General Chemistry and Hydrocarbons

The rearrangement of car-2-ene to isolimonene (mentha-2,8-diene), known since 1965, has been described again.  

The two limonene epoxides (93) behave differently when treated with lithium in ethylamine. The trans-epoxide (93a) yields exclusively trons-) -terpineol (94a), excess lithium and long reaction periods reducing the double bond, whereas the c/s-epoxide (93b) yields not only cis-)8-terpineol (94b), but also neo- (95) and isodihydrocarveol (96). The menth-l-ene epoxides [e.g. (97)] behave similarly.  

The stereochemistry of the epoxidation of cjs-menth-2-ene (98) has been discussed. From the two epoxides obtained, lithium aluminium hydride reduction gives isocarvomenthol (99a), neoisocarvomenthol (99b) neoisomenthol (100), and isomenthol (101) in the ratio 36 22 24 18. The hydroboration of a- 

Limonene (88) is known to react with one or two equivalents of hydrogen chloride the monohydrochloride (105) is conveniently prepared in carbon disulphide. The dihydrochloride (106) reacts with chlorine in a complex way Carman and Venzke have shown how the trichloride (107) and tetrachloride (108) are formed, the latter being identical with the product obtained from terpinolene (89) and iodobenzene dichloride. Zinc-alcohol reduction of the tetrachloride leads to the 1,2-cts-dichloride (109), which will, in turn, add hydrogen chloride to give a second trichloride (110). Carman and Venzke have shown that the 

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Menthanes.—General Chemistry and Hydrocarbons. A review on a-terpinene containing 123 references, only 18 of which are post-1960 (the latest 1968 ) has appeared. 

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