The phytotoxin diplopyrone is considered to be the main phytotoxin in a fungus that is responsible for cork oak decline. Highly stereoselective syntheses of the enantiomer of the structure originally proposed for the phytotoxin diplopyrone and structural analogs have been achieved using carbohydrate starting materials in two complementary approaches. Key steps in the synthesis of putative (-)-diplopyrone are a highly stereoselective pyranose chain-extension based on methyltitanium, preparation of a vinyl glycoside via Isobe C-alkynylation-rearrangment/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 analogs were prepared for biological evaluation, the most promising being a pyranopyran nitrile synthesized in an alternative approach by Ferrier cyanoglycosidation, Wittig chain extension, and lactonization. Biological assays revealed potent antibacterial activity for the nitrile analog against common bacterial pathogens E. ictaluri and F. 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 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.