A novel metabolite from the Chilean mollusk Siphonaria lessoni

ARTIGO

A novel metabolite from the Chilean mollusk Siphonaria lessoni

Juana Rovirosa; Aurelio San-Martín^*

Departamento de Química, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile

ABSTRACT

A new and two previously known metabolites possessing a polypropionate carbon skeleton have been isolated from the marine gastropod mollusk Siphonaria lessoni, collected at Chilean coasts. Their structures have been determined by spectroscopical methods.

Keywords: mollusk; Siphonaria; polypropionates.

INTRODUCTION

Mollusks of the marine pulmonate genus Siphonaria are air breathing intertidal herbivores, often referred to as false limpets. During the high tides, siphonariids remain firmly fixed to depressions or "home scars" in the rock surface. As the tide recedes these mollusks leave their home scars to feed on algae and microorganisms thus exposing themselves to predation by terrestrial predators, in addition to the aquatic predators e.g. tidepool fish encountered when they are submerged. Shortly after being disturbed by a potential predator, siphonariids produce a white mucus, containing polypropionate metabolites, from lateral pedal glands¹. A feature of Siphonaria polypropionate compounds is their frequent cyclization to yield furanone, pyrone and hemi-acetal functionalities². Surprisingly, the role of these compounds in the chemical ecology of Siphonaria species is poorly understood² .

Siphonariid limpets have been shown to contain branched chain plypropionate metabolites of considerable biosynthetic interest³. For example, the air – breathing gastropod S. diemenensis contains the diemenensins A and B, compounds that display antimicrobial properties⁴. On the other hand, the pulmonate S. denticulata Quay and Gaimard biosynthesizes the denticulatins A and B, polypropionate compounds that have attracted considerable attention from both synthetic and bioorganic chemists^5-7. Chilean specimens of S. lessoni yielded norpectinatone 1, norsiphonarianone 2, norsiphonarienolone⁸3 and a mixture of furanone 4 and other isomer⁹ . The nature of this isomer is no clear because originally was suggested that is the Z- isomer but further it was assigned to the epimer of carbon 2. In addition, norsiphonarianone and norsiphonarienolone were found as a mixture. The isolation of mixture of polypropionates in these mollusks is not an unusual situation¹⁰.

Over the last thirty years, many structurally novel polypropionate metabolites have been isolated from marine organisms as phylogenetically diverse as marine bacteria and sponges. The most important source of marine polypropionate compounds are the Mollusca.

In continuation of our search for new bioactive compounds from Chilean marine mollusks, we have reexamined specimens of S. lessoni from the central coast of Chile.

RESULTS AND DISCUSSION

The acetone extract of S. lessoni was subjected to open column chromatography on silica gel, using increasing proportions of ethyl acetate in petrol ether to afford a new polypropionate 5 and the mixtures of the known compounds 3 and 4. The IR spectrum of compound 5 presented bands at 3460 and 1700 cm^-1 indicating the presence of an hydroxyl and carbonyl groups. The ¹³C – NMR spectral data (see table 1) of 5 indicated 19 carbons and, together with an accurate mass spectra measurement (LREIMS: 296.2571), indicated the molecular formula C₁₉H₃₆O₂ with two sites of unsaturation for 5. The ¹³C NMR decoupled spectrum of 4 showed well resolved resonances for all 19 carbons. ¹³C DEPT analysis using a rotation angle of 90º indicated one sp² methine carbon at d 137.0 and five saturated methines at d 81.2; 48.8; 30.1; 29.7; and d 28.6. The DEPT 135º spectrum showed four methylene (d 45.8; 45.4; 36.8 and 29.7) and seven methyl carbons (d 22.1; 20.5; 20.0; 14.6; 11.6; 11.1; 7.9) indicating, after comparison with the decoupled spectrum, that the carbons at d 216.3 and 133.2 were non-hydrogenated.

