NATURAL CYCLOPENTANOID CYANOHYDRIN GLYCOSIDES .13. STRUCTURE DETERMINATION OF NATURAL EPOXYCYCLOPENTANES BY X-RAY CRYSTALLOGRAPHY AND NMR-SPECTROSCOPY

Two stereoisomeric epoxycyclopentane cyanohydrin glucosides were isolated, along with several previously described cyclopentene cyanohydrin glucosides, from Passiflora suberosa L. (Passifloraceae) and Kiggelaria africana L. (Flacourtiaceae); they represent new members of a rare class of natural nonannellated cyclopentane derivatives. The new glucosides were shown, by NMR spectroscopy (including NOE measurements), X-ray crystallography, and enzymatic hydrolysis to the corresponding cyanohydrins, to be (1R,2R,3R,4R)- and (1S,2S,3S,4S)-1-(beta-D-glucopyranosyloxy)-2,3-epoxy-4-hydroxycyclopentane-1-carbonitrile and named suberin A and B, respectively. The crystal structure of suberin A was determined at 110 K and refined to R = 0.036 for 2198 unique reflections; the cyclopentane ring forms an envelope with C5 placed 0.41 angstrom away from the plane of the remaining four carbon atoms, and to the same side as the three oxygen substituents. In addition to the glucosides, two amides, (1S,2S,3R,4R)-2,3-epoxy-1,4-dihydroxycyclopentane-1-carboxamide and (1S,4R)-1,4-dihydroxy-2-cyclopentene-1-carboxamide, were isolated from P. suberosa and characterized; the amides are probably artefacts, and their formation represents a novel enzymatic transformation of plant cyanohydrins.