The crystal and molecular structure of grayanotoxin XIX. A new minor diterpene from Leucothoe grayana Max.

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The molecular structure of grayanotoxin XIX, C20H30O3, has been determined by means of X-ray crystal analysis. The crystals are triclinic, with two molecules in a unit cell with dimensions of a=10.406, b=10.648, c=8.003 Å, α=90.00, β=94.25, and γ=102.76°; the space group is P1. 2671 unique intensity data were collected on a four-circle diffractometer with Ni-filtered Cu Kα radiation. The structure was elucidated by the Monte Carlo direct method, using the 50 strongest reflections as the starting set. The block-diagonal least-squares refinement reduced the R value to 0.038. The structure obtained corresponds to 14-deoxygrayanotoxin VII. The two independent molecules exist in different conformations: one has a half-chair A-ring, and the other, an envelope A-ring. In both molecules, the B-ring takes a conformation intermediate between the chair and twist-chair forms, and the C-ring adopts a boat conformation. The crystal consists of infinite hydrogen-bonded molecular chains along the c axis.