Synthesis of 7-¹⁵N-Oroidin and Evaluation of Utility for Biosynthetic Studies of Pyrrole−Imidazole Alkaloids by Microscale ¹H−¹⁵N HSQC and FTMS

Numerous marine-derived pyrrole−imidazole alkaloids (PIAs), ostensibly derived from the simple precursor oroidin, 1a, have been reported and have garnered intense synthetic interest due to their complex structures and in some cases biological activity; however very little is known regarding their biosynthesis. We describe a concise synthesis of 7-¹⁵N-oroidin (1d) from urocanic acid and a direct method for measurement of ¹⁵N incorporation by pulse labeling and analysis by 1D ¹H−¹⁵N HSQC NMR and FTMS. Using a mock pulse labeling experiment, we estimate the limit of detection (LOD) for incorporation of newly biosynthesized PIA by 1D ¹H−¹⁵N HSQC to be 0.96 μg equivalent of ¹⁵N-oroidin (2.4 nmole) in a background of 1500 μg of unlabeled oroidin (about 1 part per 1600). 7-¹⁵N-Oroidin will find utility in biosynthetic feeding experiments with live sponges to provide direct information to clarify the pathways leading to more complex pyrrole−imidazole alkaloids.