A facile access to condensed and spirosubstituted pyrimidine phosphor esters

Treatment of alloxan-5-oximes with different types of phosphonate carbanions gave moderate to high yields of condensed and spiro bis-heterocyclic systems bearing a phosphonate substituent. Mechanisms for the formation of the five-and six-membered rings were provided. A marked resemblance between alloxan-5-oxime and 1,3-dimethyl derivative is observed through the studied reactions. The bioassay indicated that some of the products have good selective anti-tumor activity and the structure-activity relationship (SAR) of new phosphonates has also been studied.


Introduction
Recently, a novel heterocyclic transformation reaction has been described involving an additionelimination, and rearrangement sequence of violuric acid (1) with saturated, unsaturated and active alkylidenephosphoranes leading to a series of oxazolo-pyrimidenes as well as pyrimidooxazines (Scheme 1). 1 It has been pointed out that the reactions of such ylides proceeded only when the latter were generated in situ from the corresponding salts (see Scheme 1) in the presence of ~3-fold excess LiOH (aqueous).The findings reflected the inertness of molecule 1, which was attributed to the low reactivity of the oxime function and the amidic carbonyl group toward nucleophilic attack.The thermal condition, coupled with the presence of an excess of a strong base that used for the generation of ylide, however, can deprotonate either the ylides or the oxime 1 promoting thus the further reaction.Furthermore, the adjacent carbonyl group re-enforces the electrophilic character of the oximino moiety.Preliminary screening of the products showed appreciable potency in antagonizing hypothermia and catalepsy caused by reserpine. 1n contribution of this work, the present study has been focused on synthesis of bioactive heterocyclic systems bearing phosphonate substituent.The methodology centered on the application of phosphonyl carbanions (Wittig-Horner (WH) reagents) to violuric acid (1) and the dimethyl derivative 2. Pharmacological evaluation of the produced phosphonates is discussed.Furthermore, a comparative study on the behavior of 1 toward alkylidenephosphoranes counterparts is also undertaken.

O
2,4,6-Trioxo-5-oximinohexylhydropyrimidine (1, also known as violuric acid, barbituric acid-5-oxime, and alloxan-5-oxime) is an example of 6-membered ambident heterocycle possessing several tautomeric structures (Figure 1) and display dual reactivity. 2,3Violuric acid is almost colorless, or at best only a pale yellow color, in the solid state due to its existence in the oximino-ketonic formula 1a. 3 In an aqueous solution it exists mainly in form 1a with only a small proportion of the nitroso-enolic structure 1b.Nevertheless, as soon as an alkali like NaOH is added to the solution, the color turns to deep brown indicating that the original weekly acidic oximino-ketonic character of violuric acid tautomerizes to the more acidic nitroso-enolic form 1b.And in this form it becomes fixed by salt formation with sodium hydroxide.

