Intramolecular Hetero Diels–Alder Routes to γ-Carboline Alkaloids

Concise and efficient routes to the carboline alkaloids isocanthine (3), isocanthin-6-one (4), 1-methylisocanthine (5), and 1-methylisocanthin-6-one (6) are reported. In each case, the key synthetic step was an intramolecular hetero Diels–Alder reaction of a 1-aza-1,3-diene with an acetylenic dienophile.