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Synlett 2016; 27(20): 2799-2802
DOI: 10.1055/s-0036-1588312
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© Georg Thieme Verlag Stuttgart · New York

Synthesis of Polyhydroxylated Conidine Alkaloid as a Conformationally Restricted Azasugar

Sujit Pal
a   Division of Organic Chemistry, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India   Email: sujit.pal@bilkent.edu.tr
,
Shrinivas G. Dumbre*
b   KU Leuven, Laboratory of Medicinal Chemistry, Rega Institute for Medical Research, University of Leuven, Minderbroedersstraat 10, 3000 Leuven, Belgium   Email: shrinivas.dumbre@rega.kuleuven.be
› Author Affiliations
Further Information

Publication History

Received: 20 April 2016

Accepted after revision: 25 August 2016

Publication Date:
06 September 2016 (online)

 
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§ These authors contributed equally to this study
Dedicated to Prof. Ganesh Pandey on the occasion of his 62nd birthday

Abstract

A conformationally restricted polyhydroxylated 1-azabicy­clo[4.2.0]octane core has been synthesized in search for a potent selective glycosidase inhibitor. The key feature of the synthesis involves the high stereoselective photoelectron-transfer-promoted cyclization of the strained α-trimethylsilylmethylazetidine moiety to the tethered π functionality.


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Key words

glycosidase inhibitor - conformational rigidity - amine radical cation - PET cyclization - iminosugar

A widespread interest has been observed in the synthetic as well as naturally occurring polyhydroxylated iminosugars[2] for the increasing applications as therapeutics for the treatments of AIDS,[3] diabetes,[4] cancer,[5] and viral infections. The glycosidase inhibition activity of these glycomimetics are stimulated at the physiological pH value by the protonation of the basic nitrogen site which subsequently becomes resemble to either positive charge or flattened half-chair shape of the oxocarbenium ion[6] 2 (Scheme [1]).

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Scheme 1 Glycosidase inhibition mechanism at physiological pH

Thus, if any glycomimetic possesses lesser degree of freedom, it would bind more efficiently to the active site of the specific enzyme. This understanding has motivated chemists to design selective glycomimetics.[7] [8] [9] [10] [11] As a part of our long-standing research interest to design and synthesize glycosidase inhibitors, we have reported a series of iminosugars containing six-six and six-five 1-aza bicyclic frameworks as the core skeleton.[12] During the biological studies of these designed iminosugars, it was occurred to us that due to the conformational flexibility, the respective iminosugar losses selectivity towards a particular glycosidase. This hitch urged us to design and synthesize conformationally rigid six-four-fused β-lactam-azasugar hybrid framework 5.[13] It was envisioned that the conformational rigidity of β-lactam ring which compels the polyhydroxylated ring to adopt a nearly half-chair conformation to mimic the shape and the carbonyl group of the β-lactam ring may provide an additional hydrogen-bonding site for specific enzyme–substrate interactions. Although, it came up with low activity (K i = 900 μM), but it showed selective inhibition against β-galactosidase. This observation encouraged us to extend our research to improve further activity by incorporating certain structural variation to its core. To the best of our knowledge conformationally rigid six-four or six-three bicyclic ring-fused structural cores have been rarely recognized as potent glycosydase inhibitors.[14] Thus considering unexplored SAR of the six-four bicyclic class of azasugars, we herein report the design, synthesis, and glycosidase inhibition study of (3S,4R,5S,6R)-1-azabicyclo[4.2.0]octane-3,4,5-triol (3). It was assumed that under physiological pH after the protonation at nitrogen centre, the highly conformationally constrained azetidine moiety of 4 might force six-membered polyhydroxylated ring to adpot near half-chair or boat conformation which is prerequisite for the mimicking of 2 by both charge as well as shape during the glycosidase inhibition process (Scheme [1]).