The ¹H NMR spectrum (see table 1) of 5 showed signals corresponding to an olefinic proton at d 5.06 (1H, d, J = 9.6 Hz), and a singlet at d 1.56 (3H, s) complete the double bond system. A doublet at d 4.03 (d, 1H, J= 9.3 Hz) was attributed to geminal proton on carbon bearing a hydroxyl group. The ¹³C NMR spectra data also confirmed the existence of one trisubstituted double bond in a linear system: d 137.0 (CH), 133.2 (C) and 11.1 (CH₃) and the existence of a secondary alcohol (C-6) at d 81.2 (CH). The ¹H COSY spectrum showed a correlation between the doublet at d 4.03 and the signal at d_H 2.70 (dq, 1H, J = 9.3; 6.4 Hz; d _C 48.8) and this one was connected to a methyl group d_H 0.86 (q, 3H, J = 6.4 Hz; d_C 14.6). An ethyl group vicinal to a carbonyl group was deduced by the following signals: a methylene at d_H 2.48 (q, 2H, J = 7.2Hz; d_C 36.8) coupled to the methyl group at d_H 0.99 (q, 3H, J = 7.2 Hz; d_C 7.9 ) was observed in the ¹H COSY spectrum. Four additional methyl groups were observed at d_H 0.84 (d, 3H, J = 7.4 Hz; d _C 20.5); d_H 0.73 (d, 3H, J = 6.5 Hz; d_C 20.0); d_H 0.74 (d, 3H, J = 6.5 Hz; d_C 22.1) and d_H 0.76 (t, 3H, J = 5.0 Hz; d_C 14.6). Finally, the ketone group was defined by ¹³C-NMR signal at d 216.3 (C-3). Other overlapped signals were not assigned. The data shown above together with analysis of ¹H COSY, HMQC and HMBC experiments are in agreement with a polypropionate alcohol with 19 carbon atoms instead of 20 which are presents in siphonarienolone¹¹. The NMR data of compound 5 are in agreement with those of siphonarienolone (see Table 1) however, there is a difference between the values the optical rotations even its has the same sign (compound 5 [a]₂₄^D = 52.6º and siphonarienolone [a]₂₄^D = 19.6º) The stereochemistry shown in 5 was proposed by analogy with other polypropionates isolated previously from the same mollusk^8,9 and by the ROESY NMR spectroscopy cross-peak that was found between H-4 and Me-5. No other valuable information from this method was obtained. The same stereochemistry relationship were found in two analogous compounds isolated previously^11,12 Once again, a norpolypropionate was isolated from S. lessoni but in this opportunity 5 was a pure compound instead of a mixture.

EXPERIMENTAL PART

The ¹H (300 MHz) and ¹³C (75.5 MHz) spectra were recorded on a Bruker AMX-300 spectrometer, chemical shifts are reported relative to Me₄Si and coupling constants are given in Hertz. Mass spectra were recorded on a V. G. Micromass, ZAB-2R. Infrared spectra were measured on a Bruker IFS-25 spectrometer. Optical rotation was determined for solution in CHCl₃ with a Perkin-Elmer Mod. 241 polarimeter. Silica gel chromatography was performed on silica gel Merck Nº 7734 and 7741.The tlc plates were developed by spraying with H₂SO₄-H₂O-AcOH (1:4:20) and heating. Sephadex LH-20 obtained from Pharmacia was used for gel filtration chromatography.

Collection and extraction of Siphonaria lessoni

650 specimens of S. lessoni were collected from the intertidal zone at Las Cruces, V Región and stored in acetone until work up. The extract was decanted and the specimens were re-extracted with acetone. The acetone extracts were pooled, concentrated and partitioned between EtOAc and water. The polypropionate compounds obtained from the concentrated EtOAc partition layer by silica chromatography EtOAc /n-hexane (1:2), normal HPLC EtOAc/n-hexane (1:4) phase were compound 3 and 4. Polar fractions of the chromatography were rechromatographed on SiO₂, obtaining a oil. The oil was filtered through a silica gel using EtOAc/n-hexane (3:1) as eluant to obtain the compound 5.

Compound 5.- oil [a]₂₄^D = 52.6º (c 0,076g/100 mL CHCl₃) IR KBr cm^-1: 3460, 2980, 2900, 1700, 1450, 1370, 1000, 970, 860. MS: m/z (%) 296 (C₁₉H₃₆O₂), 279 (2), 221 (1), 211 (3), 155 (12), 141 (1), 137 (4), 115 (4), 97 (12), 57 (100). NMR spectral data: ¹H NMR (CDCl₃,) and ¹³C-NMR: see Table 1.

ACKNOWLEDGEMENTS

The research was supported by grants from DID (Universidad de Chile) and Fondecyt Nº 1040651.

Recebido em 12/11/04; aceito em 29/4/05; publicado na web em 3/10/05

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Publication Dates

• Publication in this collection
  03 Mar 2006
• Date of issue
  Feb 2006

History

• Received
  12 Nov 2004
• Accepted
  29 Apr 2005