Synthesis and Intramolecular Regioselective Cyclization of 2-Cyano-3,10-dioxo-1,10-seco-1,25-dinorlupan-28-yl Benzoate

Abstract

The paper describes the synthesis of a new 1,10-seco-triterpenoid with a 2-cyano-3,10-diketone fragment and its further regioselective cyclization under acidic and basic conditions with the formation of 2S-cyanopyran-3-one derivative or A-pentacyclic alkene β-ketonitrile, respectively.

1. Introduction

Dicarbonyl has been a common structural component in many natural products and medicines [1,2,3,4]. Simultaneously, the compounds containing a diketone moiety are frequently employed as building blocks in designing novel structurally complex molecules [5,6]. When in synthesis, 1,4-diketones most often serve as universal precursors to produce a variety of carbocyclic and heterocyclic compounds. For example, they can be used to produce five-membered heterocycles in the Paal–Knorr reaction [7]. The significance of 1,4-dicarbonyls in the evolution of chemistry of heterocyclic compounds necessitates the search for new methods for the synthesis of these motifs. Recently, we described the synthesis of 1,10-seco-lupane triterpenoid 1, whose structure enables obtaining a new triterpene derivative with a 1,4-diketone fragment [8]. Intermediate 3-Hydroxy-10-ketone, being formed as a result of ozonolytic oxidation of the ∆^10,25 bond of 1, was evinced to in situ undergo chloroformylation of the hydroxyl group (compound 2) and intramolecular cyclization to furanoterpenoid 3 (Scheme 1).

Herein, methods are presented for the synthesis of 1,4-diketone and its cyclization products using the semi-synthetic triterpenoid 1 as a starting material.

2. Results and Discussion

A two-step synthesis of lupane 1,4-diketone 5 involved initial oxidation of a 3-hydroxy group of compound 1 using a Jones reagent, which gave 3-oxo derivative 4 with a 67% yield (Scheme 2). The characteristic proton signals of the H₂-2 methylene group were detected in the ¹H NMR spectrum of compound 4 as two doublets at 3.45 and 3.59 ppm with J = 19.0 Hz. The signals of C-1 and C-3 carbon atoms were recorded in ¹³C NMR spectrum at 114.30 and 203.10 ppm, respectively.

Further ozonolytic oxidation of the ∆^10,25 bond of lupane 3-oxo derivative 4 gave 3,10-dioxo derivative 5 with a 59% yield (Scheme 2). The doublet of doublets at 2.45 and at 2.95 ppm in the ¹H NMR spectrum of compound 5 were identified as vicinal H-5 and H-9 protons, and two doublets with J = 19.1 Hz at the downfield positions at 3.71 and at 3.91 ppm — assigned to geminal H₂-2 protons. The signals of the C-3 and C-10 carbon atoms in the ¹³C NMR spectrum were detected at 213.36 and at 204.04 ppm, respectively.

Next, we had planned to obtain A-cyclic furanoterpenoid 6 as a result of the intramolecular cyclization of the 1,4-diketone fragment of compound 5, under the acidic conditions of the Paal–Knorr reaction [7] (Scheme 3).

However, when boiled in benzene with TsOH, compound 5 gave 2-cyanopyran-3-one 7 instead of the expected furan 6 (Scheme 4). The heterocyclization of compound 5 to compound 7 is likely caused by the acid-catalyzed enolization of the 10-oxo group of compound 5 and the formation of an oxonium ion. This ion serves as a hydride ion acceptor and can further interact with the C-2 electrophilic center that is formed as a result of the synergistic effect of two strongly electron-withdrawing groups in the β-oxonitrile fragment of compound 5 [9]. The ¹H NMR spectrum of compound 7 contained signals of protons H-2, H₁-11, and H-5 at 4.26, 2.79, and 2.35 ppm, respectively. The signals of carbon atoms C-3, C-9, and C-10 in the ¹³C NMR spectrum were recorded at 182.89, 116.32, and 146.70 ppm, respectively.

