Oxidation of ursolic acid by ozone

Ursolic acid-based 24-nor- and A-seco-24-oxy-triterpenoids

Using methyl 2-cyano-3,4-seco-12(13),4(23)-diene-ursolate as a starting scaffold a series of 3-oxo-24-nor-ursolate and A-seco-ursanes holding hydroxy-, furoyloxy-, p-tosyloxy- as well as aldehyde fragments at C24 that possess cytotoxic activity has been synthesised. The structures of the new ursanes were confirmed by detailed spectral data analysis. The chemoselectivity of methyl 2-cyano-3,4-seco-12(13),4(23)-diene-ursolate oxidation involving the double bond in the A cycle was observed.

A simple but unusual rearrangement of an oleanane to a taraxerane-28,14 β -olide

Reaction of 3-O-acetyl-oleanolic acid (3) with formic acid/hydrogen peroxide at 100 °C for several hours provides an extraordinary but simple pathway to a taraxeran-28,14 β -olide type triterpenoid while the same reaction at 0 °C occurred without re-arrangement of the carbon skeleton, and an oleanane-28,13 β -olide was obtained instead. The products from these reactions were subjected to a cytotoxicity screening employing several human tumor cell lines showing the latter compound not cytotoxic while the former was cytotoxic especially for MCF-7 (breast adenocarcinoma), and FaDu (hypopharyngeal carcinoma) cells. The highest cytotoxicity, however, was observed for 3 β, 12α, 13 β -trihydroxy-oleanan-28-oic acid (6) holding with EC₅₀ = 4.2 μM for MCF-7 tumor cells.

Diastereoselective Synthesis of Triterpenoid 1,2,4-Trioxolanes by Griesbaum Co-ozonolysis

Diastereoselective synthesis of triterpenoid 1,2,4-trioxolanes by Griesbaum co-ozonolysis was shown for the first time. Ozonolysis of 2-methoxyoximes (syn-anti-isomers mixture) of allobetulin or methyl oleanoate with CF₃-ketones resulted in asymmetrical spiro-1,2,4-trioxolanes as mixtures of diastereomers in yields up to 80-85%. The configuration of the spiro-C-2 center of individual ozonides was determined by 2D NMR spectra and X-ray crystallographic analysis. The products of ozonolysis of triterpenoid 3-methoxyoximes were mixtures of regioisomeric N-methoxylactams. Thus, the fundamental differences in the oxidation of homologous triterpenoid 2- or 3-methoxyoximes with ozone have been established. These results may afford a new stage in the development of the Griesbaum method as applied to natural compounds and biologically active peroxides.

Synthesis of A-ring quinolones, nine-membered oxolactams and spiroindoles by oxidative transformations of 2,3-indolotriterpenoids

This paper describes an access to new nitrogen-containing heterocyclic triterpenoids by the reaction of 2,3-indolotriterpenoids with ozone and dimethyldioxirane. The oxidation of indolo-fused 28-oxo-allobetulin or methyl platanoate with ozone led to a mixture of a quinolone as the major product and a nine-membered 2,3-seco-2-oxolactam and three different types of spiroindoles as byproducts. The formation of quinolone and 2,3-seco-2-oxolactam derivatives could be explained by the standard 1,3-dipolar cycloaddition of ozone to the C2(3)-double bond of the triterpene core similar to the products observed in the ozonolysis of indoles in the Witkop-Winterfeldt oxidation (WWO). The formation of spiroindoles was unexpected and could be explained through the 1,2-cycloaddition of ozone to the C2(3)-double bond with consecutive intramolecular rearrangements of the 2,3-epoxy-intermediate. These spiroindoles seem to be novel structures observed in the WWO reaction. The formation of only two isomeric triterpene spiroindolinones was achieved by the oxidation of 2,3-indolo-28-oxo-allobetulin with dimethyldioxirane that could be explained by the rearrangement of the 2,3-epoxy-intermediate. 19β,28-Epoxy-18α-olean-28-oxo-2-nor-2,3-4'(1H)-quinolone was the most active against HPV-11 with EC50 0.45 μM and SI50 322 in a primary assay and SI90 < 10 against HPV-16 in a secondary assay. The oxidative transformations of indolotriterpenoids have great potential for further modifications towards the preparation of new biologically active compounds.

Synthesis and antimalarial activity of 3'-trifluoromethylated 1,2,4-trioxolanes and 1,2,4,5-tetraoxane based on deoxycholic acid

A series of new steroidal peroxides - 3'-trifluoromethylated 1,2,4-trioxolanes and 1,2,4,5-tetraoxanes based on deoxycholic acid were prepared via the reactions of the Griesbaum coozonolysis and peroxycondensation, respectively. 1,2,4-Trioxolanes were synthesized by the interaction of methyl O-methyl-3-oximino-12α-acetoxy-deoxycholate with CF₃C(O)CH₃ or CF₃C(O)Ph and O₃ as the mixtures of four possible stereoisomers at ratios of 1:2:2:1 and in yields of 50% and 38%, respectively. The major diastereomer of methyl 12α-acetoxy-5β-cholan-24-oate-3-spiro-5'-(3'-methyl-3'-trifluoromethyl-1',2',4'-trioxolane) was isolated via crystallization of a mixture of stereoisomers from hexane and its (3S,3'R)-configuration was determined using X-ray crystallographic analysis. Peroxycondensation of methyl 3-bishydroperoxy-12α-acetoxy-deoxycholate with CF₃C(O)CH₃ or acetone led to 1,2,4,5-tetraoxanes in yields of 44% and 37%, respectively. Antimalarial activity of these new steroidal peroxides was evaluated in vitro against the chloroquine-sensitive (CQS) T96 and chloroquine-resistant (CQR) K1 strains of Plasmodium falciparum. Deoxycholic acid 3'-trifluoromethylated 1,2,4,5-tetraoxane demonstrated a good IC₅₀ value against CQR-strain (IC₅₀ (K1) = 7.6 nM) of P. falciparum. Tetraoxane with the acetone subunit demonstrated the best results among all tested peroxides with an IC₅₀ value of 3 nM against the CQ-resistant K1 strain. In general, 1,2,4-trioxolanes of deoxycholic acid are less active than 1,2,4,5-tetraoxanes.

Asymmetric synthesis of 2-substituted oxetan-3-ones via metalated SAMP/RAMP hydrazones

2-Substituted oxetan-3-ones can be prepared in good yields and enantioselectivities (up to 84% ee) by the metalation of the SAMP/RAMP hydrazones of oxetan-3-one, followed by reaction with a range of electrophiles that include alkyl, allyl, and benzyl halides. Additionally, both chiral 2,2- and 2,4-disubstituted oxetan-3-ones can be made in high ee (86–90%) by repetition of this lithiation/alkylation sequence under appropriately controlled conditions. Hydrolysis of the resultant hydrazones with aqueous oxalic acid provides the 2-substituted oxetan-3-ones without detectable racemization.