Bioreversible protection for the phospho group: bioactivation of the di(4-acyloxybenzyl) and mono(4-acyloxybenzyl) phosphoesters of methylphosphonate and phosphonoacetate

Abstract

The di(4-acetoxybenzyl) ester of methylphosphonate 4(X = H, R = Me) and the di(4-acyloxybenzyl) esters of methoxycarbonylmethylphosphonate 4(X = MeO₂C, R = Me, Et, Pr, Prⁱ, Bu or Bu^t) were prepared from the appropriate benzyl alcohol and phosphonic dichloride. At pD 8.0 and 37 °C, both series of compounds hydrolyse with half-lives greater than 24 h to the corresponding mono(4-acyloxybenzyl) esters 5(X = H or MeO₂C, R = Me, Et, Pr, Prⁱ Bu or Bu^t) which were prepared by treatment of the di(4-acyloxybenzyl) esters 4 with sodium or lithium iodide. The mono(4-acyloxybenzyl) esters 5(X = H, R = Me) and 5(X = MeO₂C, R = Me, Et, Pr, Prⁱ or Bu^t) undergo chemical hydrolysis to methylphosphonate 6(X = H), and methoxycarbonylmethylphosphonate 6(X = MeO₂C) respectively, together with 4-hydroxybenzyl alcohol and the appropriate acylate anion. The rates of hydrolysis of the mono(4-acyloxybenzyl) esters decrease as the length and steric bulk of the acyl group increases, with half-lives ranging from ∼ 150 h for the acetyl analogues to 2240 h for the pivaloyl derivative. The hydrolyses of the di- and mono-(4-acyloxybenzyl) esters were catalysed by porcine liver carboxyesterase (PLCE), and in all cases the acylate anion was formed. The rate of enzymatic hydrolysis was most rapid for the 4-butanoyloxybenzyl and 4-isobutanoyloxybenzyl analogues. The methoxycarbonyl ester of the phosphonoacetate analogues was not cleaved by PLCE. The methylphosphonate generated from the reaction of 4(X = H, R = Me) in the presence of esterase and H₂¹⁸O, did not contain ¹⁸O attached directly to phosphorus. These results suggest that both the chemical and enzymatic hydrolyses of the mono(4-acyloxybenzyl) esters and the PLCE-catalysed hydrolyses of the di(4-acyloxybenzyl) esters proceed via hydrolysis of the acyl group to give the acylate anion and the unstable 4-hydroxybenzyl esters. The electron-donating 4-hydroxy group facilitates the cleavage of the benzyl-oxygen bond with the formation of the 4-hydroxybenzyl carbonium ion 9, which readily reacts either with water or the phosphate buffer. The 4-acyloxybenzyl phosphoesters provide the first example of a protecting group which will enable the bioactivation of phosphonate prodrugs at rates appropriate to biological systems.