Microwave-Assisted Synthesis of Tetrahydro-β-carbolines and β-Carbolines

A microwave-mediated Pictet-Spengler procedure utilizing 1,2-dichloroethane (DCE) and trifluoroacetic acid (TFA) was developed to provide tetrahydro--carboline salts in high yields. Reactions are complete in 20 minutes or less and the product precipitates from solution in high yields and purity, negating the need for liquid-liquid extraction or column chromatography. This method tolerates a wide range of functionality and can be performed on milligram to gram scales. A subsequent microwave-mediated aromatization of the synthesized tetrahydro--carbolines to -carbolines was also developed utilizing catalytic Pd/C. The aromatization is complete in 60 min or less with most substrates requiring minimal purification.