2-Cyano-6-hydroxybenzothiazole

2-Cyano-6-hydroxybenzothiazole (CBTOH) is a critical intermediate in D-luciferin biosynthesis, also used for its chemical synthesis and as luciferin bioluminescence precursor for chemical analysis. Firefly light emission involves the enzymatic oxidation of D-luciferin to yield electronically excited oxyluciferin, which is the bioluminescent molecule. Oxyluciferin is afterwards enzymatically oxidized to thioglycolic acid and CBTOH, which regenerates luciferin by condensation with cysteine[1].

2-Cyano-6-hydroxybenzothiazole serves as a synthetic precursor for firefly luciferin via condensation with cysteine, and acts as an enhancer of light emission in the luminol/hydrogen peroxide system[3][4].

A new synthesis of 2-cyano-6-hydroxybenzothiazole has been realized starting from the reaction of 1,4-benzoquinone with l-cysteine ethyl ester, followed by oxidation-cyclization of the intermediate ethyl (R)-2-amino-3-(2,5-dihydroxyphenylsulfanyl)propan-oate hydrochloride to 2-carbethoxy-6-hydroxybenzothiazole. A suitable protection of this intermediate and conversion to the corresponding nitrile gave, after deprotection, 2-cyano-6-hydroxybenzothiazole (32% yield from 1,4-benzoquinone)[2].

[1] Ankita S. Jadhav . “Firefly luciferin precursor 2-cyano-6-hydroxybenzothiazole: Fluorescence à la carte controlled by solvent and acidity.” Dyes and Pigments 177 (2020): Article 108285.
[2] G. Meroni. “A new synthesis of 2-cyano-6-hydroxybenzothiazole, the key intermediate of D-luciferin, starting from 1,4-benzoquinone.” Synlett 29 1 (2009): 2682–2684.
[3] SHAHMORADI, G., AMANI, S. (2018). Synthesis, spectroscopic characterization, and computational studies of 2-cyano-6-hydroxybenzothiazole: a key synthetic intermediate of firefly luciferin. TURKISH JOURNAL OF CHEMISTRY, 42(6), 1499–1517. https://doi.org/10.3906/kim-1801-76
[4] Thorpe, G. H., Kricka, L. J., Gillespie, E., Moseley, S., Amess, R., Baggett, N., Whitehead, T. P. (1985). Enhancement of the horseradish peroxidase-catalyzed chemiluminescent oxidation of cyclic diacyl hydrazides by 6-hydroxybenzothiazoles. Analytical Biochemistry, 145(1), 96–100. https://doi.org/10.1016/0003-2697(85)90332-x