Cerium(IV) ammonium nitrate for the tandem nitration and oxidative rearrangement of 2-acetyl-1-naphthol benzoylhydrazones into 1,2-diacylnaphthalenes; synthesis of benzo[ f ]phthalazines

A novel nitration and oxidation reaction sequence of 2-acetyl-1-naphthol benzoylhydrazones with CAN is presented. There is strong indication that nitration precedes an oxidative rearrangement to 1,2-diacyl-4-nitronaphthalenes or oxidative electrocyclisation to 3-methyl-5-nitronaphtho[2,1-d ]isoxazole. Condensation of 1,2-diacyl-4-nitronaphthalenes with hydrazine hydrate yields 1,4-disubstituted benzo[ f ]phthalazines.


Introduction
Cerium(IV) ammonium nitrate (CAN) has received considerable attention as a powerful one-electron oxidant with many uses in organic synthesis.Its extensive use has been dictated by its high reduction potential (1.61 eV), +3 and +4 oxidation states of cerium, low cost, stability in air, easy and simple handling, low toxicity, solubility in many organic solvents and a versatile reactivity profile.Reactions of CAN include, oxidation, oxidative addition, oxidative catalysis, nitration, photo-oxidation, carbon-carbon and carbon-heteroatom bond formation, carbon-carbon, carbon-heteroatom and Si-O bond cleavage, fragmentation, alkoxylation, esterification and transesterification, dehydrogenation, catalysis of multicomponent syntheses and polymer grafting, as described in recent review articles. 1,2CAN has played a significant role in oxidative cyclisations giving access to various heterocycles, although the synthesis of five-membered rings with two heteroatoms is limited to benzimidazoles, 3 pyrazoles, 4 benzothiazoles 5,6 and isoxazoles. 7CAN has been reported to cleave semicarbazones to the corresponding aldehydes and ketones 8 but hydrazones are not affected.Furthermore, the reagent has not been used on 2-hydroxyaryl ketone acylhydrazones.A transformation of the latter to 1,2diacylbenzenes was first reported by Kotali and Tsoungas 9,10 using lead tetraacetate (LTA) and its mechanism was studied by Katritzky et al. 11 The use of iodobenzene diacetate (IBD), 12 polystyrene-supported IBD 13 or cross-linked poly[styrene(iodoso diacetate)] 14 as oxidants in this transformation, gives comparable results.Other methods of synthesizing 1,2-diacylbenzenes are: palladium(II)-catalyzed direct acylation of acetophenone N-Boc hydrazones with aldehydes via C-H bond activation 15 and Pd-catalysed oxidative C-H bond coupling of acetophenone O-methyl oximes and aldehydes to give 1,2-diacylbenzene O-methyl oximes which are then hydrolysed, 16 oxidation of benzhydrols with selenium dioxide, 17 of benzofurans with LTA, 18 and of 2-ethylacetophenone with potassium permanganate, 19 or acylating benzene with 2-acetylbenzoyl chloride. 20It is worth noting that 2,3-diacylnaphthalenes are known, 21 while their 1,2-diacyl regioisomers are hitherto unreported.
Hydrazone functional groups endow molecules with unique physical and chemical properties.Their nitrogen atoms are nucleophilic while the imine carbon centre has a dual electrophilic and nucleophilic character. 41Aspects of their structure and reactivity have been reviewed. 42There are no reports on 2-acetyl-1naphthol benzoylhydrazones other than those presented in this study.However, 1-naphthaldehyde benzoylhydrazones are known in the literature.There are examples of their effectiveness against a variety of drug resistant HIV-1 RT mutants 43 and their use as parasitic protease inhibitors. 44Due to their chelation with a variety of metal ions, 45 spectrophotometric methods have been developed for the determination of several of these metal ions in solution. 46In one report, their chelation with iron formed complexes were found to possess antimalarial activity. 47

