Abstract
The pyran-opyran amide (2S,4aR,8aR)-6-oxo-2,4a,6,8a-tetra-hydro-pyrano[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 mol-ecular sieves, in the presence of acetaldoxime. Compound 3 belongs to a new class of pyran-opyrans that possess anti-bacterial and phytotoxic activity. Crystallographic analysis of 3 shows a bent structure for the cis-fused bicyclic pyran-opyran, similar to nitrile 2. Evidence of an intra-molecular hydrogen bond involving the amide group and ring oxygen was not observed; however, two separate inter-molecular hydrogen-bonding inter-actions were observed between the amide hydrogen atoms and adjacent carbonyl oxygen atoms along the b- and a-axis directions. The latter inter-action may also be supported by an inter-molecular 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.
Highlights
In 2019, our laboratory published the synthesis and biological evaluation of pyranopyran analogs based on the structure of diplopyrone (Lazzara et al, 2019)
A consequence of the cis ring fusion is that the molecule has more of a bent shape than it would if trans-fused, which is demonstrated by the O1—C8A—C4A—O5 torsion angle of 72.95 (15) versus 177 for a comparable trans-fused pyranopyran (Yu et al, 2017)
A packing diagram of 3 (Fig. 2a) shows the N—HÁ Á ÁO hydrogen-bonding interactions forming molecular planes defined by the crystallographic a- and b-axes; packing of these hydrogen-bonded layers appears to be a function of solvent exclusion and van der Waals contact alone, lacking any hydrogen bonding
Summary
The phytotoxin diplopyrone 1 was isolated from the fungus Diplodia mutila and reported in 2003 (Evidente et al, 2003). In 2019, our laboratory published the synthesis and biological evaluation of pyranopyran analogs based on the structure of diplopyrone (Lazzara et al, 2019) These enantiomeric analogs showed antibacterial and phytotoxic activity, in one case exceeding the activity of a commercially used antibiotic that is used to treat bacterial diseases in pond-raised catfish, which is the largest segment of aquaculture in the United States. A consequence of the cis ring fusion is that the molecule has more of a bent shape than it would if trans-fused, which is demonstrated by the O1—C8A—C4A—O5 torsion angle of 72.95 (15) versus 177 for a comparable trans-fused pyranopyran (Yu et al, 2017) Both rings adopt half-chair conformations, placing the amide group in a near 1,3-diaxial interaction with H4A. 762 Greene et al C9H9NO4 consistent with the observed vicinal coupling constant of 4.5 Hz for H4A—H8A in the associated 1H NMR spectrum
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