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.

The pyranopyran amide (2S,4aR,8aR)-6-oxo-2,4a,6,8a-tetrahydropyrano[3,2-b]pyran-2-carboxamide, C9H9NO4, 3, was prepared by a chemoselective hydration of the corresponding nitrile, 2, using a heterogeneous catalytic method based on copper(II) supported on molecular sieves, in the presence of acetaldoxime. Compound 3 belongs to a new class of pyranopyrans that possess antibacterial and phytotoxic activity. Crystallographic analysis of 3 shows a bent structure for the cis-fused bicyclic pyranopyran, similar to nitrile 2. Evidence of an intramolecular hydrogen bond involving the amide group and ring oxygen was not observed; however, two separate intermolecular hydrogen-bonding interactions were observed between the amide hydrogen atoms and adjacent carbonyl oxygen atoms along the b- and a-axis directions. The latter interaction may also be supported by an intermolecular C—H...O hydrogen bond. The lattice is filled out by close-packed layers of this hydrogen-bonded network along the c-axis direction, related from one to the next by a 21 screw axis.

An efficient method for the conjugate addition of indoles to a variety of chalcones using BiBr 3 in ethanol (190 proof) is reported. Products are isolated by a simple procedure that avoids an aqueous work up and extensive chromatography, thus minimizing waste. Bismuth (III) compounds are especially attractive from a green chemistry perspective because they are remarkably nontoxic, non-corrosive and relatively inexpensive.