Microbial transformations of some monoterpenoids and sesquiterpenoids

The absolute configurations of fragrances, flavours and drugs are often important for their special properties. The growing interest of organic chemists in chiral synthons has stimulated work on biotransformations, for which readily available and inexpensive compounds can be used as substrates. Microbial transformations of 1-menthenes like gamma-terpinene, alpha-terpinene, limonene and alpha-phellandrene give the corresponding 1,2-trans-diols with high stereospecificity. Because of the volatility and toxicity of these substrates, and their low solubility in aqueous solutions, a special fermentation technique has been developed in which the terpenes are fed continuously to extended cultures of Corynespora cassiicola or Diplodia gossypina. (4R)-Limonene is transformed by Gibberella cyanea to (1S,2S,4R)-p-menth-8-en-1,2-diol, but 3,3,5,5-tetramethyl-limonene yields a 6-monohydroxylated product and a 6,10-dihydroxylated product with a 6-hydroxy-8,10-epoxy structure as the main metabolite. Vicinal diols are also formed from aliphatic terpenes, by reaction at the terminal isoprenoid groups. Some oxirane structures are found as intermediates. Acyclic sesquiterpenes often form complex mixtures when they are metabolized further. The products of the transformation of trans-nerolidol by several fungi are given as examples. Cyclic sesquiterpenes, with less flexible structures, are oxidized more specifically. Whereas longifolene is a very poor substrate for Corynespora cassiicola, isolongifolene is always hydroxylated at one of the methyl groups attached to C-7. The 14- or 15-hydroxy compounds are further oxidized, very fast, in the 3 position or 4 position.