Synthesis of Marine Sponge Alkaloids Oroidin, Clathrodin, and Dispacamides. Preparation and Transformation of 2-Amino-4,5-dialkoxy-4,5-dihydroimidazolines from 2-Aminoimidazoles

The preparation and transformation of 2-amino-4,5-dialkoxy-4,5-dihydroimidazolines A from 2-aminoimidazoles (AIs) is described. The oxidation of 2-aminoimidazole 8 with NCS in methanol affords cyclic guanidine adduct 9 which, upon heating, affords vinylogous AI derivative 3 and 2-aminoimidazolinone (glycocyamidine) 13 . Olefin 3 comprises the core structure found in the oroidin alkaloids. Furthermore, oxidation of 8 with Br 2 and DMSO affords directly α,β-unsaturated imidazolinone 14 which is the key structural unit comprising the dispacamides ( 2 ). A highly facile and practical synthesis of the C 11 N 5 marine sponge alkaloids oroidin ( 1a ), clathrodin ( 1c ), and dispacamides ( 2 ) is outlined.