Results and Discussion
Herein, we describe the unprecedented reaction of 2-acetyl-1-naphthol benzoylhydrazones 3a-e with CAN (Scheme 1).The reaction of 3a-e with 1 equivalent of CAN produced three products, 2-acetyl-4-nitro-1naphthol benzoylhydrazones 4a-e, 1,2-diacyl-4-nitronaphthalenes 5a-d and 3-methyl-5-nitronaphtho[2,1-d]isoxazole (6).Starting materials 3a-e were prepared in good yields (70-85%) by the reaction of 2-acetyl-1naphthol (1) with the corresponding hydrazides 2a-e, in refluxing propan-2-ol, containing glacial acetic acid as catalyst.Hydrazones 3a-e, in acetonitrile, were subjected to a slight excess of CAN for 1 hour at ambient temperature.In all reactions, TLC examination confirmed complete conversion of starting material into three products.One of the products precipitated out of the reaction mixture, collected at 0 o C, crystallised from either acetonitrile or a mixture of DMSO and water in moderate yields (47-52%) and identified as 4nitrobenzoylhydrazones 4a-e.Column chromatography allowed the isolation of the remaining two products.The less polar compound, obtained in low yield (15-30%), was the same product in all reactions.It was crystallized from a mixture of dichloromethane and hexane and identified as the fused isoxazole 6.The more polar compound, also crystallized from a mixture of dichloromethane and hexane, was isolated in slightly better yields (28-32%) and was identified as the 1,2-diacylnaphthalene 5a-d, Table 1.Derivative 5e was not isolated possibly because intermediate III R = Me (Scheme 2) lacking an aryl group is not stable enough to be formed.Nevertheless, 4e was obtained in 50% yield and the corresponding 6 in 30% yield.
Unambiguous confirmation of the structure of compounds 5a and 6 was obtained from their single crystal X-ray analysis, shown in Figs 1 and 2 (and see the Supporting Information).The 1 H NMR spectra of 4a-e show the broad singlets of the OH protons, unexpectedly shifted to 16.32-16.64ppm, the NH protons as broad singlets at 11.35-12.03ppm and the H-3 protons as sharp singlets at 8.65-8.73ppm.The benzoyl carbonyl   At this point, the question arose whether 4 is a precursor of 5 and 6.To that end, the reaction of 3a-d with one equivalent of CAN was repeated and after one hour having established the presence of 4a-d, 5a-d and 6 in the reaction mixture by TLC examination, 1 more equivalent of CAN was added that resulted in the isolation of only 5a-d and 6 (Table 2).The reaction was also tested by using a two-fold excess of CAN on 3a-d from the start of the reaction, which provided 5a-d and 6 directly (Table 3).These experiments therefore confirm that the reaction indeed proceeds via an initial nitration of 3a-d to 4a-d, followed by oxidative transformation of the isolable intermediates 4a-d into 5a-d and 6.The yields of 5a-d and 6 by the two-step method are slightly higher than those obtained by the direct method (Tables 2 and 3) while the yields of compounds 5a-d, useful as doubly electrophilic precursors, are reasonably good, 63-68% by the two-step method and 60-64% by the direct method.The formation of the isolated products can be rationalized as follows (Scheme 2).Key intermediate 4 is initially formed by an electrophilic aromatic substitution reaction on 3 by CAN, acting as a nitronium carrier, by analogy to a report on N,N-dialkylanilines. 48Thus, the first step is probably the addition of the activated C-4 of 3 onto the N=O group of CAN to form an intermediate species I, from which elimination of Ce(OH)(NO3)5 2-and re-aromatization leads to 4. The following steps of the mechanism are analogous to the oxidative transformation of 2-hydroxy aryl ketone acylhydrazones into 1,2-diacylbenzenes by Pb(OAc)4, studied by Katritzky et al. 11 An oxidative cyclisation of aromatic aldehyde acylhydrazones to unsymmetrically 2,5disubstituted 1,3,4-oxadiazoles, 49 by CAN, lends further support to our proposed pathway.Therefore, we propose that CAN initiates free radical oxidation of 4 to give an intermediate β-azo-o-quinone methide II, which undergoes intramolecular cyclisation to form the 1,3,4-oxadiazoline intermediate III.In intermediate III the naphthoxide oxygen atom adds to the carbocation of the oxadiazoline ring to form the tetracyclic epoxynaphthoxadiazepine species IV.Elimination of nitrogen from IV leads to the formation of unstable naphthoxirenofuran V, which undergoes electrocyclic rearrangement to form 5. Scheme 2. Plausible mechanism for the formation of 4 and 5 from 3.
The formation of 6 dictates that a competing free radical oxidation of 4 seems to be running in parallel (Scheme 3).This is rationalised as follows.In the H-bonded "locked" conformation shown, 4 is oxidised to βazo-o-quinone methide II followed by its electrocyclisation to isoxazole-N-acylimide VI.It is reasonable to assume that the next step, under the pertaining reaction conditions, is a N-N bond cleavage in VI to give 6.The released benzoyl nitrene species then rapidly undergoes the Curtius rearrangement to produce an aryl isocyanate.

