Synthesis and heterocyclization of triterpenic 1,3-diketones

Procedures for the synthesis of some new lupane and 18αН-oleanane pyrazole and isoxazole derivatives from betulin are reported. The synthetic scheme for the preparation of 1,2-azoles involves aldol condensation of triterpenic aldehydes with acetone as a key stage.


Introduction
The strategy for the design of hybrid molecules, including those from natural compounds, is based on the methods of modern organic and medicinal chemistry and provides the preparation of chemical entities with two (or more than two) structural domains with different biological functions 1 .In accord with the molecular hybridization concept, heterocyclic modification of triterpenic skeleton can provide new hybrid molecules with high biological potential and bioavailability [2][3][4][5] .Concurrently, β-dicarbonyl compounds have shown themselves as useful building blocks for the construction of heterocycles with one, two, or more heteroatoms [6][7][8] .So, 1,3diketones obtained from 3-oxo derivatives of allobetulin, betulinic, dihydrobetulonic, 23-hydroxybetulinic, oleanolic, maslinic, and glycyrrhetinic acids were successfully used as convenient and promising objects in the synthesis of N,O-based five-membered heterocycles, such as pyrazoles, oxazoles, and isoxazoles condensed with triterpenic skeleton at the 2,3-position [9][10][11][12][13][14][15][16][17][18][19] .Herein, we describe a convenient synthetic route for the preparation of lupane and oleanane 1,3-diketones with use of the aldol condensation.Further heterocyclization of the synthesized triterpenic 1,3-diketones to derivatives with a 1,2-azole fragment (pyrazole and isoxazole) in A or E cycles of triterpenoid has also been evinced as possible.
The aldol reaction of compounds 10-12 with acetone was discontinued at first signs of the formation (TLC) of a croton's by-product produced by the water elimination of their β-hydroxy ketone fragment.In turn, the aldol condensation of betulinal 5 with acetone carried out at room temperature and using t-BuOK−t-BuOH led to the formation of a new α,β-unsaturated methyl ketone 13 as a single product in an excellent 80% yield.The 1 H NMR spectrum of α,β-unsaturated methyl ketone 13 showed CH 3 -33 protons of the methyl ketone moiety recorded as a singlet at 2.27 ppm.The E-configuration of the С-28-С-31 double bond was confirmed by a large coupling constant (16.5 Hz) between two olefinic protons at 6.16 and 7.07 ppm.The signals of С-28, С-31 carbon atoms and carbonyl group were observed in 13 C NMR spectrum of compound 13 at 130. 19, 149.62 and  198.58 ppm, respectively.
The different location of aldehyde group in the structure of compounds 5-9 enabled the introduction of a 1,3-diketone fragment at C-2, C-28 and C-30 positions of the triterpenic core.According to TLC, chromium (VI) oxide in anhydrous pyridine, normally used as an oxidation reagent, was not suitable for β-hydroxy ketones 10 and 11 because in both cases the reaction proceeded with the formation of a multicomponent hard-toseparate product mixture.Lupane 1,3-diketones 14, 15 were obtained in 30% yields by treating compounds 10 and 11 with PCC in anhydrous CH 2 Cl 2 or with the Jones reagent in acetone (Scheme 2).18αH-Oleanane 1,3diketone 16 was obtained by oxidation of β-hydroxy ketone 12 with the Jones reagent (other oxidizing systems failed to lead to a good result).Lupane 1,3-diketones 14 and 15 are completely enolized at the C-30 atom; that was confirmed by the data of 1 H and 13 C NMR spectra with characteristic signals: (1) a singlet of protons H 3 -33 at 2.11-2.12ppm and a signal of the carbon atom C-32 at 193. 13-193.17ppm were assigned to the methyl ketone fragment; (2) a singlet of enol hydroxyl in the downfield area at 15.65-15.67ppm and a signal of proton H-31 at 5.85-5.87ppm were identified as an enol fragment.Two enol forms A and B for 18αH-oleanane 1,3-diketone 16 in solution at the ratio 7:3 were registered on the basis of the integrated intensity of the signals of protons H 3 -34 (1.94 and 2.13 ppm) and proton H-19 (3.53 and 3.54 ppm), as well as the proton of enolic hydroxyl (15.63 and 15.70 ppm) in the 1 H NMR spectrum.Lupane 1,3-diketone 17 was obtained by the Claisen condensation of α,β-unsaturated methyl ketone 13 with HCOOC 2 H 5 in 65% yield (Scheme 3).The enolization of the ketone group C-34 of compound 17 was confirmed by the registration of doublet signals of olefinic protons Н-28 and Н-31 (5.97To obtain pyrazole derivatives, compounds 14, 16, 17 were heated with hydrazine hydrate in an alcohol−acetic acid mixture (1:1) for 1.5 h (Scheme 4).
The 1 H NMR spectra of compounds 18 and 19 showed the presence of two singlet signals at 2.28 and 6.12-6.23 ppm which conjointly with a broad signal at 8.20-8.22ppm were assigned to protons Н 3 -6′, Н-4′ and the proton of NH group of a substituted pyrazole cycle, respectively, whereas the 13 H NMR spectra revealed the characteristic signals of heterocyclic fragment at 102.02-102.62 and 141.25-147.12ppm.In the 1 H NMR spectrum of compound 20, the signals of H-4' and H-5' protons of the pyrazole fragment were recorded as two doublets at 6.33 and 7.49 ppm with a coupling constant of 1.7 Hz and the NH proton signal at 5.79 ppm.The treatment of triterpenic 1,3-diketones 14, 16 with hydroxylamine hydrochloride in aqueous EtOH in the presence of CH 3 COONa at reflux afforded isoxazole derivatives 21, 22 in 86% and 73% yields, respectively (Scheme 5).In the 1 H NMR spectra of the compounds 21 and 22, the characteristic singlet signals of protons Н 3 -6' and Н-4' of isoxazole fragment were recorded at 2.30-2.31and 6.12-6.61ppm, respectively.

