Head-space volatiles obtained from male mountain pine beetles, Dendroctonus ponderosae, were analyzed by coupled CC-MS and chiral gas chromatography. 5-Ethyl-7-methyl-6,8-dioxabicyclo[3.2.1] (6) was found as a new naturally occurring isomer of brevicomin (1). In addition, several stereoisomers of 7-ethyl-5-methyl-6,8-dioxabicyclo[3.2.1] (11) and 1-(5-methyl-6,8-dioxabicyclo[3.2.1]octyl)ethanol (12) could be identified. Relative and absolute configurations of the compounds were determined by unambiguous syntheses, which are described. Copyright (C) 1996 Elsevier Science Ltd