The methyl salicylate (wintergreen oil) is the methyl ester of salicylic acid.
Methyl salicylate is produced by esterifying salicylic acid with methanol, the reaction scheme is attached to this answer. In the past, it was commonly distilled from the twigs of Betula lenta (sweet birch) and Gaultheria procumbens (eastern teaberry or wintergreen).
