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General method: GP3-1: A solution of 5-nitrobenzothiazole-2-thiol (1.0 eq.) with potassium ethylxanthate (1.2 eq. or 2.2 eq., 2.2 eq. is usually used) in 10 volumes of anhydrous DMF was heated for 4 hr. at 100 °C or 120 °C in a nitrogen atmosphere. The reaction progress was monitored by TLC. Upon completion of the reaction, the mixture was cooled to room temperature, diluted with water (10 volumes), and neutralized with 1 M HCl solution to pH 5. The precipitate formed was collected by filtration, washed with water, and dried using a rotary evaporator followed by vacuum drying.GP3-2: Thionyl chloride (SO2Cl2, 1 volume) was slowly added to a mixture of 2-mercaptobenzothiazolyl chloride (2.2 eq.) containing 2-mercaptobenzothiazole (1.0 equivalent) in 2 volumes of anhydrous DCM solution. The mixture was stirred at room temperature for 1 h. The reaction progress was monitored by TLC. After the raw materials were consumed, the reaction mixture was diluted with 30 volumes of ether and the reaction was quenched by careful addition of water. Stirring was continued for 1 hour to ensure complete consumption of thionyl chloride and product release. The organic layer was separated, neutralized with saturated NaHCO3 solution, dried over Na2SO4 and subsequently purified by silica gel column chromatography to give the pure product. The final product was characterized by LC-MS and NMR.

[1] Journal of Heterocyclic Chemistry, 2005, vol. 42, # 4, p. 727 - 730
[2] Bioorganic and Medicinal Chemistry, 2015, vol. 23, # 24, p. 7661 - 7670
[3] Patent: WO2016/207785, 2016, A1. Location in patent: Page/Page column 45; 46