Enzymatic synthesis of furanoterpenoids from beta-myrcene and related monoterpenes was observed using a solubilised enzyme fraction of mycelium lyophilisates of several Pleurotus species. The initial enzymatic step, the incorporation of molecular oxygen into the conjugated 1,3-diene moiety, was similar to a 2 + 4 cycloaddition of 1,3-dienes with dienophilic (1)O(2), and was followed by a non-catalysed degradation sequence leading to the furans. The cyclic peroxides 3,6-dihydro-4-(2-(3,3-dimethyloxiran-2-yl)ethyl)-1,2-dioxine and 5-(3,6-dihydro-1,2-dioxin-4-yi)-2-methylpentan-2-ol were identified as key intermediates. Biotransformation of beta-myrcene in (18)O-labelled HEPES-buffer did not yield a detectable label in perillene, so a water addition to 3,10-epoxy-beta-myrcenes as an alternative was ruled out. The pathway suggested presents a substantiated biogenetic scheme for the formation of monoterpenoid furans and opens biotechnological access to valuable natural flavour compounds, such as perillene and rosefurane. Only one metabolite, identified as the new natural compound 6-methyl-2-methylene-hept-5-enal, carried the (18)O-label. The enzymatic formation of this compound through a 1,2-endoperoxide (3-(5methyl-l-methylene-hex-4-enyl)-[1,2]-dioxetane) is suggested. The label may simply result from a chemical oxygen exchange between the carbonyl group and the (18)O-labelled water. (c) 2009 Elsevier Ltd. All rights reserved.