Letter Cite This: Org. Lett. 2018, 20, 28−31
pubs.acs.org/OrgLett
Rare Sulfur-Containing Compounds, Kujounins A1 and A2 and Allium Sulfoxide A1, from Allium f istulosum ‘Kujou’ Masashi Fukaya,† Seikou Nakamura,† Ryota Nakagawa, Souichi Nakashima, Masayuki Yamashita, and Hisashi Matsuda* Kyoto Pharmaceutical University, Misasagi, Yamashina-ku, Kyoto 607-8412, Japan S Supporting Information *
ABSTRACT: Three sulfur-containing compounds, kujounins A1 (1) and A2 (2) and allium sulfoxide A1 (3), were isolated from the acetone extract of Allium f istulosum ‘Kujou’. Their chemical structures were elucidated on the basis of physicochemical evidence, including X-ray crystallographic data. Compounds 1 and 2 possess three rings and an acetal structure and were obtained as complex compounds having disulfide and monosaccharide moieties. On the other hand, compound 3 has a thiolane skeleton derived from allicin. Naturally occurring compounds 1−3 have rare molecular skeletons. This study is the first to determine the absolute configuration of thiolane-type compounds.
Allium species are common components found in vegetables and spices around the world, and their extracts show several pharmacological activities, including anticancer, anti-inflammatory, and antidiabetic activities.1−3 In particular, A. sativum, A. cepa, and A. schoenoprasum were designated as anticancer foods by the Designer Foods Project of the National Cancer Institute of the United States. It is known that these plants produce sulfur-containing compounds.4,5 Sulfur-containing single-ring compounds were isolated from Allium plants.6−8 However, there are few studies of the isolation of sulfur-containing compounds with multiple rings from the plants.9 Sulfur compounds from Allium have been found to block tumor necrosis factor alpha (TNF-α)-induced inflammatory pathways by inhibiting nuclear factor kappa B (NF-κB) activation.10 The purpose of this study is to explore biofunctional compounds containing a sulfur atoms. Surprisingly, we were able to isolate compounds having a rare skeleton from A. f istulosum ‘Kujou’, which is one of the cultivars of welsh onion and mainly cultivated in the southern area of Kyoto. This molecular skeleton is very interesting; it is made up of three rings with an acetal structure. However, it has not been reported that the structure containing sulfur atoms was isolated from familiar foods. In this paper, we discuss the structure elucidation of the three new compounds, including their absolute configurations and biosynthetic pathways. Kujounin A1 (1) was isolated as colorless crystals and showed a positive optical rotation ([α]25D +61.1, in MeOH) (Figure 1). Its IR spectrum featured absorption bands at 3437 and 1785 cm−1, which are due to hydroxy and ester groups, respectively. In the ESIMS measurement of 1, the pseudomo© 2017 American Chemical Society
Figure 1. Structures of kujounins A1 (1) and A2 (2) and allium sulfoxide A1 (3).
lecular ion peak [M + Na]+ was observed at m/z 317. Its molecular formula was C10H14O6S2, which was determined on the basis of the HRESIMS peak and 13C NMR data. The 13C NMR spectra of 1 showed signals corresponding to a secondary methyl group at δC 13.8; a methyl attached to the disulfide moiety at δC 24.2; a methine at δC 49.5; a diastereotopic oxygen-bearing methylene at δC 74.4; an oxygen-bearing methine at δC 74.2; two methines neighboring the electronwithdrawing atom at δC 88.6 and δC 96.9; an oxygen-bearing quaternary carbon at δC 81.6; a two-oxygen-bearing quaternary carbon at δC 116.8; and a lactone carbonyl carbon at δC 171.5 (Table 1). DQF-COSY correlations were observed between 6CH3, H-6, and H-7 and also between H-2, H-3, and H-3a (Figure 2). The HMBC spectrum revealed key correlations between the following proton−carbon pairs. Namely, the correlation of H-2 to C-8a, H-7 to C-5a and H-3a to C-8a indicated an acetal Received: October 17, 2017 Published: December 11, 2017 28
DOI: 10.1021/acs.orglett.7b03234 Org. Lett. 2018, 20, 28−31
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Organic Letters Table 1. 13C NMR (125 MHz) and 1H NMR (500 MHz) Spectroscopic Data for Compound 1 in CDCl3 1 position
δC
δH (J, Hz)
2
74.4
a 4.11 (dd, 5.0, 10.0) b 4.22 (dd, 3.5, 10.0) 4.44 (m) 4.64 (d, 1.5)
3 3a 5 5a 6 7 8a 6-CH3 SSCH3
74.2 88.6 171.5 81.6 49.5 96.9 116.8 13.8 24.2
2.74 (m) 5.00 (d, 4.0) 1.23 (d, 7.5) 2.54 (s)
Figure 3. Hypothetical biosynthetic pathway to compound 1.
