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Total synthesis of gobiusxanthin stereoisomers and their application to determination of absolute configurations of natural products: revision of reported absolute configuration of epigobiusxanthin.

Yamano Y, Ematsu K, Kurimoto H, Maoka T, Wada A - Mar Drugs (2014)

Bottom Line: The validity of the reported stereochemistry of gobiusxanthin was demonstrated by the fact that the reported spectral data of natural gobiusxanthin were in agreement with those of synthetic (3R,3'S,6'R)-gobiusxanthin (1a).On the other hand, the reported CD spectral data of natural epigobiusxanthin, which has been assigned as (3R,3'R,6'R)-isomer (3'-epigobiusxanthin), were identical with those of synthetic (3R,3'S,6'S)-isomer 1d (6'-epigobiusxanthin) rather than those of the corresponding synthetic 3'-epi-isomer 1b.It was found that the stereochemistry at C3-position has little effect on the shape of their CD spectra.

View Article: PubMed Central - PubMed

Affiliation: Department of Organic Chemistry for Life Science, Kobe Pharmaceutical University, Motoyamakita-machi, Higashinada-ku, Kobe 658-8558, Japan. y-yamano@kobepharma-u.ac.jp.

ABSTRACT
(3R)-Gobiusxanthin stereoisomers (1a-d) were synthesized by stereoselective Wittig reaction of the (3R)-C15-acetylenic tri-n-butylphosphonium salt 7 with C25-apocarotenal stereoisomers 5a,b and 14a,b bearing four kinds of 3,6-dihydroxy-ε-end groups. The validity of the reported stereochemistry of gobiusxanthin was demonstrated by the fact that the reported spectral data of natural gobiusxanthin were in agreement with those of synthetic (3R,3'S,6'R)-gobiusxanthin (1a). On the other hand, the reported CD spectral data of natural epigobiusxanthin, which has been assigned as (3R,3'R,6'R)-isomer (3'-epigobiusxanthin), were identical with those of synthetic (3R,3'S,6'S)-isomer 1d (6'-epigobiusxanthin) rather than those of the corresponding synthetic 3'-epi-isomer 1b. It was found that the stereochemistry at C3-position has little effect on the shape of their CD spectra. Thus, in order to reinforce the validity of the absolute configurations at C3-position of natural specimens, (3S,3'S,6'R)- and (3S,3'S,6'S)-stereoisomers 1e and 1f were also synthesized and a HPLC analytical method for four stereoisomers was established by using a column carrying a chiral stationary phase. The HPLC analysis has proven that the stereochemistry of the natural epigobiusxanthin is 3R,3'S,6'S.

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Structures of stereoisomers of gobiusxanthin (1a–f) and other related carotenoids.
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marinedrugs-13-00159-f001: Structures of stereoisomers of gobiusxanthin (1a–f) and other related carotenoids.

Mentions: Gobiusxanthin (1a) (Figure 1), which bears a novel 3',6'-dihydroxy-ε-end group, was first isolated from the common freshwater goby Rhinogobius brunneus [1] and then from the salmon Oncorhynchus keta [2]. Its structure was determined to be 7,8-didehydro-β,ε-carotene-3,3',6'-triol by MS and 1H-NMR spectroscopies and the absolute configuration was tentatively assigned as 3R,3'S,6'R from the resemblance of its CD spectrum to the calculated one of half (3S,6S,3'S,6'S)-tunaxanthin and half (3R,3'R)-alloxanthin according to the additivity rule of CD spectra [3]. From the salmon Oncorhynchus keta, salmoxanthin and deepoxysalmoxanthin, possessing the same 3',6'-dihydroxy-ε-end group, were also isolated together with gobiusxanthin [2]. Their absolute configurations were similarly postulated by comparing their CD spectra with those of analogous compounds. Recently, we accomplished the first total synthesis of these two carotenoids and consequently confirmed that their proposed configurations are correct [4]. The stereoisomer of gobiusxanthin, 3'-epigobiusxanthin (1b) was isolated from the crown-of-thorns starfish Acanthaster planci [5]. Its trans-configuration of the two hydroxy groups at C3' and C6' was determined by NOESY experiment and a 6'R configuration was estimated from the fact that it showed the negative Cotton effect around 280 nm in the CD spectrum [3]. In order to obtain an additional proof on the stereochemistries of gobiusxanthin (1a) and 3'-epigobiusxanthin (1b), we expected that efficient combination of a sterically-defined synthesis of authentic stereoisomers, spectroscopic analyses including NMR and CD, and a HPLC separation using a chiral column could be beneficial.


