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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
a   Department of Chemistry, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan
,
Kohei Azami
a   Department of Chemistry, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan
,
Yusuke Fujiki
a   Department of Chemistry, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan
,
Reina Dohi
a   Department of Chemistry, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan
,
Tomokazu Yasuike
b   Department of Liberal Arts, The Open University of Japan, 2-11 Wakaba, Mihama-ku, Chiba 261-8586, Japan
c   Elements Strategy Initiative for Catalysis and Batteries, Kyoto University, 1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8520, Japan
,
Ken Ohmori  ∗
a   Department of Chemistry, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan
,
Keisuke Suzuki  ∗
a   Department of Chemistry, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan
› Author AffiliationsThis work was supported by Grants-in-Aid from the Japan Society for the Promotion of Science (JSPS) (Grants JP16H06351, JP16H04107, JP18H04391, JP21H04703) and by the Nagase Science Technology Foundation.
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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

Supporting Information

    Supporting information for this article is available online at https://doi.org/10.1055/s-0040-1719836.
  • Supporting Information


Publication History

Received: 02 August 2021

Accepted after revision: 20 August 2021

Article published online:
21 September 2021

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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); Rf = 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.