Bu3SnH.11,12 However, to the best of our knowledge, the radical reaction in biosynthetic precedent has not been reported. Finally, compound 1 was presumed to be obtained via cyclization and disulfide formation. Kujounin A2 (2) was isolated as a colorless oil and showed a negative optical rotation ([α]25D −74.1, in MeOH). Its IR spectrum featured absorption bands at 3436 and 1786 cm−1, which are due to hydroxy and ester groups, respectively. In the ESIMS measurement of 2, the pseudomolecular ion peak [M + Na]+ was observed at m/z 317, and the molecular formula was determined as C10H14O6S2 on the basis of the HREIMS peak and the 13C NMR data. The 1H NMR and 13C NMR spectra of 2 showed signals corresponding to a secondary methyl group, a methyl attached to the disulfide moiety, a methane, a diastereotopic oxygen-bearing methylene, an oxygen-bearing methane, two methines neighboring the electron-withdrawing atom, an oxygen-bearing quaternary carbon, a two-oxygenbearing quaternary carbon, and a lactone carbonyl carbon (Table 2). The planar structure of 2 was estimated in the same
Figure 2. X-ray crystalline structure and key correlations in the 2D NMR and NOESY measurements of compound 1.
structure, the correlation of H-3a to C-5 and H-6 to C-5 indicated a lactone, and the correlations of 6-CH3 to C-6 and C-7 indicated a methyl moiety (Figure 2). According to this evidence, the planar structure of 1 was estimated. The NOESY spectrum of 1 showed correlations between Hb-2 and H-3; Ha-2 and H-3a; H-3a and 6-CH3; H-6 and 6-CH3; H-7 and 6-CH3; and 6-CH3 and SSCH3 (Figure 2). The results of NOESY correlations suggested relative stereochemistry. Namely H-3 is on the opposite side of H-3a and 6-CH3 is on the opposite side of SSCH3, and the NOESY correlation between 6-CH3 and H3a proves the configuration at C-6. In addition, 1 was obtained as a monocrystal in EtOAc, and hydrous methanol and Cu Kα X-ray diffraction measurements clarified its absolute configuration to be 3S,3aR,5aS,6S,7R,8aR (Figure 2). On the basis of these pieces of evidence, the chemical structure of 1 was determined to be (3S,3aR,5aS,6S,7R,8aR)-3,5a-dihydroxy-6methyl-7-(methyldisulfanyl)tetrahydro-2H-difuro[3,2-b:2′,3′c]furan-5(5aH)-one. The hypothetical biosynthetic pathway to 1 is proposed as shown in Figure 3: thioacrolein was obtained from allicin via 1propene sulfenic acid. Intermediate i derived from monosaccharide and intermediate ii derived from thioacrolein might be combined to form hemiacetal (intermediate iii). This intermediate was presumed to convert into intermediate v via intermediate iv. On a radical cyclization onto a vinyl group, similar radical cyclic reactions can be carried out using
Table 2. 13C NMR (125 MHz) and 1H NMR (500 MHz) Spectroscopic Data for Compound 2 in CDCl3 2 position
δC
2
75.0
3 3a 5 5a 6 7 8a 6-CH3 SSCH3
73.8 87.7 172.4 78.5 46.2 95.4 117.2 7.7 24.8
δH (J in Hz) 4.11 4.27 4.48 4.85
(dd, 4.5, 10.5) (dd, 2.0, 10.5) (br s) (br s)
2.76 (m) 5.19 (d, 10.0) 1.19 (d, 6.5) 2.51 (s)
way as for compound 1 by HMBC and DQF-COSY correlations (Figure 4). The NOESY spectrum of 2 showed correlations between H-2 and H-3; H-3a and H-6; H-3a and 6CH3; H-6 and 6-CH3; and H-7 and SSCH3. The NOESYs between 6-CH3 and both H-3a and H-7 for compound 1 to prove its relative stereochemistry, which is then contrasted with compound 2 for which the NOESY between H-6 and H-3a prove the configuration at C-6. There is also a NOESY between 29
DOI: 10.1021/acs.orglett.7b03234 Org. Lett. 2018, 20, 28−31
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Organic Letters
Figure 4. Key correlations in 2D NMR and NOESY of experimental compound 2.
SSCH3 and H-3a to indicate the configuration of C-7 in compound 2. The values of J between H-6 and H-7 in compound 2 also support the change in configurations at C-6 and C-7 in compound 2 relative to compound 1. The configurations of C-8a, C-3, C-3a and C-5a in compound 2 are assumed based on the similarity of the chemical shifts and coupling constants to compound 1 (Figure 4). On the basis of these findings, the relative configuration of kujounin A2 (2) was determined, and its chemical structure was presumed to be (3S*,3aR*,5aS*,6R*,7S*,8aR*)-3,5a-dihydroxy-6-methyl-7-(methyldisulfanyl)tetrahydro-2H-difuro[3,2b:2′,3′-c]furan-5(5aH)-one. Allium sulfoxide A1 (3) was isolated as colorless crystals and had a positive optical rotation ([α]25D +5.4, in MeOH). Its IR spectrum indicated an absorption band at 2961, 1220, 1099 cm−1 due to methyl, ether, and sulfoxide groups. In the ESIMS measurement of 3, a pseudomolecular ion peak [M + Na]+ was observed at m/z 231, and the molecular formula was determined as C8H16O2S2 on the basis of the HREIMS peak and the 13C NMR data. The 13C NMR spectra of 3 showed signals corresponding to two methyl groups at δC 12.9 and δC 14.3; four methine groups at δC 45.7, δC 44.6, δC 72.9, and δC 94.2; and a methoxy group at δC 56.8. As the 13C NMR spectra showed peaks appearing at δC 36.0, respectively, we concluded that 3 had a sulfoxide-bearing methyl moiety (Table 3). DQF-
Figure 5. X-ray crystalline structure and key correlation of 2D NMR and NOESY of compound 3.
