Oxidative Dimerization of 2-Aminoimidazoles by Molecular Bromine. Synthesis of Parazoanthoxanthin A

摘要

Zoanthoxanthins are a collective group of highly fluorescent marine pigments isolated from marine coelenter-ates which are closely related to sea anemones and stony corals. The linear parazoanthoxanthin A (1) and angular pseudozoanthoxanthin A (2) form the basis of this group of tricyclic pigments in which a number of N-methylated derivatives have been identified to date. All contain a bis-fused 2-aminoimidazole (AI) nucleus that embodies a ten-electron azulene ring system. Both parazoanthoxanthin A (1) and pseudozoanthoxanthin A (2) have been previously synthesized. The cleverly crafted synthesis by Buchi was the first to demonstrate that 1 and 2 can be obtained in a biomimetic fashion from the acid-promoted dimerization of various C_5N_3 precur-sors, namely AI derivatives 3—5. From subsequent research in our lab, a one-step synthesis of 1 and 2 was accomplished using the C_3N_3 metabolite, 2-aminoimidazole (AI). This forerunner served as an immediate precursor to the Buchi intermediate. The novel feature of the synthesis involved the facile addition of aldehydes to AI via an aldol or Mannich type process. In this report, we describe the oxidative dimerization of 2-aminoimidazoles by molecular bromine. In particular, oxidative dimerization of a C_5N_3 precursor produced the tricyclic C_(10)N_6 marine pigment, parazoanthoxanthin A (1).