Synthesis of Analogues of Cercidin,an Antimicrobial Compound of Cercidiphyllum japonicum : Glycosylation of Methyl Gallate with Ulosyl Bromides

Abstract

Background: Plant materials of Cercidophyllum japonicum, a tree native in East Asia, have long been used in Asian traditional medicine, owing to their antiinflammatory, antioxidative, antifungal, antimicrobial, and hair growth promoting activities. Pharmacologically interesting ingredients include a biphenyl phytoalexin, a galloylflavonol glycoside, anomeric tannins and flavonoids, and phenolic glycosides. Of the latter, cercidin 1 has antimicrobial activity and contains a glycosidic tricyclic structure with a central 1,4-dioxan ring which is also a key structural component of the antibiotic spectinomycin 2 and the cardenolide gomphoside 3. To date, no total synthesis of cercidin 1 has been reported and therefore its stereochemistry is still unknown. Methods: Reaction processes and products were detected on thin-layer chromatography (TLC). Preparative chromatographic separations were carried out on columns with Merck silica gel 60 (15-40 µm) as well as Merck precoated silica gel plates 60 F254, 20 × 20 cm, 0.25 mm. Structures were elucidated by NMR spectra, which were measured on a Bruker 300, 500 and 700 MHz spectrometer at 300 K. Mass spectra were run on a Varian MAT 311 and Bruker Impact II spectrometer. Results: We synthesized the tricyclic pyran-dioxan-benzene ring framework of 1 for the first time, thereby generating novel cercidin analogues. The configuration and conformation of these phenolic glycosides, 13 to 19 as well as 24, 28 and 29 were proven on the basis of ¹H and ¹³C NMR data. Conclusion: These novel cercidin analogues will enable future structure-activity-relationship studies and thus may contribute to the development of novel cercidin-based antibiotics.

Keywords

Natural product synthesis HMBC ADEQUATE phenolic glycosides cercidin bioactivity

Introduction

Cercidophyllum japonicum Siebold et Zucc¹ (Cercidiphyllaceae, Japanese name: katsura) is found in the forests of East Asia at altitudes between 600 and 2700 meters. The long-lived tree has dull green colored leaves and reaches heights up to 30 meters with trunk diameters of over 2 meters (Figure 1). In autumn, the leaves change color to salmon pink, later to golden yellow and shortly before foliage fall they smell like freshly baked cake. Plant materials of C japonicum have long been used in Asian traditional medicine, owing to their antiinflammatory, antioxidative, antifungal, antimicrobial, and hair growth promoting activities, as well as arresting convulsion.² In light of these usages, numerous pharmacologically interesting ingredients have been identified in various above-ground parts of C japonicum. In 1986, the first publication reports on the isolation of a biphenyl phytoalexin from cortical tissue of C japonicum twigs.³ More than 30 years later, a galloylflavonol glycoside together with anomeric tannins and flavonoids, isolated from the same plant part, were published.⁴ In addition, maltol and phenolic glycosides were detected in the leaves,^5,6 bark,^7-9 flowers,² and fruits.¹⁰ Furthermore, the whole methanol extract of the heartwood of C japonicum has been shown to stimulate proliferation of mouse hair epithelial cells.¹¹

Figure 1.

Cercidophyllum japonicum, Whose Plant Materials are Used in Asian Traditional Medicine. (A) Whole Tree, (B) Trunk and Branches, and (C) Leaves of C japonicum. Shown is Europe's Tallest Specimen with a Height of Over 20 Meters, Located in the Botanical Garden of the Technical University of Darmstadt, Germany. Photos Were Taken by the Author in July 2020.

In 1991, a compound which possessed antimicrobial activity against Bacillus subtilis and Escherichia coli was isolated by Tada and Sakurai¹² using methanol extraction of fresh leaves of C japonicum. Structural analysis by high resolution mass spectrometry and ¹H and ¹³C NMR spectroscopy revealed that the natural product, which they named cercidin 1, possessed a tricyclic structure with a central 1,4-dioxan ring fused to a pyran-4-one ring on the left side and methyl gallate on the right side. Similar tricyclic pyranodioxan ring structures are also realized in the antibiotic spectinomycin 2, produced by Streptomyces spectabilis,¹³ and gomphoside 3, a cardenolide glycoside isolated from Gomphocarpus fruticosus¹⁴ a milkweed of the Asclepiadaceae family (see Figure 2).

Figure 2.

The Natural Products Cercidin 1, Spectinomycin 2, and Gomphoside 3 are Characterized by a Tricyclic Pyranodioxan Ring Structure (Marked in Red Color).

To date, no total synthesis of cercidin 1 or a further isolation from Cercidiphyllum japonicum have been reported. Thus, the stereochemistry of 1 is still unknown, but due to ring tension only one pair of enantiomers with α- or β-cis-annulation of pyran and dioxan ring should be possible. Here, we confirmed this assumption by synthesizing the tricyclic pyran-dioxan-benzene ring framework for the first time, and generating novel cercidin analogues. These analogues are useful for future structure-activity-relationship studies and thus for the development of novel cercidin-based antibiotics.

Methods

Instruments

Thin-layer chromatography (TLC) was performed on POLYGRAM SILG/UV254 (Macherey Nagel & Co.). Preparative chromatographic separations were carried out on columns with Merck silica gel 60 (15-40 µm) and Merck precoated silica gel plates 60 F254, 20 × 20 cm, 0.25 mm. Melting points were determined on a Bock-Monoskop VS or on a Büchi SMP-20 and are uncorrected. Specific optical rotations were determined on a Perkin-Elmer Polarimeter 241 in 1 dm cuvettes at a wavelength of 589 nm. NMR spectra were measured on a (a) Bruker WM 300 spectrometer, (b) Bruker DRX 500 spectrometer, (c) Bruker 300 MHz spectrometer with Avance-II console and BBO probe, (d) Bruker 500 MHz spectrometer with DPX console and BBFO probe, and (e) Bruker Avance III HD 700 MHz spectrometer with a QCI cryo probe at 300 K using trimethylsilane as internal reference or by calibration with the shift of the solvent of the sample. Multiplicities of the shifts are abbreviated as s for singlet, d for doublet, t for triplet, q for quartet, sxt for sextet, oct for octet, m for multiplet, br for broad, and p for pseudo. Mass spectra were run on a Varian MAT 311 and Bruker Impact II spectrometer. Elemental analyses were performed on a Perkin Elmer 240 Elementar Analyser. NMR shifts with an asterisk * can be reversed.

Syntheses

The synthesis of the title compounds 13, 14, 15, 18, 19, 22, 24, and 28 are described below. Practical preparations of their starting materials, the phenolates 11, 12, and, 20, and the synthesis of the two derivatives, 16 and 17, of main product 13, as well as for side-product 29 are described in the Supplemental Material.

