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Summer Research Fellowship Programme of India's Science Academies

C-H activation of SP3 carbon using oxime as directing group

Athira Yoyakki

IMSC, Central University of Tamilnadu, Thiruvarur 610005

Dr. S Chandrasekhar

Director IICT, Indian Institute of Chemical Technology, Tarnaka, Hyderabad, Telangana 500007

Abstract

This work was done as a part of my summer internship during the period of 13th May 2019 to 7th July 2019 under the supervision of Dr. S. Chandrashekhar, Indian Institute of Chemical Technology, Hyderabad. Oxime directed C-H activation is an efficient strategy in various field of organic synthesis owing to a better yield. In spite of being acting as a better directing group oxime could serve as an internal oxidant in several transformations which in turn decreases the need of extra oxidant. O-Acetyl Oximes act as an efficient directing group for Pd- catalyzed sp2 and sp3 C-H functionalization reactions, whose products can be converted into ortho or β functionalized ketones, alcohols, amines and heterocycles. Recent literatures show that a sulfonamide moiety acts as major structural components in several domains of medicinal and pharmaceutical chemistry. Antibiotic and anticancer profiles of sulfonamide moieties were known earlier. These specialties make sulfonamide moieties to mark their own position in above discussed field of chemistry. Considering above biological profiles LSF strategy was used to synthesize several GA-Sulfonamide derivatives using 18β-glycyrrhetinic acid as lead molecule and their anticancer properties were figured out. Chang’s protocol was used to monitor the anticancer properties of GA-sulfonamide moieties. A ketoester was found to be formed on subjecting 18β-glycyrrhetinic acid to Jone’s oxidation followed by esterification, which in turn leads to the formation of ketoxime on treatment with methoxylamine hydrochloride followed by C-H amidation of the obtained compound using iridium as catalyst. Spectral analysis of the compound helped to have a clear cut conclusion and confirmation about the obtained structure of the compound making it more vivid. Cationic iridium was formed in presence of silver catalyst. Structural modifications on synthesized analogues paved a way to have a great understanding on the anticancer activity of several functional groups.

Keywords:

INTRODUCTION

I (Athira Yoyakki) worked under the guidance of Dr. S. Chandrashekar at Indian Institute of Chemical Technology from 13th May 2019 to 7th July 2019. On the course, I was guided to work under the “Oxime directed C-H activation of sp2 and sp3 carbons”, which is an emerging strategy in the field of organic total synthesis. Oxime based compounds can be marked independently in this field owing to their special directing properties and their efficiency to act as an internal oxidant. In concern with medicinal and pharmaceutical chemistry sulfonamide moieties play’s an adamant role. Antibiotic properties of these moieties can vividly represented by the first antibiotic sulpha drug, prontosil, which is chemically identified as sulfanilide. Anticancer profiles were shown by the sulfonamide derivatives such as sulofenur, E70101 etc. which possess unique anti tumor profiles. In discussion with the alteration of pharmacokinetic profiles, such as absorption, distribution, metabolism and excretion, it was shown that the incorporation of sulfonamide plays an important role. The property of -SO2NH group to mimic several essential components in several functional biomolecules such as -COOH, -CONH- and PO(OR)OH2 and this marks the importance of sulfonamide moieties in the emerging world. On the basis of the above discussed biological profiles 18β-glycyrrhetinic acid was used as lead molecule for synthesis of sulfonamide derivatives using late stage functionalization and further followed by C-H amidation. Anticancer profiles could be analyzed and the structural conformation was done using spectral analysis.

LITERATURE REVIEW

Glycyrrhetinic acid is a known triterpenoic acid which was identified earlier to show several pharmacological effects, and was most renowned for its anti-tumor activity, however to a weak extent. This paved a way for the synthesis of new analogues and thereby enhancing the potency and anti-tumor activity. Introduction of fused heterocycles at C-2 and C-33 positions was reported and lead to the formation of nearly 18 novel GA derivatives and was shown an enhanced anti-tumor potency on compared with that of the lead molecule taken. In this background our objective was to synthesize GA-sulfonamide derivatives which would be expected to show a greater anti-tumor potency due to the known anticancer activities of sulfonamide derivatives such as sulofenur, E7010 and ER-344101. The mechanism for C-H amidation was proposed on the basis of Chang’s protocol4. Involvement of a cationic species in the proposed mechanism is depicted by the species 1 in the presence of the silver catalyst. A five membered heterocycle 2 was formed with the loss of acetic acid on C-H bond activation of methyl group on coordination with that of oxime. Intermediate 3 was formed by further coordination of five membered heterocycle with that of organic azide which further leads to formation of 4 by transfer of amide via N2 elimination. Formation of the desired product can be explained by the immediate protonation of the intermediate 4 by proton transfer.

