An unexpected conversion of 2E-furfurylidene-3-oxo-24-nor-allobetulin to 23-nor-allobetulins

A series of 2E-furfurylidene-23-nor- and 24-nor-allobetulins has been synthesized by the Claisen-Schmidt condensation and conditions of their formation were studied in detail. It was found that among an expected 2E-furfurylidene-3-oxo-24-nor-allobetulin 4 two byproducts holding 3-oxo-4α-hydroxy- 5 and 3β,4α-dihydroxy- 6 substituents were formed, which could become the main products under the change of reaction time and amount of the base solution. Moreover, a conversion of individual 2E-furfurylidene-23-nor-3-oxo-4α-hydroxy- 5 into 2E-furfurylidene-23-nor-3β,4α-dihydroxy-derivative 6 under the treatment with the base solution was observed. An inversion of the configuration at C4 from 24-nor- to 23-nor-allobetulins for compounds 5 - 7 was proved by the NMR spectra. The probable explanation of compound 5 formation includes oxidation by atmospheric oxygen to 4-hydroperoxide, which was further transformed into 4-hydroxy-group. In the presence of the base the reduction C3(=O)-function of compound 5 occurs like Meerwein- Ponndorf-Verley reaction to give compound 6. As a result, a difference in the reactivity of native allobetulin scaffold and 24-nor-allobetulin in the Claisen-Schmidt condensation was observed and a first case of conversion 24-nor- to 23-nor-allobetulin derivatives was described.

24-Nor-allobetulins possess strong α-glucosidase inhibitory activity

A series of 24-nor-allobetulin derivatives holding 3β-hydroxy-, oxime, methoxyoxime, lactame and 4-bromobenzylidene substituents have been synthesized and their differences in the NMR spectra were studied in detail. It was revealed that 3-oxo-24-nor-allobetulin loses selectivity in the reaction of oximation and forms a mixture of Z/E oximes (and methoxyoximes) in contract to the related derivatives of native scaffold (that forms only E-isomers). The screening of α-glucosidase inhibitory activity revealed that 24-nor-allobetulins are more active than allobetulins. The lead 3-oxo-24-nor-allobetulin with IC50 0.49 µM was more than 60-fold and 500-fold active than acarbose and 3-oxo-allobetulin, respectively. We can conclude that the removal of the C-24 methyl group significantly increased the antidiabetic effect and 24-nor-allobetulins should be identified as the new and promising scaffolds as α-glucosidase inhibitors on the basis of triterpenoids.

Synthesis of erythrodiol C-ring derivatives and their activity against Chlamydia trachomatis

To develop new potential agents against Chlamydia trachomatis among oleanane type triterpenoids the synthesis, spectral and X-ray analysis as well as antimicrobial screening of C-12 oxygen and nitrogen derivatives of erythrodiol is presented. The reduction of methyl 3β-acetoxy-12-oxo-oleanoate with LiAlH₄ led to isomeric erythrodiol 12β- and 12α-hydroxy-derivatives, their stereochemistry with respect to the position of hydroxyl-group at C-12 was determined based on the multiplets splitting patterns, the magnitude of the spin-spin interaction, and NOESY interactions. Methyl 3β-acetoxy-12-oxo-oleanoate was transformed to 12E-hydroxyimino- and 12E-methoxyimino-derivatives by the interaction with NH₂OH∙HCl or CH₃ONH₂∙HCl, respectively. By Beckmann rearrangement with SOCl₂ in dioxane 12E-oxime was converted to C-lactame and its following reduction with LiAlH₄ in THF or dioxane led to erythrodiol C-azepanone or C-azepane derivatives. The structure 3-O,12-N-bis-acetyl-derivative of C-azepane-erythrodiol was confirmed by the single crystal X-ray analysis. Erythrodiol 12β-hydroxy- and C-azepane derivatives were found to be lead compounds with significant activity against C. trachomatis with MIC 1.56 and 3.125 μg/mL. Molecular docking was employed to suggest potential binding interaction, the tested compounds are likely to act as Cdu1 protein inhibitors while 12β-hydroxy-erythrodiol exhibited the highest affinity towards this respective target protein. These results indicated that C-ring oxygen and nitrogen erythrodiol derivatives might be considered for further research in the design of antibacterial agents against Chlamydia trachomatis.

Synthesis of messagenin and platanic acid chalcone derivatives and their biological potential

The chalcone derivatives of 20-oxo-lupanes have been synthesised and screened for some types of biological activity. Ozonolysis of lupanes afforded 20-oxo-derivatives with the following condensation using different aromatic aldehydes by Claisen‒Schmidt reaction to the target compounds. The E configuration of 19-[3-(pyridin-3-yl)-prop-2-en-1-one]-fragment was established by X-ray analysis. Screening of cytotoxic activity against NCI-60 cancer cell line panel revealed, that messagenin derivative 9 has the highest activity with GI₅₀ value ranged from 0.304 to 0.804 μM. A colorimetric SRB assay revealed for the 2,30-bis-furfurylidene derivative 11 and 30-bromo-20-oxo-29-nor-3,28-diacetoxy-betulin 16 cytotoxic activity against breast carcinoma MCF-7 and ovarian carcinoma A2780 cell lines. Compounds 11 and 13 acted also as inhibitors of the enzyme α-glucosidase (from S. saccharomyces) with IC₅₀ values of 1.76 μM and 3.3 μM thus being 97- and 52-fold more active than standard acarbose. Antiviral potency of compounds 12 and 14 against HCMV, HSV-1 and HPV is also discussed.