The 700 and 500 MHz NMR spectra (¹H, ¹³C, DEPT, HSQC, HMBC, COSY, and NOESY) for compounds 13, 14, 15, 18, and 19 are also illustrated in the Supplemental Material (Figure S1 to Figure S38).

(2R,3R,4S,4aS,10aS)-2-((benzoyloxy)methyl)-4a,9-dihydroxy-7-(methoxycarbonyl)-3,4,4a,10a-tetrahydro-2H-benzo[b]pyrano[2,3-e][1,4]dioxine-3,4-diyl dibenzoate 13

With triethylamine in dichloromethane: A mixture of methyl gallate 4 (1.84 g, 10.0 mmol) and triethylamine (1.4 mL, 1.0 g, 10.0 mmol in anhydrous dichloromethane (50 mL) was stirred for 15 min in the presence of freshly desiccated molecular sieve (4 Å). Ulosyl bromide 8^15,16 (5.53 g, 10.0 mmol) was added in one portion and stirring was continued for another 1.5 h. The molecular sieve was filtered off and the reaction mixture was diluted with dichloromethane (200 mL) and washed successively with 2N HCl (2 × 50 mL) and water (2 × 50 mL). The organic phase was dried with Na₂SO₄ and evaporated in vacuo to form an amorphous solid residue (6.26 g) which showed 4 spots on TLC with R_f 0.12, 0.24, 0.29, and 0.47 (toluene/ethyl acetate 4:1). The mixture was separated by column chromatography (20 × 5 cm, toluene/ethyl acetate 2:1) and the most polar fraction with R_f 0.12 was evaporated in vacuo: 2.17 g (33%) of glycoside 13. Crystallization from n-hexane/diethyl ether 1:1 gave analytical pure 13 with m.p. 182 to 183 °C and [α]_D²⁰ = –19.3 (c = 0.9, CHCl₃).

C₃₅H₂₈O₁₃ (656.6): calcd. C 64.02, H 4.30; found C 63.88, H 4.19.

MS (FD, 0-20 mA): m/z = 656 (100%, M⁺).

¹H NMR (700 MHz, DMSO-d6) δ 3.84 (s, 3H, CO₂CH₃),), 4.34 (dd, 1H, 2'-H_A), 4.42 (dd, 1H, 2'-H_B), 4.62 (m, 1H, 2-H), 5.61 (pt, 1H, 3-H), 5.81 (s, 1H, 10a-H), 5.93 (d, 1H, 4-H), 7.13 (d, 1H, 8-H), 7.15 (d, 1H, 6-H), 7.42, 7.45, 7.49 (each 2H-pt, 6 meta-C₆H₅CO), 7.61 (m, 3H, 3 para-C₆H₅CO), 7.79 (pt, 4H, 4 ortho-C₆H₅CO), 7.92 (d, 2H, 2 ortho-C₆H₅CO), 8.02 (1H, 4a-OH), 9.96 (1H, 9-OH); J_2,2'A = 3.98, J_2’,2'B = 2.80, J_2'gem = 12.48, J_2,3 = 9.90, J_3,4 = 9.68, J_6,8 = 2.05 Hz.

NOE between 10a-H (5.81) and 2-H (4.62), 4-H (5.93), as well as 4a-OH (8.02).

¹³C NMR (175 MHz, DMSO-d6) δ 52.04 (CO₂CH₃), 62.27 (C-2’), 68.34 (C-3), 71.04 (C-2), 73.97 (C-4), 91.21 (C-4a), 91.76 (C-10a), 109.23 (C-6), 110.28 (C-8), 122.80 (C-7), 128.51, 3 × 128.57, 128.61, 128.72 (each m-C₆H₅CO), 2 × 129.07, 129.19, 129.21, 129.27, 129.46 (each o-C₆H₅CO), 132.74 (C-9a), 133.42, 133.46, 134.77 (each p-C₆H₅CO), 140,51 (C-5a), 146.26 (C-9), 164.82 (3-C₆H₅CO), 165.07 (2’-C₆H₅CO), 165.19 (4-C₆H₅CO), 165.68 (CO₂CH₃); ¹J_C10a,H10a = 175.7 Hz.

The synthesis of 13 in anhydrous dimethyl formamide as well as the ¹H NMR data of compound 13 in CDCl₃ are described in the Supplemental Material.

Synthesis of the compounds 14 and 15 by further processing of the fractions of the above column chromatography:

(2R,3R,4S,4aS,10aS)-2-((benzoyloxy)methyl)-4a,6-dihydroxy-8-(methoxycarbonyl)-3,4,4a,10a-tetrahydro-2H-benzo[b]pyrano[2,3-e][1,4]dioxine-3,4-diyl dibenzoate 14

The next fraction of the synthesis of compound 13 with R_f 0.24, 0.29, and 0.47 yielded a colorless, amorphous residue (2.82 g) after evaporation in vacuo which was divided into two parts, and each part was separated by column chromatography (20 × 5 cm, toluene/ethyl acetate 4:1). The combined fractions with R_f 0.29 yielded 250 mg (4%) of glycoside 14, which had m.p. 135 to 137 ° C (diethyl ether) and [α]_D²⁰ = -23.9 (c = 0.8, chloroform).

C₃₅H₂₈O₁₃ (656.6): calcd. C 64.02, H 4.30; found C 64.02, H 4.13.

MS (FD, 0-20 mA): m/z = 656 (100%, M⁺).

¹H NMR (500 MHz, CDCl₃) δ 3.85 (s, 3H, CO₂CH₃), 4.29 (ddd, 1H, 2-H),), 4.43 (dd, 1H, 2'-H_A), 4.58 (dd, 1H, 2'-H_B), 5.45 (s, 1H, 10a-H), 5.60 (d, 1H, 4-H), 5.91 (pt, 1H, 3-H), 6.02 (broad m, 1H, 4a-OH), 7.32 (d, 1H, 9-H), 7.34 (d, 1H, 7-H), 7.25–8.05 (m, 15H, 3 C₆H₅CO); J_2,2'A = 5.14, J_2’,2'B = 3.15, J_2'gem = 12.27, J_2,3 = 9.95, J_3,4 = 9.61, J_7,9 = 1.99 Hz.

Weak NOE between 10a-H (5.45) and 2-H (4.29).

¹³C NMR (125 MHz, CDCl₃) δ 52.22 (CO₂CH₃), 62.86 (C-2’), 68.07 (C-3), 72.63 (C-2), 77.20 (C-4), 91.86 (C-4a), 93.54 (C-10a), 111.07 (C-9), 111.45 (C-7), 124.38 (C-8), 128,35, 128,57, 128,67 (each m-C₆H₅CO), 129.30, 129.67, 129.85 (each o-C₆H₅CO), 131.52 (C-5a), 133.35, 133.93, 134.44 (each p-C₆H₅CO), 139.65 (C-9a), 144.72 (C-6), 165,50 (3-C₆H₅CO), 166.22 (2’-C₆H₅CO), 166.49 (CO₂CH₃), 168.13 (4-C₆H₅CO); ¹J_C10a,H10a = 172.6 Hz.

