The isolation and structural elucidation of euparotin acetate, a novel guaianolide tumor inhibitor from Eupatorium rotundifolium.

156-157” (vac); [ ( u I 3 O ~ -191O (c 0.54, EtOH); end absorption at 210 mp (e 18,400); A:,”: 2.91, 5.67, 5.74, 5.85, 6.03, 6.04, 6.06, 7.49, 7.95, 8.90, 9.75, 10.20 p ; and nmr signals (in CDCl,) at T 3.62 and 4.33 (2 H, doublets, J = 3.5 cps, I), 3.93 (1 H, q, J = 7 cps, vinyl H), 4.28 (3 H, multiplets, vinyl H and 2 >CH-0), 5.18 (1 H, d, J = 8 cps, >CH-0), 5.78 (1 H, m), 7.12 (1 H, br s, OH), 7.30 (2 H, s, II), 7.97 (3 H, s, -O-COCH3), 8.02 and 8.18 (9 H, multiplets, vinyl methyls). It should be emphasized that “hardening” the potential functions to account for the observed isotope effect in acetyl chloride and t-butyl chloride inevitably leads to grossly overestimated effects in systems where hyperconjugation is impossible. In summary, we conclude: (1) in ordinary systems with hyperconjugation possible, less than 10%probably 2-5 %--of the observed isotope effect is due to nonbonded interactions; reasonable estimates of the nonbonded isotope effect might be obtained by using Bartell’s procedure with the Scott and Scheraga functions.