Asymmetric synthesis of exo-isobrevicomin and exo-brevicomin via conjugated addition of primary alkyl iodides to α,β-unsaturated ketones

(-)-exo-Isobrevicomin (1) and (+)-exo-brevicomin (2) are volatile substances produced by males of the beetles Dendroctonus ponderosae, which inhabit pine trees found in the northern hemisphere, frequently causing the death of their host. In order to obtain these aggregation pheromones, which present the 6,8-dioxabicyclo[3.2.1]octane structure, the synthetic strategies utilized in this work had as key steps the Sharpless asymmetric dihydroxylation and the conjugated addition, promoted by the Zn(Cu) couple in aqueous medium and accelerated by ultrasound. The conjugated addition of acetonides 13 and 14 to the respective unsaturated ketones (methyl vinyl ketone and ethyl vinyl ketone) furnished the adducts 15 and 16. The intramolecular catalyzed cyclization of compounds 15 and 16 with phosphotungstic acid (H3PW12O40) produced exo-isobrevicomin (1) and exo-brevicomina (2).