The structural features of the A-cycle of compound 7 were determined by analyzing the data derived from 2D NMR correlation spectra. Among the most informative signals in the HMBC spectrum of compound 7, the corresponding cross-peaks were identified for C-3/H-2, C-3/H₃-23, C-3/H₃-24, C-5/H₃-23, C-5/H₃-24, C-9/H₃-26, C-4/H-5, C-9/H-11, C-10/H-11, and C-10/H-5 (Figure 1). The NOESY experiment showed a correlation between protons H-2 and H₃-23, as well as H₃-23 and H-5. This identified compound 7 to be a 2β-cyano derivative and confirmed the regio- and stereoselectivity of 1,4-diketone heterocyclization.

Next, we investigated the regiospecificity of the base-catalyzed reaction of activated β-oxonitrile 5. Intramolecular nitrile-anionic cyclization of metalated nitrile 5 into A-pentacyclic cyano-enone 8 was observed upon boiling under basic conditions of t-BuOH/t-BuOK (Scheme 4). The signals of protons H-5 and H-9 were present in the ¹H NMR spectrum of compound 8 at 2.77 ppm and at 2.37 ppm, respectively. The ¹³C NMR spectrum contained the characteristic signals of carbon atoms: C-1 (113.53 ppm), C-2 (194.34), C-3 (206.65 ppm), and C-10 (111.63 ppm) [10]. Homologous five-membered oxonitriles were synthesized by a domino ozonolysis-aldol cyclization of ω-alkenyl-β-ketonitriles owing to their tendency to keto-enol tautomerization [11]. It should be noted that, according to NMR spectroscopy, triterpenoids 4 and 5 existed only in their keto form. Thus, in our experiment, base-induced cyclization was necessary to obtain the desired product 8.

3. Materials and Methods

3.1. General

The spectra were recorded and analytical research performed at the Center for Collective Use of the PFRC Ural Branch of the Russian Academy of Sciences “Investigation of materials and substances”. ¹H and ¹³C NMR spectra were recorded on a Bruker AVANCE II 400 (Bruker, Rheinstetten, Germany) using CDCl₃ as a solvent and hexamethyldisiloxane as an internal standard at 400 MHz and 100 MHz, respectively. Mass spectra were obtained on a gas chromatograph-mass spectrometer, model Agilent Technologies 6890N (Agilent Technologies, Wilmington, NC, USA), with column HP-5 ms (15,000 mm × 0.25 mm), an evaporator temperature of 240 °C with temperature programming within 20–40 deg/min, helium as the carrier gas, and electron ionization as a sample ionization method. Melting points were detected on an OptiMelt MPA100 (Stanford Research Systems, Sunnyvale, CA, USA) at a heating rate of 1 °C/min. FTIR spectra (ν, cm⁻¹) were recorded on a spectrophotometer, model IFS 66/S (Bruker, Ettlingen, Germany) in chloroform. Optical rotations were measured on model 341 (Perkin Elmer Corporation, Waltham, MA, USA), with wavelength 589 nm. The thin-layer chromatography analyses were performed on Sorbfil plates, using petroleum ether-ethyl acetate as a solvent system. The substances were detected by 10% solution of H₂SO₄ with subsequent heating to 100–120 °C for 2–3 min. The new synthesized derivatives were purified by column chromatography using silica gel 60–200 μm (Macherey Nagel, Germany).

3.2. Synthesis of 2-Cyano-3-oxo-1,10-seco-1-norlup-10(25)-en-28-yl benzoate 4

A Jones reagent (3 mmol) was added to a solution of compound 1 (0.85 mmol) in acetone (20 mL) at RT. The reaction mixture was stirred for 4 h. The solvent was distilled off. H₂O was added to residue, and the reaction products were extracted using CHCl₃ (3 × 20 mL). The organic layer was separated and dried over anhydrous MgSO₄. The solvent was distilled off, and the residue purified by column chromatography on SiO₂, eluent: petroleum ether-EtOAc (15:1). Compound 4 was obtained as a white powder with 67% yield. Mp 94.8 °C. ${\left[\mathrm{\alpha }\right]}_D^{22}$–23.8 (c 0.45, CHCl₃). IR (CHCl₃): 1717, 2260. ¹H-NMR (CDCl₃, δH, ppm, J, Hz): 0.81 (d, 3H, J = 6.7, CH₃), 0.86 (d, 3H, J = 6.8, CH₃), 0.88 (s, 3H, CH₃), 1.08 (s, 3H, CH₃), 1.17 (s, 3H, CH₃), 1.25 (s, 3H, CH₃), 2.09 (dd, 1H, J = 3.5, 12.1), 2.17 (dd, 1H, J = 3.5, 12.1), 3.45 (d, 1H, J = 19.0, H-2), 3.59 (d, 1H, J = 19.0, H-2), 4.09 (d, 1H, J = 11.2, H-28), 4.51 (d, 1H, J = 11.2, H-28), 4.58 (s, 1H, H-25), 4.65 (s, 1H, H-25), 7.43 (t, 2H, J = 7.7, Ph), 7.53 (t, 1H, J = 7.4, Ph), 8.03 (d, 2H, J = 7.0, Ph). ¹³C-NMR (CDCl₃, δC, ppm): 203.10, 166.91, 150.99, 132.83, 130.58, 129.54 (2C), 128.36 (2C), 114.30, 107.70, 63.18, 51.69, 50.49, 48.10, 47.03, 45.26, 44.56, 43.29, 41.94, 37.73, 34.87, 33.37, 29.87, 29.54, 28.27, 27.23, 26.95, 25.94, 25.51, 24.50, 22.88, 21.73, 17.83, 14.92, 14.88, 14.61. GC-MS: 557.4 ([M+], C₃₇H₅₁NO₃; calcd 557.39).