(2R,3R,4S,4aR,10aR)-2-((benzoyloxy)methyl)-4a,6-dihydroxy-8-(methoxycarbonyl)-3,4,4a,10a-tetrahydro-2H-benzo[b]pyrano[2,3-e][1,4]dioxine-3,4-diyl dibenzoate 15

The combined fractions of the synthesis of compound 13 with R_f 0.24 (toluene/ethyl acetate 4:1) gave, after evaporation in vacuo and crystallization from n-hexane/diethyl ether 1:1, compound 15 in 335 mg (5%) yield as colorless crystals with m.p. 130 to 132 °C and [α]_D²⁰ = + 30.5 (c = 1.0, chloroform).

C₃₅H₂₈O₁₃ (656.6): calcd. C 64.02, H 4.30; found C 64.16, H 4.29.

MS (FD, 0-20 mA): m/z = 656 (100%, M⁺).

¹H NMR (500 MHz, CDCl₃) δ 3.85 (s, 3H, CO₂CH₃), 4.52 (dd, 1H, 2'-H_A), 4.69 (m, 2H, 2-H, 2'-H_B), 5.40 (s, 1H, 10a-H), 5.73 (d, 1H, 4-H), 5.98 (pt, 1H, 3-H), 7.15 (d, 1H, 9-H), 7.34 (d, 1H, 7-H), 7.20–8.20 (m, 15H, 3 C₆H₅CO); J_2,2'A = 5.31, J_2,2'B covered, J_2'gem = 13.25, J_2,3 = 9.95, J_3,4 = 9.62, J_6,8 = 1.82 Hz.

As expected no NOE between 10a-H (5.51) and 2-H (4.80).

¹³C NMR (125 MHz, CDCl₃) δ 52.43 (CO₂CH₃), 62.72 (C-2’), 68.48 (C-3), 70.04 (C-4), 70.93 (C-2), 92.99 (C-4a), 94.72 (C-10a), 111.11 (C-7, C-9), 124.95 (C-8), 128,53, 128,58, 128,60 (each m-C₆H₅CO), 129.96, 129.98 130.12 (each o-C₆H₅CO), 132.63 (C-5a), 133.39, 133,78, (each p-C₆H₅CO), 140.14 (C-9a), 145.04 (C-6), 165,14 (CO₂CH₃), 165.38 (2’-C₆H₅CO), 166.47 (3-C₆H₅CO), 166.60 (4-C₆H₅CO); ¹J_C10a,H10a = 180.2 Hz.

(2R,3R,4S,4aS,10aS)-2-((benzoyloxy)methyl)-4a-hydroxy-9-methoxy-7-(methoxycarbonyl)-3,4,4a,10a-tetrahydro-2H-benzo[b]pyrano[2,3-e][1,4]dioxine-3,4-diyl dibenzoate 18

Methyl 3-O-methylgallate 11 (1.0 g, 5.0 mmol) dissolved in anhydrous dichloromethane (25 mL) containing triethylamine (0.7 mL, 0.5 g, 5.0 mmol) was stirred in the presence of freshly desiccated molecular sieve (4 Å) for 15 min at 20 °C. Ulosyl bromide 8^15,16 (2.77 g, 5.0 mmol) was added in one portion and stirring was continued for another 1.5 h. The molecular sieve was filtered off and the filtrate diluted with dichloromethane (200 mL) and washed successively with 2N HCl (2 × 50 mL) and water (2 × 50 mL). Drying over sodium sulfate, removal of the solvent under reduced pressure and crystallization of the brown solid residue (3.18 g) from n-hexane/diethyl ether 1:1 gave 2.45 g (73%) of 18 as colorless crystals with R_f 0.23 (toluene/ethyl acetate 4:1), m.p. 217 to 218 °C and [α]_D²⁰ = –26,2 (c = 1.0, CHCl₃).

C₃₆H₃₀O₁₃ (670.6): calcd. C 64.47, H 4.51; found C 64.12, H 4.44.

MS (FD, 0-20 mA): m/z = 670 (100%, M⁺).

¹H NMR (500 MHz, CDCl₃) δ), 3.84 (s, 3H, CO₂CH₃), 3.90 (s, 3H, (aromat-OCH₃), 4.32 (ddd, 1H, 2-H),), 4.47 (dd, 1H, 2'-H_A), 4.59 (dd, 1H, 2'-H_B), 5.54 (s, 1H, 10a-H), 5.58 (d, 1H, 4-H), 5.73 (broad s, 1H, 4a-OH), 5.96 (pt, 1H, 3-H), 7.16 (d, 1H, 8-H), 7.49 (d, 1H, 6-H), 7.28–8.03 (m, 15H, 3 C₆H₅CO); J_2,2'A = 5.56, J_2,2'B = 2.99, J_2'gem = 12.28, J_2,3 = 9.79, J_3,4 = 9.54, J_6,8 = 1.90 Hz.

Weak NOE between 10a-H (5.54) and 2-H (4.32), as well as 8-H (7.16) and aromat-OCH₃ (3.90).

¹³C NMR (125 MHz, CDCl₃) δ 52.34 (CO₂CH₃), 56.43 (aromat-OCH₃), 63.26 (C-2’), 68.30 (C-3), 72.73 (C-2), 77.26 (C-4), 91.18 (C-4a), 93.48 (C-10a), 106.81 (C-8), 112.78 (C-6), 123.68 (C-7), 128,40, 2 × 128,62, (each m-C₆H₅CO), 129,88, 129,95, 130,42 (each o-C₆H₅CO), 133.21, 133,76, 134.21, (each p-C₆H₅CO), 133.47 (C-9a), 139.78 (C-5a), 148,54 (C-9), 165,16 (3-C₆H₅CO), 166.22 (2’-C₆H₅CO), 166.66 (CO₂CH₃), 167.95 (4-C₆H₅CO); ¹J_C10a,H10a = 171.8 Hz.

