N -Propargylation of Indolo-Triterpenoids and Their Application in Mannich Reaction

: The introduction of the alkynyl moiety to the triterpenic core through a linkage to the indole nitrogen is described. The reaction of N -propargylindoles with N -methylpiperazine using Mannich reaction led to propargylaminoalkynyl-triterpenoids, whose structures were established by NMR spectroscopy. Propargyl bromide, DMF, 0-5


Introduction
During the last few years, the synthetic transformation of natural compounds to prepare biologically active analogues has become a topical course of bioorganic chemistry. Triterpenoids are a large group of natural compounds that possess a broad-spectrum pharmacological activity and represent a biologically active scaffold for chemical transformations because of several key positions available on the molecule [1,2]. One of the trending topics in the chemistry of triterpenoids is the synthesis of various heterocyclic derivatives by the condensation with A-ring, modification of carboxylic group and C3 position [3].
The indole cycle can be converted into other ring systems leading to further privileged structures. According to our recent studies, the oxidized 28-oxo-indolo-allobetulone derivatives possess antiviral activity [12]. Oxidation of an aromatic moiety by H 2 O 2 provides the ursane indoloquinone formation [13].
At the same time, there are no reports about the synthesis of N-substituted indole-fused triterpenoids, which in turn opens up the possibility for obtained of new conjugates. For example, in the last decade one of the priority topics in the chemistry of triterpenoids is the synthesis of various alkynyl derivatives with subsequent modification by click-chemistry to produce biologically active compounds bearing a 1,2,3-triazolyl fragment [14,15]. The conjugates obtained by click or Mannich reaction of C-28-propargyl amide or ester derivatives of 2,3-indolo-triterpenic acid possess the anticancer activity [16,17].
In this work, the first example of N-propargylation of indolo-triterpenoids is described and their application for Mannich reaction is presented.
Then the reaction of E-ring modified indoles 3 and 4 with propargyl bromide and NaH in DMF provided the N-substituted derivatives 5 and 6 with 68 and 70% yields. The structure of compounds 5 and 6 was ascertained by NMR spectroscopy. Thus, the disappearance of signals of amine group protons of indolo-fragment at δ 7.71 and 7.76 ppm was observed. The signals of the acetylene group at δ 72.2-72.3 ppm ( 13 C-NMR) and δ 1.88 and 2.38 ppm ( 1 H-NMR), as well as methylene (δ 5.02-5.04 ppm ( 1 H-NMR)) were characteristic (see Supplementary Materials).
The reaction of N-propargylindoles 5 and 6 with N-methylpiperazine using Mannich reaction (secondary amine, paraformaldehyde, NaOAc, CuI) gave indolo-N-methylpiperazine conjugates 7 and 8 with 72 and 77% yields. The 1 H-NMR spectra of Mannich bases 7 and 8 showed typical signals of the N-methylpiperazine fragment: the methyl group as a singlet at δ 2.20-2.40 ppm and the methylene groups as a multiplet at δ 2.38-2.78 and 2.44-2.80 ppm. The signals of the acetylene were observed at δ 70.9-81.0 ppm ( 13 C-NMR).
Then the reaction of E-ring modified indoles 3 and 4 with propargyl bromide and NaH in DMF provided the N-substituted derivatives 5 and 6 with 68 and 70% yields. The structure of compounds 5 and 6 was ascertained by NMR spectroscopy. Thus, the disappearance of signals of amine group protons of indolo-fragment at δ 7.71 and 7.76 ppm was observed. The signals of the acetylene group at δ 72.2-72.3 ppm ( 13 C-NMR) and δ 1.88 and 2.38 ppm ( 1 H-NMR), as well as methylene (δ 5.02-5.04 ppm ( 1 H-NMR)) were characteristic.

Materials and Methods
The spectra were recorded at the Center for the Collective Use 'Chemistry' of the Ufa Institute of Chemistry, part of the Ufa Federal Research Centre of the Russian Academy of Sciences. 1 H and 13 C-NMR spectra were recorded on a "Bruker AM-500" (Bruker, Billerica, MA, USA, 500 and 125.5 MHz respectively, δ, ppm, Hz) in CDCl3, internal standard tetramethylsilane. Mass spectra were obtained on a liquid chromatograph-mass spectrometer LCMS-2010 EV (Shimadzu, Kyoto, Japan). Melting points were detected on a micro table "Rapido PHMK05" (Nagema, Dresden, Germany). Optical rotations were measured on a polarimeter "Perkin-Elmer 241 MC" (PerkinElmer, Waltham, MA, USA) in a tube length of 1 dm. Elemental analysis was performed on a Euro EA-3000 CHNS analyzer (Eurovector, Milan, Italy); the main standard is acetanilide. Thin-layer chromatography

Materials and Methods
The spectra were recorded at the Center for the Collective Use 'Chemistry' of the Ufa Institute of Chemistry, part of the Ufa Federal Research Centre of the Russian Academy of Sciences. 1 H and 13 C-NMR spectra were recorded on a "Bruker AM-500" (Bruker, Billerica, MA, USA, 500 and 125.5 MHz respectively, δ, ppm, Hz) in CDCl 3 , internal standard tetramethylsilane. Mass spectra were obtained on a liquid chromatograph-mass spectrometer LCMS-2010 EV (Shimadzu, Kyoto, Japan). Melting points were detected on a micro table "Rapido PHMK05" (Nagema, Dresden, Germany). Optical rotations were measured on a polarimeter "Perkin-Elmer 241 MC" (PerkinElmer, Waltham, MA, USA) in a tube length of 1 dm. Elemental analysis was performed on a Euro EA-3000 CHNS analyzer (Eurovector, Milan, Italy); the main standard is acetanilide. Thin-layer chromatography analyses were performed on Sorbfil plates (Sorbpolimer, Krasnodar, Russian Federation), using the solvent system chloroform-ethyl acetate, 40:1. Substances were detected by a 10% solution of a sulfuric acid solution with subsequent heating at 100-120 • C for 2-3 min. Compounds 1 [9] and 4 [9], 2 [11] were obtained according to the methods described previously.

Conclusions
The first synthesis of triterpenic N-propargylindoles and their modification using a Cu(I)-catalyzed Mannich reaction were achieved.