The synthesis of α,β-unsaturated 18αH,19βH-ursane methyl ketones

An efficient and facile synthetic technique of a new α,β-unsaturated ketones of 18αH,19βH-ursane type from betulin and a possibility of their further heterocyclization to C20 pyrazoline derivative are reported. The synthetic scheme involves aldol condensation of 18αH,19βH-urs-20(21)-ene 30-aldehyde with acetone as a key stage.


Introduction
Pentacyclic triterpenoids are a group of secondary plant metabolites therapeutically promising due to their prevalence in nature, unique skeleton arrangements, and versatile biological activities, along with the absence of toxicity [1][2][3][4][5][6] .The interest indicated by medicinal chemists in triterpenic carbon scaffolds is caused by attractive opportunities of improving physicochemical, pharmacokinetic, and pharmacodynamic properties of triterpenoids through their simple structural transformations, modifications of functional groups or the introduction of new reaction centers [7][8][9][10] .Considering that triterpenic oxo-derivatives are favorable objects for various chemical modifications, including the so-called aldol condensation forming new carbon-carbon bonds 11 , triterpenic α,β-unsaturated ketones were synthesized by a simple reaction of some aromatic and heterocyclic aldehydes as carbonyl component with triterpenic 3-ketones as a methylene component 7,[12][13][14][15][16] , while lupane 28-aldehyde was used as a carbonyl reactant in condensation with acetophenone 17 .The introduction of α,β-unsaturated oxo-fragment into triterpenic structures frequently enables to enhance synthetic 18,19 and biological 20,21 potentials of polycyclic triterpenoids.So, the transformations of the lupane α,β-unsaturated 3-ketones in reactions of reduction, oxidation, and cyclopropanation were investigated 15,16 .An additional structural modification of triterpenic skeleton proceeding with the creation of new reaction centers prospective for the realization of aldol condensation is regarded as a promising approach to expansion of the spectrum of triterpenic α,β-unsaturated ketones.For example, saturated alicyclic systems of lupane triterpenoids with a five-membered ring E and trisubstituted double bond in the side chain can selectively be converted to a six-membered ring E with a methyl substituent at the double bond, subsequent oxidation of which leads to α,β-unsaturated aldehyde 22 .The consistent conversion of betulin through allobetulin to heterobetulin is an efficient method for the synthesis of l8αH,19βH-ursane derivatives 23,24 .Herein, we describe a convenient synthetic route for the preparation of the 18αH,19βH-ursane α,β-unsaturated methyl ketones using the aldol condensation as a key stage of betulin's transformations.The possibility of further heterocyclization of the synthesized ketones with formation of corresponding C20 pyrazoline derivatives has also been demonstrated.

