Microwave-assisted cleavage of phosphate, phosphonate and phosphoramide esters

Microwave-Accelerated McKenna Synthesis of Phosphonic Acids: An Investigation

Phosphonic acids represent one of the most important categories of organophosphorus compounds, with myriad examples found in chemical biology, medicine, materials, and other domains. Phosphonic acids are rapidly and conveniently prepared from their simple dialkyl esters by silyldealkylation with bromotrimethylsilane (BTMS), followed by desilylation upon contact with water or methanol. Introduced originally by McKenna, the BTMS route to phosphonic acids has long been a favored method due to its convenience, high yields, very mild conditions, and chemoselectivity. We systematically investigated microwave irradiation as a means to accelerate the BTMS silyldealkylations (MW-BTMS) of a series of dialkyl methylphosphonates with respect to solvent polarity (ACN, dioxane, neat BTMS, DMF, and sulfolane), alkyl group (Me, Et, and iPr), electron-withdrawing P-substitution, and phosphonate-carboxylate triester chemoselectivity. Control reactions were performed using conventional heating. We also applied MW-BTMS to the preparation of three acyclic nucleoside phosphonates (ANPs, an important class of antiviral and anticancer drugs), which were reported to undergo partial nucleoside degradation under MW hydrolysis with HCl at 130-140 °C (MW-HCl, a proposed alternative to BTMS). In all cases, MW-BTMS dramatically accelerated quantitative silyldealkylation compared to BTMS with conventional heating and was highly chemoselective, confirming it to be an important enhancement of the conventional BTMS method with significant advantages over the MW-HCl method.

Fmoc-chemistry of a stable phosphohistidine analogue

We report the synthesis of the phosphohistidine analogue, Fmoc-4-diethylphosphonotriazolylalanine 5 and its incorporation into peptides. Our synthesis of 5 has enabled us to demonstrate that the analogue is compatible with Fmoc-solid phase peptide synthesis (SPPS) conditions. Standard cleavage conditions yield the diethyl phosphonate-protected peptide, however this can be subsequently deprotected using trimethylsilyl bromide to yield the free phosphonic acid-containing peptides.

A one step/one pot synthesis of N,N-bis(phosphonomethyl)amino acids and their effects on adipogenic and osteogenic differentiation of human mesenchymal stem cells

The one pot reaction of amino acids with diethylphosphite and formaldehyde yielded N,N-bis(phosphonomethyl)amino acids. This synthetic route does not require harsh reagents to cleave the ester group. The molecular structures of the new compounds were determined by X-ray diffraction methods. By employing DFT calculations the hydrolysis of the intermediate phosphonic esters to the respective acids could be explained by the decreasing P-OEt bond strength for C(alpha)-bisalkylated amino acids. Biological evaluation on the adipogenic and osteogenic differentiation of mesenchymal stem cells revealed no modification of the adipocyte differentiation, but inhibition of osteoblast formation at concentrations without detectable cytotoxicity.

A remote controlled system for the preparation of 7 alpha-[18F]fluoro-17 alpha-methyl 5 alpha-dihydrotestosterone ([18F]FMDHT) using microwave

For non-invasive imaging of the prostate cancer, we synthesized 7 alpha-fluoro-17 alpha-methyl 5 alpha-dihydrotestosterone ([(18)F]FMDHT) for androgen receptor mediated PET imaging. Preliminary in vitro and in vivo evaluations of this compound show promise. We designed and implemented a remote controlled system for reliable, efficient, and safe handling of radioactivity during the radiochemical synthesis of [(18)F]FMDHT. The key features of this report are the microwave assisted radiochemical synthesis, increased radiochemical yields, improved radiochemical purity, reduced overall synthesis time, and remote controlled automation of the entire synthesis. The overall synthesis using microwave reaction took 60-70 min and provided the desired product in 20-30% radiochemical yields with >99% radiochemical purity.