INSA 1.JPG
    Plausssible mechanism of Ir-catalyzed C-H amidation

    METHODOLOGY

    Late stage functionalization was used to synthesize several sulfonamide derivatives taking 18β-glycyrrhetinic acid as a lead molecule (Scheme 1) and hence followed by the analysis of their anticancer profiles.

    INSA 2.JPG
      Synthesis of sulfonamide derivative via late stage functionalization

      Chang’s protocol was used for the incorporation of several sulfonamide moieties by the aid of site selective late stage functionalization at the gem dimethyl group of 18β-glycyrrhetinic acid. Ketoester (3.1.3) was formed on subjecting 18β-Glycyrrhetinic acid (3.1.2) to Jone’s oxidation followed by esterification5. Further treatment of ketoester (3.1.3) with methoxylaminehydrochloride furnishes ketoxime (3.1.4), a prerequisite for selective C-H amidation (Scheme 2).

      INSA 3.JPG
        Scheme 2

        Tosyl azide (6a) was used as amine source for Ir-catalyzed C-H amidation of ketooxime (Scheme 3). Amidation takes place at equatorial methyl group of compound 6 in a selective manner.

        INSA 4.JPG
          Scheme 3

          Experimental Section

          Preparation of Jone’s reagent

          Jone’s reagent, diluted chromium trioxide in sulphuric acid, acts as an oxidant for organic compounds diluted in acetone 2.5M reagent can be prepared by dissolving chromiumtrioxide (25g, 25mol) in water (75 ml) which was taken in a 500 ml beaker, followed by the addition of concentrated sulphuric acid (25 ml), which was kept in an ice-bath, slow addition with constant stirring is appreciated, keeping temperature in a range of 0°C–5°C.

          Preparation of ketone

          INSA 5.JPG
            Ketone prepaeration

            Followed by the dilution of 18β-Glycyrrhetinic acid in Tetra hydro furan the mixture was kept in ice bath along with slow, drop wise addition of Jone’s reagent maintaining a temperature of 0°C for nearly for a time period of one hour.

            Preparation of ketoester

            INSA 6.JPG
              Ketoester prepaeration

              Followed by the dissolution of the ketone compound (302g, 645 mmol) in acetone, potassium carbonate (134g, 968 mmol) was added which was then followed by the slow addition of methyl iodide (60 mL, 968 mmol). And an overnight stirring was carried at room temperature. The solution obtained was then poured into water (10 mL), and chloroform (8 mL) was added, the obtained solution is then stirred and separated. Obtained organic layer was then dried over anhydrous sodium sulfate, which was then filtered and concentrated under reduced pressure to obtain the desired compound.

              Synthesis of ketoxime

              INSA 7.JPG
                Ketoxime synthesis

                Methoxylamine hydrochloride (677mg, 8.1 mmol), NaOAc (1.08g, 13.2 mmol) were placed in a RB flask and 10 ml water was added continuously stirring the mixture for 5h, keeping temperature at 80°C. Once the reaction is completed, TLC was checked to confirm the completion of the reaction. Followed by the dilution of reaction mixture with 10ml of CH2Cl2, aqueous layer was extracted with 10 ml of CH2Cl2. Obtained organic layers was combined and dried over Na2SO4, followed by filtration and concentration under vacuum, which promises an environment of reduced pressure. Purification of the residue was done by column chromatography aiding silica as the stationary phase and 10 o/o of EtOAc in hexane as the eluent, which gives a colorless solid (5).

                General procedure for the Ir-Catalyzed amidation of ketoximes with azides

                INSA 8.JPG
                  Ir-Catalyzed amidation of ketoximes with azides

                  Ketoxime (1.0 eqiv), azide (2.0 eqiv), [IrCp*Cl2]2 (5mol o/o), AgNTf2 (20 mol o/o), AgOAc (0.02 mmol, 10 mol o/o) and 1,2-DCE (0.5mL) was added into a screw capped vile with magnetic stirrer in N2 atmosphere. The reaction mixture was stirred at 60°C for 24h. Concentration of solvent was done under the reduced pressure on completion of the reaction. Obtained residue was then purified by column chromatography on silica gel (EtOAc/hexne), which further gives the desired product. Preparation of the compound 6 was done using the general procedure discussed above, which was then purified by flash chromatography (20 o/o EtOAc/hexane) which furnishes a white solid.

