The panorama of base-catalysed isomerisations of terpenes is an important part of aroma chemistry. Major contributions in this area are presented here under sections on hydrocarbons, alcohols, aldehydes, ketones, acids, esters, and epoxides.
Hydrocarbons
p-Menthenes. Pines and Eschinazi introduced sodium-organosodium catalysts, for example sodium-'benzylsodium' catalyst (prepared by treating an excess of sodium in toluene with o-chlorotoluene) for isomerising the title compounds. One of their main findings is that when (+)-p-menth-l-ene (1), trans-p-menth-2-ene (2) or p-menth-3-ene (3), is refluxed at 168-175° for 20-22 hrs with the catalyst, the isomerisate is an equilibrium mixture of (3) (63%), (1) (32%), and p-menth-8(9)-ene (4) (5%). The rate of racemisation of (+)-(1) is relatively faster than that of its rate of isomerisation and (+)-trans-(2) reorganised to (+)-(1). There is no formation of p-cymene (5). The mechanism proposed involves intermediate carbanions.
