In organosulfur chemistry, a mesylate is any salt or ester of methanesulfonic acid (CH₃SO₃H). In salts, the mesylate is present as the CH₃SO−3 anion. When modifying the international nonproprietary name of a pharmaceutical substance containing the group or anion, the spelling used is sometimes mesilate (as in imatinib mesilate, the mesylate salt of imatinib).^[1]

Mesylate esters are a group of organic compounds that share a common functional group with the general structure CH₃SO₂O−R, abbreviated MsO−R, where R is an organic substituent. Mesylate is considered a leaving group in nucleophilic substitution reactions.^[2]

Mesylate esters are generally prepared by treating an alcohol and methanesulfonyl chloride in the presence of a base, such as triethylamine.^[3]

Related to mesylate is the mesyl (Ms) or methanesulfonyl (CH₃SO₂) functional group. The shortened term itself was coined by Helferich et al. in 1938 similarly to tosyl adopted earlier.^[4] Methanesulfonyl chloride is often referred to as mesyl chloride.

Whereas mesylates are often hydrolytically labile, mesyl groups, when attached to nitrogen, are resistant to hydrolysis.^[5] This functional group appears in a variety of medications, particularly cardiac (antiarrhythmic) drugs, as a sulfonamide moiety. Examples include sotalol, ibutilide, sematilide, dronedarone, dofetilide, E-4031, and bitopertin.^{[citation needed]}

Ice core samples from a single spot in Antarctica were found to have tiny inclusions of magnesium methanesulfonate dodecahydrate. This natural phase is recognized as the mineral ernstburkeite. It is extremely rare.^[6][7]

• Tosylate
• Triflate, the fluorinated analog of mesylate.