Results and Discussion
The synthetic route to the 18αH,19βH-ursane α,β-unsaturated methyl ketones consists of two stages: (1) synthesis of 18αH,19βH-ursane aldehydes (Scheme 1), and (2) aldol condensation of triterpenic aldehyde with acetone (Scheme 2).Allobetulin 2 can be easily obtained by the Wagner-Meerwein rearrangement of betulin 1 under various acidic conditions 7 .The 3,28-dibenzoyl-heterobetulin 3 was prepared from allobetulin 2 by treatment with benzoyl chloride in refluxing toluene 23 .α,β-Unsaturated aldehyde 4 was obtained by oxidation of compound 3 with H 2 SeO 3 in 1,4-dioxane 24 .The NMR spectral data were obtained and compared with those reported 23,24 for the known compounds 3 and 4.Here we additionally confirmed the structure of aldehyde 4 by X-ray diffraction analysis (Figure 1).According to the literature 14 , alkaline hydrolysis of 3,28-diacylderivatives of 30-nitril-, or 30-carboxy-, or 30-carbomethoxy-, or 30-hydroxyheterobetulin leds to the formation of the tetrahydrofuran cycle bonded to ring E via the C17 and C21 carbon atoms.When α,β-unsaturated aldehyde 4 was boiled in an alcoholic solution of KOH, the 21β,28-epoxy aldehyde 5 was also formed.The presence of 21β,28-epoxy-moiety in the structure of the synthesized compound 5 was confirmed by detecting AB system of two doublets for the H28 protons (3.26 and 4.08 ppm) and doublet for H21 proton (4.48 ppm) in the 1 H NMR spectrum, and signals of C21 and C28 atoms at 73.94 and 65.39 ppm in the 13 C NMR spectrum, respectively.The conversion of betulin 1 to 18αH,19βH-ursane aldehydes 4 and 5 opens up attractive prospects in a wide range of their synthetic applications.Since compound 4 does not contain any proton at the α-carbon position to the aldehyde group and can be used only as a carbonyl reactant, we had selected acetone as a compound with active methyl group for the condensation reaction.In the condensation process of aldehyde 4 with acetone, the conditions of base catalysis typical of the aldol reaction were tested 25 .The reaction of aldehyde 4 in acetone using NaOH or NaH at room temperature afforded α,β-unsaturated ketone 6 as a single product in reasonable yields of 30% or 57%, respectively (Scheme 2).The attempts to force the aldol condensation of aldehyde 4 using NaOH or KOH during boiling gave rise to hydrolysis of 3,28-dibenzoyl groups with the formation of aldehyde 5, which under these conditions was inert.We were able to obtain α,β-unsaturated methyl ketone 7 in an extremely low yield (5%) when using MeONa as a base catalyst, while the main product of the reaction was 21β,28β-epoxy aldehyde 5.The 1 H NMR spectrum of the triterpenic α,β-unsaturated methyl ketones 6 and 7 showed CH 3 -33 protons of the methyl ketone moieties as singlet signal at 2.24-2.26ppm, and two characteristic peaks in the downfield area at 5.98-6.04 and 6.72-7.02ppm which can be assigned to the protons of the double bond C30−C31, the coupling constant of which at 16 Hz indicates their trans relative position and, hence, the Econfiguration of the double bond.The signals of the aromatic and the olefinic carbons (125.4-150.5 ppm) and the carbonyl group at 198.7-199.0ppm were observed in the 13  The structural features of ring E of compound 7 were finally elucidated by the analysis of the HMBC and NOESY data (Figure 2).The location of the oxo-group at C-32 (δ 199.0 ppm) and the C30-C31 double bond were determined by the HMBC correlations between Н30 (δ 6.72 ppm) and Н31 (δ 5.98 ppm) olefinic protons with oxygenated quaternary carbon atom at 199.04 ppm.A cross-peak C30/H21 (δ 150.46/4.00ppm) appeared to be the key HMBC cross-peak confirming the 17β,21β-orientation of the tetrahydrofuran cycle bonded to ring E. The 2D 1H-1H (NOESY) NMR spectrum showed cross-peaks H20/H30 (δ 1.83/6.72ppm), H20/H31 (δ 1.83/5.98ppm), H21/H30 (δ 4.00/6.72ppm), H21/H31 (δ 4.00/5.98ppm), H30/H 3 29 (δ 6.72/1.00ppm).The NOESY correlations between H20/H 3 29 (δ 1.83/1.00ppm) and H20/H21 (δ 1.83/4.00ppm) confirmed the α-orientation of H20 and H21 protons.

Experimental Section
General.All the reactions were conducted in air atmosphere.All the commercial reagents and solvents were used as received, without further purification.Column chromatography was performed using Macherey-Nagel 60 Silica (0.063-0.2 mm) as an adsorbent.Sorbfil plates used for thin layer chromatography (TLC) were at first visualized under UV light (254 nm), then treated with 5% solution of H 2 SO 4 .Melting points were determined on an OptiMelt MPA100 device at the heating rate of 1°C/min.IR spectra of the compounds dissolved in CHCl 3 were recorded on a Bruker 66/S IFS Fourier spectrometer.The 1 H, 13 C and 2D NMR spectra of compounds dissolved in CDCl 3 were recorded on a Bruker AVANCE II spectrometer at 400 MHz and 100 MHz, respectively, with chemical shift values expressed in parts per million (ppm), relative to TMS.Optical rotation was measured on a Perkin-Elmer 341 polarimeter using sodium D for CHCl 3 solutions at 589 nm.The initial compound in our experiments, allobetulin 2, was prepared from betulin 1 by the known procedure 1 .For the known compounds 3, 4 first described in the 1960s 2,3 , we herein submit updated findings of NMR spectroscopy and X-ray diffraction analysis.

Figure 1 .
Figure 1.Molecular structure of compound 4 with atoms represented as thermal vibration ellipsoids, with 50% probability.

Figure 2 .
Figure 2. The key HMBC and NOESY correlations of compound 7.

Figure 3 .
Figure 3.The key HMBC and NOESY correlations of compound 8.