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 8-aminoimidazolinone (glycocyamidine) 13. Olefin 3 comprises the core structure found in the oroidin alkaloids. Furthermore, oxidation of 8 with B-2 and DMSO affords directly alpha,beta-unsaturated imidazolinone 14 which is the key structural unit comprising the dispacamides (2). A highly facile and practical synthesis of the C11N5 marine sponge alkaloids oroidin (la), clathrodin (1c), and dispacamides (2) is outlined.