Synthetic Study on Carthamin, Part 4. Improved Synthesis of a C-Glycosyl Quinochalcone by Installation of a Side Chain through Regioselective De-O-methylation and Acyl Rearrangement

Seiya Matsuoka; Kohei Azami; Yusuke Fujiki; Reina Dohi; Tomokazu Yasuike; Ken Ohmori; Keisuke Suzuki

doi:10.1055/s-0040-1719836

Synlett, Table of Contents

Synlett 2021; 32(20): 2046-2052
DOI: 10.1055/s-0040-1719836

letter

Synthetic Study on Carthamin, Part 4. Improved Synthesis of a C-Glycosyl Quinochalcone by Installation of a Side Chain through Regioselective De-O-methylation and Acyl Rearrangement

Seiya Matsuoka

^aDepartment of Chemistry, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan

,

Kohei Azami

^aDepartment of Chemistry, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan

,

Yusuke Fujiki

^aDepartment of Chemistry, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan

,

Reina Dohi

^aDepartment of Chemistry, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan

,

Tomokazu Yasuike

^bDepartment of Liberal Arts, The Open University of Japan, 2-11 Wakaba, Mihama-ku, Chiba 261-8586, Japan

^cElements Strategy Initiative for Catalysis and Batteries, Kyoto University, 1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8520, Japan

,

Ken Ohmori ∗

^aDepartment of Chemistry, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan

,

Keisuke Suzuki ∗

^aDepartment of Chemistry, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan

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Abstract

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In memory of the late Professor Daisuke Uemura.

Abstract

We report an improved synthesis of a C-Glycosyl quinochalcone that is a key intermediate in our total synthesis of carthamin, a natural red pigment of traditional heritage. The C-glycosyl quinochalcone is prepared by regioselective de-O-methylation of a C-glycosyl bromodienone, and installation of a p-coumaroyl side chain through an O→C acyl rearrangement.

Key words

carthamin - glycosides - chalcones - natural products - pigments

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References

References and Notes
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20 C-Glycosyl Quinochalcone 2 To a solution of 29 (90.0 mg, 0.101 mmol) in THF (4.0 mL) 0.5 M aq NaOH (4.0 mL) was added at 0 °C, and the mixture was stirred for 3 h at 0 °C. The reaction was then stopped by addition of EtOAc and 1.0 M aq HCl. The crude products were extracted with EtOAc (×3), and the combined organic extracts were washed with brine, dried (Na₂SO₄), and concentrated in vacuo to afford the crude product 30, which was used in the next step without further purification. To a solution of the crude material in CH₂Cl₂ (1.1 mL) was added Boc₂O (39.7 mg, 0.182 mmol) and DMAP (1.2 mg, 9.1 mmol), and the mixture was stirred for 1 h at RT. The mixture was then concentrated in vacuo, and the residue was purified by preparative TLC [silica gel, hexane–acetone (6:4)] to afford the C-glycosyl quinochalcone 2 as a yellow amorphous material; yield: 57.9 mg (59%); mp 126–128 °C; [α]_d ²⁰ –94.2 (c 1.12, CHCl₃); R_f = 0.34 (hexane–acetone, 6:4). IR (ATR): 3603, 2981, 2938, 1759, 1672, 1529, 1510, 1370, 1281, 1256, 1149, 854, 754 cm^–1. ¹H NMR (600 MHz, CDCl₃): δ = 1.41 (s, 9 H), 1.44 (18 H, overlapped), 1.47 (s, 9 H), 1.56 (s, 9 H), 3.06 (d, J = 2.1 Hz, 1 H), 3.63 (ddd, J = 9.8, 6.3, 3.1 Hz, 1 H), 3.73 (d, J = 9.5 Hz, 1 H), 3.86 (s, 3 H), 3.97–4.04 (3 H, overlapped), 4.61 (dd, J = 9.8, 9.6 Hz, 1 H), 4.87 (dd, J = 9.6, 9.4 Hz, 1 H), 5.62 (s, 1 H), 7.21 (d, J = 8.6 Hz, 2 H), 7.65 (d, J = 8.6 Hz, 2 H), 7.88 (d, J = 15.8 Hz, 1 H), 8.09 (d, J = 15.8 Hz, 1 H), 18.49 (s, 1 H). ¹³C NMR (150 MHz, CDCl₃): δ = 27.7 (3 C), 27.78 (3 C), 27.83 (6 C), 27.9 (3 C), 57.0, 65.4, 70.2, 71.3, 75.9, 78.44, 81.9, 82.4, 82.8, 83.2, 84.1, 84.4, 100.8, 107.3, 121.8 (2 C), 122.4, 130.2 (2 C), 132.8, 144.2, 150.8, 151.4, 152.3, 152.9, 153.1, 153.2, 169.7, 185.1, 189.9, 191.6 (one signal missing, possibly due to overlapping). HRMS (ESI): m/z [M – H]^– calcd for C₄₇H₆₃O₂₁: 963.3867; found: 963.3844.

Supplementary Material

Supporting Information