Hexahydrocyclopenta[C]Pyran-4-Carboxylate Iridoid from Viburnam Cylindricum

Introduction

Viburnum cylindricum belong to the family Capriofoliaceae evergreen shrubs with grey bark, leaves oblong lanoceolate or ovate glaucous green above occurs in moist shaded oak forest 1200-2500 mt.(1) From leaves of V. Cylindricum  Neochlorogenic acid methyl ester, cryptochlonogenic acid ester and chlorogenic acid methyl ester are isolated(2). From leaves of V. Pronifolium 2- acetyldihydropenstemide, 2′- trans-p-caumrayl dihydropenstemide, 2- acetylpatrinoside and patrinosid are isolated(3). From leaves of V. dilatatum p- hydroxyphenyl-6-0-trans-caffeoyl-b-D-glucoside, p-hydroxyphenyl-6-O-transcaffeoyl- b-D-apiosyble [1-6]- b-D-glucoside are isolated(4). From leaves of V. orientale Acyclic monoterpendiglycosides was isolated(5). The structure of compounds have been elucidated through. mass, ¹H, ¹³C NMR and 2 D-NMR spectra and their biological activities.

Experimental

General

¹H-NMR at (400 MHz),¹³C-NMR at (75 MHz) TMS as internal standard, using DMSO as solvent column chromatography was carried out on silica-gel 60-120 mesh (Merck). TLC was performed on percolated silica-gel. The eluting solvent was CHCl­₃-MeOH spots were visualized by 7% H₂SO₄ followed by heating.

Plant material        

The whole plant of Viburnam cylindricum were collected from Bacchear District. Chamoli Uttrakhand in the month of October and identified by Department Botany, P.G. College Gopeshwar where vaucher specimen was deposited.

Extraction and isolation

The air dried whole plant (3kg) was exhaustively extracted with  90% aqueous  EtOH for 72 hours. The ethanol extract was concentrated to dryness. The dry ethanolic extract was chromatographic over silica-gel using Methanol Chloroform (70:30) as elution solvent which afforded the compound.

Result

Compound was refluxed with 5% aqueous HCl (5 ml) at 80⁰C for  3 h, after cooling the reaction mixture was neutralized with AgNO_3. The aqueous layer after concentration under reduced pressure was subjected to PC using BuOH:AcOH-H₂O

(4:1:5) with authentic sugars. The Rf values of sugars were identical with authentic sugars were identical with those of D-glucose.

The elemental analysis of compound found values, C=50.23%, H=6.38% required values for C17H26O11; C=50.25%, H=6.40% corresponded to molecular weight 406. The IR spectrum of compound displayed characteristic absorption maxima at  3500 cm^-1   for a chelated OH group at 2900 cm^-1 for C-H stretching of saturated carbon atom and at 1700 and 1650 cm^-1 for α-β-unsaturated carbonyl function. Its UV-spectrum showed absorption bands characteristic to an iridoid enol showed presence of five double bond equivalence in the molecule.

¹H-NMR (400MHz, C₅D₅N): δ 6.71 (1H, d, J=1.2 Hz, H-1), 6.53 (1H, d, J=6.4 Hz, H-3), 5.15(1H, dd, J=6.4, 1.2 Hz, H-4), 4.03 (1H, d, J=4.4Hz, H-6), 2.06 (1H, dd, J=14.8, 4.4Hz, H-7α), 2.54 (1H,d, J=14.8Hz, H-7β), 3.54 (1H, brs, H-9), 1.59 (3H, s, H-10), 1.85(3H, s, OAc), 5.30 (1H,d,J=8Hz, H-1’), 4.00 (1H, dd, J=8.0, 8.8 Hz, H-2’), 3.99 (1H, t, J=8.8Hz, H-3’), 4.23 (1H, m, H-4’), 4.26 (1H, dd, J=11.6, 2.4Hz , H-5’), 4.50 (1H, t, J=11.6, 2.4Hz, H-5’), 4.50 (1H, dd, J=11.6, 2.4 Hz, H-6’a), 4.32 (1H, dd, J=11.6, 5.2Hz, H-6’b). The 1H-NMR spectrum of compound coupled with detailed analysis of 1H-1H COSY indicated presence of two integrated protons signals each for 1H at  δ 6.53 (d, J=6.4 Hz, H-3), and 5.15 (dd, J=6.4, 1.2Hz, H-4), two oxygen bearing methine signal at  δ 6.71 (d, J=1.2 Hz, H-1), methylene protons at δ2.06 (1H,dd, J=14.8, 4.4 Hz, H-7α), and 2.54 (1H, d, J=14.8 Hz, H-7β), one oxygen bearing methine signal at δ 4.03 (1H, d, J=4.4 Hz, H-6) and methine proton signal at 3.54 (1H, brs, H-9). In addition to this a three protons singlet at δ 1.85 assignable for acetoxy group, methoxy protons singlet at δ 1.59. and seven protons due to the sugar moiety were observed. A methylene proton at δ 2.06 (dd) showed coupling with another methylene proton at δ 2.54 and with an oxygen bearing methine proton at δ 4.03. A methine proton signal (brs) showed weak coupling with the proton signal appeared at δ 6.71 which might due to a methine proton bearing two oxygen atoms and is compatible with the H-1 proton signal of most of the iridoids having O-glycosylation at C-1 carbon(6-8).