(2R,3R,4S,4aS,10aS)-2-((benzoyloxy)methyl)-4a-hydroxy-9-isopropoxy-7-(methoxycarbonyl)-3,4,4a,10a-tetrahydro-2H-benzo[b]pyrano[2,3-e][1,4]dioxine-3,4-diyl dibenzoate 19

A solution of methyl 3-O-isopropylgallate 12 (1.12 g (5.0 mmol) and triethylamine (0.7 mL, 5.0 g, 5.0 mmol) in anhydrous dichloromethane (25 mL) was allowed to react (1.5 h, 20 °C) with ulosyl bromide 8^15,16 (2.77 g, 5.0 mmol) as described for the preparation of compound 18. Work up furnished a brown solid residue which was purified through a short silica gel column (10 × 4 cm) with toluene/ethyl acetate 4:1 as eluent. The obtained crude product (2.80 g) with R_f 0.49 was crystallized from n-hexane/diethyl ether 1:1 to yield 1.99 g (57%) of 19 as colorless crystals with m.p. 172 to 174 °C, and [α]_D²⁰ = -24.8 (c = 0.7, CHCl₃).

C₃₈H₃₄O₁₃ (698.7): calcd. C 65.32, H 4.90; found C 65.31, H 4.91.

MS (FD, 0-20 mA): m/z = 698 (30%, M⁺), 697 (100%, M ⁺ -1).

¹H NMR (500 MHz, CDCl₃) δ 1.38 and 1.39 (each 3H-d, 2 isopropyl-CH₃), 3.84 (s, 3H, CO₂CH₃), 4.29 (ddd, 1H, 2-H),), 4.47 (dd, 1H, 2'-H_A), 4.58 (m, 1H, isopropyl-CH), 4.60 (dd, 1H, 2'-H_B), 5.53 (s, 1H, 10a-H), 5.60 (d, 1H, 4-H), 5.61 (broad s, 1H, 4a-OH), 5.96 (pt, 1H, 3-H), 7.18 (d, 1H, 8-H), 7.45 (d, 1H, 6-H), 7.32–8.20 (m, 15H, 3 C₆H₅CO); J_{isoprpyl−Me,CH} = 6.06, J_2,2'A = 5.33, J_2,2'B = 3.20, J_2'gem = 12.16, J_2,3 = 9.80, J_3,4 = 9.55, J_6,8 = 1.87 Hz.

Weak NOE between 10a-H (5.55) and 2-H (4.31).

¹³C NMR (125 MHz, CDCl₃) δ 22.18, 22.21 (each isopropyl-CH₃), 52.26 (CH₃CO), 63.23 (C-2’), 68.36 (C-3), 72.21 (isopropyl-CH), 72.69 (C-2), 77.25 (C-4), 91.08 (C-4a), 93.39 (C-10a), 110.24 (C-8), 112.52 (C-6), 123.64 (C-7), 128,17, 128,20, 128,39 (each m-C₆H₅CO), 129.87, 129.95, 130.46 (each o-C₆H₅CO), 132.18, 133.68, 134.11, (each p-C₆H₅CO), 134.52 (C-9a), 139.92 (C-5a), 147.03 (C-9), 165.16 (3-C₆H₅CO), 166.22 (2′-C₆H₅CO), 166.70 (CO₂CH₃), 167.95 (4-C₆H₅CO); ¹J_C10a,H10a = 171.6 Hz.

Methyl 3-O-(2′S,6′S)-4′-benzoyloxy-6′-benzoyloxymethyl-3′-oxo-3′,6′-dihydro-2H-pyran-2-yl)oxy)-4,5-isopropylidenoxybenzoate 22

A solution of methyl gallate acetonide 20 (0.60 g, 2.68 mmol) and triethylamine (374 μl, 271 mg, 2.68 mmol) in anhydrous dichloromethane (20 ml) was allowed to react with ulosyl bromide 8^15,16 (1.48 g, 2.68 mmol) at 20 °C for 2 h, as described for the preparation of compound 19. Work up furnished a brown solid residue (1.77 g) that was crystallized from n-hexane diethyl ether 1:1 to yield 0.40 g (21%) of 22 as colorless crystals with R_f 0.62 (toluene/ethyl acetate 4:1), m.p. 119 to 120 °C and [α]_D²⁰ = -105.9 (c = 0.8, CHCl₃).

MS: HR-ESI 575.1548 [M + 1]⁺ (calcd. for C₃₁H₂₆O₁₁ + 1, 575.1552).

¹H NMR (300 MHz, CDCl₃) δ 1.67 and 1.69 (each 3H-s, 2 isopropyl-CH₃), 3.80 (s, 3H, CO₂CH₃), 4.66 (pd, 2H, 6'-CH₂OBz), 5.23 (sxt, 1H, 6'-H), 5.99 (s, 1H, 2'-H), 7.04 (d, 1H, 5'-H), 7.11* (d, 1H, 6-H), 7.49* (d, 1H, 2-H), 7.30–8.20 (m, 10H, 2 C₆H₅CO); J₅’_,6'=3.6, J_2,6 = 1.0 Hz.

¹³C NMR (75 MHz, CDCl₃) δ 25.78 and 25.86 (each isopropyl-CH₃), 52.07 (CO₂CH₃), 65.98 (6'-CH₂OBz), 71.69 (C-6’), 96.65 (C-2’), 105.25* (C-6), 113.33* (C-2), 120.61 (isopropyl-CMe₂), 123.71 (C-1), 132.81 (C-5’), 138.87, 140.38, 142.06, 148.98 (C-4’, C-3, C-4, C-5), 163.7, 165.8, 166.0 (CO₂CH₃, 2 C₆H₅CO), 180.38 (C-3’); ¹J_C2’_,H2’ = 174.1 Hz.

Methyl (2S,4aS,10aS)-4a-(benzoyloxy)-2-((benzoyloxy)methyl)-6-hydroxy-4-oxo-3,4,4a,10a-tetrahydro-2H-benzo[b]pyrano[2,3-e][1,4]dioxine-8-carboxylate 24

The solution of open-chain glycoside 22 (70 mg, 0.12 mmol) in a mixture of chloroform, trifluoroacetic acid, and water (5 ml, 50:10:1) was refluxed at 20 °C for 6 h and then stirred over night at 20 °C. The reaction mixture was diluted with dichloromethane (200 ml), washed with saturated NaHCO₃ solution (2 × 50 mL) and water (2 × 50 mL), dried (Na₂SO₄) and concentrated under reduced pressure. The remaining colorless foam (63 mg) was crystallized from a mixture of n-hexane/diethyl ether: 46 mg (72%) of 24 as colorless crystals with m.p. 103 to 105 °C and [α]_D²⁰ = -42,8 (c = 0.8, CHCl₃).

MS (FD, 15 mA): m/z = 534 (100%, M⁺).

¹H NMR (300 MHz, CDCl₃) δ 2.88 and 3.17 (each 1H-dd, 3-H_e and 3-H_a), 3.87 (s, 3H, CO₂CH₃), 4.43 (m, 1H, 2-H), 4.47 (m, 2H, 2'-H_A and 2'-H_B), 5.92 (s, 1H, 10a-H), 6.23 (broad s, 1H 6-OH ; exchangeable with D₂O), 7.33* and 7.39* (each 1H-d, 7-H and 9-H), 7.20–8.10 (m, 10H, 2 C₆H₅CO); J₂,_3a = 11.1, J_2,3e = 2.3, J_3gem = 15.1 Hz.

