Synthesis of some new 2,4,6-trisubstituted phenyl pyrimidines using 4-nitro and 4-fluorophenacyldimethylsulfonium bromides with aromatic aldehydes

: 4-Nitrophenacyldimethylsulfonium bromide and 4-fluorophenacyldimethylsulfonium bromide have been prepared by the reaction of dimethyl sulfide with 4-substitutedphenacyl bromide in benzene at reflux temperature under nitrogen atmosphere. These sulfonium salts on treatment with NaOH gave 4-nitrophenacylidenedimethylsulfuranc and 4-fluorophenacylidenedimethylsulfurane. The reaction of these sulfonium salts and sulfuranes with various aromatic aldehydes is carried out in presence of ammonium acetate and acetic acid at reflux in an atmosphere of nitrogen to give 2,4,6-triarylpyrimidines in 35-80% yields. Ammonium acetate in acetic acid was used as aza cyclization agent. The structures of new pyrimidines were confirmed on the basis of IR and NMR spectral data.

Pyridinium, phosphonium, arsonium and isoquinolinium ylides have gained considerable importance in the synthesis of acyclic, cyclic and heterocyclic compounds 1 . As reported earlier sulfonium salts and sulfuranes are also better potential reagents than corresponding salts and ylides of V -th group elements for the synthesis of cyclic and heterocyclic system 2 . Further extensions of reaction of sulfonium ylides leading to the pyrimidine nucleus seems to be pertinent with a view to test the domain of applicability of sulfonium ylides. In the present investigation 4-nitrophenacyldimethylsulfonium bromide and 4fluorophenacyldimethylsulfonium bromide as well as their corresponding sulfuranes have been coupled with a wide range of aromatic aldehydes in the presence of ammonium acetate and acetic acid at reflux temperature leading to ring closure to form pyrimidine nucleus. 1690 cm-1 for carbonyl group. The diagnostic ~bsorption bands in the region 3300-3000 cm-1 were observed due to C-H stretching vibrations of methylene group attached to sulfur atom 4 .

Results and discussion
Reaction of dimethylsulfide with 4-nitrophenacyl bromide and 4-fluorophenacyl bromide in benzene at reflux temperature gave 4-nitrophenacyl bromide (la) 4fluorophenacyldimethylsulfonium bromide (lb) in fair to good yields. The structure of sulfonium salts (la-b) were confirmed by comparision of melting points of salts with those reported in the literature 3 and by spectral data. The IR spectra of salts (la-b) showed a characteristic absorption band due to C-0 stretching vibrations in the region 1670-The treatment of these salts (la-b) with aqueous sodium hydroxide/potassium carbonate gave 4-nitrophenacyldimethysulfurane (2a) and 4-fluorophenacyldimethylsulfurane (2b) which are highly unstable. The reaction was, therefore, carried out by generating the ylide intermediates (2a-b) in situ from the corresponding salts Further attempts were made to synthesize symmetrical (la-b). Heating the mixture of sulfonium salts (la-b) with pyrimidines having identical substituems at 2,4,6-positions. substituted benzaldehyde (3a-l) in presence of ammonium For this purpose 4-nitrophenacyldimethylsulfonium acetate and glacial acetic acid at reflux temperature gave bromide (la) with 4-nitrobenzaldehyde and 4-2,4,6-triarylpyrimidines (Sa-l, 6a-l) in 35-80% yields fluorophenacyldimethylsulfonium bromide (lb) with 4-(Scheme 1).
AcONH, AcOH The reaction takes place through Mannich type reaction. The methylene group of salt (la-b) with aromatic aldehydes (3a-l) in presence of ammonium acetate forms Mannich base sulfonium to form sulfonium salt (4a) which, in turn, undergoes condensation with another molecule of benzaldehyde in presence of ammonia to form sulfonium salt intermediate (4b). The later, undergoes elimination of dimethylsulfonium hydrobromide to form 2,4,6-triarylpyrimidines (Sa-l, 6a-l).
A number of 2,4,6-triarylpyrimidine (Sa-l) and (6a-1) synthesized by the above route are listed in Table 1. All the pyrimidines gave satisfactory elemental and spectral analysis. The IR spectral data ~hawed (Table 2) characteristic absorption bands in the region 3100-3000 cm-1 which were assigned due to C-H stretching mode of pyrimidine ring. The bands in the region 1600-1500 cm-1 were due to interaction between C=C and C = N vibrations of the ring. The NMR spectra (Table 3) of pyrimidines showed pyrimidyl protons at C 5 -H in the range 8 6.60-6.80 and aromatic protons at 8 6.60-8.40.

Experimental
All the reagents were obtained from commercial source

Preparation of 4-substituted phenacyldimethyl sulfonium bromide (la-b) :
A solution of 100 mmol of 4-nitrophenacyl bromide and 100 mmol of dimethyl sulfide in 100 ml of anhydrous acetone was stirred for 6-8 h at room temperature in an atmosphere of nitrogen gave solid mass which was filtered, washed twice with acetone and crystallized from benzene pet. as detailed below.  m,ArH).