O. B. Flekhter

Biological activity and pharmacological prospects of lupane terpenoids: I. natural lupane derivatives

The biological activity of natural and semisynthetic lupane triterpenoids is discussed in a two-part review. The first part is devoted to the pharmacological properties of natural lupane triterpenoids. Betulinic acid has proven to be the most effective antitumor agent among more than fifty natural lupanes.

Antiviral activity of betulin, betulinic and betulonic acids against some enveloped and non-enveloped viruses

Antiviral properties of betulin, betulinic and betulonic acids were investigated in cell cultures infected with herpes simplex type I, influenza FPV/Rostock and ECHO 6 viruses. All studied triterpenes were active against herpes simplex virus. Betulin and especially betulinic acid also suppressed ECHO 6 virus reproduction.

Biological activity and pharmacological prospects of lupane terpenoids: II. Semisynthetic lupane derivatives

The discussion of lupane triterpenoids as prospective medicinal preparations is continued, and semisynthetic triterpenoids are being discussed. Acyl derivatives that mainly exhibit high anti-HIV, antitumor, and organoprotective activities are described.

Synthesis of Betulinic Acid from Betulin Extract and Study of the Antiviral and Antiulcer Activity of Some Related Terpenoids

3 -Hydroxy-lup-20(29)-en-(28)-oic (betulinic) acid (I) was isolated at the beginning of the 20th century and originally called gratiolone [1]. In the past decade, some special biological properties of this triterpenoid has drawn the attention of researchers. In 1994, betulinic acid was found to exhibit anti-HIV-1 activity in H9 lymphocyte cell culture [2]. The most promising inhibitor of HIV-1 replication is 3-O-(3 ,3 -dimethylsuccinate) of betulinic acid [3]. In 1995, it was established that betulinic acid is capable of selectively inhibiting the growth of malignant human melanoma [4]. In addition, betulinic acid and its derivatives possess antibacterial, antimalarial, antiinflammatory, bile-expelling, and other properties [5 – 7]. Since the content of betulinic acid in plants is very low, isolation of this compound from raw plant material is poorly profitable. A more effective method of obtaining betulinic acid is synthesis from betulin (II) and betulonic acid (III). The first synthesis of betulinic acid was suggested by Ruzicka et al. [8]. This original procedure involved (i) protection of the hydroxy groups of II by acetylation, (ii) selective hydrolysis of the acetyl group at C-28, (iii) oxidation of the oxymethyl group to carboxy group, and (iv) removal of the protective acetyl group at C-3. The total yield of betulinic acid from this process did not exceed 30%. In the late 1990s, betulinic acid was synthesized (I) in two steps by oxidizing betulin (II) with Jones reagent or chromic anhydride in acetic acid, followed by selective oxidation of the resulting betulonic acid (III) with NaBH4 [9, 10], whereby the total yield of the target compound reached up to 60%. We used Jones reagent to oxidize the extract obtained by treating birch bark with aqueous isopropanol in an autoclave at 80°C. The initial extract contained up to 90% of betulin and about 5% of lupeol (IV). Betulonic acid (III) formed as a result of this oxidation process was isolated from the reaction mixture in the form of a potassium salt (V) with a yield of 82%, which allowed us to exclude the stage of intermediate product purification by column chromatography [10]. Upon acidification of an ethanol solution of the potassium salt V with a 5% hydrochloric acid solution, betulonic acid (III) of 95% purity was isolated with a yield of 85%. The physicochemical parameters of the product corresponded to the published data [2, 10, 11]. The total yield of betulonic acid (III) calculated for the initial betulin extract was 67%. Column chromatography of the organic fraction yielded 3-keto-lupeol (VI) with a yield of 1.7%.

Synthesis and Pharmacological Activity of Betulin, Betulinic Acid, and Allobetulin Esters

A series of new triterpene hemisuccinates, hemiphthalates and nicotinates have been obtained with 52 – 95% yields via reactions of monoacetates of betulin, betulinic acid, and allobetulin with succinic and phthalic anhydrides and with nicotinic acid chloroanhydride. 3-O-Acetylbetulin-28-O-hemiphthalate showed the most pronounced antiinflammatory activity comparable with the effect of ortophen (diclofenac). 3-O-Acetylbetulin-28-O-hemisuccinate exhibited the most pronounced antiulcer activity comparable with that of carbenoxolone.

Synthesis of Ursolic Phosphonate Derivatives as Potential Anti-HIV Agents

In order to search for new anti-tumor and anti-viral agents, a series of α-aminophosphonate conjugates of 3-O-β-acetyl ursolic acid were prepared. Their biological activities as cytotoxic and anti-HIV agents were evaluated. The preliminary bioassays indicate that synthesized compounds 7a–j have anti-HIV activity (targeting HIV-1 gp120 and CD4) and no cytotoxicity on HT-29 cells (human colon adenocarcinoma cell line).