Total synthesis of gobiusxanthin stereoisomers and their application to determination of absolute configurations of natural products: revision of reported absolute configuration of epigobiusxanthin.

Yamano Y, Ematsu K, Kurimoto H, Maoka T, Wada A - Mar Drugs (2014)

Structures of stereoisomers of gobiusxanthin (1a–f) and other related carotenoids.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4306930&req=5

marinedrugs-13-00159-f001: Structures of stereoisomers of gobiusxanthin (1a–f) and other related carotenoids.
Mentions: Gobiusxanthin (1a) (Figure 1), which bears a novel 3',6'-dihydroxy-ε-end group, was first isolated from the common freshwater goby Rhinogobius brunneus [1] and then from the salmon Oncorhynchus keta [2]. Its structure was determined to be 7,8-didehydro-β,ε-carotene-3,3',6'-triol by MS and 1H-NMR spectroscopies and the absolute configuration was tentatively assigned as 3R,3'S,6'R from the resemblance of its CD spectrum to the calculated one of half (3S,6S,3'S,6'S)-tunaxanthin and half (3R,3'R)-alloxanthin according to the additivity rule of CD spectra [3]. From the salmon Oncorhynchus keta, salmoxanthin and deepoxysalmoxanthin, possessing the same 3',6'-dihydroxy-ε-end group, were also isolated together with gobiusxanthin [2]. Their absolute configurations were similarly postulated by comparing their CD spectra with those of analogous compounds. Recently, we accomplished the first total synthesis of these two carotenoids and consequently confirmed that their proposed configurations are correct [4]. The stereoisomer of gobiusxanthin, 3'-epigobiusxanthin (1b) was isolated from the crown-of-thorns starfish Acanthaster planci [5]. Its trans-configuration of the two hydroxy groups at C3' and C6' was determined by NOESY experiment and a 6'R configuration was estimated from the fact that it showed the negative Cotton effect around 280 nm in the CD spectrum [3]. In order to obtain an additional proof on the stereochemistries of gobiusxanthin (1a) and 3'-epigobiusxanthin (1b), we expected that efficient combination of a sterically-defined synthesis of authentic stereoisomers, spectroscopic analyses including NMR and CD, and a HPLC separation using a chiral column could be beneficial.

Bottom Line: The validity of the reported stereochemistry of gobiusxanthin was demonstrated by the fact that the reported spectral data of natural gobiusxanthin were in agreement with those of synthetic (3R,3'S,6'R)-gobiusxanthin (1a).On the other hand, the reported CD spectral data of natural epigobiusxanthin, which has been assigned as (3R,3'R,6'R)-isomer (3'-epigobiusxanthin), were identical with those of synthetic (3R,3'S,6'S)-isomer 1d (6'-epigobiusxanthin) rather than those of the corresponding synthetic 3'-epi-isomer 1b.It was found that the stereochemistry at C3-position has little effect on the shape of their CD spectra.

View Article: PubMed Central - PubMed

Affiliation: Department of Organic Chemistry for Life Science, Kobe Pharmaceutical University, Motoyamakita-machi, Higashinada-ku, Kobe 658-8558, Japan. y-yamano@kobepharma-u.ac.jp.

ABSTRACT
(3R)-Gobiusxanthin stereoisomers (1a-d) were synthesized by stereoselective Wittig reaction of the (3R)-C15-acetylenic tri-n-butylphosphonium salt 7 with C25-apocarotenal stereoisomers 5a,b and 14a,b bearing four kinds of 3,6-dihydroxy-ε-end groups. The validity of the reported stereochemistry of gobiusxanthin was demonstrated by the fact that the reported spectral data of natural gobiusxanthin were in agreement with those of synthetic (3R,3'S,6'R)-gobiusxanthin (1a). On the other hand, the reported CD spectral data of natural epigobiusxanthin, which has been assigned as (3R,3'R,6'R)-isomer (3'-epigobiusxanthin), were identical with those of synthetic (3R,3'S,6'S)-isomer 1d (6'-epigobiusxanthin) rather than those of the corresponding synthetic 3'-epi-isomer 1b. It was found that the stereochemistry at C3-position has little effect on the shape of their CD spectra. Thus, in order to reinforce the validity of the absolute configurations at C3-position of natural specimens, (3S,3'S,6'R)- and (3S,3'S,6'S)-stereoisomers 1e and 1f were also synthesized and a HPLC analytical method for four stereoisomers was established by using a column carrying a chiral stationary phase. The HPLC analysis has proven that the stereochemistry of the natural epigobiusxanthin is 3R,3'S,6'S.

Show MeSH