Reaction of monoterpenoids with hydrogen sulphide to form thiols and epi-sulphides of potential organoleptic significance



Reactions of limonene, α-pinene, α- and γ-terpinene, terpinolene, car-3-ene and pulegone with hydrogen sulphide in the presence of aluminium trichloride or tribromide at c. 0°C yielded mixtures of thiols and sometimes additionally bridged epi-sulphides of the monoterpenoids in yields up to 20%. Reaction of limonene with elemental sulphur, in contrast, produced only the bridged compounds together with complex polysulphides.

In total, nineteen sulphur-containing terpenoids were characterized, of which eight were new compounds. Four-, five- and six-membered rings containing sulphur were present in the epi-products, the most favoured ring-size being five. The main product from reaction of limonene or α-pinene with hydrogen sulphide–aluminium trihalide was p-menth-1-en-8-thiol which is responsible for the characteristic flavour of grapefruit. The main product from pulegone was 8-mercapto-truns-p-menth-3-one an important component of Buchu leaf oil. A major, unexpected product from the reaction of car-3-ene was cis-1,3,3,6-tetramethyl-2-thia-bicyclo[2.2.1]heptane, in which ring contraction had occurred to yield the iridoid monoterpenoid skeleton.