Towards this endeavour, it was initially envisioned that by following intramolecular N-alkylation over 14 [15] should give us the desired 1-azabicyclo[4.2.0]octane core of 3 (Scheme [2]). Thus, to test the feasibility of our proposed strategy, we aimed to functionalize the tethered free hydroxyl group of 14 into a good leaving group, followed by in situ intramolecular N-alkylation under different conditions. However, to our dismay, the expected six-four fused framework of 3 was not isolated, even after by following several protocols.[16] This unanticipated failure in cyclization may be due to the high constrain experienced during the formation of 1-azabicyclo[4.2.0]octane framework.

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Scheme 2 Possible retrosynthetic analysis for the synthesis of 1-azabicyclo[4.2.0]octane structural core

Therefore, to overcome this limitation, we modified our intramolecular cyclization sequence so as to build azetidine ring by intramolecular N-alkylation, prior to the polyhydroxylated piperidine ring construction as shown in the revised retrosynthetic approach (Scheme [2]). The key step involves the addition of azetidine α-amino radical (19)[17c] [d] [e] [f] from the top face of the tethered π functionality in exo-6-dig fashion, as illustrated in Scheme [3].

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Scheme 3 Photocatalytic cyclization of α-TMS-group-stabilized amine radical cation moiety to tethered π functionality

To this premise, the synthesis of 3 was carried out through the steps as outlined in Scheme [4]. The synthesis began with synthesizing the PET precursor 10 by intramolecular N-alkylation of 8 by using MsCl and triethylamine in toluene. The preparation of 8 was accessed from 6 and 7, described earlier by our group.[15] The photoinduced electron transfer (PET) cyclization was carried out by irradiating (>280 nm) a dilute solution of 10 (1.5 mmol) and 1,4-dicyanonaphthalene (DCN, 0.4 mmol) in 2-PrOH using 450 W Hanovia medium-pressure mercury vapor lamp without removing the dissolved oxygen from reaction medium.[17] [18] Usual workup and purification of the photolysis reaction mixture produced 12 as a single diastereomer in 35% yield. The cyclized product 12 was fully characterized by extensive 1H NMR, 13C NMR, and 1H-1H 2D spectral analyses. The NOESY cross correlation between H5–H7 suggested that they are cis to each other (Figure [1]).

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Scheme 4 Synthesis of 3·HCl
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Figure 1 Graphical summary of NOESY spectrum observed for 12, 15, and 3·HCl

After constructing 1-azabicyclo[4.2.0]octane framework successfully, we turned our attention towards functionalizing the exo-olefinic bond. Osmium tetroxide dihydroxylation of 12 afforded 15 in 52% yield. The stereochemical outcome of 15 was adjudged by 1H NMR, 13C NMR, COSY, and NOESY analysis. The diol 15 upon sodium periodate oxidation afforded the corresponding ketone 16, which was immediately reduced by sodium borohydride to obtain the expected 17 as a single diastereomer. Based on our previous experience[15] the hydride attack to the carbonyl of 16 was assumed to be occurred from the bottom face. Finally the removal of acetonide protecting group of 17 by treating with 1 N HCl afforded 3 as a hydrochloride salt. The targeted azasugar 3 was extensively characterized by 1D as well as 2D 1H NMR spectral studies to confirm the stereochemical outcome (Figure [1]). Due to instability of 3 as free base, it was utilized in its hydrochloride salt form for further biological studies.

Table 1 Inhibition (K i in μM) of Various Glycosidases by 3·HCla

Glycosidase

β-Gal

α-Gal

β-Glu

α-Glu

β-Man

α-Man

Inhibition ability of 3·HCl (K i = μM)

114

n.i.

7.6

n.i.

n.i.

n.i.

a The enzyme inhibition study was carried out under similar conditions as reported in ref. 15 and the same is detailed in the Supporting Information; n.i.: no inhibition at 1 mM.

