Idiospermolide a new triterpene lactone

From the dried leaves of Idiospermum australiense (Diels) S.T. Blake was extracted a new triterpene lactone that was given the name idiospermolide (0.037% of dried weight).

Idiospermolide crystallized from methanol as colourless plates,m.p. 243-245°, (Found: C, 79.5%; H, 9.8%; M.W. 438.315 ± 0.003. C29H42O3 requires C, 79.4%; H, 9.6%; M.W. 438.3134. O.R.D. (c 0.132 in methanol) : [α]589 + 34.7°, [α]350 + 87.6°, [α]304 + 362°, [α]294 0°, [α]270 - 1645°, [α]260 - 1090°; νmax 1705, 1448, 1380, 1348, 1140 cm-1; u.v. spectrum transparent above 210 nm; mass spectrum: 438 (P, C29H42O3, 22), 423 (C28H39O3, 12), 339 (C24H35O, 14), 315 (3), 314 (6), 299 (C21H31O, 12), 298 (12), 243 (3), 233 97), 175 (13), 173 (13), 161 (15), 147 (21), 133 (25), 123 (21), 121 (32), 119 (31), 111 (37), 107 (42), 105 (33), 95 (68), 94 (39), 93 (57), 91 (39), 83 (37), 81 (48), 79 (41), 77 (22), 69 (26), 67 (43), 55(100), 53 (38).

Idiospermolide was shown to be a 9,19 cyclotriterpenoid with a single methyl group at C4. It shares these features with cycloeucalenol (J.S.G. Cox, F.E. King and T.J. King, J. chem.. Soc., 1956, p1384), 29-norcycloartenol (M. Devys, A. Alcaide, F. Pinte and M. Barbier, Tetrahedron Lett., 1970, p4621) and 29-norcycloartanol (G. Berti, F. Bottari, A. Marsili, I. Morelli and M. Palvani, Tetrahedron Lett., 1967, p125 and K.N.N. Ayengar and E. Rangaswami, Tetrahedron Lett., 1967, p3567).

The other structural feature is the functionalization of the side chain as an α, β – unsaturated δ–lactone. The i.r. spectrum had a broad carbonyl absorption at 1705 cm-1. Idiospermolide readily gave a dihydro derivative whose i.r. spectrum showed carbonyl absorptions at 1730 cm-1 and 1705 cm-1. It also formed a mono-ethylenethioketal whose i.r. spectrum had a carbonyl absorption at 1710 cm-1. This indicates that there are at least two carbonyl groups present, one of which is conjugated to a double bond.

Idiospermolide had a cyclopropane ring at C9 and C10, a ketone at C3, an equatorial methyl group at C4, an α, β – unsaturated δ–lactone at C22.

The n.m.r. spectrum established the presence of a cyclopropane ring by an AB quartet at δ 0.48 and δ 0.62, each of one proton (J 4.0 Hz). The other signals observed in the n.m.r. spectrum were consistent with the structure proposed and were assigned as follows:

Broad doublet, 1H, δ 6.62, C24-H (J 6Hz)
Doublet of doublets, 1 H, δ 4.51, C22-H (J 12.8 Hz, J 3.5 Hz)
Broad singlet, 3H, δ 1.92, C25-Me
Singlet, 3H, δ 1.02, C13-Me
Doublet, 3H, δ 0.99, C20-Me (J 7.0 Hz)
Doublet, 3H, δ 0.98, C4-Me (J 7.0 Hz)
Singlet, 3H, δ 0.95, C14-Me
Multiplet, 23H, δ 2.80- 0.90 the remaining protons

The assignments of C13-Me and C14-Me were based on the assignment of the methyls of cycloartane derivatives (C. Iavarone, G. Paincatelli, E. Mincione and G. Niuta, Gazz. Chim. Ital., 1970, 100, p 888).

Confirmation of some of these assignments were obtained by double irradiation experiments. Thus, irradiation of the olefinic proton, C24-H, at δ 6.62 resulted in the collapse of the signal from the C25-Me at δ 1.92 to a doublet (J 2.0 Hz).

