Oxidation of Methyl 2-Cyano-3,4-seco-4(23)-Ene-Ursolate by Ozone

Abstract

oxetane. The last was formed by oxidation of intermediate 12-oxoursolate. Ozonolysis of the ursolic acid derivative with two unsaturated bonds in the C-4(23) and C-12(13) positions was non-selective, in contrast with that reported previously for an analogous oleanolic-acid derivative.The triterpenoids oleanolic and ursolic acids are widespread in the plant world and exhibit various biological activities including antidiabetic, anticancer, antiviral, etc. [1][2][3]. Oxygenation of ring C is an important technique for modifying their structures in order to prepare new biologically active compounds. Examples include the synthesis from oleanolic acid of bardoxolone methyl (2-cyano-3,12-dioxooleane-1(2),9(11)-dien-28-oic acid methyl ester), which passed phase I clinical trials for treating cancer and phase II for treating chronic kidney disease in type 2 diabetes patients [4]. 3E,12D-Dihydroxyoleane-13E,28-lactone is a key intermediate in the synthesis of endothelin A receptor antagonists and a promising antidiabetic agent [5]. 12E-Hydroxyurs-11-ene-13E,28-lactone was produced by oxidative transformation of ursolic acid by ruthenium porphyrins and was active against A431 human skin cancer cells [5,6].The structures of the starting substrates are known to affect considerably the oxidation of triterpenoids [7][8][9][10][11][12][13][14][15][16][17][18][19][20]. For example, the reactivities of oleanolic and ursolic acid derivatives were found to differ during oxidation by dimethyldioxirane although these triterpenoids differ only by the location of the methyl in ring E [21]. Proposed schemes for separating mixtures of ursolic and oleanolic acids, which occur together in natural sources, were based on the inertness of the C-12(13) unsaturated bond of the former to the action of m-CPBA and HCOOH/H 2 O 2 [21,22]. We found earlier that oleanolic acid methyl ester was oxidized by ozone to methyl 12-oxoolean-28-oate [23] whereas oxidation of the analogous ursolic acid derivative gave 12-oxo-11S,13R-oxetane on ring C [24]. Limitations of methods for synthesizing such chiral non-racemic oxetanes were noted [25].Considering the aforementioned examples and continuing research on oxidative transformations of plant terpenoids [11,12,14,17,18,[26][27][28], we decided to determine the direction of the oxidation by ozone of 2-cyano-3,4-seco-4(23)-ene ursolic acid methyl ester (1) and to compare the results with those published for oxidation of the analogous oleanolic acid derivative [23].Oxidation of 1 with two unsaturated bonds in the C-4(23) (ring A) and C-12(13) (ring C) positions by ozone in CH 2 Cl 2 formed the three 2-cyano-3,4-seco-4-oxo derivatives 12-oxours-11S,13R-oxetane (2, 68%), 11-oxours-12-ene (3, 12%), and 12-hydroxy-4,11-dioxours-12-ene (4, 8%) (Scheme 1). Thus, oxidation of methyl 2-cyano-3,4-seco-4(23)-ene ursolate by ozone was non-selective, in contrast with the oxidation of the analogous oleanolic acid derivative, which enabled selective transformations of rings A and C to be carried out [23].