Reticulatins A and B and hyrtioreticulin F from the marine sponge Hyrtios reticulatus

Three new alkaloids, reticulatins A (1) and B (2) and hyrtioreticulin F (3), were isolated from the water-soluble fraction of an EtOH extract of the marine sponge Hyrtios reticulatus. Compounds 1 and 2 were found to be novel 1,3-dimethyl-5-(methylthio)imidazolium alkaloids. The structure of 3 was determined to be an indole alkaloid related to hyrtioreticulin E (8) and hyrtioerectine B (9), which were isolated previously from the n-BuOH-soluble fraction of the sponge extract. The presence of three NH units in 3 was indicated by the 1H–15N HSQC spectrum in D2O/H2O (5:95) containing 0.05% TFA measured at 5 °C. Compound 3 is likely biosynthesized from l-tryptophan, two units of l-alanine, and glycine by the Pictet–Spengler reaction. The absolute configurations of 1–3 were determined by the methods based on ECD spectra. In addition, the result for 1 was further confirmed by the PGME method.