The inhibitory activities of 3·HCl were screened against β-galactosidase (Aspergillus oryzae), α-galactosidase (coffee beans), β-glucosidase/β-mannosidase (almonds), α-glucosidase (yeast), and α-mannosidase (jack beans). The results are summarized in the Table [1]. The targeted azasugar 3 executes strong inhibition against only β-glucosidase along with weak β-galactosidase inhibition. These results clearly reflect that the presence of conformational inflexibility in the structural framework of current azasugar enhance the activity and selectivity for a particular glycosidase.

In conclusion, we have successfully synthesized 3·HCl from template 12 via PET cyclization. The unique feature of the synthesis is avoiding any predetermined carbohydrate-based starting material to control the stereochemistry. Notably, the structurally rigid small-ring-fused 1-aza bicyclic framework was achieved via PET cyclization. A versatile approach for preparing libraries of azasugars could be turned up by following simple functionalization of exocyclic double bond of 12. Fairly good and specific glycosidase inhibition exhibited by 3·HCl is expected to encourage further research towards improving its potency by incorporating minor structural variation to its core or by increasing the polarity of the molecule.


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No conflict of interest has been declared by the author(s).

Acknowledgment

We thank DST, New Delhi for financial support. The entire work has been carried out under unconditional support and guidance of Prof. Ganesh Pandey in CSIR-National Chemical Laboratory, Pune. Special thanks to Prof M. I. Khan and K. S. S for biological studies at Biochemistry Department, CSIR-NCL, Pune. S. G. D and S. P. thanks to CSIR, New Delhi for their research fellowship.

Supporting Information

    Supporting information for this article is available online at http://dx.doi.org.accesdistant.sorbonne-universite.fr/10.1055/s-0036-1588312.
  • Supporting Information

Primary Data

    Primary Data for this article are available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/s-0040-1707215. Please note that the DOI for the Primary Data associated with this article was updated on April 21, 2021 and is now 10.4125/pd0000th.
  • Primary Data

  • References and Notes

  • 1 New address: S. Pal, Bilkent University, Department of Chemistry, 06800, Ankara, Turkey.
  • 5 Laver WG, Bischofberger N, Webster N. Sci. Am. 1999; 280: 78
    CrossrefPubMedSearch in Google Scholar
    • 9a Håkansson AE, van Ameijde J, Horne G, Nash RJ, Wormald MR, Kato A, Besra GS, Gurcha S, Fleet GW. J. Tetrahedron Lett. 2008; 49: 179
      Search in Google Scholar
    • 9b Torres-Sánchez MI, Borrachero P, Cabrera-Escribano F, Gómez-Guillén M, Angulo-Álvarez M, Álvarez E, Favre S, Vogel P. Tetrahedron Asymmetry 2007; 18: 1809
      Search in Google Scholar
    • 9c García-Moreno MI, Díaz-Pérez P, Mellet CO, García Fernández JM. Chem. Commun. 2002; 848
      PubMed
    • 10a Carmona AT, Whigtman RH, Robina I, Vogel P. Helv. Chim. Acta 2003; 86: 3066
      Search in Google Scholar
    • 10b Zhang W, Sato K, Kato A, Jia Y.-M, Hu X.-G, Wilson FX, van Well R, Horne G, Fleet GW. J, Nash RJ, Yu C.-Y. Org. Lett. 2011; 13: 4414
      PubMedSearch in Google Scholar
    • 10c D’Adamio G, Goti A, Parmeggiani C, Moreno-Clavijo E, Robina I, Cardona F. Eur. J. Org. Chem. 2011; 7155
  • 11 Isabel Torres-Sánchez M, Borrachero P, Cabrera-Escribano F, Gómez-Guillén M, Angulo-Álvarez M, Diánez MJ, Estrada MD, López-Castro A, Pérez-Garrido S. Tetrahedron: Asymmetry 2005; 16: 3897
    Search in Google Scholar
  • 13 Pandey G, Dumbre SG, Khan MI, Shabab M. Tetrahedron Lett. 2006; 79: 4523
    Search in Google Scholar
  • 15 Pandey G, Dumbre SG, Khan MI, Shabab M. J. Org. Chem. 2006; 71: 8481
    PubMedSearch in Google Scholar

