Solvent-dependent regio- and stereo-selective reactions of 3-formylchromones with 2-aminobenzothiazoles and transacetalization efficiency of the product 3-((benzo[d]thiazol-2-ylimino)butyl)-4H-chromen-4-one |
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Authors: | Owais Ahmed Vandana Cherkadu Praveen Kumar Kalavagunta Jing Shang |
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Institution: | School of Life Sciences and Technology, China Pharmaceutical University, Nanjing 211198 China ; State Key Laboratory of Natural Medicines, Jiangsu Key Laboratory of TCM Evaluation and Translational Research, School of Traditional Chinese Pharmacy, China Pharmaceutical University, Nanjing 211198 China.; R&D Centre, CJEX Biochem, 7&8, 2nd Cross, Muniswamappa Layout, Hosur Road, Bommanahalli, Bangalore 560 068 India |
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Abstract: | Using 2-propanol as the solvent, 3-formylchromones and 2-aminobenzothiaoles formed corresponding imines, while 1° and 2°-alcohols formed the corresponding 2-alkoxy-3-enamines with selectivity for the Z-isomer. Changing the substrates with similar molecules such as 3-formylchromone with quinoline-, quinolone- and indole-3-carbaldehydes sometimes resulted in the formation of the corresponding imines, whereas replacing 2-aminobenzothiazole with amides resulted in the formation of acetals. Considering the effect of the solvent, replacing alcohols with the aprotic solvents THF and CH2Cl2 resulted in the formation of imines and enamines, which are the characteristic reactions of 2-propanol and other 1° and 2°-alcohols, respectively. 2-Alkoxy-3-enamines were found to undergo transacetalization with both short and long chain alcohols. The novelty of these reactions is that they did not require an external catalyst, all the reactions were performed at the same temperature, and purification was achieved by filtration. The transacetalization we performed herein is a new concept, which has not been reported to date. In contrast, other similar reactions, such as transalkoxylation, transalkylation, and transetherification, are performed on a commercial scale using expensive catalysts such as Otera''s catalyst. The highly sensitive nature of 3-formylchromones towards variations in the substrates and solvents to form different products and the reason behind the selective formation of the Z-isomer of 2-alkoxy-3-enamines and its transacetalization efficiency need further studies to understand the reaction mechanism and possibly other factors such as solvent effects.2-Propanol forms lmine (4), other alcohols forms enamine (3) which can undergo transacetalization and replacing 2 with amides forms acetals (5). |
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