The phytotoxin diplopyrone is considered to be the main phytotoxin in a fungus that is responsible for cork oak decline. A carbohydrate-based synthesis of the enantiomer of the structure proposed for diplopyrone has been developed from a commercially available derivative of d-galactose. Key steps in the synthesis are a highly stereoselective pyranose chain-extension based on methyltitanium, preparation of a vinyl glycoside via Isobe C-alkynylation-rearrangement/reduction, and RCM-based pyranopyran construction. Crystallographic and NMR analysis confirms an earlier report that the structure originally proposed for diplopyrone may require revision. Structural analogues were prepared for biological evaluation, the most promising being a pyranopyran nitrile synthesized from tri-O-acetyl-d-galactal by Ferrier cyanoglycosidation, Wittig chain extension, and lactonization. Biological assays revealed potent antibacterial activity for the nitrile analogue against common bacterial pathogens Edwardsiella ictaluri and Flavobacterium columnare that cause enteric septicemia (ESC) and columnaris disease, respectively, in catfish. The IC50 value of 0.002 against E. ictaluri indicates approximately 100 times greater potency than the antibiotic florfenicol used commercially for this disease. Phytotoxic activity for all three target compounds against duckweed was also observed. The antibiotic and phytotoxic activities of the new pyranopyrans synthesized in this study demonstrate the potential of such compounds as antibiotics and herbicides.

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The synthesis of 7,9-di-O-methyl-ll-oxosibiromycinone (8) is described. Nitration of methyl 4-methyl-3,5dimethoxybenzoate (18) gave the corresponding nitro derivative 20 which was converted to 4-methyl-3,5-dimethoxy-2-nitrobenzoyl chloride (24). Ethyl 4-formylpyrrole-2-carboxylate (10) was treated with ethylmagnesium bromide and the resulting secondary alcohol 25 heated in dimethyl sulfoxide to afford ethyl (£)-4-(lpropenyl)pyrrole-2-carboxylate (26). Amide bond formation between acid chloride 24 and the sodium salt of pyrrole derivative 26 gave 29. Reduction of the nitro group of 29 with triirondodecacarbonyl gave the corresponding amine 31 which cyclized to the desired compound 8 on heating with p-toluenesulfonic acid in toluene. Preliminary attempts to convert 8 to sibiromycinone were unsuccessful.