The mono-ethylenethioketal derivative of idiospermolide

The n.m.r spectrum of the mono-ethylenethioketal derivative of idiospermolide revealed that under the acidic reaction conditions the cyclcopropane ring had opened to give a compound with a 9,11 double bond. 9,19-cyclotriterpenoids are known to cleave predominantly to the Δ9(11) isomer under acid treatment (G. Ourisson, P. Crabbé and O.R. Rodig, Tetracyclic Triterpenes (Herman: Paris 1964)).

Apart from the broad doublet at δ 6.61 (J 6 Hz) for C24-H and the broad doublet at δ 4.47 (J 13 Hz) for C22-H, the characteristic features of the n.m.r. spectrum were a broad doublet at δ 5.30 (J 5Hz) for C11-H, as four proton singlet at δ 3.22 for the ethylenedithioketal protons, a singlet at δ 0.72 for C14-Me and a singlet at δ 0.66 for C13-Me. The signals for C13-Me and C14-Me were assigned by comparison with the signals for C13-Me and C14-Me in grandisolide (J.P. Kutney, D.S. Grierson, G.D. Knowles, N.D. Westcott and I.H. Rogers, Tetrahedron, 1973, vol. 29, p13); the chemical shift of C11-H at δ 5.30 is characteristic of Δ9(11) triterpenoids.

The other signals of the n.m.r. spectrum were consistent with the structure proposed and were assigned as follows:

Broad singlet, 3H, δ 1.90 C25-Me
Doublet, 3H, δ 1.15, C4-Me (J 7 Hz)
Singlet, 3H, δ 0.98, C10-Me
Doublet, 3H, δ 0.96, C20-Me (J 7 Hz)
Multiplet, 21H, the remaining protons

Studies with the shift reagent Eu(dpm)3 gave further information. The reagent apparently bonded to the lactone carbonyl. For a 5.9 x 10-5M chloroform solution of the ethylenethioketal which was 1.8 x 10-5 M with respect to Eu(dpm)3, the following shifts were observed:

Doublet, δ 7.05, C24-H (J 6 Hz)
Singlet, δ 2.75, C25-Me
Doublet, δ 1.15, C4-Me (J 7 Hz)
Doublet, δ 1.11, C20-Me (J 7 Hz)
Singlet, δ 0.98, C10-Me
Singlet, δ 0.79, C14-Me
Singlet, δ 0.67, C13-Me