Long-chain phenols. Part 18. Conversion of anacardic acid into urushiol

Abstract

(15 : 0)-Anacardic acid (6-pentadecylsalicylic acid), prepared by reduction of unsaturated anacardic acid from Anacardium occidentale, has been converted into anacardic alcohol (6-pentadecylsalicyl alcohol) and thence by oxidation at carbon into anacardaldehyde. Phenolic oxidation of anacardic alcoholled to 8-pentadecyl-1-oxaspiro-[2.5]octa-5,7-dien-4-one, itself readily convertible photochemically, but less so thermally, into anacardaldehyde. Reaction of thionyl chloride with anacardic acid led mainly to the anhydride, which by hydride reduction gave anacardaldehyde less satisfactorily. Dakin oxidation of anacardaldehyde furnished (15 : 0)-urushiol (3-pentadecylcatechol) identical chemically and from argentation t.l.c. with the hydrogenated natural product from Rhus vernicifera. (15 : 0)-Cardanol (3-pentadecylphenol) has been detected in hydrogenated urushiol. The composition of the unsaturated constituents of urushiol from Rhus vernicifera and Rhus toxicodendron and its mode of formation have been discussed. An improved synthesis of (15 : 0)-urushiol has been devised based on an organolithium route. Aromatic methyl ether and ester formation in this series is greatly facilitated by phase-transfer catalysis.