Conformation of an eleven-membered lactone: structure of (E,13R,14S)-13-hydroxy-17-nor-8,9-secolabd-9(11)-en-8,14-olide

Abstract

C19H3203, Mr=308.49, orthorhombic, P212121, a = 7-072 (3), b = 9389 (3), c = 27-888(11)A, v=1852(2)A 3, z=4, Ox = I. 11 Mg m - 3, ,t(Mo Ka) = 0.71069 )k, /z = 0.08 mm-1, F(000) = 680, T = 293 K, R = 0.050 for 1475 independent reflections with I> 2-5o'(/). The conformation of the l l-membered lactone ring is essentially the same as the calculated minimum- energy conformation of trans, trans-cycloundeca- 1,6-diene with differences between corresponding ring torsion angles of only 1-15 ° (mean 6°). This conformation is found in several cytochalasans and dolabellane diterpenoids but not in the pyrrolizidine alkaloids with l l-membered dilactone rings. The cyclohexane ring is distorted to alleviate 1,3-diaxial repulsion between methyl substituents (C(18)-C(20) = 3.291 (7) A); the smallest torsion angles (-47.4 (5), 48.1 (5) °) are at ring bonds between the methyl groups, the largest torsion angles (-55-6 (6), 56.4 (6) °) at bonds remote from these substituents. Introduction. Eleven-membered rings are features of several classes of natural products, e.g. the cyto- chalasans (Tamm, 1980), dolabellane diterpenoids (Matsuo, Kamio, Uohama, Yoshida, Connolly & Sim, 1988) and pyrrolizidine alkaloids (Robins, 1982). Examination of crystallographic results reveals two conformations for the trans, trans- cycloundeca-l,5-diene system in humulene deriva- tives (Khan, MacAlpine, Porte & Sim, 1983) and a number of dolabellane diterpenoids (Matsuo, Uohama, Yoshida, Nakayama, Hayashi, Connolly &