      • We also followed few different protocols like:
    • 16a MsCl, Et3N, CH2Cl2;
    • 16b Ph3P, I2, CH2Cl2;
    • 16c DEAD, Ph3P, Et3N, CH2Cl2.
  • 18 Typical Procedure for PET Cyclization of 10 A solution containing 10 (0.5 g, 1.87 mmol) and 1,4-dicyanonaphthalene (0.09 g, 0.56 mmol) in 2-PrOH (200 mL) was irradiated in an open specially designed irradiation vessel using a 450 W Havonia medium-pressure mercury lamp. The lamp was immersed in a Pyrex water-jacketed immersion well which allowed only wavelength greater the 300 nm to pass through. After 3 h of irradiation, the consumption of the starting material was found to be almost complete (monitored by GC and TLC) and at this stage the irradiation was discontinued. The solvent was removed under reduced pressure, and the residue was column chromatographed (1% MeOH–EtOAc, silica, Rf = 0.21) to afford pure 12 (0.13 g, 35%) as a colourless liquid. [α]D 27 + 91.1 (c 0.75, CHCl3). 1H NMR (200 MHz, CDCl3): δ = 1.44 (s, 3 H), 1.45 (s, 3 H), 1.84–1.87 (m, 1 H), 2.49–2.52 (m, 1 H), 2.76 (dd, J = 13.3, 11.1 Hz, 1 H), 2.94–2.98 (m, 1 H), 3.30–3.34 (m, 1 H), 3.46–3.54 (m, 1 H), 3.67–3.74 (m, 1 H), 3.80–3.82 (m, 1 H), 4.07–4.09 (m, 1 H), 4.77 (t, J = 1.23 Hz, 1 H), 5.19 (t, J = 1.24 Hz, 1 H). 13C NMR (50 MHz, CDCl3): δ = 22.9, 26.9, 27.1, 51.8, 52.0, 63.1, 75.4, 80.2, 103.6, 110.6, 146.9. MS: m/z (%) [M + H] calcd for C11H18NO2: 196.1338; found: 196.1331. GC-MS: 195, 180, 152, 138, 108, 95, 81, 67, 42. GC: t R = 7.44 min; conditions: column CP – Sil 5 CB, 100 °C (1 min) 10 °C/min – 150 °C (0 min) °C Deg/min – 250 °C (9 min) on Varian CP-3800.

  • References and Notes

  • 1 New address: S. Pal, Bilkent University, Department of Chemistry, 06800, Ankara, Turkey.
  • 5 Laver WG, Bischofberger N, Webster N. Sci. Am. 1999; 280: 78
    CrossrefPubMedSearch in Google Scholar
    • 9a Håkansson AE, van Ameijde J, Horne G, Nash RJ, Wormald MR, Kato A, Besra GS, Gurcha S, Fleet GW. J. Tetrahedron Lett. 2008; 49: 179
      Search in Google Scholar
    • 9b Torres-Sánchez MI, Borrachero P, Cabrera-Escribano F, Gómez-Guillén M, Angulo-Álvarez M, Álvarez E, Favre S, Vogel P. Tetrahedron Asymmetry 2007; 18: 1809
      Search in Google Scholar
    • 9c García-Moreno MI, Díaz-Pérez P, Mellet CO, García Fernández JM. Chem. Commun. 2002; 848
      PubMed
    • 10a Carmona AT, Whigtman RH, Robina I, Vogel P. Helv. Chim. Acta 2003; 86: 3066
      Search in Google Scholar
    • 10b Zhang W, Sato K, Kato A, Jia Y.-M, Hu X.-G, Wilson FX, van Well R, Horne G, Fleet GW. J, Nash RJ, Yu C.-Y. Org. Lett. 2011; 13: 4414
      PubMedSearch in Google Scholar
    • 10c D’Adamio G, Goti A, Parmeggiani C, Moreno-Clavijo E, Robina I, Cardona F. Eur. J. Org. Chem. 2011; 7155
  • 11 Isabel Torres-Sánchez M, Borrachero P, Cabrera-Escribano F, Gómez-Guillén M, Angulo-Álvarez M, Diánez MJ, Estrada MD, López-Castro A, Pérez-Garrido S. Tetrahedron: Asymmetry 2005; 16: 3897
    Search in Google Scholar
  • 13 Pandey G, Dumbre SG, Khan MI, Shabab M. Tetrahedron Lett. 2006; 79: 4523
    Search in Google Scholar
  • 15 Pandey G, Dumbre SG, Khan MI, Shabab M. J. Org. Chem. 2006; 71: 8481
    PubMedSearch in Google Scholar