                  RESULTS AND DISCUSSION

                  Ketone

                  INSA A.JPG
                    Ketone derivative

                    Melting point: 308–310°C

                    1H NMR (300MHz, CDCl3) : δ 75(s,1H), 2.97(m, 1H), 2.64(m,

                    1H), 2.41(s, 1H), 2.41-0.87(m,

                    40H)

                    13C NMR(75MHz, CDCl3) : δ217.3, 199.7, 181.2, 169.8, 128.4

                    , 76.6, 61.0, 55.4, 48.2, 47.8,

                    45.3, 43.8, 43.3, 40.9, 39.7, 37.7,

                    36.7, 34.2, 32.1, 31.9, 30.9, 28.6,

                    2 8.4, 26.5, 26.3, 23.3, 21.4, 18.8,

                    18.5, 15.6

                    Ketoester

                    insa b.JPG
                      Ketoester derivative

                      Melting point: 248–250°C

                      1H NMR (300MHz, CDCl3) : δ 68(s, 1H), 3.69(s, 3H), 2.97(m,

                      1H), 2.62(m, 1H), 2.40(s, 1H),

                      2.37-0.83(m, 39H)

                      13C NMR (75MHz, CDCl3) : δ 215.6, 198.4, 176.1, 168.7,

                      128.3, 60.9, 55.3, 51.5, 48.2,

                      47.5, 45.1, 43.8, 43.2, 41.1

                      39.6, 37.7, 36.6, 33.9, 32.1,

                      31.7, 31.0, 28.5, 28.2, 26.5,

                      26.4, 26.3, 23.3, 21.3, 18.7,

                      18.5, 15.5

                      Ketoxime

                      insa c.JPG
                        Ketoxime derivative

                        Melting point: 256–258°C

                        [α]20D : +108.6(c=0.79,CHCl3).

                        1H NMR(400 MHz,CDCl3) : δ 5.67(s,1H),3.81(s,3H),3.68(s,3H),2.93

                        (ddd, J=15.6,5.0,3.8Hz,1H),2.81(ddd, J=

                        13.3, 5.7, 3.7 Hz, 1H), 2.11-1.76(m,5H)

                        1.72-1.37(m,6H), 1.34(s, 3H), 1.30(d

                        J= 9.6Hz, 2H),1.21-1.18(m, 1H), 1.23(s,

                        3H), 1.14 (s, 6H), 1.06 (s, 3H), 1.05-0.98

                        (m, 3H),0.8(s,3H).

                        13C NMR(101 MHz, CDCl3) : δ 200.0, 177.0, 169.5, 165.9, 128.6, 61.5,

                        61.2, 55.8, 51.9, 48.5, 45.5, 44.2, 43.4,

                        41.2, 40.3, 39.2, 37.9, 37.1, 32.6, 32.0,

                        31.3, 28.7, 28.5, 27.4, 26.6, 26.5, 23.6,

                        23.4, 18.8, 18.3, 17.9, 15.8

                        HRMS (ESI) : m/zcalcd for C32H50NO4[M+H]+ :512.3740;

                        Found512.3739

                        Compound 6 (Sulfonamide Derivative)

                        insa d.JPG
                          Sulfonamide derivative

                          Melting point: 210–212°C

                          [α]20 D : +69.15(= 0.98, CHCl3)

                          1H NMR (500MHz, CDCl3) : δ 7.74(d, J = 8.2 Hz, 2H), 7.32 (d, J =

                          8.1Hz, 2H), 5.67(s, 1H), 5.21 (dd, J =

                          9.8, 4.4Hz, 1H), 3.74 (s, 3H), 3.69 (s,

                          3H), 3.05-2.82(m, 4H), 2.44(s, 3H)

                          2.41(S, 1H), 2.12-1.69 (m, 8H),

                          1.55-1.38(m, 5H), 1.36 (s, 3H), 1.34-

                          1.30(m, 2H), 1.26(s, 3H), 1.15 (s, 3H),

                          1.14(s, 3H), 1.60-1.00(m, 1H), 0.96(s,

                          3H), 0.91-0.84(m, 2H), 0.80(s, 3H)