3.3. Synthesis of 2-Cyano-3,10-dioxo-1,10-seco-1,25-dinorlupan-28-yl benzoate 5

Ozone was passed through a solution of compound 4 (1.0 mmol) in CH₂Cl₂ (50 mL) at −50 °C for 2 h. The solvent was evaporated, and the residue purified by column chromatography on SiO₂ and eluted with petroleum ether-EtOAc-CHCl₃ (7:1:1) to afford compound 5 as a white powder with 59% yield. Mp 95.6 °C, ${\left[\mathrm{\alpha }\right]}_D^{22}$–61.71 (c 0.7, CHCl₃). IR (CHCl₃): 1715, 2262. ¹H-NMR (CDCl₃, δH, ppm, J, Hz): 0.79 (d, 3H, J = 6.6, CH₃), 0.86 (d, 3H, J = 6.6, CH₃), 0.84 (s, 3H, CH₃), 1.05 (s, 3H, CH₃), 1.07 (s, 3H, CH₃), 1.20 (s, 3H, CH₃), 2.45 (dd, 1H, J = 2.8, 12.7), 2.95 (dd, 1H, J = 6.8, 12.6), 3.71 (d, 1H, J = 19.1, H-2), 3.91 (d, 1H, J = 19.1, H-2), 4.04 (d, 1H, J = 11.2, H-28), 4.50 (d, 1H, J = 11.2, H-28), 7.42 (t, 2H, J = 7.7, Ph), 7.54 (t, 1H, J = 7.4, Ph), 8.03 (d, 2H, J = 6.9, Ph). ¹³C-NMR (CDCl₃, δC, ppm): 213.36, 204.04, 166.86, 132.86, 130.49, 129.52 (2C), 128.35 (2C), 114.34, 63.01, 58.87, 51.92, 47.90, 47.06, 46.95, 45.27, 44.42, 41.89, 37.52, 34.83, 31.52, 29.78, 29.44, 29.26, 27.70, 24.95 (2C), 22.83, 22.71, 21.70, 21.16, 19.15, 15.69, 14.87, 14.52. GC-MS: 539.3 ([M − H₂O, –2H], C₃₆H₄₉NO₄; calcd 559.37).

