Synthesis of fluorescent (benzyloxycarbonylamino)(aryl)methylphosphonates

Summary The synthesis of a library of structurally variable aromatic esters of (benzyloxycarbonylamino)(aryl)methylphosphonic acids is described by means of the Oleksyszyn reaction. The library was enlarged by the application of a Suzuki–Miayra approach and by preparation of mixed esters.


Triaryl phosphites 1
To the solution of apprioprate phenol (50 mmol) in dry acetonitrile (200 mL) phosphorus trichloride (1.47 mL, 16 mmol) was added dropwise and the obtained mixture was refluxed for 5 h. After cooling the product precipitated or deposited as an oil. It was washed with acetonitrile and dried in dessicator. Phosphites were characterized by their 31 P NMR spectra (presence of only a single phosphorus signal) and used directly after synthesis.

Diaryl (benzyloxycarbonylamino)(phenyl)methylphosphonates 2a-k
Benzyl carbamate (4.53 g, 30 mmol), triarylphosphite (30 mmol) and benzaldehyde (4.53 mL, 45 mmol) were dissolved in acetic acid (100 mL) and the obtained mixture was S4 refluxed for 2 h. Then the acetic acid was evaporated and the oily residue was dissolved in a small volume of acetone (10-20 mL depending on product), several drops of hexane were added and left for crystallization at 4 °C.

Miyaura-Suzuki reaction
Phosphonate ester 2e or 2k (5 mmol) was dissolved in dioxane/water mixture  (4) Diphenyl (benzyloxycarbonylamino)(phenyl)methylphosphonate (5.0 g, 10 mmol) and potassium hydroxide (5.8 g, 100 mmol) were suspended in a mixture of 1 M sodium hydroxide solution (20 mL) and dioxane (20 mL) and several crystals of 18-crown-6 were added (5-10 mg). The resulting mixture was refluxed for 10 min and left at room temperature for 24 h while stirring. Then the mixture was concentrated with a rotary evaporator, the aqueous residue was acidified to pH 1 with concentrated hydrochloric acid and extracted three times with 10 mL-portions of ethyl acetate. The organic fraction was dried over anhydrous sodium sulfate and the organic solvent was evaporated in vacuo.

Phenyl (benzyloxycarbonylamino)(phenyl)methylphosphonate
The resulting oil was dissolved in acetone (3 mL) and left for crystallization in the refrigerator (4 °C). In this manner 3.2 g of the desired product was obtained (81% yield);

Phenyl (benzyloxycarbonylamino)(phenyl)methylphosphonic bromide (6)
This compound was obtained by using the procedure identical as described above for the synthesis of compound 5. 0.5 g (42% yield) of the desired bromide as a yellowish oil was  Phosphonic chloride 5 (1.07 g, 2.5 mmol) was dissolved in dry chloroform (20 ml) and corresponding alcohol (5 mmol) was added. This solvent was heated up to boiling and then triethylamine (0.18 ml, 2.6 mmol) was added dropwise. The resulting solution was additionally refluxed for 5 h. Then the solvents were evaporated under reduces pressure and the resulting brown oil was dissolved in chloroform and left for crystallization. Second portion of product was purified by flash chromatography using a gradient of hexane and chloroform (solvent containing 5% of chloroform more after each 10 minutes of elution) as eluent.

Fluorescence studies
Solutions of the studied compounds in DMSO (0.05-0.1 mmole) were prepared in Eppendorf tubes (300 μL). These solutions were transferred to wells of CELLSTAR geiner black 96 wells plate and were irradiated with 254 and 366 nm UV light. The fluorescence was observed visually and presented schematically in Figure S1. Photographic documentation (Olympus MJU) of the results is not fully reasonable and therefore also negative pictures have been analyzed with moderate success ( Figure S1). S23 Figure S1. Examination of fluorescent properties of synthesized compounds