      • We also followed few different protocols like:
    • 16a MsCl, Et3N, CH2Cl2;
    • 16b Ph3P, I2, CH2Cl2;
    • 16c DEAD, Ph3P, Et3N, CH2Cl2.
  • 18 Typical Procedure for PET Cyclization of 10 A solution containing 10 (0.5 g, 1.87 mmol) and 1,4-dicyanonaphthalene (0.09 g, 0.56 mmol) in 2-PrOH (200 mL) was irradiated in an open specially designed irradiation vessel using a 450 W Havonia medium-pressure mercury lamp. The lamp was immersed in a Pyrex water-jacketed immersion well which allowed only wavelength greater the 300 nm to pass through. After 3 h of irradiation, the consumption of the starting material was found to be almost complete (monitored by GC and TLC) and at this stage the irradiation was discontinued. The solvent was removed under reduced pressure, and the residue was column chromatographed (1% MeOH–EtOAc, silica, Rf = 0.21) to afford pure 12 (0.13 g, 35%) as a colourless liquid. [α]D 27 + 91.1 (c 0.75, CHCl3). 1H NMR (200 MHz, CDCl3): δ = 1.44 (s, 3 H), 1.45 (s, 3 H), 1.84–1.87 (m, 1 H), 2.49–2.52 (m, 1 H), 2.76 (dd, J = 13.3, 11.1 Hz, 1 H), 2.94–2.98 (m, 1 H), 3.30–3.34 (m, 1 H), 3.46–3.54 (m, 1 H), 3.67–3.74 (m, 1 H), 3.80–3.82 (m, 1 H), 4.07–4.09 (m, 1 H), 4.77 (t, J = 1.23 Hz, 1 H), 5.19 (t, J = 1.24 Hz, 1 H). 13C NMR (50 MHz, CDCl3): δ = 22.9, 26.9, 27.1, 51.8, 52.0, 63.1, 75.4, 80.2, 103.6, 110.6, 146.9. MS: m/z (%) [M + H] calcd for C11H18NO2: 196.1338; found: 196.1331. GC-MS: 195, 180, 152, 138, 108, 95, 81, 67, 42. GC: t R = 7.44 min; conditions: column CP – Sil 5 CB, 100 °C (1 min) 10 °C/min – 150 °C (0 min) °C Deg/min – 250 °C (9 min) on Varian CP-3800.

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Zoom Image
Scheme 1 Glycosidase inhibition mechanism at physiological pH
Zoom Image
Scheme 2 Possible retrosynthetic analysis for the synthesis of 1-azabicyclo[4.2.0]octane structural core
Zoom Image
Scheme 3 Photocatalytic cyclization of α-TMS-group-stabilized amine radical cation moiety to tethered π functionality
Zoom Image
Scheme 4 Synthesis of 3·HCl
Zoom Image
Figure 1 Graphical summary of NOESY spectrum observed for 12, 15, and 3·HCl