Conclusions
Triterpenic derivatives with a pyrazole or isoxazole fragment in the A or E cycle of triterpenoids were synthesized from the lupane and 19β,28-epoxy-18αH-oleanane 1,3-diketones obtained from commercially available pentacyclic triterpenoid betulin.The synthetic route to triterpenic 1,2-azoles involved the reaction of aldol condensation of α, β-unsaturated aldehydes with acetone, the products of which as β-hydroxyketone and methyl ketone were converted to 1,3-diketones, whose participation in reaction with hydrazine hydrate and hydroxylamine led to target heterocyclic derivatives.

Experimental Section
General.The 1 H, 13 C and 2D NMR spectra (HMBC) (δ, ppm; J, Hz) were recorded for solutions in CDCl 3 using a Bruker AVANCE II spectrometer (400 MHz and 100 MHz, respectively), relative to HMDS.IR spectra (ν, cm -1 ) were recorded on a Bruker 66/S IFS Fourier spectrometer using a thin film obtained by evaporation from the solution of the substance in CHCl 3 .Melting points were determined on an OptiMelt MPA100 device at the heating rate 1°C/min.Optical rotation was measured on a Perkin-Elmer 341 polarimeter using sodium D for CHCl 3 solutions at 589 nm.Elemental analysis was preformed using a vario EL cube elemental analyzer.Chromato-mass spectra were analyzed using Agilent Technologies 6890N, capillary column HP-5ms 15000 x 0.25 mm, electronic ionization as sample ionization method.Thin layer chromatography (TLC) on "Sorbfil" plates was used to control the reaction course and substance purity by visualization under UV light (254 nm).The samples were then subjected to treatment with a 5% solution of H 2 SO 4 and heating at 95-100 °C for 2-3 min.Column chromatography (CC) procedure was performed using Macherey-Nagel 60 Silica (0.063-0.2 mm) as an adsorbent.For each compound, eluents were selected individually.3,28-Betulin dibenzoate 2 and 3,28betulin diacetate 3 synthesized by treating technical betulin with an acylating agent (benzoyl chloride or acetic anhydride) in pyridine 21,24 .Betulinal 5 was obtained by oxidation of the C28 hydroxyl group of betulin by PPC in pyridine 25 .