Isopropyl Cresols: Synthesis and Herbicidal Activity

Isopropyl cresols were prepared by a two‐step process: monoterpene oxidation catalyzed by CrO3 and terpenone isomerization catalyzed by 13X molecular sieves. 5‐Isopropyl‐3‐methylphenol and carvacrol were prepared from 3‐carene, whereas thymol and 4‐isopropyl‐3‐methylphenol were synthesized from α‐pinene. The two reactions followed an identical procedure, but the reaction of 3‐carene was more efficient, benefiting from the three‐membered ring in the structure. 5‐Isopropyl‐3‐methylphenol, thymol, and carvacrol were found to show higher herbicidal activity than glyphosate toward the root growth of barnyard grass. However, they exhibited slightly lower activity in preventing shoot growth. Moreover, 5‐isopropyl‐3‐methylphenol showed higher herbicidal activity than its isomers, which indicates that the relative position of substitutes is significantly associated with the herbicidal activity of isopropyl cresols. Four isopropyl cresols were prepared from two monoterpenes by a simple process. The herbicidal activity of 5‐isopropyl‐3‐methylphenol (11) was considerably higher than those of glyphosate and the isomers 10, 14, and 15 toward the root growth of barnyard grass.