OCH3 (Figure 5). The results of NOESY correlations proved the relative stereochemistry. Namely the relative configurations between H-2 and H-3 and between H-4 and H-5 are in the syndirection; on the other hand, the relative configuration between H-3 and H-4 is in the anti-direction. In addition, allium sulfoxide A1 (3) was obtained as a monocrystal in EtOAc and methanol, and Cu Kα X-ray diffraction indicated that its absolute configuration is 2S,3R,4R,5R (Figure 5). On the basis of these findings, the chemical structure of allium sulfoxide A1 (3) was determined to b e ( 2 S , 3 R , 4 R , 5 R ) - 2 -m e t h o x y - 3 , 4 - d i m e t h y l - 5- ( ( S ) methylsulfinyl)tetrahydrothiophene. The hypothetical biosynthetic pathway to 3 is proposed as shown in Figure 6. Allicin might be transformed into 1-
Table 3. 13C NMR (125 MHz) and 1H NMR (500 MHz) Spectroscopic Data for Compound 3 in CDCl3 3 position
δC
2 3 4 5 2-OCH3 3-CH3 4-CH3 5-SOCH3
94.2 45.7 44.6 72.9 56.8 12.9 14.3 36.0
δH (J in Hz) 5.00 2.27 2.65 3.87 3.36 1.01 1.40 2.40
Figure 6. Hypothetical biosynthetic pathway to compound 3.
(d, 4.5) (m) (m) (d, 7.0) (s) (d, 7.0) (d, 7.0) (s)
propenyl 1-propenethiosulfinate and subsequently might be converted into 2,3-dimethylbutanedithial 1-oxide via a [3,3]sigmatropic rearrangement. Next, the intermediate might be obtained by cyclization.9 Finally, compound 3 was presumed to be obtained by oxidation and methylation. In conclusion, three sulfur-containing compounds, kujounins A1 (1) and A2 (2) and allium sulfoxide A1 (3), were isolated from Allium f istulosum ‘Kujou’. The molecular skeletons of kujounins are interesting because they might arise biosynthetically from a monosaccharide. This is the first report of the absolute stereostructure of a thiolane-type compound. Allium plants exhibit several biofunctional activities. In this regard, the biofunctional activities of these compounds should be studied further.
COSY correlations were observed between H-2, H-3, H-4, and H-5; H-4 and 4-CH3; and H-3 and 3-CH3 (Figure 5). The HMBC experiment revealed key correlations between the following proton−carbon pairs. Namely, the correlation of 2OCH3 to C-2 indicated a methoxy, the correlation of H-2 to C5 indicated a sulfide, and the correlations of 5-SOCH3 to C-5 indicated a methyl sulfoxide (Figure 5). According to this evidence, the planar structure of 3 was estimated. The NOESY spectrum of 3 showed correlations between H2 and H-3; H-2 and 3-CH3; H-2 and 2-OCH3; H-4 and H-5; 3CH3 and 4-CH3; 4-CH3 and 5-SOCH3; and 3-CH3 and 230
DOI: 10.1021/acs.orglett.7b03234 Org. Lett. 2018, 20, 28−31
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Organic Letters
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(11) Middleton, D. S.; Simpkins, N. S.; Begley, M. J.; Terrett, N. K. Tetrahedron 1990, 46, 545−564. (12) Shanmugam, P.; Rajasingh, P. Tetrahedron Lett. 2005, 46, 3369− 3372.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03234. 1 H and 13C NMR data of compounds 1−3, 2D NMR spectra, optical rotations, MS, HRMS, experimental procedure, plant information, and X-ray crystallographic data for compounds 1 and 3 (PDF) Accession Codes
CCDC 1586352−1586353 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +81 75 959 4633. Fax: +81 75 595 4768 ORCID
Hisashi Matsuda: 0000-0003-4217-065X Author Contributions †
M.F. and S.N. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported in part by a Ministry of Education, Culture, Sports, Science and Technology (MEXT)-Supported Program for the Strategic Research Foundation at Private Universities 2015−2019. This work was supported by JSPS KAKENHI Grant No. 17K08354 (S.N.). We thank Koto Kyoto Co., Ltd., for supplying materials.
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DOI: 10.1021/acs.orglett.7b03234 Org. Lett. 2018, 20, 28−31
