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-(β-d-glucopyranosyloxy)-2,3-epoxy-4-hydroxycyclopentane-l-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 Å 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, (lS,2S,3R,4R)-2,3-epoxy-l,4-dihydroxycyclopentane-l-carboxamide and (lS,4R)-l,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.