¹³C NMR (75 MHz, CDCl₃) δ 41.10 (C-3), 52.20 (CO₂CH₃), 64.94 (C-2′), 70.27 (C-2), 92.60 (C-10a), 94.31 (C-4a), 111.72* (C-7), 110.86* (C-9), 125.32 (C-8), 128.4–130.6 (C-5a, C₆H₅CO), 139.03 (C-9a), 144.82 (C-6), 164.40, 2 × 165.94 (CO₂CH₃, 2 C₆H₅CO), 192.64 (C-4).

Methyl (2S,4aR,10aS)-2-((benzoyloxy)methyl)-4a,9-dihydroxy-4-oxo-3,4,4a,10a-tetrahydro-2H-benzo[b]pyrano[2,3-e][1,4]dioxine-7-carboxylate 28

Tetrabutylammonium acetate (687 mg, 2.28 mmol) was added to a stirred solution of 13 (500 mg, 0.76 mmol) in aqueous acetonitrile (25 mL, prepared by adding of 3 drops of water to 100 ml of acetonitrile). After 18 h at room temperature, the mixture was diluted with dichloromethane (200 ml), washed with water (3 × 50 ml), dried (Na₂SO₄), and evaporated in vacuo. The remaining brown syrup was liberated from tetrabutylammonium acetate by filtration over a column of silica gel (10 × 3 cm, toluene/ethyl acetate 2:1 → ethyl acetate) and evaporation of the filtrate in vacuo: 218 mg (66%) of colorless and amorphous 28 with R_f 0.42 (chloroform/methanol 9:1), [α]_D²⁰ = -24.8 (c = 1.0, CHCl₃).

MS (FD, 0-20 mA): m/z = 430 (100%, M⁺).

MS: HR-ESI 431.0973 [M + 1]⁺ (calcd. for C₂₁ H₁₈ O₁₀ + 1, 431.0973).

¹H-NMR (300 MHz, CDCl₃) δ 2.69 (dd, 1H, 3-H_e), 3.10 (dd, 1H, 3-H_a), 3.83 (s, 3H, CO₂CH₃), 4.19 (m, 1H, 2-H), 4.39 and 4.46 (each 1H-dd, 2'-H_A and 2'-H_B), 5.26 (s, 1H, 10a-H), 7.17 (d, 1H, 8-H), 7.30 (d, 1H, 6-H), 7.20–8.10 (m, 5H, C₆H₅CO); J_2,2'A = 5.3, J_2,2'B = 3.8, J_2'gem = 12.0, J_2,3a = 12.2, J_2,3e = 0, J_3gem = 14.5, J_6,8 = 1.9 Hz.

¹³C-NMR (75 MHz, CDCl₃) δ 39.73 (C-3), 52.45 (CO₂CH₃), 65.30 (C-2’), 70.12 (C-2), 90.36 (C-4a), 94.26 (C-10a), 111.63* (C-6), 111.12* (C-8), 124.27 (C-7), 131.94 (C-9a), 139.59 (C-5a), 145.04 (C-9), 166.38, 166.63 (CO₂CH₃, C₆H₅CO), 198.32 (C-4); ¹J_C10a,H10a = 174.0 Hz.

Results

Based on our previous work on the annulation of 2-ketosugars to glycol,¹⁷ the synthesis of linear fused pyran-dioxan-cxclohexan tricycles,¹⁸ of spectinomycin 2^19,20 and of gomphoside 3,²⁰ we here report the synthesis of analogues of cercidin 1 for the first time. The synthesized compounds had the same tricyclic backbone as the natural products 2 and 3 with the difference of a benzene ring instead of a cyclohexane moiety on the right side of the molecule.

Reactions of Ulosyl Bromides with Methyl Gallates

Glycosyl bromides are the most widely used glycosyl halides in one-step aromatic O-glycosylations with general yields of less than 60%. But their main advantage is their simplicity of generation as the thermodynamically favored α-anomers. Glycosylation generally results in the inversion of the stereochemistry and yields the β-products.²¹

Thus, 2-keto glycosyl bromides, so-called ulosyl bromides, are easily synthetically accessible as well.^15,16 The key reaction to construct cercidin analogues was the installation of the benzene moiety in the desired tricycle by aromatic O-glycosylation.^21-25 Due to the lower nucleophilicity of phenols, phenolates of methyl gallate 4 have to be used, which could be achieved in the presence of triethylamine as a basic reaction partner. Under these reaction conditions, the triphenolic aglycon 4 is in equilibrium with the para- and meta-phenolates 5 and 7. Because of the additional mesomeric stabilization of resonance structure 5 to its para-quinoid counterpart 6,²⁶ an electrophilic attack of ulosyl bromides should preferentially occur at the para-disposed hydroxyl group of 4 (see Scheme 1).

With this knowledge, we first analyzed the conversion of methyl gallate 4 with ulosyl bromide 8 in the presence of triethylamine and dichloromethane as solvent (s. Scheme 2). As expected, a preferential S_N-2 attack of the para-disposed hydroxyl group of 4 occurred at the anomeric center of bromide 8. Subsequent in situ ketalization led to the β-cis-configured tricyclic glycoside 13 which after column chromatography could be obtained in 33% yield. The side products, regioisomer 14 and α-anomer 15, were isolated in 4% and 5% yield, respectively.

The same glycosylation reaction of 4 with 8 in anhydrous dimethylformamide gave similar results and did not improve yield and selectivity.

We also studied the reaction of methyl gallate 4 with the ulosyl bromides 9 and 10. They differ from 8 at C-5 of the pyran ring by the presence of a methyl group or hydrogen atom instead of a benzoyloxymethyl group. Again, the β-cis-configured glycosides 16 and 17 were obtained as main products in 29% and 27% yield, respectively.

The para-selectivity was remarkably increased by blocking one of the two symmetric meta-hydroxyl groups of methyl gallate 4. Thus, the methylether 11²⁷ and the isopropylether 12^28,29 resulted in the corresponding glycosides 18 and 19 in 73% and 57% yield, respectively.

By blocking the vicinal hydroxyl groups of methyl gallate 4 by isopropylidenation to its acetonide 20,³⁰ an exclusive meta-glycosylation was achieved. The reaction of 20 with bromide 8 (see Scheme 3) gave the crystalline open-chain product 22 in 21% yield. Due to the basic reaction conditions applied, ulosyl intermediate 21 was primarily formed, eliminating one molecule of benzoic acid and thereby generating enolester 22. Subsequent cleavage of the isopropylidene protecting group under standard conditions with trifluoroacetic acid in aqueous chloroform gave the tricyclic cercidin analogue 24 in 72% yield. In this two-step reaction, the deprotected intermediate 23 was formed, which cyclizes via simultaneous benzoyl group migration as shown by the arrows.

