Regioselectivity of Pictet–Spengler cyclization reactions to construct the pentacyclic frameworks of the ecteinascidin–saframycin class of tetrahydroisoquinoline antitumor antibiotics

The regiochemical outcome of Pictet–Spengler cyclization reactions directed toward the preparation of the pentacyclic core of the ecteinascidin class of antitumor antibiotics has been investigated on two different phenolic substrates. In one substrate, the assistance of an incipient benzylamine group at C-4 is postulated to direct the cyclization in favor of the pentacyclic framework of ET-743, which bears a hydroxyl group at C-18. Conversely, cyclization of an alternative substrate lacking a heteroatom at C-4 favors the opposite regiochemical outcome, primarily affording an unnatural pentacyclic core bearing a hydroxyl group at C-16.