3.4. Synthesis of (2S)-2-Cyano-2,10-epoxy-3-oxo-1,10-seco-1,25-dinorlup-5(10)-en-28-yl benzoate 7

TsOH (0.5 mmol) was added to a solution of compound 5 (0.4 mmol) in toluene (20 mL). The reaction mixture was boiled under reflux with a Dean-Stark trap for 4 h. The solvent was evaporated in vacuo, and dry residue dissolved in EtOAc. The solution was washed with H₂O until attaining neutral reaction of the medium. The organic layer was separated, dried over anhydrous MgSO₄, and evaporated in vacuo. The residue was purified by column chromatography on SiO₂, eluent: petroleum ether-EtOAc (15:1). Compound 7 was obtained as white powder with 73% yield. Mp 101.2 °C, ${\left[\mathrm{\alpha }\right]}_D^{22}$–114.4 (c 0.13, CHCl₃). IR (CHCl₃): 1645, 1717, 2217. ¹H-NMR (CDCl₃, δH, ppm, J, Hz): 0.79 (d, 3H, J = 6.8, CH₃), 0.87 (d, 3H, J = 6.8, CH₃), 0.93 (s, 3H, CH₃), 0.97 (s, 3H, CH₃), 1.19 (s, 3H, CH₃), 1.26 (s, 3H, CH₃), 2.35 (dt, 1H, J = 8.5, 11.2, H-5), 2.79 (dd, 1H, J = 4.2, 14.9, H-11), 4.08 (d, 1H, J = 11.0, H-28), 4.58 (d, 1H, J = 11.0, H-28), 4.26 (s, 1H, H-2), 7.43 (t, 2H, J = 7.8, Ph), 7.54 (t, 1H, J = 7.4, Ph), 8.03 (d, 2H, J = 7.2, Ph).¹³C-NMR (CDCl₃, δC, ppm): 182.89, 166.90, 146.70, 132.83, 130.58, 129.54 (2C), 128.35 (2C), 116.32, 116.08, 65.49, 63.11, 48.62, 46.82, 46.09, 44.84, 44.42, 42.76, 42.06, 37.67, 34.78, 30.48, 30.08, 29.54, 28.37, 25.85, 23.84, 23.11, 22.86, 21.76, 21.04, 20.88, 18.83, 17.02, 14.86. GC-MS: 541.4 ([M − O]⁺, C₃₆H₄₇NO₄; calcd 557.35).

3.5. Synthesis of 28-Hydroxy-3-oxo-2,10-cyclo-1,10-seco-1,25-dinorlup-2(10)-en-1-yl cyanide 8

t-BuOK (0.40 mmol) was added to a solution of compound 5 (0.38 mmol) in t-BuOH (10 mL). The reaction mixture was boiled for 1 h. A 10% HCl solution was added to the reaction mixture until attaining acidic reaction of the medium. The formed precipitate was extracted with EtOAc. The extract was washed with H₂O until attaining neutral reaction of the medium. The organic layer was separated, dried over anhydrous MgSO₄, and evaporated in vacuo. The residue was purified by column chromatography on SiO₂, eluent: petroleum ether-EtOAc (5:1). Compound 8 was obtained as white powder with 60% yield. Mp 89.7 °C, ${\left[\mathrm{\alpha }\right]}_D^{22}$–187.20 (c 0.13, CHCl₃). IR (CHCl₃): 1609, 1719, 2227, 3421. ¹H-NMR (CDCl₃, δH, ppm, J, Hz): 0.78 (d, 3H, J = 6.8, CH₃), 0.86 (d, 3H, J = 6.9, CH₃), 0.99 (s, 3H, CH₃), 1.02 (s, 3H, CH₃), 1.09 (s, 3H, CH₃), 1.10 (s, 3H, CH₃), 2.37 (dd, 1H, J = 6.2, 12.5, H-9), 2.77 (dd, 1H, J = 3.4, 12.8, H-5), 3.35 (d, 1H, J = 10.8, H-28), 3.75 (d, 1H, J = 10.8, H-28). ¹³C-NMR (CDCl₃, δC, ppm): 206.65, 194.34, 113.53, 111.63, 60.31, 55.42, 47.90, 47.79, 46.61, 46.44, 46.15, 14.82, 44.44, 42.29, 37.08, 34.03, 31.64, 29.57, 29.11, 27.96, 26.67, 26.57, 25.40, 24.81, 22.85, 21.77, 19.56, 15.85, 14.88. GC-MS: 435.3 ([M − 2H]⁺, C₂₉H₄₃NO₂; calcd 437.33).

The ¹H-NMR, ¹³C-NMR, 2D correlation spectra ¹H-¹H (NOESY), ¹H-¹³C (HSQC, HMBC), IR spectra and mass spectra of the synthesized compounds are present in Supplementary Materials.

4. Conclusions

We developed the method for preparation of 1,10-seco-triterpenoid 5 with a 2-cyano-3,10-diketone fragment. Stereoselective heterocyclization of compound 5 proceeded under conditions of acidic catalysis with formation of a 2S-cyano-pyran-3-one ring fused to the C-5−C-10 bond of the triterpenoid. Regioselective intramolecular cyclization of compound 5 to form A-pentacyclic alkene β-ketonitrile proceeded under conditions of basic catalysis.