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.