Structure and spectral properties of β-carbolines. Part 4. Synthesis of the new ring system: 9,10,15,15b-tetrahydroindolo[1′,2′ : 4,3]pyrazino[2,1-a]-carbolin-7(6H)-one
1-Indol-2-yl-1,2,3,4-tetrahydro-β-carboline and its 5-methoxy derivative were synthesized by the Pictet-Spengler reaction between tryptamine or 5-methoxytryptamine and indole-2-carbaldehyde, with a high yield. A step-by-step oxidation of the indolyl carbolines led to formation of the appropriate dihydrocarboline derivatives and two analogues of neoeudistomine, the fully unsaturated carbolines. Condensation of the indolyltetrahydrocarbolines with bromoacetyl chloride yielded the appropriate 2-substituted derivatives, which were cyctized to yield the new ring system compounds 9,10,15,15b-tetrahydroindolo[1′,2′ : 4,3]pyrazino[2,1-a]carbolin-7(6H)-ones, the structures of which were determined by using a high-resolution 1H NMR technique.