Aminoketone, oxazole and thiazole synthesis. Part 15. 1 2-[4-(4-Halobenzenesulphonyl)-phenyl]-5-aryloxazoles

Acylaminoacylation of aromatic hydrocarbons (benzene, toluene, meta-xylene, mesitylene) with 2-[4-benzenesulfonyl-(4-halophenyl)]-5-oxazolones in the presence of anhydrous aluminum chloride leads to 2-aza-1,4-diones 5 which cyclize under the action of phosphorus oxychloride yielding the corresponding 2-[4-(4-halobenzenesulphonyl)-phenyl]-5-aryloxazoles 6 . The para-halogens are chloro or bromo atoms. Electronic absorption, vibrational, 1 H-NMR and 13C-NMR spectral data are presented. The UV and NMR spectra provide evidence for the non-coplanarity of the oxazole and mesityl rings.


Introduction
In continuation of the previous part in this series, 1 we now report the preparation of new 2,5diaryloxazoles wherein the 2-aryl group is 4-[4-chloro-or 4-bromo-benzenesulfonyl)-phenyl].Such 2,5-diaryloxazoles 6a-h (Scheme 1) which are potential fluorescent sensors, laser dyes, and scintillators for detecting nuclear radiations 3 have been prepared by extending our earlier method 2 by reacting 2-oxazolones (azlactones) 4a-b with benzene, toluene, m-xylene and mesitylene under Friedel-Crafts reaction conditions to afford the corresponding ketones 5a-h, in The structures were confirmed by their analytical and spectral data (see Table 2 for analytical data).The electronic spectra of 6a-h were also recorded, and their absorption bands and extinction coefficients are described in Table 3.An evident hypsochromic effect for the longest-wavelength band (about 20 nm) can be observed for the two mesityl derivatives 6g and 6h.Its origin is the non-coplanarity of the 5mesityl and oxazole rings due to steric hindrance.One ortho-methyl group as in the meta-xylyl derivatives 6e and 6f causes only a small hypsochromic shift (about 5 nm, if one takes into account also the extra electron-donating effect of methyl groups as evidenced by comparing the phenyl compounds 6a, 6b with the p-tolyl congeners 6c, 6d); a similar trend was observed for the congeneric systems devoid of halogen atoms. 1 The 1 H-NMR data for azadiketones 5a-h are presented in Table 4.The 1 H-NMR data for oxazoles 6a-h are presented in Table 5.Data for oxazoles 6a-h are described in Table 6.Tables 4-6 are placed before the Experimental Part.
From Tables 4 and 5 one can observe that there is a slight deshielding of the protons in the halogen-substituted ring (especially H-17) on increasing the electronegativity of the halogen, i. e. on changing the bromo to the chloro substituent.][11][12][13] There is an interesting trend in the chemical shifts of the oxazolic proton H-4.On going from a 5-phenyl to a 5-para-tolyl group, one can observe a deshielding of H-4 by 0.35 ppm.On increasing the number of methyl groups attached to the 5-phenyl ring (in the series para-tolylmeta-xylyl -mesityl), the oxazolic proton H-4 becomes increasingly shielded; at the same time, the ortho-methyl protons in the 5-aryl group also become increasingly shielded.These observations are consistent with the non-coplanarity of the oxazolic and 5-aryl rings due to the presence of ortho-methyl substituents.

Experimental Section
General Procedures.NMR data were obtained with a Varian Gemini-300 instrument at 300 MHz for 1 H-NMR spectra and at 75 MHz for 13 C-NMR spectra.IR spectra were recorded with an FT-IR instrument, and UV spectra with a Perkin-Elmer Lambda-2 spectrophotometer.

General procedures for the synthesis of 4-(4-halobenzenesulfonyl)-benzoyl chlorides 2a and 2b
The acid chlorides 2a and 2b were prepared from their respective benzoic acids 1a and 1b using Vilsmeier procedure (thionyl chloride and dimethylformamide in benzene). 4The bromo compound 2a was obtained in about 90 % yield, had mp 154º C, IR bands at 1781 and 1937(C=O), 1331 and 1159 cm -1 (SO 2 ).Similarly the chloro product 2b was obtained in about 90 % yield, had mp 138º C and IR bands at 1781 and 1738 (C=O), 1332 and 1163 cm -1 (SO 2 ).Reaction of glycine with acid chlorides 2a and 2b.Formation of 4-(4-halobenzenesulfonyl)hippuric acids 3a and 3b.Glycine (20mmol) in 20ml of 1N sodium hydroxide was cooled at 0-5º C and the cold solution was added dropwise to a solution of 20 mmol of acid chlorides 2a or 2b in 30ml of chloroform.The reaction mixture was continued under stirring for an additional one hour.The aqueous layer was separated and acidified with 2N hydrochloric acid.The products 3a and 3b were collected by filtration and recrystallized from 80% ethanol as colorless needles.

General method for preparation of 2-aza-1-[4-(4-halobenzenesulfonyl)-phenyl]-4-phenyl-1,4-butanediones 5a-h
The azlactones 4a and 4b each 5mmol in 25ml of appropriate hydrocarbon (benzene, toluene, mxylene or mesitylene) in excess was treated portionwise with 2.0g (15mmol) of anhydrous aluminum chloride at room temperature.After the addition, the reaction mixture was continued under stirring for 20hrs.The reaction mixture was then poured over crushed ice with hydrochloric acid and the organic component was extracted with methylene chloride, washed (H 2 O) and dried.The solvent was removed to yield the crude azadiketones 5a-h, which were

Table 3 .
UV Spectra of oxazoles 6 in methanol