Stereoselective syntheses of new pyranopyrans that are related to the natural product diplopyrone, which is a phytotoxin implicated in cork oak decline, have been achieved from carbohydrate starting materials in two approaches that are based on C-glycosides as key intermediates. A C-alkynyl glycoside prepared by Ferrier rearrangement was used as the precursor to a new pyranopyran alkyne that showed potent antibacterial activity against the common bacterial pathogen Edwardsiella ictaluri that causes enteric septicemia in catfish. The C-alkynyl glycoside also showed herbicidal activity. New bioassay data for the pyranopyran nitrile (4aR,6S,8aR)-6-cyano-6,8a-dihydropyrano-[3,2-b]pyran-2(4aH)-one, the most potent of the pyranopyrans synthesized to date, were obtained in greenhouse studies that revealed additional herbicidal activity. Other new analogues that were synthesized included desmethylpyranopyrans that were prepared by Isobe C-alkynylation-rearrangement/reduction and RCM-based pyranopyran construction. The antibiotic and phytotoxic activities of the new pyranopyrans synthesized in this study highlight the importance of substituents on the nonlactone ring and demonstrate the potential of such compounds as antibiotics and herbicides.
Keywords: C-alkynyl glycoside; cork oak; diplopyrone; enteric septicemia; phytotoxin; pyranopyrans.