Potassium (1R,4R,5S,8S)-4,5,8-trihy­droxy-3-oxo-2,6-dioxabicyclo­[3.3.0]octane-4-sulfonate dihydrate

The title salt, K(+)·C6H7O9S(-)·2H2O, formed by reaction of dehydro-l-ascorbic acid with potassium hydrogen sulfite in water, crystallizes as colourless plates. The potassium ion is coordinated by eight O atoms arising from hy-droxy or sulfonate groups. The sulfonate group is bonded to the C atom neighbouring that of the lactone carbonyl group. As is commonly observed in crystalline l-ascorbic acid derivatives, the O atom of the primary hy-droxy group is linked to the second C atom from the lactone C atom, forming a hemi-acetal function, thereby creating a bicyclic system of two fused five-membered rings, both of which have envelope conformations with one of the shared C atoms as the flap. Addition of the sulfur nucleophile occurs from the less hindered face. One of the two independent lattice water mol-ecules has hydrogen bonds to sulfonate O atoms of two different anions and is the acceptor of bonds from hy-droxy groups of two further anions; the second lattice water mol-ecule donates to the carbonyl and a hy-droxy O atom in different anions, and accepts from a hy-droxy O atom in a further anion. Thus, through K-O coordination and hydrogen bonds, the potassium cations, sulfonate anions and water mol-ecules are linked in a three-dimensional network.