Synthesis and Bioactivity of a-Aminophosphonates Containing Fluorine

Twenty-one a-aminophosphonates containing fluorine were synthesized by Mannich-type reactions. Their structures were established by elemental analysis, IR, 1H- NMR and MS. In field tests, some of these new compounds display high antiviral activity against the tobacco mosaic virus (TMV). The molecular geometry of compound 4f was determined by X-ray diffraction structure analysis.


Introduction
It is known that a -aminophosphonic acids and their derivatives display interesting bioactivities such as antiphytoviral, antitumor, herbicidal and fungicidal activity, therefore they have attracted considerable attention in pesticide and medicine formulation. A large volume of research on their synthesis and biological activities has been reported during the last twenty years [1][2][3][4][5][6][7][8], but little attention has been paid to the synthesis of a-aminophosphonates bearing fluorine. Considering that fluorine has special biomimetic, electronegativity, blocking and lipophilic effects, once introduced into organic compounds, there may be profound and unexpected changes in biological activity of these compounds. In our search for new classes of compounds with high biological activity and low toxicity, a series of simple a-aminophosphonates containing fluorine were synthesized by one-pot Mannich-type reactions. (Figure 1) and their structures were confirmed by 1  R1=H, o-F, p-F, p-Cl; R2=CF3, CH3, H, NO2; R=Me, Et, i-Pr, n-Pr, n-Bu The biological activity of some of the compounds synthesized was tested in the field against the tobacco mosaic virus (TMV). The test results are shown in Table 1.

Reaction Conditions
The reaction temperature should be controlled between 95-100°C. The byproduct s increase and the yield decreases if the temperature is allowed to exceed 120°C. If the reaction temperature is below 90°C the reaction time will be 1.5 times longer. Toluene is a good solvent for the reaction and other solvents such as ethanol or dichloromethane are not so good, mainly because their reflux temperatures are lower that that of toluene. [9] To further confirm the structure of the compounds synthesized single-crystals of compound 4f were obtained by recrystallization from anhydrous ethanol at room temperature. Determination of the unit cell and the data collection were performed with Moka radiation (λ=0.71073Å) on an Enraf-Nonions CAD4 four -circle diffractometer. Crystallographic data (excluding structure factors) for the structure(s) reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 201543 [9]. The crystallography parameters and conditions of data collection of intensity are summarized as follows: C 18 H 20 F 4 NO 3 P; (M.W. 405.32). The crystal data are triclinic, space group P -1 with unit cell dimensions a=8.301(3), b=9.358(4), c=12.731(5) Å, α=79.8°(6), β=89.2°(7), γ=87.4° (7), z=2, Dx=1.384mg/m-3 , V=172.4Å 3 , F(0000)=420, scan type

X-Ray diffraction data
The corrections for Lp factors and semi empirical absolution were applied to the intensities. The other non-hydrogen atoms were determined with successive difference Fourier syntheses. The final refinement by full matrix least-square method with isotrop ic thermal parameters for F(2) atom was converged with outweighed and weighted agreement factors of 0.061 and 0.097. The highest peak on the final difference Fourier map had a height of 0.62/Å 3 .
The distance of P(1)-C(5) is 1.814(4) Å which is longer than the sum of Van der Waals radii between the atoms of phosphorus and carbon which shows that there is no coordinated bond from the carbonyl oxygen to the central phosphorus atom. Comparison with the difference between the standard tetrahedral geometry and the bond angle of O(1)-P-O(3)(98.91) shows that the compound is 4-coordinated triclinic with a distorted tetrahedral geometry. The distances of the two N-C bonds are 1.398 and 1.447. It may be the result of the steric effect of the proximity to the bulky phosphoruscontaining substituent. The molecular structure is shown in Figure 2. The packing diagram of the unit cell is shown in Figure 3.

Biological activities against TMV
The bioassays show that all compounds tested had significant toxicity against TMV in the field. The activity showed large variability depending on the substituent groups. The best results were obtained when R is Et. The inhibitory rate of compound 4f towards TMV was 84.92% at the concentration of 500 mg L -1 .

Acknowledgements
We thank the National Natural Science Foundation of China for financial support of this work (No: 29662002).

General
Melting points were measured on a Tech X-4 melting point apparatus and they are not corrected. 1 H-NMR spectra was recorded on a Varian INOVO400 (400MHz) pulse Fourier-transform NMR spectrometer in CDCl 3 using tetramethylsilane as an internal standard. The IR spectra were recorded on a Bruker VECTOR22 spectrometer. The X-ray diffraction data recorded on an Enraf-Nonious CAD4 four-circle diffractometer. The mass spectra were taken on an HP5988A spectrometer. Elemental analysis was performed with a Vario -II CHN analyzer. The reagents were all analytical reagent grade or or of high chemical purity.
Typical experimental procedure for synthesis of 4a-s.
A substituted aryl benzaldehyde (1, 5mmol), 4-aminobenzotrifluoride (2, 5.3mmol) and toluene (15mL) were placed in a three-necked 25mL flask. Dialkyl phosphate (3, 5.3mmol) was added with stirring and the reactants were refluxed for 2-5hrs. The progress of reactions was checked by TLC (petroleum ether-ethyl acetate = 1:3). Toluene was evaporated under vacuum and the residue was recrystallized from ethanol and water to give the target compound 4.

Biological tests
A field trial was carried out at Guizhou University using spring tobacco, cv.Hung. The experiment was arranged with 4 replicates per concentration per test compound on randomized plots of 60.0m 2 . The compounds 4a-4h, 4j and 4o were formulated as a 30% WP which was then diluted with water to give AI concentrations of 500 mg litre -1 . Disease assessment of TMV was estimated 7 days and 14 days after application when untreated control plants had 10-15% infected leaf area. Results are expressed as the inhibition rate calculated on the basis of % infected leaf area.