¹³C-NMR (100 MHz, C5D5N): δ 94.7 (C-1), 142.2 (C-3), 108.1 (C-4), 73.2(C-5), 78.7 (C-6), 45.8 (C-7), 87.2 (C-8), 55.1 (C-9), 22.6 (C-10), 99.1 (C-1’), 74.8 (C-2’), 78.4 (C-3’), 71.6 (C-4’), 76.8 (C-5’), 62.8(C-6’),22.1 (CH₃COO). 170.9 (CH₃COO). The ¹³C- NMR spectrum and distortionless enhancement by polarization transfer (DEPT) spectrum showed 17 carbon signals; three quaternary carbon, 10 methine carbon, two methylene carbon and two methyl carbon.

Acid hydrolysis of compound with 5% HCl gave as a sugar which was identified as D-glucose by paper chromatography. The glycoside nature of the compound was supported by a doublet at δ 5.30 (J=8.0 Hz) assignable to the anomeric proton of β-D-glucose. The usual location of sugar moiety at position O-1 of the agycone was shown by the downfield shifted signal of H-1 (δ 6.71, d, J=1.2 Hz) (6). The ¹³C-NMR chemical shift of anomeric carbon atom (C-1’) at δ 99.1 and the chemical shift of the carbon atom of sugar moiety [δ 74.8 (C-2’), 78.4(C-3’), 71.6(C-4’), 76.8 (C-5’), 62.8 (C-6’)] are in agreement with the NMR spectrum and thus confirmed the presence of glucose in the molecule. The ¹³C-NMR spectrum confirmed the presence of methyl function (δ 22.6), a methylene carbon [δ 45.8 (C-7)], two methine carbons [(δ 78.7 (C-6) having oxygen function and 55.1 (C-9)], a secondary carbonyl carbon [δ 94.7 (C-1)], two quaternary carbons having an oxygen function [δ 73.2 (C-5) and 87.2 (C-8)], a di-substituted double bond [δ 142.2 (C-3), 108.1 (C-4)] and an acetate function [(δ 22.1 (CH₃COO), 170.9 (-COO-)]. These spectral data was strongly reminiscent of that reported for 8-acetylharpagide(6).  On the basis of above discussed spectral data compound was identified as 8-acetylharpagide that was further confirmed comparison of spectral data with that of reported data(6).

References

1. Gaur; R.D. Flora of District Garhwal” Trans Media, Srinagar Garhwal 1999.
2. Zurich-AG, Helvetica Chemica. Acta 2005, 88 : 2, 339-342.
3. Tomassiru-L, Cometa-MF, Foddai-S, Nicoletti-M, Planta Medica 1999, 65 : 2, 195.
4. Machida-K, Nakona-Y, Kikuchi-M, Phytochmistry, 1991, 30 : 6, 2013-2014.
5. Calis-I, Yuruker-A, Ruegger-H, Wright-AD, Stricher-U, Helvetica chemical Acta 1993, 76 : 1, 416-424.
6. Takeda, Y., Tsuchida, S. and Fujita, T., Phytochemistry, 26(8), 2303
7. Stricher, O., Helv. Chem.Acta., 53, 2010 (1970).
8. Chaudhari, R.K., Afifi-Yazar, F.U. and Stricher, O., Tetrahederon,36,2317 (1980).

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