In an n.O.e. experiment, irradiation of the C25-Me signal at δ 1.92 resulted in an 11% increase of the intensity of the signal from the C24-H at δ 6.62, thus confirming their structural relationship.

Studies with the shift reagent Eu(fod)3 gave further information. The reagent apparently bonded to the C3=O rather than to the lactone ring oxygens. For a 1.1 x10-4M chloroform solution of idiospermolide which was 2.5 x 10-5M with respect to Eu(fod)3, the following shifts were observed:

Multiplet, δ 4.8 – δ 4.15, protons on C2 and C4
Doublet, δ 2.71, C4-Me (J 7.0 Hz)
Singlet, δ 2.50, C25-Me
Doublet, δ 1.24, C20-Me
Singlet, δ 1.24, C13-Me
Singlet, δ1.16, C14-Me

The mass spectrum of an ethylenedithioketal derivative of idiospermolide had its base peak at m/e 131. This gave good evidence for the presence of a 3-oxo function (compare the behaviour of 5α-androstan-3-one ethylene ketal – H. Budzikiewicz, C. Djerassi and D.H. Williams, Structure Elucidation of Natural Products by Mass Spectrometry, vol. 2 (Holden Day: San Francisco 1964)).

The o.r.d. curve of idiospermolide showed a position Cotton Effect (a = +1907°). This is analogous to lophenone and lophanone which both have a single methyl group at C4 which is in the equatorial or thermodynamically favoured conformation and which both show positive Cotton Effects: a = + 1700° and a = + 1300° respectively. This supports the conclusion that there is only one methyl group at C4 which is in the equatorial conformation.

Sodium borohydride reduction of idiospermolide gave a mixture of epimeric alcohols from which only the major product could be isolated. It had the hydroxyl group in the 3β position. This is in agreement with the observation that cycloeucalenone, lophenone and cirtrostanone, each of which has a single equatorial methyl group at C4, give 3β-alcohols on reduction with lithium aluminum hydride (G. Ourisson, P Crabbé and O.R. Rodig, Tetracyclic Triterpenes (Hermann: Paris 1964)).

The n.m.r. spectrum of this alcohol as well as Eu(dpm)3 shift studies were consistent with the proposed structure for idiospermolide.

It was decided to degrade idiospermolide and correlate it with a known substance. The reaction of idiospermolide with potassium t-butoxide in refluxing t-butanol resulted in ring opening of the δ-lactone function to give a diene acid. This was characterized as both the methyl ester and the saturated methyl ester. The saturated ester was reduced with lithium aluminium hydride to the diol and the primary alcohol selectively tosylated. Reduction with lithium aluminium hydride then gave 29-norcylcoartnaol. The latter was found to be identical with a sample prepared by the ozonolysis of the acetate of cylcoeucalenol followed by sodium borohydride reduction, tosylation and reduction with lithium aluminium hydride.

With respect to the absolute configuration at C22, the proton on C22 has vicinal coupling constants of 12.8 Hz and 3.5 Hz (cf parasorbic acid which has vicinal coupling constants of 10.3 Hz and 5.4 Hz).

If this proton were pseudo equatorial, the coupling constants would have been nearly equal. Hence the large constituent at C22 is pseudo equatorial. Now the sign of the ene-lactone n →π* Cotton Effect near 250 nm in the C.D. curve of α,β-unsaturated δ-lactones have been shown to give the absolute configuration at the carbon atom adjacent to the ring oxygen (G. Snatzke, Angew. Chem. Int. Ed., 1968, 7, p14; A.F. Beecham, Tetrahedron, 1972, 28, p5543). Since the C.D. curve of idiospermolide has Δε253 = -2.2, the chirality in the pentenolide ring is opposite to that in parasorbic acid and the absolute configuration at C22 is S. This absolute configuration at C22 of idiospermolide is opposite to that found in the withanolides.