THE CHEMICAL HISTORY OF THE TERPENES 55 



isomerides of limonene, in which the double bond is otherwise 

 placed. Theoretically there may be three such compounds in 

 which the A is in the ring — 



CMe 



CMe 



CH 

 Me.C~CH 2 



CH 

 MeC = CH, 



CMe 



CH 

 MeC = CH 



and some ten or twelve others all having the para distribution 

 of the side chains. 



It is somewhat remarkable that the great majority of the 

 terpenes occurring in nature belong to this type, for, as already 

 mentioned, most of them may be converted into p cymene by 

 comparatively simple operations. 



Terpenes of the ortho- type have not at present been recog- 

 nised among natural terpenes, but a dihydro-o-terpene or 

 o menthene has been prepared synthetically by Ray and 

 Perkin. 1 



The idea that terpenes may exist in which the meta position 

 is occupied by the side chains has also been put to the test by 

 Perkin, who has succeeded in producing synthetically not only 

 a number of compounds of this structure, but one hydrocarbon 

 which is identical with the sylvestrene originally isolated from 

 Swedish turpentine by Atterberg in 1878. 2 The method used 

 was similar to that which has already been described in the case 

 of the para compounds. 



Proceeding from A 1 cyclohexenescarboxylic or A 1 tetrahydro- 

 meta-toluic acid, 



CMe CH 



CH 



>CH . CO,H 



CH 2 CH 2 



the a? acid was first isolated from the inactive mixture by means 

 of brucine and the ester made to interact with magnesium 

 methyl iodide. The resulting d dihydrocarvestrenol is the 



1 Trans. Chem. Soc. [1905], 1066. 



2 Ber. 10, 1202. 



