Elsevier

Organic Geochemistry

Volume 120, June 2018, Pages 12-18
Organic Geochemistry

Amberene and 1-methylamberene, isolated and identified from Kuji amber (Japan)

https://doi.org/10.1016/j.orggeochem.2018.02.014Get rights and content

Abstract

Abundant labdatriene derivatives, named amberene [15,19,20-trinor-5,7,9-labdatriene (or 1,6-dimethyl-5-isopentyltetralin) (1)] and 1-methylamberene [15,20-dinor-5,7,9-labdatriene (or 1,1,6-trimethyl-5-isopentyltetralin) (2)] were isolated from the methanol extract of Kuji amber by HPLC. The structures were elucidated by spectral analyses including EIMS, 1D and 2D NMR and some previous assignments corrected.

Introduction

Ambers are fossilized resins from ancient plants and are important organic geochemical archives (Anderson, 2006, Lambert et al., 2008). The molecular composition of amber is commonly determined by Fourier transform infrared spectroscopy (FTIR), solid state 13C nuclear magnetic resonance (13C NMR) spectroscopy and (pyrolysis) gas chromatography-mass spectrometry [(Py-)GC–MS] (Drzewicz et al., 2016). Both FTIR and solid state 13C NMR analyses have some limitations, however, in that different ambers sometimes show rather similar molecular distributions. It has therefore also become increasingly common to elucidate the molecular composition of ambers by comparison of data from GC–MS and/or GC analyses with those of standards and with library data (Mills et al., 1984, Anderson, 2006, Lambert et al., 2008, Bray and Anderson, 2009, Shi et al., 2014, Bechtel et al., 2016). GC–MS is quite effective for the identification of known compounds, but mass spectra are not always conclusive for structural elucidations of new compounds, particularly sometimes, of stereochemical aspects (Pereira et al., 2009, Menor-Salván et al., 2010, Nohra et al., 2015). In contrast, only a few attempts have been made of the characterization of compounds isolated from ambers using solution NMR spectroscopy (Menor-Salván et al., 2016) and very few using two-dimensional (2D) NMR (Jossang et al., 2008, Kimura et al., 2012, Abe et al., 2016, Wang et al., 2017).

Recently, for example, 2,5,8-trimethyl-1-butyltetralin, a hypothetical diagenetic product from labdenoic acids, was identified by GC–MS in an extract of amber found in the Cretaceous Basque-Cantabrian Basin (Spain). The structure was subsequently revised to that 1,6-dimethyl-5-isopentyltetralin (so-called ‘amberene’) after analysis by one-dimensional (1D) NMR (Menor-Salván et al., 2016).

It is important for geochemical studies of amber that, where possible, pure compounds are isolated and structures fully elucidated, so that any geochemical relationships between the constituents of the original amber and the older geochemical products can be established. For example, it has been reported that quesnoin might be formed as a byproduct of the polymerization process from Oise amber (55 Ma) (Jossang et al., 2008). Similarly, 15-nor-cleroda-3,12-diene was isolated from Dominican amber and a pathway proposed for its formation (Wang et al., 2017).

Kuji amber (Japan) has never been the subject of a detailed geochemical study (Yoshihara et al., 2009), but studies of the alcohol soluble fraction for biologically active compounds have been made (Shitamukai et al., 2000, Chanklan et al., 2008, Ogasawara et al., 2008). New compounds named kujigamberol (15,20-dinor-5,7,9-labdatrien-18-ol), kujiol A (13-methyl-8,11,13-podocarpatrien-19-ol) and kujigamberol B (15,20-dinor-5,7,9-labdatrien-13-ol) were studied using activity guided fractionation (Kimura et al., 2012, Ye et al., 2012, Uchida et al., in press).

To examine Kuji amber from the point of view of organic geochemistry, we focused herein on other abundant, but biologically-inactive compounds, in the methanol (MeOH) extract.

Using high performance liquid chromatography (HPLC), we isolated two abundant compounds and identified their structures as ‘amberene’ and ‘1-methylamberene’ by electron ionization MS and by 1D and 2D NMR spectroscopy. Some previous assignments were revised.

Section snippets

General Experimental Procedures

Kuji amber was excavated from the mines of Kuji Kohaku Co. Ltd. located in the upper part of the Tamagawa Formation of the Kuji Group of Kuji city, Iwate Prefecture, northeastern Japan. Baltic, Dominican and Burmese ambers were imported by Kuji Kohaku Co. Ltd. Unless otherwise stated, chemicals used were of the best commercially available grade. Structure elucidations of amberene (1) and 1-methylamberene (2) were performed using high resolution EIMS (HREIMS; JMS700, JEOL Ltd., Tokyo, Japan),

Structural elucidation of amberene (1) and 1-methylamberene (2)

The numbering systems for 1 and 2 used here follow that published for kujigamberol (Kimura et al., 2012). These are different from those used elsewhere in the literature (Menor-Salván et al., 2016). Amberene and 1-methylamberene were each isolated as pure colorless oils by HPLC (Fig. S1) and purities were determined by HPLC and GC (Fig. S2). The elemental formulae of 1 and 2 were determined to be C17H26 and C18H28, respectively, by HREIMS, with m/z 230.2035 and 244.2188 assigned as the

Conclusions

The biomarkers amberene (1) and 1-methylamberene (2), were isolated from Kuji amber and their structures elucidated using EIMS, UV, 1D and 2D NMR spectra. The mass spectrum of 2 was different from that published previously, but that of 1 was as published. Both compounds were detected in extracts of Kuji and Burmese ambers, but not in Baltic and Dominican ambers by HPLC. This study shows that the isolation of individual compounds and structural analysis using NMR are important, in addition to

Notes

The authors declare no competing financial interest.

Acknowledgments

We are grateful to Mrs. Shizuko Nakajyo of the center for Regional Collaboration in Research and Education of Iwate University for HREIMS and to Prof. Hisao Ando, Ibaraki University, for personal communications about Kuji amber. We thank Emeritus Professor Don R Phillips of La Trobe University for critical reading of the manuscript. We are indebted to Prof. S. J. Rowland co-Editor-in-Chief for valuable comments and suggestions on an earlier manuscript submission. We also thank Dr. C.

References (21)

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