This investigation describes the synthesis and biological evaluation of a series of oxabicyclic analogues related to the helminthosporins. Four oxabicycles were prepared by [4+3] cycloaddition of an oxyallyl carbocation, generated in situ from 2,4-dibromopentan-3-one, with selected furans. Functional group manipulations of the oxabicyclic architecture generated nine further derivatives. The phytotoxic properties of these oxabicycles were evaluated as their ability to interfere with the growth of Sorghum bicolor and Cucumis sativus seedlings. In both species, the most active compounds were oxabicycles possessing a carbonyl group conjugated with a double bond.