A one-pot synthesis of 4-aryl-2-methyl-1,2,3,4-tetrahydro-γ-carbolines from 5-aryloxazolidines and indoles via a Mannich/Friedel–Crafts sequence

Benzaldehydes react smoothly with the nonstabilized azomethine ylide derived from sarcosine and formaldehyde to form 5-aryloxazolidines, which undergo ring-opening to give 2-(indol-3-ylmethylamino)-1-arylethanols in 69–79% yields on reaction with indoles in acetic acid. Their subsequent acid-catalyzed cyclization into 4-aryl-2-methyl-1,2,3,4-tetrahydro-γ-carbolines was performed in polyphosphoric acid in moderate yields. The latter can also be prepared directly from 5-aryloxazolidines and indoles in polyphosphoric acid.