxazol-7-one (7) (13%). This reaction was performed in analogy to our recent work on the synthesis of fused oxazolocoumarins by the treatment of o-hydroxynitrocoumarins with benzyl alcohol catalyzed by Au/TiO 2 or Ag/TiO 2 or FeCl 3 [21]. During this process, a simultaneous reduction of nitro-to amine-group and oxidation of benzyl alcohol to benzaldehyde occurred, followed by imine formation from the amine and benzaldehyde, cyclization by addition The reduction of nitro-derivative 6 with NaBH 4 as hydride ion donor, in the presence of the catalyst Au/TiO 2 , according to a recent publication for the use of Au-NPs in the reduction of nitroarenes to anilines [38], resulted to the chemoselective preparation of 4-amino-2-(p-tolyl)-7H-chromeno[5,6-d]oxazol-7-one (7) in 94% yield. This is a new compound with absorptions in FT-IR at 3446, 3356 cm −1 for the NH 2 group. There are two doublets at 6.29 (1 H, J = 9.6 Hz) and 8.26 (1 H, J = 9.6 Hz) for the 3-H and 4-H, respectively, in the 1 H-NMR spectrum of 7, a broad singlet at 4.50 ppm for the NH 2 protons and one singlet at 6.61 (1 H) for the 8-H, see Supplementary Materials. This upfield shift is consistent with the structure of 7 with the oxazole-ring fused at the 5,6-position and the NH 2 group at the 7-position of the coumarin moiety. If the oxazole ring is at the 6,7-position and the amine group at the 5-position of the coumarin (in a structure isomeric to 7), the 8-H would be expected to be above 7.0 ppm. In the case of 2-phenyl-6H-chromeno[6,7-d] [1,3]oxazol-6one the 8-H is at 7.54 ppm [21]. The p-tolyl group gives rise to two doublets at 7.36 (1 H, J = 7.9 Hz) and 8.15 (1 H, J = 7.9 Hz) and one singlet at 2.46 (3 H). In the 13 C-NMR, there is the upfield peak for the 8-C of the coumarin moiety at 98.1 ppm in comparison to the carbons of nitro-compound 6, see Supplementary Materials. This peak is consistent with the analogous peak (98.9 ppm) for 7-aminocoumarin [39]. The HR-MS is m/z [M + Na] + calcd for C 17 H 12 NaN 2 O 3 : 315.2778, found: 315.2784.

Biology
Preliminary biological experiments were performed in vitro. Compounds 6 and 7 were tested as possible antioxidant agents and inhibitors of soybean lipoxygenase according to our previous published assays [10,25]. They did not present any interaction with DPPH at 100 µM after 20 and 60 min under the reported experimental conditions. The anti-lipid peroxidation activity was very low at 100 µM (less than 1% for compound 6 and 23% for compound 7), as tested by the 2,2 -azobis(2-amidinopropane) dihydrochloride (AAPH) protocol. No inhibition of soybean lipoxygenase was observed.

Materials
All the chemicals were procured from either Sigma-Aldrich Co. or Merck & Co., Inc. (St. Louis, MO, USA) Melting points were determined with a Kofler hotstage apparatus and are uncorrected. IR spectra were obtained with a Perkin-Elmer Spectrum BX spectrophotometer as KBr pellets. NMR spectra were recorded with an Agilent 500/54 (DD2) (Santa Clara, CA, USA) (500 MHz and 125 MHz for 1 H and 13 C, respectively) using CDCl 3 as solvent and TMS as an internal standard. J values are reported in Hz. Mass spectra were determined with a LCMS-2010 EV Instrument (Shimadzu, Kyoto, Japan) under electrospray ionization (ESI) conditions. HRMS (ESI-MS) were recorded with a ThermoFisher Scientific model LTQ Orbitrap Discovery MS. Silica gel No. 60, Merck A.G. was used for column chromatography.

Biological Experiments: In Vitro Assays
The compounds were dissolved in DMSO.
preliminary biological assays pointed that compound 7 presents low anti-lipid peroxidation activity.