Keto Group Generation in the Pyran Ring

A pronounced structural feature of cercidin 1 is the keto group in the sugar part. This group was generated in compound 13 as shown in Scheme 4. Stirring a solution of 13 with the weak base tetrabutylammonium acetate in aqueous acetonitrile (reaction path a) gave pure 28 after column chromatography (66% yield). During this remarkable reaction, the intermediates 25, 26 and 27 are passed through. Thus, the primarily formed 25, the open keto form of 13, cleaved off one molecule of benzoic acid and formed the enolester 26. This was saponified to form enol 27 which cyclized by intramolecular ketalization to the tricyclic compound 28 as shown by the arrows.

The same reaction sequence already took place in part during the synthesis of compound 17 starting from methyl gallate 4 and ulosyl bromide 10 and in the presence of triethylamine in anhydrous dichloromethane (reaction path b). Under these reaction conditions, the enol ester 26 was not saponified and cyclized directly to the tricycle 29 under benzoyl group displacement.

All synthesized tricyclic phenol glycosides 13 to 19 as well as 24, 28, and 29 are novel structural analogues of the antimicrobial cercidin 1.

Structural Elucidations

The successful conversion of ulosyl bromides 8, 9, and 10 with methyl gallates 4, 11, and 12 to the glycosides 13 to 19 was evidenced by elemental analysis, field desorption mass spectrometry (FD) or high-resolution electrospray ionization technique (HR-ESI). Configuration and conformation were proven on the basis of ¹H and ¹³C NMR data (COSY, HSQC, NOE, partly HMBC and ADEQUATE).

In a first step, the structure of the main product 13 was analyzed in detail by 700 MHz NMR spectra in DMSO-d₆ (see Figures 3, 4, and 5).

The 700 MHz ¹H NMR spectrum of compound 13 is characterized by sharp and well separated shifts (Figure 3). At high field, the peak of solvent DMSO-d_6. (2.50 ppm) together with its typical water singlet (3.32 ppm) can be identified, while the sharp 3H-singlet (3.84 ppm) belongs to the ester methyl group.

Figure 3.

700 MHz ¹H NMR Spectrum of Compound 13 in DMSO-d₆.

The group of shifts around 4.5 ppm (see also enlarged section of Figure 3) are part of the exocyclic methylene group 2′-CH₂ and of pyran ring proton 2-H. As expected, the 2 geminal protons 2′-H_A and 2′-H_B are characterized by 2 separate double doublets with large geminal coupling constants (12.48 Hz) and smaller ones of 3.98 and 2.80 Hz, respectively. The latter 2 coupling constants are also present in the multiplet of 2-H, due to its further coupling with 3-H. The shift of 3-H, a pseudo triplet, is located in the middle of the spectrum (5.61 ppm) together with the sharp singlet of anomeric proton 10a-H (5.81 ppm) and the doublet of 4-H (5.93 ppm).

At the lower field of the spectrum, the phenolic part is represented by 2 very closely adjacent doublets of 8-H (7.13 ppm) and 6-H (7.15 ppm), both with the small meta-coupling constant of 2.05 Hz, respectively. At the lowest field of the spectrum, 2 singlets for the aliphatic hydroxyl proton 4a-OH (8.02 ppm) and 9-OH (9.96 ppm) are present, while the aromatic shifts of the 3 benzoyl groups are found as a characteristic broad multiplet between 7.4 to 7.9 ppm.

The assignment of the hydrogen-bearing carbon atoms of the pyran moiety (C-2, C-2′, C-3, C-4, and C-10a) could be easily confirmed via HSQC spectrum.

The 2 very closely adjacent doublets of 8-H (7.13 ppm) and 6-H (7.15 ppm,) could be assigned beyond any doubt by ¹H-¹³C HMBC spectrum analysis (see Figure 4). The hydroxyl proton of 9-OH correlates with C-8 and therefore also determines C-6 and the shifts of the protons H-6 and H-8. Their HMBC correlations, ³J_C8,H6 and ³J_C6,H8, confirm these findings.

Figure 4.

700 MHz ¹H-¹³C HMBC Spectrum of Compound 13 in DMSO-d₆. The Correlation to Determine the Closely Adjacent Shifts C-6, C-8, H-6, and H-8 and the Quaternary Chemical Shifts of C-4a and C-9a are Highlighted in Blue Color.

Because of the sharp singlet shift of anomeric proton 10a-H, the quaternary carbon atom C-9a could be easily assigned on its ³J_C9a,H10a correlation as well with its further correlations with 6-H, 8-H and 9-OH) (³J_C9a,H6, ³J_C9a,8a, ³J_C9a,9OH,). Carbon C-4a could be determined in a similar way by two-bond correlation (²J_C4a,H10a, ²J_C4a,4OH).

The correlations for the 3 quaternary carbon atoms C-5a, C-7 and C-9 were not very pronounced in the HMBC-spectrum, therefore, a 700 MHz 1,1-ADEQUATE spectrum^31,32 of compound 13 in DMSO was included (see Figure 5). The ester group carrying carbon C-7 correlates with its neighboring phenolic protons 6-H and 8-H. Quaternary phenolic carbon C-5a correlates with neighboring proton 6-H and the hydroxyl group carrying carbon C-9 correlates with proton 8-H.

Figure 5.

Section of the 700 MHz ¹H-1,1-ADEQUATE Spectrum of Compound 13 in DMSO-d₆ for the Determination of Chemical Shifts of Quaternary Carbons C-5a, C-7, and C-9.

Compound 13 possesses gluco-configuration with a nearly chair conformation of the pyran ring (see Figure 6). This finding is substantiated by the observed large coupling constants of 2-H, 3-H and 4-H with J_2,3 = 9.90 and J_3,4 = 9.68 Hz and thus trans-arrangement of the benzoyloxymethyl and the 2 benzoyloxy substituents. The β-cis-configuration of the pyran and aromatic ring are clearly evidenced by the measured NOE's between the axial anomeric proton 10a-H with the axial pyran protons 2-H and 4-H. In line with this observation is the further NOE between10a-H and the hydroxyl proton of equatorial 4a-OH.

Figure 6.

Configuration and Conformation of Glycoside 13 Proved by NOE's and ¹J_C,H Coupling Constant and the Action of an Anomeric Effect.

Scheme 1.

Conversion of Methyl Gallate 4 to the Para- and Meta-Phenolates 5 and 7 by the Action of Triethylamine. In Contrast to 7, Phenolate 5 is Mesomerically Stabilized Due to its Para-Quinoid Resonance Structure 6.²⁶ For Clarity, the Benzoid and Para-Quinoid Structural Systems are Highlighted in Blue Color.

