Synthesis and herbicidal activity of 2α,4α-dimethyl-8-oxabicyclo[3.2.1]oct-6-en-3-one derivatives

The catalytic oxidation of 2 alpha,4 alpha-dimethyl-8-oxabicyclo[3.2.1]oct-6-en-3-one with osmium tetraoxide and excess hydrogen peroxide resulted in the formation of 2 alpha,4 alpha-dimethyl-6,7-exo-isopropylidene- dioxy-8-oxabicyclo[3.2.1]octan-3-one (2), with 91% yield. Addition of aryllithium reagents to this compound resulted in the formation of the aromatic alcohols (6a-h) with 48-76% yield. These alcohols were treated with thionyl chloride in pyridine, and the corresponding alkenes (7a-h) were obtained with 46-80% yield. The effect of compounds 6a-h and 7a-h on the root growth of Sorghum bicolor was evaluated at a concentration of 6.6 mu g g(-1). The alcohols 6a-h caused an inhibitory effect (8-100%) on the S. bicolor radicle growth. The three most active compounds were 6e (aryl = p-methylphenyl), 6g (aryl = p-chlorophenyl), and 6h (aryl = p-fluorophenyl) and caused 100% inhibition. The effect of alkenes 7a-h was less pronounced and varied from 15% to 46% inhibition. Another experiment was carried out in a greenhouse to evaluate the effect of alcohols 6e, 6g, and 6h, at a 6.6 mu g g(-1) dose, against Cucumis sativus, S. bicolor and the weeds Bidens pilosa, Desmodium tortuosum, and Pennisetum setosum. All three compounds showed an inhibitory effect on the development of the aerial parts (26-73%) and roots (13-79%) of the weeds and crops.