Scheme 3. Plausible mechanism for the formation of 6 from 4.
A common feature in both oxidation pathways (Schemes 2 and 3) is the intermediacy of transient β-azo-oquinone methide II΄ that adopts a conformation dependent on the orientation taken up by the benzoylhydrazone substituent in 4. In these reactions, strong electron withdrawing p-NO2 substitution in 4 is expected to reduce its susceptibility to further oxidation pathways and thus increase the likelihood of alternative competing reactions.This, perhaps, can serve as a rationale for both isolated 5 and 6.Furthermore, the "locked" orientation adopted by 4 tends to be favoured by the intramolecular O-H…N hydrogen bonding between the phenol and imine sites.Its resonance-assisted stabilisation, 50 however, is rather diminished by the effect of the p-NO2 substituent and therefore may explain the lead of 5 over 6.
1,2-Diacylnaphthalenes 5a-d, upon reaction with 1 equivalent of hydrazine hydrate in propan-2-ol, at room temperature for 1 hour, via consecutive intermolecular condensation and cyclodehydration reactions, led, after crystallization from propan-2-ol, to the corresponding 1,4-disubstituted benzo[f]phthalazines 7a-d, in excellent yields (89-98%) (Scheme 4).The most downfield signal in the 1 H NMR spectra of 7a-d is the singlet of H-5, adjacent to the nitro group, at 8.52-8.57ppm.In the 13 C NMR spectra of these compounds the peaks of C-1, bonded to the aryl group, are at 155.13-157.16ppm while those of C-4, bonded to the methyl group, are at 156.99-160.64 ppm.The synthesis of phthalazines 7, besides serving as a further confirmation to the structure of 5, is a useful contribution to existing synthetic routes towards this important heterocycle.

Experimental Section
General.All reactions were carried out under a N2 atmosphere.Solvents and reagents were used as received from the manufacturers (Aldrich, Acros, Fluka, and Alfa Aesar) except for THF, DCM, MeOH, EtOAc, hexane and toluene that were purified and dried according to recommended procedures.Organic solutions were concentrated by rotary evaporation at 23-40 °C under 15 Torr.Melting points were taken on a Büchi 510 apparatus. 1H and 13 C NMR spectra were measured in CDCl3 or DMSO-d6 on a 250 or 400 MHz Brüker spectrometer. 1 H chemical shifts are reported in ppm from an internal standard TMS, residual chloroform (7.26 ppm) or DMSO-d6 (2.50 ppm). 13C NMR chemical shifts are reported in ppm from an internal standard TMS, residual chloroform (77.16 ppm) or DMSO-d6 (39.43 ppm).High resolution ESI mass spectra were measured on a ThermoFisher Scientific Orbitrap XL system or with a resolution of 10000 on a Kratos MS50TC or a Kratos Mach III system.Low resolution ESI spectra were measured with an Agilent 1100 LC-MS/MS spectrometer.IR spectra were acquired on a Perkin-Elmer GX FTIR spectrophotometer as liquids between NaCl discs and are reported in wave numbers (cm -1 ).Analytical thin layer chromatography was performed with Merck 70-230 mesh silica gel TLC plates.Purification of reaction products was generally done by dry-column flash chromatography using Μerck silica gel 60 and/or flash chromatography using Carlo Erba Reactifs-SDS silica gel 60.

