Add time:08/08/2019         Source:sciencedirect.com

The acid-catalyzed Pictet–Spengler reaction of d-tryptophan methyl ester with piperonal in acetic acid has been reported, the best stereoselectivity (cis/trans = 92:8) was obtained with benzoic acid as the catalyst. The Pictet–Spengler reaction of d-tryptophan methyl ester hydrochloride with piperonal in various solvents has been extensively studied, the solvent-dependence of stereoselectivities could be principally attributed to the solubility-difference between cis and trans products 5-HCl in the used solvent, the best stereoselectivity (cis/trans = 99:1) was obtained using nitromethane or acetonitrile as the solvent. A base-catalyzed epimerization at 12a-position of Cialis 1 (tadalafil) in a DMSO-containing solvent was also exploited. Cialis, 12a-epi-Cialis 2, deuterium-labeled 3,3,12a-d3-Cialis 3, and 3,3,12a-d3-12a-epi-Cialis 4 were efficiently synthesized from d-tryptophan methyl ester hydrochloride.

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