Synthesis and Antiviral Activity of Betulonic Acid Amides and Conjugates with Amino Acids

Betulonic acid amides with aliphatic and heterocyclic amines and with L-amino acids were synthesized by the acid chloride method. Betulonic acid amide and L-methionine derivatives of betulonic acid and its 3-oxime effectively inhibit the influenza A virus. Betulonic acid octadecylamide is active against the herpes simplex Type 1 virus. The conjugate of betulonic acid 3-oxime with L-methionine is also active toward HIV-1. The tested compounds mainly show no activity toward the ECHO6 virus, which is devoid of a coat.

SYNTHESIS AND PHARMACOLOGICAL ACTIVITY OF ACYLATED BETULONIC ACID OXIDES AND 28-OXO-ALLOBETULONE

In recent years, the chemical transformations and biological activity of triterpenoids of the lupane group have received much attention. It was found that betulin acylates possess antitumor properties [1], while lupeol esters with palmitic and linolic acids produce antiarrhythmic action [2]. The most promising inhibitors of HIV replication include 3-O-(3,3-dimethylsuccinate) of betulinic acid and 3-O(3,3-dimethylsuccinate)-28-O-(2,2-dimethylsuccinate) of betulin [3]. Recently, Kashiwada et al. [4] reported on the synthesis of derivatives of 3-alkylamino-3-deoxo-betulinic acid possessing anti-HIV-1 properties [4]. Another promising compound is betulin 3,28-di-O-nicotinate, which shows hepatoprotector, antiulcer, antiinflammatory, wound-healing, anti-HIV, and immunomodulant activity [5]. An analysis of published data suggests that the class of lupane triterpenoids and related compounds containing acyl groups is a promising source of new biologically active substances. Below we report on the first synthesis of acylated oximes based on betulonic acid (Ia), its methyl ester (IIa), and 28-oxo-allobetulone (IIIa). In the first step, boiling 3-oxo-triterpenoids (Ia, IIa, IIIa) with hydroxylamine hydrochloride in anhydrous pyridine led to a quantitative yield of the corresponding 3-oximes (Ib, IIb, IIIb). By acylating these compounds in anhydrous benzene with excess acetic, succinic, and phthalic anhydrides at room temperature in the presence of triethylamine, we obtained acylated oximes (Ic – Ie, IIc – IIe, IIIc – IIIe) with a yield of 64 – 78% after purification of the products by column chromatography (Table 1).

The synthesis and hepatoprotective activity of esters of the lupane group triterpenoids

Hemisuccinates, hemiphthalates, acetylsalicylates, cinnamates, andp-methoxycinnamates of lupeol, betulin, and 3-O-acetylbetulin were synthesized via interaction with corresponding acid anhydrides or acid chlorides. A number of betulin esters in position 3 and 28 were shown to exhibit a pronounced hepatoprotective effect similar to that of betulin and silibor. These experimental data were in a good agreement with the computer prediction of their biological activity. Betulin 3,28-bishemiphthalate was more effective than carsil in models of experimental hepatitis caused by carbon tetrachloride, tetracycline, and ethanol.

Synthesis and Antiviral Activity of Lupane Triterpenoids and Their Derivatives

It was reported that derivatives of lupane triterpenoids (lupeol, betulin, betulinic acid) exhibit antiviral activity, in particular, with respect to human immunodeficiency virus (HIV) [1], herpes simplex virus (HSV), and Epstein – Barr virus (EBV) [2 – 7]. Previously, we have also studied the anti-HIV properties of betulin 3,28-di-O-nicotinate [8] and the antiviral activity of some betulin oximes [9] and 28-oxo-allobetulone [10]. In continuation of our investigations of the structure – antiviral activity relationship in the series of lupane triterpenoids, we have synthesized a series of new derivatives and studied their activity in comparison to that of the known compounds. Lupeol 3-O-nicotinate (II) was obtained with 94% yield by acylating lupeol (I) with nicotinic acid chloroanhydride in a mixture of anhydrous pyridine and tributylamine. 2-Furfurylidene methylbetulonate (IV) was synthesized via interaction of methylbetulonate (III) with furfurol in an alcohol solution of alkali (after purification by column chromatography, the product yield was 80%). The proposed structures of compounds II and IV were confirmed by NMR data (see experimental part below). N-formylamine (V) was obtained with a 62% yield using the Leuckart reaction between methylbetulonate (III) and formamide in formic acid. The C NMR spectrum of this compound displays a shift of the signal from the C3 nuclei toward stronger field ( = 56.3 ppm) and reveals a signal due to C1 atom at = 164.1 ppm. In the H NMR spectra, the signals from ptotons of the formyl group is manifested by a singlet at = 8.15 ppm and the NH proton, by a broad signal at = 8.00 ppm.