Preparation of Fluorescent Materials from Biomass-Derived Furfural and Natural Amino Acid Cysteine through Cross-Coupling Reactions for Extended π-Conjugation

Dedicated to Professor Peter Vollhardt for his great contribution to Synlett

Abstract

Preparation of 2-furylthiazole-4-carboxylic acid methyl ester is achieved in four steps from biomass-derived heteroaromatic compound furfural and a natural amino acid l-cysteine. One-pot bromination and following palladium-catalyzed arylation with arylboronates of the thus obtained furylthiazole at the furan ring gives arylated furylthiazole in excellent yields. Further arylation at the C–H bond of the thiazole ring (5-position) in the presence of AgF as an additive leads to di­arylated furylthiazoles, which show strong photoluminescence. Homocoupling at the C–H bond of thiazole is also carried out with AgF to afford the corresponding further conjugated product composed of eight (hetero)aromatic rings.

Key words

furfural - l-cysteine - cross-coupling - π-conjugation - photoluminescence

• References and Notes

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• 14 Preparation of 2-Furylthiazole-4-carboxylic Acid Methyl Ester (3) To a solution of 2-cyanofuran (4) in MeOH–H₂O (2:1, 43 mL) were added l-cysteine (1.82 g, 15 mmol) and K₂CO₃ (2.07 g, 15 mmol). The solution warmed to 60 °C and stirred for 21.5 h under nitrogen atmosphere. After cooling to r.t., the reaction mixture was diluted with MeOH, and the solution was concentrated under reduced pressure to leave a crude solid, which was purified by short column chromatography on silica gel (MeOAc) to afford furylthiazoline carboxylic acid (5) as orange crude solid. To a solution of the crude solid of 5 in DMF (100 mL) were added K₂CO₃ (4.14 g, 30 mmol) at r.t. under an nitrogen atmosphere. After cooling to 0 °C, MeI (1.87 mL, 30 mmol) was added dropwise. After stirring for 1.5 h at 0 °C, the mixture was quenched by H₂O, and the solution was poured into the mixture of Et₂O–H₂O to result in separation into two phases. The aqueous phase was extracted with Et₂O repeatedly, and the combined organic layer was dried over anhydrous Na₂SO₄ and concentrated under reduced pressure to leave a crude oil, which was purified by column chromatography on silica gel (hexane–MeOAc, 3:1) to afford 1.33 g of furylthiazoline carboxylic acid methyl ester (5′, 63%). To a 20 mL Schlenk tube equipped with a magnetic stirring bar were added 5′ (105.6 mg, 0.5 mmol) and toluene (1.5 mL). To the solution was added activated carbon (105.6 mg, 100 wt%), and stirring was continued at 100 °C for 22.5 h under oxygen atmosphere. After cooling to r.t., the mixture was diluted with CHCl₃ and passed through a Celite pad, which was washed with CHCl₃ repeatedly. The filtrate was concentrated under reduced pressure to leave the crude solid, which was purified by column chromatography on silica gel to afford 98.3 mg of 3 as a yellow solid (94%). The reaction was also performed in a larger scale under similar conditions with 5′ (2.58 g, 12.2 mmol) and activated carbon (12.2 g, 100 wt%) in toluene (40 mL) to afford 1.51g of 3 (59% yield); mp 88.6–89.5 °C. ¹H NMR (300 MHz, DMSO-d ₆): δ = 3.97 (s, 3 H), 6.56 (dd, J = 1.8, 3.5 Hz, 1 H), 7.17 (dd, J = 0.7, 3.5 Hz, 1 H), 7.53 (dd, J = 0.7, 1.8 Hz, 1 H), 8.14 (s, 1 H). ¹³C NMR (125 MHz, DMSO-d ₆): δ = 53.1, 111.3, 113.8, 129.3, 146.4, 147.5, 148.4, 158.6, 162.0. IR (ATR) 3143, 3117, 3103, 1729, 1623, 1594, 1505, 1482, 1463, 1436, 1345, 1258, 1231, 1212, 1159, 1103, 1026, 1006, 978, 879, 859, 840, 775, 751, 619 cm^–1. HRMS (ESI⁺): m/z calcd for C₉H₇NO₃SNa [M + Na]⁺: 232.0044; found: 232.0045.