STUDY OF CONFORMATIONAL PREFERENCES OF ERYTHRO- CAROLIGNAN E ASSESSED BY THE COUPLING BETWEEN H7'-H8' IN DIFFERENT NMR SOLVENTS Rudiyansyah

Erythro-carolignan E (1) has been obtained from the ethanol extract of the wood bark of Durio affinis Becc. This research was conducted in order to prove that conformational preferences of compound 1 were solvent dependent. On the basis of 1 HNMR data, the relative configuration of compound 1 was characterized by a coupling constant ( 3 JHH) value of 3.3 Hz at H-7' in CDCl3. The coupling constant ( 3 JHH) values of H7' in compound 1 has changed to 4.1 Hz and 5.3 Hz in pyridine-d5 and acetonitrile-d3 respectively. As a result, the conformation of compound 1 at C7'-C8' has changed in different NMR solvents. In conclusion, structure of erythro-carolignan E that contains a dihydroxy group at C7'-C8' is able to change in different NMR solvent.


INTRODUCTION
Durio or durian, well known as the King of Fruits, has an economic value as a source of timber and fruits.This plant grows only in tropical region of Southeast Asia.Indonesia has 28 species of Durio, of which 19 are endemic species grown in Borneo Island, hence Borneo is known as thecentre of Durio in the world (Subhadrabandhu and Ketsa, 2001;Morton, 1987).Durio affinis is anon-edible fruit species and endemic to Kalimantan.This plant is also known as bird Durio.The wood of D. affinis can be used for making furniture (Uji, 2005).
In previous studies, six carolignan compounds, namely erythro-carolignan E (1), threo-carolignan E, erythro-carolignan X, threo-carolignan X, erythro-carolignan Y and threo-carolignan Y, have been isolated from the wood bark of D. zibethinus, D. carinatus and D. oxleyanus (Rudiyansyah et al., 2006(Rudiyansyah et al., , 2010;;Lim, 2012).Rudiyansyah et al. (2010) have reported the conformational preferences for erythro-and threo-carolignan X in the NMR solvents CDCl 3 and CD 3 OD.The conformation for erythro-carolignan X changed from conformer EI to EII (Figure 2) since coupling constant ( 3 J HH ) for proton H-7' changed from 2.6 Hz in CDCl 3 to 6.1 Hz in CD 3 OD.This research was conducted in order to prove that conformational preferences of compound 1were solvent dependent, which it has been reported previously in literature (Rudiyansyah et al., 2010).As part of our concern in the structures and conformations of carolignans, in this paper, we report the conformation changing for erythro-carolignan E (1) by studying the coupling constants ( 3 J HH ) of proton H-7' in CDCl 3 and other NMR solvents, pyridine-d 5 and acetonitrile-d 3 .Thus, the conformational preferences for compound 1 will be characterized.In this study, we also isolated other known compounds boehmenan and boehmenan X (Rudiyansyah et al., 2006(Rudiyansyah et al., , 2010(Rudiyansyah et al., , 2014)).

DISCUSSION
The 13 C NMR data at 100 MHz exhibited four methylenes, eighteen methines, fourteen quaternary carbons including two carbonyl resonances at  167.1 (C-9) and 167.3 (C-9"') and four methoxy carbons at  55.9 (OCH 3 -3"'/OCH 3 -3'/OCH 3 -3) and at  56.8 (OMe-3").All of these data were consistent with erythro-carolignan E (Figure 1) in which this compound has been previously reported by Rudiyansyah etal., (2010), Huang et al., (2012), Paula et al.,(1995) and by Wu et al., (2005).The relative configuration of 1 was elucidated by a triplet signal for H-7' with a coupling constant of 3.3 Hz (in CDCl 3 ) which is diagnostic of an erythro diastereomer (Braga et al., 1984).Braga et al., (1984) stated that the erythro isomer would have 3.2 Hz coupling since intramolecular hydrogen bonding would favour conformation EI (Figure 2).Braga also noted that conformer EIII, in which the hydrogens are also gauche, might contribute to the conformational equilibria for the erythro compound.Moreover, Karplus(1963) mentioned that the coupling constant between H-7' and H-8' could be used to determine the conformation around the C-7'/C-8' bond.Furthermore, Bifulco et al.,(2007) stated that the conformational relationship in acyclic systems between adjacent stereocentres is explained by staggered conformers.Additionally, Riccio et al., (2003) andMatsumoriet al., (1999) have developed a method to determine the relative configuration of acyclic compounds on the basis of proton-proton coupling constants together with proton-carbon coupling constants.For erythro compounds with C 7 C 8 -dioxy substituents, the vicinal coupling constants in an individual rotamer can be in the range 0-4 Hz (gauche) or 7-10 Hz (anti); these values are described as small or large, respectively.From our previous study, the NMR data for erythro-carolignan X were solvent dependent.When the NMR spectrum of the erythro compound was re-run in MeOH-d 4 , a coupling constant of proton H-7' changed from 2.6 Hz to 6.1 Hz (Rudiyansyah et al., 2010).These data suggested a change in conformation for the erythro compound.

Conformational Preferences of erythro-carolignan E
Recent study also proved that the conformation of compound 1 changed when the spectral data were run in either pyridine-d 5 or acetonitrile-d 3 .In acetonitrile-d 3 , with a coupling constant of 5.3 Hz, the hydroxyl group is now hydrogen-bonded to the solvent in preference to the adjacent OAryl group.
Conformational preferences will be decided by steric effects and by dipole repulsion effects.Consequently conformer EII in which the H-7'/H-8' is diaxial will be important.On the other hand, in pyridine-d 5 , a coupling constant of 4.1 Hz was found, indicating that the Newman projection has changed insignificantly.Compound 1 both in CDCl 3 and pyridine-d 5 showed similar coupling constants that were 3.3 Hz and 4.1 Hz respectively.
Figure 2. Three possible conformers for compound 1 This study showed that it was important to consider solvent effects on conformational preferences when determining relative configuration.The value of the coupling constant ( 3 J H7'-H8 ) for erythro-carolignan E (1) differed in different NMR solvents.In order to study more about preferred conformation for erythro compounds, 1 H-NMR experiment in cool condition should be conducted.Recently, Ardá et al., (2010) have measured the 3 J HH coupling by doing low-temperature NMR analysis of flexible acyclic systems in order to get the existence of multiple conformer.Other chemical structures from this plant, boehmenan and boehmenan X, were also characterized by comparison between their spectroscopic data with data from the literature [Rudiyansyah et al., 2014;Paula et al., 1995;Secaet al., 2001).
The wood bark of D. affinis Becc.was collected in Arus Deras village, sub-district of Teluk Pakedai, district of Kubu Raya, Province of West Kalimantan in March 2009, air dried, and powdered.The voucher specimens were identified and stored at the Bogoriense Herbarium in Cibinong as 460/IPH.1.02/If.8/V/2009.All solvents were distilled prior to use.
Establishment of structure of erythro-carolignan E Compound 1 was obtained as a white amorphous solid.The positive-ion HRESIMS of 1 gave an adduct [M+Na] + ion at m/z 753.2524, corresponding to a molecular formula C 40 H 42 O 13 .Compound 1 has a specific rotation [] 25 D +25.2 (c 0.15, CHCl 3