Transesterfication: Which is a suitable acid that does not contaminate the polar product?

In my PhD Project I am doing a transesterfication of a phosphonic ester with methanol. If you use HCl 37 % you introduce water necessarly which slows the reaction. Sulfuric acid is more or less dry, but in LCMS after a couple of days I observed methylbisulfate formation (111 m/z -TIC ESI). This is a problem, because my product is also very polar and can not easyly purified (DCM/water might work, but the previous reaction contained a lot of white precipitate and this made the workup problematic). Will methanolic HCl work better (premix without water)? It can only form methyl Chloride which is volatile, but, I am wondering if HCl gas will just evaporate quickly under reflux conditions. Or can you recommend others acids for transesterfication? Methanesulfonic acid for example will form Methyl methanesulfonate which is like chlormethan carciogenic. Phosphoric acid would show the same problem.