                          13C NMR (101Hz, CDCl3) : δ 200.0, 177.1, 170.1, 164.0, 143.3,

                          137.3, 129.9, 128.4, 127.0, 61.6,

                          61.4, 51.9, 49.1, 48.6, 48.2, 45.6,

                          44.2, 43.5, 41.2, 38.7, 37.9, 36.9,

                          32.1, 32.0, 31.3, 28.7, 28.5, 26.5,

                          23.6, 21.7, 20.8, 19.0, 18.1, 17.9,

                          16.1, 14.3
                          :

                          HRMS (ESI) : m/z calcd for C39H57N2O6S[M+H]+:

                          681.3937; found 681.3932

                          NMR and Mass Data's

                          NMR 1.JPG
                            Proton NMR spectra 
                            NMR 2.JPG
                              Proton NMR spectra
                              NMR 3.JPG
                                Proton NMR spectra
                                NMR 4.JPG
                                  13C NMR spectra
                                  NMR 5.JPG
                                    13C NMR spectra
                                    NMR 6.JPG
                                      13C NMR spectra
                                      NMR 7.JPG
                                        13C NMR spectra
                                        MASS 1.JPG
                                          Mass data
                                          MASS 2.JPG
                                            Mass data
                                            MASS 3.JPG
                                              Mass data
                                              HRMS 1.JPG
                                                HRMS data
                                                HRMS 2.JPG
                                                  HRMS data
                                                  HRMS 3.JPG
                                                    HRMS data

                                                    CONCLUSION AND RECOMMENDATIONS

                                                    Need of modernistic and innovative strategies for the synthesis of efficient anticancer drugs are seemed to have a greater escalation day by day. On the basis of the potent anticancer profiles which were observed in 18β-Glycyrrhetinic acid, but were not expressed in an efficient extent in its natural form, a study was done by preparation of sulfonamide moieties on the basis of their known anticancer profiles, taking GA as lead molecule. Oxime directed C-H amidation was done to reach the target. Ability of oximes to act as a sophisticated directing group and its ability to act as an internal oxidant which in turn diminishes the need of an external oxidant makes oxime unique as a directing group compared to others with the same mechanistic property. Initially 18-βGlycyrrhetinic acid was oxidized in to corresponding ketone (Jone’s oxidation). Obtained ketone was then subjected to esterification which in turn lead to the formation of keto ester (3.1.3) followed by this the synthesis of ketoxime was carried out as per the procedure discussed above (3.1.4). Further Ir-catalyzed C-H amidation was done by considering Chang’s protocol as the reference. A general procedure was followed for this (3.1.5). Obtained GA- sulfonamide derivative is expected to show advanced anticancer properties compared to its starting material. Structural conformation of each compounds was done using spectrometric and spectroscopic techniques such as 13C and 1H NMR along with mass spectrometry. Column chromatography along with thin layer chromatography was used as the main aid for the purification of the compounds.


                                                    REFERENCES

                                                    1. (a) Thornber C W.; Chem. soc. Rev. 1979, 8, 563. (b) Colinas, P. A. Expert Opin Ther. Patents 2013, 23, 761;

                                                    (c) Casini, A; Scozzafava A.; Mastrolorenzo, A.; Supuran C. T Current Cancer Drug Targets 2002, 2, 55.

                                                    2. Owa, T.; Nagasu. T. Exp. Opin. Ther. Patents 2000, 10, 1725.

                                                    3. Gao, Cheng.; Dai, F-J.; Cui, H-W.; Peng, S-H.; He, Y.; Wang X; Yi; Z-F.; Qiu W-W. Chem. Biol. Drug Des. 2014, 84, 223.

                                                    4. Kang, T.; Kim, Y.; Lee, D.; Wang, Z.;Chang, S.; J. Am. Chem. Soc. 2014, 136, 4141.

                                                    5. Gao, Cheng.; Dai, F-J.; Cui, H-W.; Peng, S-H.; He, Y.; Wang X; Yi; Z-F.; Qiu W-W. Chem. Biol. Drug Des. 2014, 84, 223.

                                                    ACKNOWLEDGEMENTS

                                                    I would have immense pleasure to offer my thanks and regards to Indian Academy of Sciences, BANGALORE for endowing me such a great opportunity to work in an illustrious institution like IICT Hyderabad, and to boost up my knowledge along with the gain of a wonderful research experience which would be a mile stone and helpful in the construction of my research career. I would also like to thank Dr. S. Chandrasekhar for his guidance and support.

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