Results and Discussion
The procedure of the reactions of oximes 1 and 2 with WH reagents are analogous to that of the latter with carbonyl compounds. 4Typically, an ethanol solution of 1.5 equivalent of the phosphorus reagent was treated with a solution of sodium ethanolate (EtONa, 4.5 equiv.) to generate the carbanion, and finally the oxime substrate was added.The reaction mixture was refluxed for the proper time (TLC), poured into distilled water and acidified with HCl (1N).The product mixture was easily separated by solvent extraction, and purified by chromatography.The formed adducts are depicted in Schemes 2-6.
According to Scheme 2, carbanions 3 readily condensed with 1/2 in the tautomeric nitrosa form to give the intermediates 4 (Perkin-type condensation). 5Perkin-type reaction was previously described for the reaction of 3 with benzaldehyde. 6Intramolecular cyclization of 4 with concomitant extrusion of an alcohol molecule afforded the substituted-oxazine phosphor esters 5. On the other hand, formation of 8 might involve an initial nucleophilic attack by the phosphonyl carbanions 3 on 2-hydroxyimino carbon (2-C=NOH) in 1 and 2 7 yielding the phosphonates 6. Subsequent ring closure the spiro products 8 would be obtained via the intermediates 7.Under elimination of an appropriate alcohol moiety from 6 with concomitant Nalkylation the intermediates 7 are presumably formed.Considering the N-alkylation by WHreagent, an analogous process has been observed in the reactions of WH synthons with pyrroles, 8a quinonimines, 8b nitrosonaphthol 8c and pyrimidines.8d In a systematic study, treatment of 1 and 2 with diethyl cyanomethylphosphonate ( Fused oxazines 10 were formed most probably through cyclization and transformation of the cyano group of initially formed condensation intermediates 4A.Similar observation was previously reported by Coppola et al 9 for the reaction product of N-methyl isatoic anhydride with WH 9. Nevertheless, it should be noted that the observed behavior of 9 toward 1 is in marked disparity with the behavior of the phosphorane counterpart, cyanomethylenetriphenylphosphorane toward oxime 1.In the latter case, cycloaddition was observed, followed by an extrusion of a water molecule to give fused-2-cyano-1,3-oxazole E (R 2 = CN) (Scheme 1). 1 Next, we studied the reactions of 1 and 2 with diethyl (α-alkylthiomethyl)phosphonates 11a and 11b.By a similar treatment of oxime 1 with diethyl (α-methylthiomethyl)phosphonate (11a) diethyl (3-ethoxy-4,5,6,7-tetrahydroxazolo [5,4-d]pyrimidine-4,6-dione)-2-phosphonate (13a, 74%) was obtained as the sole reaction product according to Scheme 4. The preferential extrusion of R 2 SH (R 2 = Me or Et, as it is monitored by its characteristic smell) than HOH molecule 10 was driven from the result of the reaction of 1 with diethyl (α-ethylthiomethyl)phosphonate (11b).When 1 was caused to react with 11b, the oxazole 13a (76%) was again obtained.On the same ground, oxime 2 reacted with either 11a or 11b to give the corresponding fused substituted oxazole phosphor ester 13b (≈ 83%) (Scheme 4).The behavior of oximes 1 and 2 toward unsaturated phosphonyl carbanions 14 and 22 was then studied and the obtained products were predicted in Schemes 5 and 6.Treatment of 1 with diethyl vinylphosphonate (14)   Furthermore, the mechanisms outlined in Schemes 5 and 6 show a similar initial attack for the phosphonyl carbanions 14 and 22 and their phosphorane counterparts. 1However, further transformations are quite different.The main difference between the present reaction and the corresponding one of the Wittig reagent with the same substrate 1 1 is that, in the latter case, the formation of the products is accompanied by elimination of the phosphorus moiety (Ph 3 P).The contrasting behavior of the initial intermediate through elimination "of the phosphorus moiety" is because Ph 3 P is a much better leaving group than [(EtO) 2 PO -] moiety.There is much precedence for this difference. 12The results clearly show that oximes 1 and 2 react with phosphorus carbanions mainly in the nitrosa form, and not in the tautomeric hydroxyimino structure.

Scheme 6
The structures suggested for all new compounds are in good agreement with their analytical and spectral data (Tables 1 and 2).
a For further details, see the experimental section; b Average yield from the two experiments.related compounds would be published elsewhere.

Conclusion
Finally, the reactions of 1 and 2 with the seven WH reagents lead to a methodology for synthesis of condensed and spiro substituted pyrimidine phosphor esters with a biological activity.

Experimental Section
General Procedures.Melting points are uncorrected.The IR spectra were recorded on a Perkin Elmer 317 Grating IR spectrophotometer, using KBr.The 1 H and 13 C NMR spectra were measured on a Joel E.C.A-500 MHz instrument using SiMe 4 as an internal reference.The 31 P NMR spectra were recorded with the same instrument, relative to external H 3 PO 4 (85%).The mass spectra were performed on a Joel JMS-A X 500 spectrometer.Elemental analyses were carried out at the Microanalysis Laboratory, Cairo University, Cairo, Egypt.The appropriate precautions in handling moisture-sensitive compounds were observed.Solvents were dried by standard techniques.The substrates violuric acid (1) 13 and 1,3-dimethyl alloxan-5-oxime (2) 14 were prepared according to the reported method.  1 and 2.

Reactions of alloxan
Scheme 1

a
Assigned NH and NOH were lost after D 2 O exchange.b NMR was run in d 6 -DMSO; c NMR was run in CDCl 3 .

- 5 -
oximes 1 and 2 with WH Reagents 3a,b; 9; 11a,b; 14 and 22 General procedure To a stirred solution of 8.6 mmol of the appropriate phosphonate 3a,b; 9; 11a,b; 14 or 22 and 25 mmol of Na in 15 mL dry EtOH was added slowly to a solution of 7.64 mmol of oxime 1 (or 2)in 20 mL EtOH at 0 o C. The resulting mixture was stirred at the reflux temperature up to the consumption of the starting oxime ((≈ 24 h, TLC).After concentration of the solvent, 20 mL of distilled water was added and the solution was acidified with HCl (1N) until the pH of the reaction mixture became acidic.The resulting solution was extracted with (3 x 50 mL) isopropyl alcohol (or AcOEt).The combined organic phase was dried over anhydrous MgSO 4 and the solvent was removed under reduced pressure.The resulting residue was chromatographed on silica gel by using n-hexane / AcOEt as eluents to give the products5a,b, 8a,b, 10a,b, 13a,b, 17a,b, 21a,b, 25a,b and 27a,b.Percentage yields; physical and spectral data of the products are listed in Tables