Scheme 2.

Base-Induced Glycosylation of Ulosyl Bromides 8, 9, and 10 with Methyl Gallates 4, 11, and 12 to the Tricyclic Phenolic Glycosides 13, 14, 15, 16, 17, 18, and 19. For Clarity, the Linear Fused Tricycle of Pyran, Dioxan and Phenyl Ring is Highlighted in Red Color.

Scheme 3.

Glycosylation of Ulosyl Bromide 8 with Methyl Gallate Acetonide 20 to the Ulosyl Intermediate 21 Followed by Benzoic Acid Elimination to Enolester 22. Subsequent Acid-Induced Cleavage of the Isopropylidene Group with Trifluoracetic Acid Generates Intermediate 23 which Cyclizes Under Simultaneous Benzoyl Group Migration to the Cercidin Analogue 24, as shown by the arrows.

Scheme 4.

Basic Elimination of (a) Tricycle 13 with Aqueous Tetrabutylammonium Acetate to the Cercidin Analogue 28 by Elimination of 2 Molecules of Benzoic Acid via the Open Chain Intermediates 25, 26, and 27 and of (b) Tricycle 17 in the Presence of Triethylamine in Water Free Dichloromethane via 25 and 26 to the Cercidin Analogue 29.

The magnitude of the anomeric coupling constant ¹J_C10a,H10a with 172.6 Hz also confirms the β-configuration of compound 13. Published literature shows that pyranoses with an axial anomeric proton possess a 10 Hz lower ¹J_C,H value than the corresponding α-anomer with equatorial proton.^33-35 This was confirmed in the α-configured product 15 which possessed a ¹J_C10a,H10a coupling of 180.2 Hz (see below). Due to the aromatic aglycon, the central 1,4-dioxan ring has boat conformation which is stabilized by an anomeric effect.

As all other compounds were analyzed by NMR spectroscopy in CDCl₃, we also performed an additional set of 700 MHz NMR spectra of compound 13 in CDCl₃. These confirmed the findings of the measurements in DMSO-d₆, even though the hydroxyl protons 4a-OH and 9-OH did not show much correlation in the 2D spectra with HMBC correlations near noise. For this reason, the HMBC correlations ²J_6H,C5a and ²J_8H,C9 have been used for the determination of the shifts 6H/C6 and 8H/C8, while the shifts of C-5a and C-9 have been determined by comparison with those measured in DMSO-d₆. This was possible due to their relatively large shift differences in both solvents: DMSO-d₆ C-5a (140.51), C-9 (146.26); CDCl₃ C-5a (140.73), C-9 (144.79 ppm).

The structural determination of the derivatives 14 to 19 was less demanding due to their similarity in NMR spectra to compound 13. Thus, the observed ³J HMBC correlation of the phenolic quaternary carbon atom C-9a with only one aromatic proton (7-H) evidenced the different regiochemistry of isomeric side products 14 and 15. The α-configuration of 15 resulted from the relative high anomeric coupling constant ¹J_C10,H10a with 180.2 Hz and the loss of an NOE between anomeric 10a-H and the pyran protons.

As expected, the open-chain glycoside 22, the precursor of compound 24, revealed 2 benzoyl groups in the ¹H NMR spectrum. The enolon structure of the pyran ring was confirmed by the 1H-doublet of proton 5'-H at low field (7.04 ppm). The ¹³C NMR chemical shift of the keto group C-3’ was found at 180.4 ppm. The further protons 2'-H, 6'-H and exocyclic 6′-CH₂OBz group had chemical shifts which were in accordance with structures 13 to 19. The anomeric coupling constant ¹J_C2’_,H2’ of 174.1 Hz confirmed the β-configuration of compound 22.

The structure of the derived product of 22, the cercidin analogue 24, was proved ¹H spectroscopically by the presence of 2 double doublets for the geminal protons 3-H₂, the 2 multiplets for 2-H and the exocyclic CH₂OBz group as well as the singlet of the anomeric proton 10a-H. The same structural features, but with separate well-resolved shifts for the geminal protons of the CH₂OBz group, also evidenced the structure of compound 28. The structure of compound 29 was proved by its well-resolved shifts for the 2-CH₂ and 3-CH₂ protons.

As expected in the ¹³C NMR spectra, the shifts of the 4-keto groups of the cercidin analogues 24, 28, and 29 were revealed at 192.64, 198.32, and 198.34 ppm, respectively.

Since the phenol moiety remained unchanged in the synthesis of 24, 28, and 29, its ¹H and ¹³C NMR data were in agreement with those of isomeric analogue 14 and the underlying starting compounds 13 and 17.

In summation: Cercidin analogues 13 to 19 were synthesized from ulosyl bromides 8, 9, 10 and methyl gallates 4, 11, 12 (Scheme 2). Methyl gallate acetonide 20 gave an open-chain product 22 which after cleavage of the isopropylidene group yielded 4-keto cercidin analogue 24 (Scheme 3). Keto group generation was also be achieved with the weak bases tetrabutylammonium acetate and triethylamine (13 → 28 and 17 → 29) (Scheme 4). The 700 MHz ¹H NMR spectra of the main compound 13 (Figure 3) had sharp shifts and the primary assignment of protons was easy. By two-dimensional correlation of ¹H-¹³C HMBC and ¹H-1,1-ADEQUATE spectra (Figure 4 and 5), all ¹H and ¹³C shifts could be determined unambiguously. NOEʹs and the ¹J_C,H coupling constant at the anomeric center (Figure 6) substantiated the results. The same spectroscopic procedures for structural elucidation were successfully applied to all other compounds.

Discussion

Here, the aromatic O-glycosylation of multivalent hydroxyl phenols on α-bromo-2-keto pyranoses is reported for the first time, which yielded 10 different structural analogues of cercidin 1.

Under basic reaction conditions, methyl gallate 4 reacted with the ulosyl bromides 8, 9, and 10 to the tricyclic glycosides 13, 16, and 17. A result, which was to be expected, since the para-positioned hydroxyl group in 4 is the most reactive in the O-glycosylation step. In the synthesis of main product 13, two side products, compound 14 and 15, could be isolated. Both were formed by O-glycosylation of one of the two meta-hydroxyl groups of methyl gallate 4.

Yields of up to 33% were obtained after recrystallization, which could be doubled by blocking one of the phenolic hydroxyl group in meta-position of aglycon 4. Thus, its methyl and isopropyl ether, compounds 11 and 12, gave with bromide 8 the glycosides 18 and 19 as colorless crystals in 73% and 57% yield, respectively.

