Structure and stereochemical assignment of sphaeropsidone, a phytotoxic from Diplodia cupressi

Sphaeropsidone is a phytotoxin produced in very large amount from Diplodia cupressi. It crystallizes in the monoclinic P21 space group with two molecules in the asymmetric unit. Cell parameters are: a = 4.1280(6) Å, b = 13.161(1) Å, c = 13.333(3) Å, β = 90.14(1)°, Z = 4, Dcalc = 1.432 mg/m3 at 173 K. The final refinement converged to R1 = 0.0413, wR2 = 0.0730 for 1684 observed reflections. In sphaeropsidone, the hydroxyl group and oxirane oxygen atoms are mutually cis positioned. The absolute stereochemistry at the three chiral centres turns out to be 1S,5R,6S. The stereochemical assignment of spaeropsidone is a fundamental basis for the assignment of the absolute configuration of some of its derivatives used in structure-activity relationship studies. No intramolecular hydrogen bonds are found. In the crystal packing, the hydroxyl and carbonyl oxygen atoms are involved in head-to-tail intermolecular hydrogen bonds to form infinite linear chains of molecules running along c. The linear chains are arranged into layers of molecules stacked along a.