General Procedure for the synthesis of hydrazones 3a-e
A stirred solution of 2-acetyl-1-naphthol (0.8 g, 4.3 mmol), benzhydrazide, p-toluic hydrazide, 4methoxybenzhydrazide, 4-nitrobenzoic hydrazide or acetohydrazide (4.7 mmol) and few drops of acetic acid were heated under reflux in propan-2-ol (20 mL) for 6 hours.The reaction mixture was cooled and the precipitated solid was collected and washed with cold propan-2-ol.The solid was purified and identified as described for the individual products 3a-e.

General Procedure 1. Oxidation of 3a-e by CAN (1.1 equiv) for the preparation of 4a-e, 5a-d and 6
To a stirred solution of appropriate hydrazone 3a-e (1.5 mmol) in dry acetonitrile (20 mL) under an atmosphere of N 2 , CAN (0.88 g, 1.6 mmol) was added and the resulting mixture was stirred at room temperature for 1 hour.TLC examination revealed the disappearance of starting material spot and appearance of 3 new spots.The reaction mixture was cooled and the precipitated solid was filtered and washed with cold acetonitrile.Recrystallisation from acetonitrile or DMSO/H2O afforded products 4a-e.The filtrate was concentrated in vacuo, water (25 mL) was added and extracted with ethyl acetate (3 × 10 mL).The combined organic layers were dried over anhydrous sodium sulfate and the solvent evaporated in vacuo.The oily residue was purified by flash chromatography (ethyl acetate/hexane, 1:6) to give products 5a-d and 6.The yields of products 4a-e, 5a-d and 6 are presented in Table 1.

General Procedure 2. Oxidation of 3a-e by CAN (1.1 equiv) followed by addition of CAN (1.1 equiv) for the preparation of 5a-d and 6
To a stirred solution of appropriate hydrazone 3a-e (1.5 mmol) in dry acetonitrile (25 mL) under an atmosphere of N2, CAN (0.88 g, 1.6 mmol) was added and the resulting mixture was stirred at room temperature for 1 hour.TLC examination revealed the disappearance of starting material spot and appearance of 3 spots corresponding to 4a-e, 5a-d and 6.CAN (0.88 g, 1.6 mmol) was added and the reaction mixture stirred for 1 hour at room temperature.TLC examination revealed the appearance of spots corresponding to 5a-d and 6.The work up and purification of products 5a-d and 6 was carried out as in the General Procedure 1.They were identified by comparing their 1 H NMR spectra with the corresponding spectra of authentic samples while their yields are depicted in Table 2.

General Procedure 3. Oxidation of 3a-e by CAN (2.2 equiv) for the preparation of 5a-d and 6
To a stirred solution of appropriate hydrazone 1a-e (1.5 mmol) in dry acetonitrile (25 mL) under an atmosphere of N2, CAN (1.76 g, 3.2 mmol) was added and the resulting mixture was stirred at room temperature for 2 hours.TLC examination revealed the appearance of spots corresponding to 5a-d and 6.The work up and purification of products 5a-d and 6 was carried out as in the General Procedure 1.They were identified by comparing their 1 H NMR spectra with the corresponding spectra of authentic samples while their yields are shown in Table 3.

Condensation of diacyl derivatives 5a-d with hydrazine hydrate; preparation of 7a-d
A mixture of diacyl compound 5a-d (0.60 mmol) and hydrazine hydrate (0.03 g, 0.60 mmol) was stirred at room temperature in propan-2-ol (8 mL) for 2 h and then cooled.The resulting precipitate was filtered off, washed with cold propan-2-ol and recrystallized from the same solvent to give the corresponding benzo

Scheme 1 .
Scheme 1. Preparation of 3a-e and reaction with CAN.

Figure 1 .
Figure 1.Molecular structure of 5a showing displacement ellipsoids drawn at the 50% probability level.Only one of the two molecules in the asymmetric unit is shown.

Figure 2 .
Figure 2. Molecular structure of 6 showing displacement ellipsoids drawn at the 50% probability level.

Table 1 .
Reaction of 3 with 1 equiv. of CAN to give 4, 5 and 6, via Scheme 1 Entry Starting material Product Yield% a Product Yield% a Product Yield % a

Table 2 .
Reaction of 3 with 1 equiv. of CAN followed by addition of another 1 equiv. of CAN to give 5 and 6, via Scheme 1