Exclusive phenolic meta-glycosylation could readily be achieved by the reaction of methyl gallate acetonide 20 and bromide 8 to the open-chain glycoside 22 with enolone structure in the pyran moiety. Liberation of the 2 blocked phenolic hydroxyl groups, followed by cyclization and benzoyl group migration gave the crystalline glycoside 24. Compound 28, a regioisomer of 24, could be obtained directly from glycoside 13 by treatment with the weak base tetrabutylammonium acetate (66% yield). In this reaction, 2 benzoyl groups are saponified (25 → 26 → 27), followed by ring closure and pyran keto group generation (27 → 28). Compound 17 with a missing CH₂OBZ group is already partially converted to ketone 29 during its synthesis in the presence of triethylamine via intermediate 25 and 26.

The main product 13 was analysed by one- and two-dimensional 700 MHz ¹H and ¹³C NMR spectra. With the aid of NOE, HMBC and ADEQUATE spectra, it was possible to ascertain its structure, configuration, and conformation. The obtained structural findings for compound 13 then facilitated the structural elucidation of the cercidin analogues 14 to 19 as well as 24, 28 and 29. Their melting points, rotations and NMR data of the central 1,4-dioxan ring are summarized in Table 1.

Table 1.

Melting Points, Rotations (CHCl₃) and NMR Data (CDCl₃) of the Central 1,4-dioxan Ring of Cercidin Analogues 13 to 19 as Well as 24, 28, and 29.

	m.p (°C)	[α]_D²⁰	10a-H	C-4a	C-5a	C-9a	C-10a	¹ J _C10a,H10a
13^a	182-183	-19.3	5.54s	91.37	140.73	131,84	93.46	172.6
14^b	135-137	-23.9	5.45s	91.86	131.52	139.65	93.54	172.6
15^b	130-132	+30.5	5.40s	92.99	132.63	140.14	94.72	180.2
16^c	113-115	-82.2	5.43s	91.59	139.55	132.18	93.48	172.7
17^c	111-113	-104.0	5.35s	91.27	139.54	132.00	93.85	172.7
18^b	217-218	-26.2	5.54s	91.18	139.78	133.47	93.48	171.8
19^b	172-174	-24.8	5.53s	91.08	139.92	134.52	93.39	171.6
24^c	103-105	-42.8	5.92s	94.31	covered	139.03	92.60	174.1^d
28^c	amorph.	-24.8	5.26s	90.36	139.59	131.94	94.26	174.0
29^c	133-135	+2.3	5.18s	90.92	139.55	133.85	93.35	173.9

700 MHz, ^b500 MHz, ^c300 MHz, ^dvalue of the precursor 22; chemical shifts in ppm; coupling constants in Hz. In the literature, the only NMR shifts reported for cerdidin 1 are listed without any assignments.¹²

All compounds have well defined melting points, negative rotations and β-configuration. Only glycosides 15 and 29 have a positive rotation, compound 15 has α-configuration, and compound 28 is amorphous. The measured ¹J_C10a,H10a coupling constants are in accordance with β-configuration (171.6 to 172.7 Hz) and α-configuration (180.2 Hz).

Consistently, a sharp singlet is found for the anomeric proton 10a-H and the chemical shifts of the quaternary carbons C-4a, C-5a, C-9a, and C-10a are very close. For cercidin analogues 14, 15 and 24, the C-5a and C-9a values are inverted, due to reversed pyran and benzene ring linkage.

All glycosides described above are novel and structurally related to cercidin 1, a natural antimicrobial substance isolated from the leaves of Cercidiphyllum japonicum.

Conclusion

This study reports the total synthesis of the tricyclic pyran-dioxan-benzene ring framework of cercidin 1 for the first time. Aromatic O-glycosylation of multivalent hydroxyl phenols on α-bromo-2-keto pyranoses yielded 10 novel structural analogues of 1, whose configuration and conformation were resolved via ¹H and ¹³C NMR analyses. As cercidin 1 has antimicrobial activity, future structure-activity-relationship studies on these novel cercidin analogues are warranted. If these will identify analogues with similar bioactive properties, this would enable the development of novel cercidin-based antibiotics.

Supplemental Material

sj-docx-1-npx-10.1177_1934578X251346329 - Supplemental material for Synthesis of Analogues of Cercidin, an Antimicrobial Compound of Cercidiphyllum japonicum: Glycosylation of Methyl Gallate with Ulosyl Bromides

Supplemental material, sj-docx-1-npx-10.1177_1934578X251346329 for Synthesis of Analogues of Cercidin, an Antimicrobial Compound of Cercidiphyllum japonicum: Glycosylation of Methyl Gallate with Ulosyl Bromides by Eckehard Cuny in Natural Product Communications

Supplemental Material

sj-docx-2-npx-10.1177_1934578X251346329 - Supplemental material for Synthesis of Analogues of Cercidin, an Antimicrobial Compound of Cercidiphyllum japonicum: Glycosylation of Methyl Gallate with Ulosyl Bromides

Supplemental material, sj-docx-2-npx-10.1177_1934578X251346329 for Synthesis of Analogues of Cercidin, an Antimicrobial Compound of Cercidiphyllum japonicum: Glycosylation of Methyl Gallate with Ulosyl Bromides by Eckehard Cuny in Natural Product Communications

Supplemental Material

sj-docx-3-npx-10.1177_1934578X251346329 - Supplemental material for Synthesis of Analogues of Cercidin, an Antimicrobial Compound of Cercidiphyllum japonicum: Glycosylation of Methyl Gallate with Ulosyl Bromides

Supplemental material, sj-docx-3-npx-10.1177_1934578X251346329 for Synthesis of Analogues of Cercidin, an Antimicrobial Compound of Cercidiphyllum japonicum: Glycosylation of Methyl Gallate with Ulosyl Bromides by Eckehard Cuny in Natural Product Communications

Supplemental Material

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Supplemental material, sj-docx-4-npx-10.1177_1934578X251346329 for Synthesis of Analogues of Cercidin, an Antimicrobial Compound of Cercidiphyllum japonicum: Glycosylation of Methyl Gallate with Ulosyl Bromides by Eckehard Cuny in Natural Product Communications

Footnotes

Acknowledgments

The author thanks Prof. Dr Michael Reggelin for the opportunity to work in his group, and Dr Schmidts and Dr Fohrer for the measurements of the 700 and 500 MHz NMR spectra and for helpful discussions.

ORCID iD

Eckehard Cuny

Ethical Considerations

Ethical approval is not applicable for this article because it does not contain any studies with human subjects or animals.

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There are no human subjects in this article and informed consent is not applicable.

Funding

The author received no financial support for the research, authorship, and/or publication of this article.

Declaration of Conflicting Interests

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Statement of Human and Animal Rights

Not applicable because this article does not contain any studies with human subjects or animals.

Supplemental Material

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